551 ACUS11 KWNS 200019 SWOMCD SPC MCD 200019 NEZ000-KSZ000-200415- MESOSCALE DISCUSSION 2178 NWS STORM PREDICTION CENTER NORMAN OK 0619 PM CST WED DEC 19 2012 AREAS AFFECTED...NWRN AND N-CNTRL KS / S-CNTRL NEB CONCERNING...BLIZZARD VALID 200019Z - 200415Z SUMMARY...A CESSATION OF THE HEAVIEST SNOWFALL RATES IS EXPECTED TO OCCUR FIRST OVER NWRN KS AND ADJACENT PARTS OF SWRN NEB DURING THE 00-03 UTC PERIOD WITH SNOWFALL RATES ABATING SUBSTANTIALLY DURING THE 03-06 UTC PERIOD FARTHER E OVER MAINLY S-CNTRL NEB AND FAR N-CNTRL KS. BLIZZARD CONDITIONS SHOULD CONTINUE INTO THE EVENING HOURS ACROSS THE DISCUSSION AREA...OWING TO REDUCED VISIBILITIES AND FREQUENT WIND GUSTS AOA 35 MPH. DISCUSSION...SURFACE MESOANALYSIS PLACES A 996MB LOW ALONG THE OK/KS BORDER 40 MI SSW ICT. AS THE UPPER TROUGH AND ASSOCIATED DEEP LAYER FORCING FOR ASCENT PIVOT EWD FROM NEAR THE CO/KS/OK BORDER TO E-CNTRL KS BY 06 UTC...THE DEEPENING/DEVELOPMENT OF THE DEEP LAYER CYCLONE WILL OCCUR FROM NEAR ITS CURRENT POSITION ALONG THE KS/OK BORDER NEWD TO NWRN MO/NERN KS BY 06 UTC. AS THIS OCCURS...EXPECT THE HEAVIEST SNOWFALL RATES TO RESIDE FROM NWRN KS /0.50-LOCALLY 1 INCH PER HR FROM 00-02 UTC/ TO S-CNTRL NEB /1-1.5 INCH PER HR FROM 00-06 UTC/ AND CORRESPONDINGLY ABATING IN A GRADUAL MANNER IN A GENERAL WSW-ENE DIRECTION. IN THE MEANTIME...THE HEAVIEST SNOWFALL RATES WILL LIKELY EXIST DURING THE NEXT FEW HOURS IN THE HSI-LNK CORRIDOR WHERE A REDUCTION IN STATIC STABILITY IS GREATEST IN ASSOCIATION WITH A MID-LEVEL DRY SLOT IMPINGING TO THE S IN NERN KS. IT IS WITHIN THIS CORRIDOR THAT ISOLD LIGHTNING STRIKES HAVE BEEN DETECTED...SIGNALING WHERE CONVECTIVE ENHANCEMENT MAY LOCALLY INCREASE SNOWFALL RATES. BRISK NLY WIND GUSTS FREQUENTLY AOA 35MPH AND LOW VISIBILITIES WILL OCCUR THROUGH THE EVENING /ESPECIALLY IN THE HEAVIEST SNOW BURSTS/. ..SMITH.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...OAX...GID...LBF...GLD... LAT...LON 39730134 40919927 41299782 41079706 40449686 40149746 39759912 39140112 39730134  010 ACUS01 KWNS 200102 SWODY1 SPC AC 200100 DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0700 PM CST WED DEC 19 2012 VALID 200100Z - 201200Z ...THERE IS A SLGT RISK OF SVR TSTMS FROM THE OZARK PLATEAU TO THE ARKLATEX AND LOWER MS VALLEY... ...OZARK PLATEAU...ARKLATEX...LOWER MS VALLEY... INCREASING FORCING FOR ASCENT THROUGH THE COMBINATION OF A STRENGTHENING COLD FRONT AND MID-LEVEL DCVA ENCOUNTERING THE WRN EDGE OF A RETURNING LOW-LEVEL MOISTURE AXIS /CHARACTERIZED BY 60S SURFACE DEW POINTS FROM NEAR FSM SWD/ WILL YIELD SWATHS OF CONVECTION THIS EVENING INTO EARLY THU. LATEST SURFACE OBSERVATIONS AND 00Z FWD RAOB APPEAR SUPPORTIVE OF 18Z OPERATIONAL GUIDANCE AND THE 21Z HRRR WITH CONVECTION LIKELY TO INITIATE FARTHER WEST THAN IN 12Z CONVECTION-ALLOWING MODELS. AS SUCH...PRIMARY CHANGE TO THIS OUTLOOK IS TO EXPAND TSTM/SLIGHT RISK PROBABILITIES WWD. ALTHOUGH SBCAPE WILL REMAIN WEAK...THE STRENGTH OF THE FORCING SHOULD RESULT IN CONVECTION EVOLVING INTO A LARGER-SCALE BROKEN BAND/QLCS THAT CONTINUES EWD TO ROUGHLY A SERN MO TO SABINE RIVER LINE BY 12Z/THU. STRENGTHENING FLOW THROUGH THE TROPOSPHERE WILL LENGTHEN HODOGRAPHS TONIGHT AND ALONG WITH 850 MB SWLYS INCREASING TO AOA 50 KT...SETUP SHOULD FAVOR EMBEDDED SUPERCELL STRUCTURES AND LOCALIZED BOWING SEGMENTS WITH A RISK FOR A FEW TORNADOES AND ISOLATED DAMAGING WINDS. THE RELATIVELY MODEST NATURE OF BOUNDARY LAYER MOISTURE RETURN...ESPECIALLY WITH NERN EXTENT WHERE FORCING FOR ASCENT AND SHEAR WILL BE GREATER...WILL LIKELY LIMIT A MORE SUBSTANTIAL SEVERE RISK AS THE WARM SECTOR IS PINCHED OFF LATER TONIGHT. A NARROW OVERLAP OF STRONG LARGE-SCALE FORCING FOR ASCENT/MID-LEVEL COOLING ON THE NW FRINGE OF THE MOIST AXIS MAY PROVE SUFFICIENT TO YIELD STRONGER CONVECTION ALONG THE COLD FRONT AS FAR N AS CNTRL MO. THIS WOULD OCCUR IN A MOIST-NEUTRAL THERMODYNAMIC ENVIRONMENT...WHERE STRONG FRONTAL UPLIFT MAY COMPENSATE FOR MINIMAL BUOYANCY TO YIELD A CONDITIONAL RISK FOR LOCALLY DAMAGING WINDS. A SEPARATE AREA OF CONVECTION MAY ALSO EVOLVE IN THE CNTRL GULF COAST/LOWER MS VALLEY AREA WITHIN A LOW-LEVEL WARM THETA-E ADVECTION REGIME AHEAD OF THE COLD FRONT. WITH COMPARATIVELY GREATER POTENTIAL BUOYANCY AND SUFFICIENT LOW/MID-LEVEL SHEAR...A LOW-END DAMAGING WIND/TORNADO RISK SHOULD EXIST HERE BY 12Z/THU. ..GRAMS.. 12/20/2012  024 ACUS11 KWNS 200107 SWOMCD SPC MCD 200107 ARZ000-MOZ000-TXZ000-OKZ000-KSZ000-200200- MESOSCALE DISCUSSION 2179 NWS STORM PREDICTION CENTER NORMAN OK 0707 PM CST WED DEC 19 2012 AREAS AFFECTED...EASTERN OK/NORTHEAST TX/WESTERN AR/FAR SOUTHEAST KS/SOUTHWEST MO CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY VALID 200107Z - 200200Z PROBABILITY OF WATCH ISSUANCE...95 PERCENT SUMMARY...SEVERE TSTM POTENTIAL IS LIKELY TO QUICKLY INCREASE THIS EVENING ACROSS PORTIONS OF EASTERN OK/NORTHEAST TX INTO WESTERN AR ALONG WITH ADJACENT PARTS OF EXTREME SOUTHEAST KS/SOUTHWEST MO. A TORNADO WATCH WILL LIKELY BE NECESSARY FOR AT LEAST PORTIONS OF THIS REGION WITHIN THE HOUR /LIKELY BY 02Z/. DISCUSSION...00Z SURFACE ANALYSIS IS INDICATIVE OF A 996 MB SURFACE LOW NEAR THE I-35 CORRIDOR OF NORTH-CENTRAL OK/SOUTH-CENTRAL KS...WITH A STATIONARY FRONT OR SLOW-MOVING MOVING WARM FRONT EXTENDING EASTWARD ACROSS FAR SOUTHEAST MO...IN ADDITION TO A SOUTHEASTWARD-MOVING COLD FRONT AND A PRECEDING DRY LINE ACROSS OK/NORTH-CENTRAL TX. THE ARRIVAL OF CONSEQUENTIAL DPVA/MASS RESPONSE ASSOCIATED WITH A PLAINS UPPER TROUGH WILL LIKELY CONTRIBUTE TO INCREASING CONVECTIVE DEVELOPMENT BY THE MID/LATE EVENING HOURS...ALTHOUGH A RECENT CONVECTIVE INCREASE HAS ALREADY BEEN NOTED IN LATIMER COUNTY OK PER RADAR IMAGERY AS OF 0115Z...AND MORE ROBUSTLY IN VICINITY OF A PRE-FRONTAL TROUGH/DRYLINE NEAR THE RED RIVER OF SOUTH-CENTRAL OK/NORTH-CENTRAL TX. THE 00Z FORT WORTH OBSERVED SOUNDING WAS INDICATIVE OF RELATIVELY LITTLE CINH...WITH A RELATIVELY STEEP LAPSE RATES PROFILE AND MLCAPE AROUND 300 J/KG. SPC OBJECTIVE ANALYSIS OTHERWISE SUGGESTS THAT LOW-LEVEL CINH EROSION CONTINUES TO OCCUR WITHIN THE WARM SECTOR WHERE UPPER 50S/LOWER 60S F SURFACE DEWPOINTS ARE INCREASINGLY PREVALENT. WITH A PROBABLE INCREASE /QUICK AT THAT/ IN STORM COVERAGE AND INTENSITY BY MID-EVENING...VERY STRONG DEEP LAYER SOUTHWESTERLY WINDS ABOVE THE SURFACE AND RESULTANT 70+ KT 0-6 KM SHEAR WILL SUPPORT A MIXED-MODE OF SUPERCELLS AND THE EVOLUTION OF SMALL/FAST-NORTHEASTWARD MOVING LINEAR SEGMENTS TONIGHT...WHICH IS GENERALLY CONSISTENT BY VARIOUS HIGH-RES CONVECTION-ALLOWING GUIDANCE. WHILE SEVERE HAIL WILL BE POSSIBLE WITHIN THE FIRST FEW HOURS OF CONVECTIVE INITIATION...DAMAGING WINDS AND THE POSSIBILITY OF A FEW TORNADOES SHOULD BE THE MORE PREVALENT RISKS. ..GUYER/WEISS.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...LZK...SGF...SHV...TSA...ICT...FWD...OUN... LAT...LON 36699630 37519556 37699444 37839262 37319172 36009193 33109389 32179713 33699677 36699630  125 ACUS11 KWNS 200300 SWOMCD SPC MCD 200300 TXZ000-200500- MESOSCALE DISCUSSION 2180 NWS STORM PREDICTION CENTER NORMAN OK 0900 PM CST WED DEC 19 2012 AREAS AFFECTED...NORTH-CENTRAL/NORTHEAST TX CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE VALID 200300Z - 200500Z PROBABILITY OF WATCH ISSUANCE...40 PERCENT SUMMARY...TSTMS SHOULD CONTINUE TO INCREASE IN COVERAGE/INTENSITY ACROSS PORTIONS OF NORTH-CENTRAL/NORTHEAST TX THROUGH LATE EVENING AND INTO THE OVERNIGHT. THE EXTENT OF THE SEVERE THREAT IS A BIT UNCERTAIN...BUT ISOLATED DAMAGING WINDS/HAIL WILL BE POSSIBLE WITH THE STRONGEST TSTMS. DISCUSSION...TO THE SOUTH OF TORNADO WATCH 686...TSTMS HAVE STEADILY INCREASED SINCE 0215Z FROM NEAR THE RED RIVER TO THE DFW METRO VICINITY AS OF 0245Z. THIS IS OCCURRING INITIALLY NEAR A DRY LINE/PRE-FRONTAL THROUGH...WITH TSTM COVERAGE LIKELY TO ONLY INCREASE THE REMAINDER OF THE NIGHT AS THE COLD FRONT ACCELERATES/OVERTAKES THE PRE-FRONTAL BOUNDARY. LOW-LEVEL WINDS HAVE BEEN NOTED TO VEER WITHIN THE LOWEST FEW KM AHEAD OF THE SURFACE BOUNDARY PER SURFACE OBS AND PROFILER/WSR-88D VWP DATA. THIS MAY BE INDICATIVE OF A MORE MODEST MASS RESPONSE ON THE SOUTHERN PERIPHERY OF THE PLAINS CYCLONE. THIS MAY INFLUENCE TSTM COVERAGE/VIGOR TO A DEGREE...SUCH THAT EXTENT OF THE SEVERE THREAT TONIGHT IS A BIT QUESTIONABLE. THAT SAID...ISOLATED SEVERE TSTMS WILL BE POSSIBLE WHERE STORMS DO MATURE TONIGHT...GIVEN ADEQUATE INSTABILITY AND VERY STRONG DEEP LAYER SHEAR /70 KT 0-6 KM PER 00Z FORT WORTH SOUNDING/. WHILE AN ISOLATED TORNADO AND/OR HAIL CANNOT BE RULED OUT...INITIATION/CONVECTIVE MODE SUGGEST THAT DAMAGING WINDS WOULD BE THE PRIMARY RISK. CONVECTIVE TRENDS WILL CONTINUE TO BE MONITORED FOR A POSSIBLE WATCH. ..GUYER/WEISS.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...SHV...HGX...FWD... LAT...LON 31139791 32919685 33289624 32969438 31039522 31139791  893 ACUS11 KWNS 200332 SWOMCD SPC MCD 200332 IAZ000-MOZ000-NEZ000-200900- MESOSCALE DISCUSSION 2181 NWS STORM PREDICTION CENTER NORMAN OK 0932 PM CST WED DEC 19 2012 AREAS AFFECTED...ERN NEB / W-CNTRL INTO NERN IA CONCERNING...BLIZZARD VALID 200332Z - 200900Z SUMMARY...BLIZZARD CONDITIONS ARE BEGINNING OVER PARTS OF E-CNTRL NEB AND WILL COMMENCE FROM WSW-ENE FROM NEAR THE MISSOURI RIVER /04-06 UTC/ TO CNTRL IA /07-09 UTC/. DISCUSSION...WATER VAPOR IMAGERY SHOWS A VIGOROUS UPPER TROUGH MOVING EWD FROM THE CNTRL PLAINS TOWARDS THE LOWER MO RIVER VALLEY. AS THIS OCCURS...00Z TOP RAOB SHOWED A MID LEVEL WARM NOSE AND THIS EXTENDS INTO SRN IA/NRN MO EARLY THIS EVENING. SURFACE MESOANALYSIS SHOWS A DEEPENING 996 MB LOW OVER SERN KS THAT IS EXPECTED TO DEVELOP ENEWD INTO W-CNTRL MO DURING THE OVERNIGHT. A CONSISTENT ENVELOPE OF WIND GUSTS AOA 35 MPH ENCOMPASSES E-CNTRL NEB SWWD INTO THE WRN HALF OF KS...CREATING BLIZZARD CONDITIONS WHERE APPRECIABLE SNOWFALL RATES/FRESHLY FALLEN SNOW ARE HINDERING VISIBILITIES. MODEL GUIDANCE IS CONSISTENT IN SHOWING THE AXIS OF HEAVIEST SNOWFALL RATES /1-1.5 INCH PER HR/ IN A SWATH NEAR THE MO RIVER AND EVENTUALLY LATER OVER CNTRL AND NERN IA DURING THE 06-09 UTC PERIOD. CONVECTIVE ENHANCEMENT TO PRECIP RATES OVER THE NEXT SEVERAL HOURS IN IA /AS IMPLIED VIA OBSERVED LIGHTNING/ WILL ACT TO LOCALLY AUGMENT RATES. AS AN ASIDE...FORECAST SOUNDINGS SHOW THE COLUMN COOLING 1-2 DEG C IN THE H8-H7 LAYER DURING THE 04-06 UTC TIME FRAME NEAR DSM...TERMINATING MIXED P-TYPE /SNOW AND SLEET/ AND RESULTING IN HEAVY SNOWFALL RATES. MODEL DATA SHOW A TIME LAG ON THE ORDER OF SEVERAL HOURS FOR THE HEAVIEST SNOWFALL RATES BEGINNING AND THE STRONGER WINDS DEVELOPING...WHICH WILL LEAD TO INTERMITTENT BLIZZARD CONDITIONS BECOMING MORE COMMON BY LATE TONIGHT. TIMING OF BLIZZARD CONDITIONS SEEMINGLY WILL OCCUR BY 06 UTC NEAR AND W OF THE MO RIVER...AND APPROACH CNTRL IA BY 07-09 UTC. ..SMITH.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...DVN...ARX...DMX...EAX...OAX... LAT...LON 43149231 43019179 42679168 42419180 41519341 40629567 40309636 40309693 40529736 40979726 41429672 43149231  316 ACUS11 KWNS 200423 SWOMCD SPC MCD 200422 MOZ000-ARZ000-TXZ000-OKZ000-200545- MESOSCALE DISCUSSION 2182 NWS STORM PREDICTION CENTER NORMAN OK 1022 PM CST WED DEC 19 2012 AREAS AFFECTED...FAR EASTERN OK/EXTREME SOUTHEAST KS/NORTHEAST TX TO WESTERN AR/SOUTHERN MO CONCERNING...TORNADO WATCH 686... VALID 200422Z - 200545Z THE SEVERE WEATHER THREAT FOR TORNADO WATCH 686 CONTINUES. SUMMARY...DAMAGING WIND/ISOLATED TORNADO POTENTIAL CONTINUES WITHIN TORNADO WATCH 686...ESPECIALLY A CORRIDOR FROM NORTHEAST TX/SOUTHEAST OK INTO NORTHWEST AR/SOUTHWEST MO. ADDITIONAL PARTS OF CENTRAL/NORTHERN AR AND SOUTH-CENTRAL MO WILL BE MONITORED THROUGH LATE EVENING/OVERNIGHT FOR A NEED OF AREAL WATCH EXPANSION AND/OR AN ADDITIONAL WATCH...ALTHOUGH SUCH A NEED IS NOT CERTAIN. DISCUSSION...AS OF 04Z...A NEARLY CONTINUOUS BAND OF STRONG TO SEVERE TSTMS CONTINUES TO CONSOLIDATE AND SPREAD EAST-NORTHEASTWARD WITHIN A CORRIDOR FROM NORTHEAST TX/SOUTHEAST OK INTO NORTHWEST AR/SOUTHWEST MO...WITH A SURFACE COLD POOL GRADUALLY BECOMING MORE APPARENT ACROSS EXTREME EAST-CENTRAL/SOUTHEAST OK. A 55 KT WIND GUST WAS RECENTLY /329Z/ MEASURED AT FORT SMITH AR. LINE-LEADING SEMI-DISCRETE STORMS /SOME WITH PERIODIC ROTATION/ HAVE ALSO BEEN COMMON IN WEST-CENTRAL/NORTHWEST AR AND SOUTHERN MO. ACCENTUATED BY THE DEEPENING PHASE OF THE CYCLONE TONIGHT...INCREASING DEEP LAYER FORCING FOR ASCENT AND A HIGHLY SHEARED ENVIRONMENT WILL PROMOTE THE SUSTENANCE OF A FORCED LINE/QLCS ACROSS SOUTHERN MO/MUCH OF AR TO THE ADJACENT ARKLATEX...EVEN WITH MINIMAL BUOYANCY /AS LOW AS A FEW HUNDRED J PER KG SBCAPE/. DAMAGING WINDS WILL REMAIN POSSIBLE INTO THE OVERNIGHT...ALONG WITH THE POTENTIAL FOR A FEW TORNADOES WITH ANY EMBEDDED MESO-VORTICES/SEMI-DISCRETE SUPERCELLS. AN ISOLATED SEVERE THREAT MAY DEVELOP TO THE EAST-NORTHEAST OF CURRENT TORNADO WATCH 686...ALTHOUGH SURFACE DEWPOINTS HAVE GENERALLY BEEN DECREASING OVER THE PAST FEW HOURS ACROSS EASTERN AR/SOUTHEAST MO OWING TO RESIDUAL LOW-LEVEL ANTICYCLONIC TRAJECTORIES. WHILE THIS PUTS AVAILABLE INSTABILITY/STORM VIGOR A BIT INTO QUESTION WITH EASTWARD EXTENT...FORCING/VERTICAL SHEAR WILL BE CONSEQUENTIAL AND INCREASING STORM-SCALE ORGANIZATION MAY NONETHELESS MAINTAIN A SEVERE THREAT ACROSS AR/SOUTHERN MO OVERNIGHT. CONVECTIVE TRENDS WILL CONTINUE TO BE MONITORED INTO THE OVERNIGHT FOR AN ADDITIONAL WATCH. ..GUYER.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...PAH...MEG...LSX...LZK...SGF...SHV...TSA...FWD... LAT...LON 33649565 35039451 36069390 37309325 37869101 37199055 35089203 33729290 33649565  568 ACUS11 KWNS 200555 SWOMCD SPC MCD 200555 MSZ000-LAZ000-ARZ000-TXZ000-200730- MESOSCALE DISCUSSION 2183 NWS STORM PREDICTION CENTER NORMAN OK 1155 PM CST WED DEC 19 2012 AREAS AFFECTED...EAST/NORTHEAST TX INTO MUCH OF LA/FAR WESTERN MS CONCERNING...SEVERE THUNDERSTORM WATCH 687... VALID 200555Z - 200730Z THE SEVERE WEATHER THREAT FOR SEVERE THUNDERSTORM WATCH 687 CONTINUES. SUMMARY...SEVERE TSTM WATCH 687 CONTINUES UNTIL 09Z WITH PRIMARY RISK OF DAMAGING WINDS...ALTHOUGH A TORNADO MAY BE POSSIBLE AS WELL. AT LEAST AN ISOLATED SEVERE THREAT WILL LIKELY CONTINUE TO SPREAD EAST-SOUTHEASTWARD...SUCH THAT AN ADDITIONAL WATCH MAY BE NEEDED OVERNIGHT FOR SOUTHEAST TX INTO LA AND PERHAPS WESTERN MS/SOUTHEAST AR. DISCUSSION...A QUASI-LINEAR CONVECTIVE SYSTEM WITH EMBEDDED BOWING STRUCTURES CONTINUES TO STEADILY MATURE/SPREAD GENERALLY EASTWARD ACROSS THE ARKLATEX REGION AS OF 0545Z. THE REGION IS ON THE SOUTHERN EDGE OF A DEEPENING PLAINS CYCLONE...WITH ATTENDANT STRONG VERTICAL SHEAR/GRADUAL HEIGHT FALLS OVERSPREADING THE REGION LATE THIS EVENING/OVERNIGHT. WHILE INSTABILITY IS NOT ROBUST...AN INFLUX OF LOW-LEVEL MOISTURE...GENERALLY CHARACTERIZED BY MIDDLE/SOME UPPER 60S F SURFACE DEWPOINTS...IS CONTRIBUTING TO SBCAPE AS HIGH AS 500 J/KG AHEAD OF THE QLCS/COLD FRONT. DEEP LAYER FORCING/FRONTAL INFLUENCE AND VERY STRONG DEEP LAYER SHEAR /70+ KT 0-6 KM PER SHV WSR-88D VWP/ WILL HELP MAINTAIN A WELL-ORGANIZED QLCS OVERNIGHT WITH A DAMAGING WIND THREAT...AND PERHAPS A TORNADO RISK WITH EMBEDDED MESO-VORTICES. AHEAD OF THE QLCS...SOME /ALBEIT SEEMINGLY LOW/ POTENTIAL EXISTS FOR SEMI-DISCRETE STORMS TO DEEPEN/MATURE WITHIN A CORRIDOR OF LOW-LEVEL CONFLUENCE ACROSS PORTIONS OF LA/MS. SHOULD THIS OCCUR...A SEVERE THREAT WOULD BE POSSIBLE...ALTHOUGH A RESIDUAL WARM LAYER ALOFT MAY PRECLUDE STRONG DEVELOPMENT UNTIL THE ARRIVAL OF THE QLCS. ..GUYER.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...JAN...LIX...LZK...LCH...SHV...HGX...FWD... LAT...LON 30739639 31869578 33049468 32919384 33369127 31499105 30119278 30739639  151 ACUS01 KWNS 200557 SWODY1 SPC AC 200555 DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 1155 PM CST WED DEC 19 2012 VALID 201200Z - 211200Z ...THERE IS A SLGT RISK OF SVR TSTMS FOR PARTS OF THE SOUTHEAST TODAY THROUGH TONIGHT... ...SYNOPSIS... LARGE-SCALE UPPER-LEVEL TROUGH WILL SHIFT FROM THE GREAT PLAINS INTO THE ERN CONUS AS A VIGOROUS SHORT WAVE IMPULSE OVER KS/OK TRACKS NEWD TO THE LOWER GREAT LAKES BY 12Z/FRI. ATTENDANT SURFACE CYCLONE OVER THE OZARK PLATEAU WILL DEEPEN DURING THE FIRST HALF OF THE PERIOD AS IT EJECTS NEWD ACROSS THE MIDWEST BEFORE REACHING THE LOWER GREAT LAKES EARLY FRI. A SHARP COLD FRONT WILL SWEEP EWD ACROSS THE OH/TN VALLEYS AND CNTRL GULF COAST THIS MORNING AND OFF THE MID/SOUTH ATLANTIC COAST TONIGHT. ...SOUTHEAST... AN EXTENSIVE BROKEN TO CONTINUOUS CONVECTIVE BAND IS EXPECTED TO BE ONGOING AT 12Z/THU ALONG THE AFOREMENTIONED COLD FRONT FROM PARTS OF THE MID-MS VALLEY TO THE SWRN LA COAST. THIS BAND SHOULD LARGELY WEAKEN DURING THE MORNING HOURS AS IT CONTINUES TO PINCH OFF THE NRN EXTENT OF THE RICHER MOIST SECTOR AMIDST A POOR LAPSE RATE ENVIRONMENT...AS SAMPLED BY REGIONAL RAOBS WED EVENING. THIS TYPE OF THERMODYNAMIC PROFILE/MARGINAL INSTABILITY WILL LIKELY REMAIN THE PRIMARY LIMITING FACTOR TO MORE ROBUST SEVERE POTENTIAL DESPITE THE PRESENCE OF STRONG SHEAR AND FORCING FOR ASCENT. MODEST DIURNAL HEATING ALONG WITH A NARROW CORRIDOR OF RICH GULF MOISTURE /CHARACTERIZED BY 60S SURFACE DEW POINTS/ AHEAD OF THE FRONT SHOULD YIELD MLCAPE OF 250-500 J/KG...LIKELY SUPPORTING AN INCREASE IN CONVECTIVE INTENSITY ACROSS PARTS OF THE DEEP SOUTH/GULF COAST DURING THE DAY. PRESENCE OF 50 KT SWLYS AT 1 KM AGL WILL YIELD POTENTIAL FOR ISOLATED DAMAGING WIND GUSTS AND PERHAPS A BRIEF TORNADO OR TWO. THIS EVENING...THE LLJ WILL STRENGTHEN ACROSS THE CAROLINAS AS SECONDARY CYCLOGENESIS OCCURS IN THE MID-ATLANTIC STATES. THIS SHOULD FAVOR A NOCTURNAL MAINTENANCE/INCREASE IN CONVECTION ACROSS THE COASTAL PLAINS WITHIN A MOIST-NEUTRAL THERMODYNAMIC ENVIRONMENT. INTENSE LOW-LEVEL S/SWLYS AND STRONG FRONTAL UPLIFT MAY COMPENSATE FOR SCANT SURFACE BASED-BUOYANCY TO YIELD A CONDITIONAL DAMAGING WIND RISK. ...LOWER OH VALLEY... VERY STRONG FORCING FOR ASCENT INFERRED THROUGH ROBUST MID-LEVEL DCVA AND SURFACE PRESSURE RISE/FALL COUPLET MAY BE SUFFICIENT TO YIELD A ROUND OF LOW-TOPPED CONVECTION IMMEDIATELY ALONG THE FRONT DURING THE LATE MORNING AND EARLY AFTERNOON IN THE WAKE OF OVERNIGHT CONVECTION. WITH SURFACE TEMPERATURES EXPECTED TO ONLY BE IN THE 50S...CONVECTION WILL BE UNLIKELY TO PRODUCE LIGHTNING. BUT ELONGATED HODOGRAPHS AND THE NATURE OF THE FORCING WILL SUPPORT POSSIBLE LOCALIZED DAMAGING WINDS FROM STRONGER SHOWERS. ..GRAMS/SMITH.. 12/20/2012  291 ACUS02 KWNS 200611 SWODY2 SPC AC 200609 DAY 2 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 1209 AM CST THU DEC 20 2012 VALID 211200Z - 221200Z ...NO SVR TSTM AREAS FORECAST... ..SYNOPSIS... CLOSED UPPER LOW WILL CONTINUE THROUGH THE ERN STATES AND OFF THE ATLANTIC SEABOARD FRIDAY WHILE UPPER RIDGE BUILDS INTO THE CNTRL STATES. FARTHER WEST...A SERIES OF IMPULSES EMBEDDED WITHIN THE BROADER SYNOPTIC TROUGH WILL AFFECT THE PACIFIC NW AND NRN CA COASTS. AT THE SFC A COLD FRONT WILL HAVE ALREADY EXITED THE ATLANTIC SEABOARD...WITH SWRN EXTENSION OF THIS BOUNDARY WELL INTO THE GULF. ...PACIFIC NW THROUGH CA COASTS.... BANDS OF RAIN AND SHALLOW CONVECTION WILL MOVE ONSHORE FROM NRN CA THROUGH WA AND OREGON. COLD AIR ALOFT IN ASSOCIATION WITH THE BROAD SYNOPTIC TROUGH WILL SUPPORT STEEP LAPSE RATES AND WEAK INSTABILITY NEAR THE COASTS. CURRENT SATELLITE IMAGERY SHOWS AN EXPANSIVE AREA OF POST FRONTAL CELLULAR CONVECTION OVER THE ERN PACIFIC...AND THIS REGIME SHOULD BEGIN TO AFFECT THE PACIFIC NW AND NRN CA COASTS LATER THURSDAY INTO FRIDAY. ..DIAL.. 12/20/2012  273 ACUS11 KWNS 200650 SWOMCD SPC MCD 200650 TNZ000-MSZ000-ARZ000-LAZ000-200745- MESOSCALE DISCUSSION 2184 NWS STORM PREDICTION CENTER NORMAN OK 1250 AM CST THU DEC 20 2012 AREAS AFFECTED...ERN AR...NRN LA...WRN MS AND WRN TN CONCERNING...SEVERE POTENTIAL...WATCH LIKELY VALID 200650Z - 200745Z PROBABILITY OF WATCH ISSUANCE...95 PERCENT SUMMARY...A NEW WW WILL BE COORDINATED AND ISSUED SOON TO THE EAST OF WW/S 686 AND 687...WITH THE NEW WW LIKELY INCLUDING ERN AR...NRN LA...WRN MS AND PERHAPS PARTS OF WRN TN. DAMAGING WINDS WILL BE THE PRIMARY THREAT...THOUGH A TORNADO OR TWO CANNOT BE RULED OUT. DISCUSSION...LATEST TRENDS IN SURFACE OBSERVATIONS AND OBJECTIVE ANALYSES INDICATED THE PRE-SQUALL LINE/COLD FRONT AIR MASS ACROSS ERN AR INTO FAR WRN TN AND SWD IS SLOWLY MODIFYING WITH SURFACE DEWPOINTS INTO THE LOWER 50 AROUND KMEM. MODIFIED 06Z LIT SPECIAL SOUNDING SUGGESTED SURFACE DEWPOINTS OF 59-60 F ARE NEEDED FOR PARCELS TO BE SURFACE-BASED. PREDOMINANT DEEP LAYER SHEAR VECTORS ORIENTED PARALLEL TO THE EWD MOVING SQUALL LINE AND STRONG FORCING FOR ASCENT WITH THE CENTRAL PLAINS STORM SYSTEM WILL TEND TO MAINTAIN A LINEAR STORM MODE WITH DAMAGING WINDS THE PRIMARY THREAT. HOWEVER...LARGE HODOGRAPH CURVATURE NOTED ON THE 06Z LIT SOUNDING ALSO SUGGESTS A TORNADO THREAT CANNOT BE RULED OUT...ESPECIALLY WITH ANY EMBEDDED MESOVORTICES IN THE LINE. STORM MOTIONS OVER NERN TX/FAR NWRN LA ARE AT 40-45 KT WHILE FARTHER N INTO NRN AR MOTIONS ARE AT 35-40 KT. DESPITE THE DIFFERENT SPEEDS...THE SSW-NNE ORIENTATION OF THE LINE WILL RESULT IN MUCH OF THE MS RIVER REGION BEING AFFECTED BY THESE STORMS AROUND 10Z. ..PETERS/EDWARDS.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...MEG...JAN...LZK...SHV... LAT...LON 31919369 34209257 36459123 35949037 35908891 33418973 32039056 31729113 31729223 31919369  869 ACUS03 KWNS 200716 SWODY3 SPC AC 200715 DAY 3 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0115 AM CST THU DEC 20 2012 VALID 221200Z - 231200Z ...NO SVR TSTM AREAS FORECAST... ...NRN CA THROUGH PACIFIC NW... STEEP LAPSE RATES RESULTING FROM COLD AIR ALOFT WILL PERSIST WITHIN BROAD CYCLONIC FLOW REGIME SUPPORTING WEAK INSTABILITY. A SERIES OF EMBEDDED VORTICITY MAXIMA WILL MOVE THROUGH THE SYNOPTIC TROUGH. SHALLOW CONVECTION WILL REMAIN LIKELY IN THIS ENVIRONMENT...SOME OF WHICH MAY PRODUCE ISOLATED LIGHTNING STRIKES FROM THE NRN CA INTO PACIFIC NW COASTAL AREAS. ..DIAL.. 12/20/2012  958 ACUS48 KWNS 200746 SWOD48 SPC AC 200745 DAY 4-8 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0145 AM CST THU DEC 20 2012 VALID 231200Z - 281200Z ...DISCUSSION... DAYS 4-8...MODELS CONTINUE TO ADVERTISE A SIGNIFICANT UPPER TROUGH WILL AMPLIFY OVER THE SWRN U.S. DAY 5...MOVING INTO THE SRN PLAINS BY DAY 6 /CHRISTMAS/ AND INTO THE ERN U.S. BY 7 OR 8. CONSIDERABLE VARIABILITY PERSISTS AMONG THE GFS...ECMWF AND THE ENSEMBLE MEMBERS REGARDING AMPLITUDE AND SPEED OF THIS FEATURE. RUN TO RUN CONSISTENCY ALSO REMAINS LOW. SOME THREAT FOR SEVERE STORMS MIGHT DEVELOP OVER ERN TX DAY 6 AND THE GULF COASTAL STATES DAY 7-8 AS PARTIALLY MODIFIED GULF AIR IS ADVECTED NWD THROUGH WARM SECTOR AHEAD OF THE TROUGH. HOWEVER...TOO MUCH UNCERTAINTY EXISTS TO DELINEATE A HIGHER COVERAGE RISK AREA AT THIS TIME. ..DIAL.. 12/20/2012  645 ACUS11 KWNS 200913 SWOMCD SPC MCD 200912 WIZ000-IAZ000-MNZ000-201315- MESOSCALE DISCUSSION 2185 NWS STORM PREDICTION CENTER NORMAN OK 0312 AM CST THU DEC 20 2012 AREAS AFFECTED...FAR NERN IA...FAR SERN MN...SRN/CNTRL WI CONCERNING...BLIZZARD VALID 200912Z - 201315Z SUMMARY...HEAVY SNOWFALL RATES OF 1-2 IN PER HR COUPLED WITH N-NELY WIND GUSTS UP TO 35 MPH /PRIMARILY AFTER 11Z/ WILL RESULT IN BLIZZARD CONDITIONS DEVELOPING NEWD INTO SRN/CNTRL WI THROUGH 13Z. DISCUSSION...DEEPENING CYCLONE CONTINUES TO DEVELOP NEWD OUT OF N-CNTRL MO INTO W-CNTRL IL...WITH PRESSURE FALLS AS HIGH AS 4 MB/2 HR OBSERVED ACROSS PARTS OF NERN IL. STRONG LOW-LEVEL WAA PRECEDING THE CYCLONE IS FORECAST TO RESULT IN A SLIGHT N-NWWD SHIFT IN THE SFC FREEZING LINE ACROSS SRN/ERN WI. FORCING FOR ASCENT ASSOCIATED WITH MIDLEVEL DCVA/HEIGHT FALLS AND UPPER DIVERGENCE...COUPLED WITH PERSISTENT LOW-LEVEL ISENTROPIC LIFT...WILL CONTINUE TO MAINTAIN SNOWFALL RATES OF 1-1.5 IN PER HR ACROSS SRN AND PARTS OF CNTRL WI. ADDITIONALLY...BOTH SLANTWISE AND UPRIGHT CONVECTIVE INSTABILITY /AS EVIDENCED BY OCCASIONAL ISOLATED LIGHTNING STRIKES/ COULD LEAD TO LOCALIZED ENHANCEMENT OF SNOWFALL RATES. LATEST HIGH-RESOLUTION MODEL GUIDANCE /INCLUDING 07Z WRF-HRRR AND 07Z RAP/ SUPPORTS THIS SCENARIO...SHOWING A NWD EXPANSION OF HEAVIER SNOW RATES INTO PARTS OF CNTRL WI. THE POTENTIAL FOR BLIZZARD CONDITIONS IS EXPECTED TO CONTINUE TO LAG ONGOING AREA OF SNOW OVER WI. THE SFC PRESSURE GRADIENT WILL TIGHTEN ACROSS SRN WI IN RESPONSE TO THE DEEPENING CYCLONE...WITH SFC WINDS ANTICIPATED TO SLOWLY BACK TOWARDS THE NNE AND EVENTUALLY THE N. AS SUCH...WIND GUSTS UP TO 35 MPH WILL BECOME MORE COMMON AFTER 11-12Z...RESULTING IN SIGNIFICANT VISIBILITY REDUCTIONS. ..ROGERS.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...GRB...MKX...DVN...ARX...MPX... LAT...LON 44288832 43678864 43338902 42919003 42649092 42609148 42959190 43579203 44299157 44779073 45018969 45028915 44958841 44688812 44288832  440 ACUS11 KWNS 201008 SWOMCD SPC MCD 201008 ALZ000-TNZ000-MSZ000-LAZ000-ARZ000-201115- MESOSCALE DISCUSSION 2186 NWS STORM PREDICTION CENTER NORMAN OK 0408 AM CST THU DEC 20 2012 AREAS AFFECTED...ERN AR/WRN TN/LA/MS/WRN AL CONCERNING...SEVERE THUNDERSTORM WATCH 688... VALID 201008Z - 201115Z THE SEVERE WEATHER THREAT FOR SEVERE THUNDERSTORM WATCH 688 CONTINUES. SUMMARY...A THREAT FOR DAMAGING WINDS AND A TORNADO OR TWO PERSISTS ACROSS WW 688...WITH RECENT TRENDS IN STORM INTENSITY/LIGHTNING DATA SUGGESTING THESE THREATS TO BE GREATEST ACROSS AR/TN/NWRN MS PORTIONS OF THIS SVR TSTM WATCH. MEANWHILE...NEW CONVECTION/TSTMS WILL CONTINUE TO DEVELOP OVER SERN LA AND SRN MS...WITH THIS ACTIVITY TRACKING ENEWD INTO CENTRAL/SRN AL. THIS WARM SECTOR ACTIVITY MAY WARRANT A SEPARATE WW. DISCUSSION...TRENDS IN REGIONAL RADAR IMAGERY/LIGHTNING DATA SHOWED SUSTAINED TSTMS/QLCS PROGRESSING EWD INTO NWRN MS...AND APPROACHING FAR WRN TN. DESPITE WEAKER INSTABILITY IN THESE AREAS /MUCAPE 200-400 J PER KG/...STRONGER FORCING FOR ASCENT/DPVA ATTENDANT TO THE STRONG SHORTWAVE TROUGH MOVING TOWARD THE MID MS/LOWER OH/TN VALLEYS WILL MAINTAIN QLCS WITH EWD EXTENT. WFO/S MAY NEED TO EXTEND THE NRN PART OF WW 688 EWD TO INCLUDE MORE OF SWRN TN AND NRN MS EARLY THIS MORNING. THE SRN EXTENT OF THE QLCS HAS BECOME LESS INTENSE /I.E. LACK OF CG LIGHTNING/ ACROSS NERN TO SWRN LA. THIS IS LIKELY DUE TO A MIDLEVEL WARM LAYER NOTED ON THE 06Z SLIDELL SPECIAL SOUNDING. ALTHOUGH THIS LINE HAS WEAKENED...GREATER CONCERN EXISTS IN THE PRE-FRONTAL WARM SECTOR ACROSS SERN LA TO SRN MS WHERE RICH MOISTURE EXISTS WITH SURFACE DEWPOINTS AROUND 70 F. RADAR IMAGERY SHOWED SOME LOW LEVEL ROTATION WITH THE CONVECTION/TSTMS TO THE SSW-NNE OF KBTR. THUS...A TORNADO THREAT MAY BE DEVELOPING WITH THIS PRE-FRONTAL ACTIVITY AS IT PROGRESSES ENEWD ACROSS SRN MS TO CENTRAL/SRN PARTS OF AL THIS MORNING. VWP DATA AT SLIDELL HAS SHOWED 40-45 KT 850 MB FLOW BEING MAINTAINED WITH LOW LEVEL SPEED SHEAR AND HODOGRAPH CURVATURE /0-1 KM SRH AROUND 200 M2 PER S2/. ..PETERS/EDWARDS.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...OHX...BMX...HUN...MOB...MEG...JAN...LIX...LZK... LCH...SHV... LAT...LON 30899297 32469200 33499115 34859055 35469039 35438881 34958751 32998767 31438793 30658887 30369048 30209152 30899297  485 ACUS01 KWNS 201251 SWODY1 SPC AC 201249 DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0649 AM CST THU DEC 20 2012 VALID 201300Z - 211200Z ...THERE IS A SLGT RISK OF SVR TSTMS SERN PORTIONS MS/LA TO ERN CAROLINAS AND PORTIONS NRN FL... ...SYNOPSIS... PROGRESSIVE AND RATHER HIGHLY AMPLIFIED SHORTWAVE/SYNOPTIC PATTERN IS FCST ALOFT...OVER MUCH OF CONUS AND DOWNSTREAM FROM SUBSTANTIAL GULF OF AK CYCLONE. STG SHORTWAVE TROUGH IS EVIDENT IN MOISTURE CHANNEL IMAGERY AND AVAILABLE UPPER-AIR DATA OVER IA...ERN MO...SERN OK AND N-CENTRAL TX. MO PORTION OF THIS FEATURE IS EVOLVING INTO 500-MB CLOSED LOW. THAT LOW WILL DEEPEN AS IT MOVES ENEWD TO NRN INDIANA BY 00Z...WITH TROUGH SWWD OVER NRN MS. BY END OF PERIOD...BROAD/PROGRESSIVE CYCLONE SHOULD BE CENTERED OVER SRN ONT...WITH STRONGEST ASSOCIATED 250-500 MB LAYER FLOW IN BELT FROM MID-MS VALLEY ACROSS NRN GA AND ERN NC. AT SFC...CYCLONE INITIALLY BETWEEN COU-UIN WILL DEEPEN/OCCLUDE AS IT MOVES NEWD ACROSS IL TO SERN LM/NRN INDIANA/SWRN LOWER MI AREA BY 00Z...THEN OVER LH BY 12Z. OCCLUSION TRIPLE-POINT LOW MAY DEVELOP OVER MID-ATLC BY END OF PERIOD. ASSOCIATED STG COLD FRONT WILL CONTINUE TO SWEEP EWD AND SEWD FROM LOWER-MID MS VALLEY. BY 00Z...FRONT SHOULD REACH FROM OH/WV TO WRN NC...WRN SC...SRN GA...ERN FL PANHANDLE AND CENTRAL GULF. BY END OF PERIOD...COLD FRONT SHOULD EXTEND FROM OCCLUSION LOW SWD ACROSS ATLC WATERS THEN SWWD OVER SRN FL PENINSULA. ...SERN CONUS... INITIAL BAND OF MOSTLY NON-TSTM CONVECTION FROM NRN MS SWWD ACROSS SRN LA REMAINS CAPABLE OF AT LEAST ISOLATED DAMAGING GUSTS...AND MAY RE-INTENSIFY THROUGH MIDDAY AS IT ENTERS PROGRESSIVELY MORE MOIST AIR MASS ACROSS CENTRAL/SRN PORTIONS MS/AL AND PART OF SERN LA. LOCALLY ENHANCED SVR THREAT MAY EXIST WHEREVER THIS BAND OF FORCED ASCENT OVERTAKES WARM-SECTOR TSTMS. ADDITIONAL CONVECTION OF MIXED MODES -- DISCRETE...CLUSTERED AND SHORT LINES -- WILL CONTINUE TO DEVELOP AND MOVE NEWD OVER WARM SECTOR WITH WEAK CINH. THIS INCLUDES SOME SHALLOW/DISCRETE/POTENTIALLY TORNADIC SUPERCELLS WITHOUT DETECTED LIGHTNING...ONE OF WHICH PRODUCED SWATH OF DAMAGE IN MOB AREA SHORTLY BEFORE 11Z. RELATED FAVORABLE MOISTURE PLUME...WITH DEW POINTS NEAR 60 F ON NRN PORTION AND NEAR 70 F ALONG GULF COAST...IS SPREADING NEWD ACROSS MS/AL AND FL PANHANDLE AMIDST 250-800 J/KG MLCAPE AND 45-55 KT EFFECTIVE SHEAR MAGNITUDES. BUOYANCY SHOULD INCREASE THROUGH EARLY AFTERNOON FROM BOUNDARY-LAYER THETAE ADVECTION AND GRADUAL DIABATIC WARMING. REF REMAINDER WW 689 AND ATTENDANT MESOSCALE DISCUSSIONS FOR MORE DETAILS ON NEAR-TERM SCENARIO OVER THIS REGION. SVR THREAT IS FCST TO SHIFT EWD ACROSS SERN AL...GA...AND FL PANHANDLE DURING AFTERNOON HOURS...ALONG/AHEAD OF COLD FRONT. PLUME OF RELATIVELY LOW-THETAE AIR -- HEATED/MIXED DIURNALLY OVER CENTRAL/NRN FL -- SHOULD ADVECT NWD TO NNEWD ACROSS PORTIONS ERN GA/SC...NARROWING WITH TIME AS IT IS BRACKETED BY MORE MOISTURE-RICH GULF AND ATLC MARINE AIR. THOUGH THIS FL PLUME MAY INHIBIT SVR POTENTIAL FOR A FEW HOURS THIS EVENING...PRIND DAMAGING-WIND THREAT WILL SHIFT ACROSS CENTRAL/SRN GA AND ERN CAROLINAS THROUGH REMAINDER PERIOD PRIOR TO FROPA. THETAE ADVECTION WILL OFFSET WEAK LAPSE RATES ALOFT ENOUGH TO PROVIDE AROUND 300-800 J/KG MLCAPE. LLJ SHOULD STRENGTHEN LOW-LEVEL SHEAR AND HODOGRAPH SIZE CONTEMPORANEOUSLY WITH INCREASE IN THETAE...SUCH THAT QLCS AND/OR SUPERCELL TORNADO THREAT WILL DEVELOP ACROSS ATLC COASTAL PLAIN PORTION OF OUTLOOK AREA BEFORE ACTIVITY SWEEPS OFFSHORE. ...OH VALLEY AND TN VALLEY REGIONS... NARROW...STRONGLY FORCED BAND OF CONVECTION SHOULD CONTINUE TO MOVE ACROSS PORTIONS OF THIS REGION THROUGH EARLY AFTERNOON...POSING RISK OF ISOLATED DAMAGING GUSTS BEFORE MOVING INTO PROHIBITIVELY COOL/STABLE AIR MASS FARTHER E. GUST POTENTIAL WILL BE RELATED TO DOWNWARD MOMENTUM TRANSFER FROM MUCH MORE INTENSE DEEP-LAYER FLOW BEGINNING JUST ABOVE BOUNDARY LAYER. FCST SOUNDINGS SUGGEST MAIN LIMITING FACTOR WILL BE VERY WEAK TO NEUTRAL INSTABILITY...AND IN PLACES...SHALLOW LAYER OF ABSOLUTE STABILITY THAT WILL INHIBIT MORE NUMEROUS STG-SVR CONVECTIVE GUSTS FROM REACHING SFC. ..EDWARDS/ROGERS.. 12/20/2012  172 ACUS11 KWNS 201310 SWOMCD SPC MCD 201309 ALZ000-FLZ000-MSZ000-LAZ000-201445- MESOSCALE DISCUSSION 2187 NWS STORM PREDICTION CENTER NORMAN OK 0709 AM CST THU DEC 20 2012 AREAS AFFECTED...ERN/SRN MS...SERN LA...CENTRAL/SRN AL...AND WRN FL PANHANDLE CONCERNING...TORNADO WATCH 689... VALID 201309Z - 201445Z THE SEVERE WEATHER THREAT FOR TORNADO WATCH 689 CONTINUES. SUMMARY...ENVIRONMENT ACROSS WW 689 WILL REMAIN CONDUCIVE FOR A TORNADO THREAT AND DAMAGING WINDS UNTIL 14-15Z IN THE MS/SERN LA PORTION OF THIS WW...AND UNTIL 15-17Z ACROSS THE CENTRAL/SRN AL AND WRN FL PANHANDLE PORTION. DISCUSSION...AT 12Z...SURFACE MESOANALYSIS SHOWED A WIND SHIFT...ALSO DEPICTED BY RADAR IMAGERY AS LOW TOPPED /NON-LIGHTNING PRODUCING/ CONVECTION...EXTENDING FROM WRN TN SWD TO ALONG THE NERN MS/NWRN AL BORDER...AND THEN SSWWD THROUGH E-CENTRAL TO SWRN MS THROUGH SERN LA BETWEEN KBTR AND KMSY. A WARM FRONT EXTENDED SEWD FROM KCBM THROUGH SWRN AL TO THE WRN FL PANHANDLE. SURFACE DEWPOINTS IN THE WARM SECTOR ARE IN THE 60S...EXCEPT FOR VALUES AROUND 70 F IN SERN LA TO THE MS COAST WHERE INSTABILITY REMAINS THE GREATEST. TRENDS IN LOW LEVEL WINDS HAVE INDICATED VEERING TO SWLY. HOWEVER...AREA WSR-88D VWPS ALSO SHOWED STRENGTHENING DEEP LAYER WIND FIELDS AS THE WELL-DEFINED SHORTWAVE TROUGH PROGRESSES EWD INTO THE MID AND LOWER MS VALLEY REGION. THIS IS MAINTAINING AND/OR SLIGHTLY ENHANCING LOW LEVEL SHEAR /SFC-1 KM OF 35-40 KT AT SLIDELL LA AND MOBILE AL/. DEEP LAYER SHEAR OF 35-45 KT WILL CONTINUE TO FAVOR THE DEVELOPMENT OF DISCRETE CELLS AHEAD OF THE AFOREMENTIONED PRIMARY WIND SHIFT...WITH THE TORNADO THREAT CONTINUING AS FORECAST IN THE SUMMARY SECTION ABOVE. ..PETERS.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...TAE...BMX...MOB...JAN...LIX... LAT...LON 30139082 32108925 33718823 33588738 33118671 33098555 31828607 30888637 30078651 29848813 29928834 30178841 30198902 30068947 30108987 30029022 29899053 29869087 30139082  092 ACUS11 KWNS 201349 SWOMCD SPC MCD 201349 ILZ000-IAZ000-MOZ000-201745- MESOSCALE DISCUSSION 2188 NWS STORM PREDICTION CENTER NORMAN OK 0749 AM CST THU DEC 20 2012 AREAS AFFECTED...N-CNTRL/NERN MO...SERN IA...W-CNTRL IL CONCERNING...BLIZZARD VALID 201349Z - 201745Z SUMMARY...SNOWFALL RATES NEAR 1 IN PER HR ARE FORECAST TO SPREAD E/NEWD INTO PARTS OF SERN IA AND W-CNTRL IL THROUGH LATE MORNING. STRENGTHENING WIND SPEEDS WILL ALSO LEAD TO SIGNIFICANT REDUCTIONS IN VISIBILITIES...CREATING BLIZZARD CONDITIONS BY 18Z. DISCUSSION...A WELL DEFINED DEEP-LAYER CYCLONE /EVIDENT ON WATER VAPOR IMAGERY/ WILL CONTINUE SHIFTING E-NEWD...WITH A DRY SLOT PROTRUDING NWD INTO SERN IA/W-CNTRL IL. 20/12Z KDVN RAOB SAMPLED A WARM NOSE OF ABOVE FREEZING TEMPERATURES WITHIN THE 900-700 MB LAYER...WITH MODERATELY STEEP MIDLEVEL LAPSE RATES SUPPORTING ISOLATED TSTM DEVELOPMENT. WITH TIME...DYNAMIC COOLING AND LOW-LEVEL CAA SHOULD RESULT IN THE ENTIRE TEMPERATURE PROFILE FALLING BELOW FREEZING /PRIMARILY AFTER 15Z ACROSS FAR NERN MO/SERN IA/...AND A SUBSEQUENT CHANGEOVER TO SNOW. ALTHOUGH THE STRONGEST FORCING FOR ASCENT WILL BE SHIFTING E OF THE HIGHLIGHTED AREA...SNOWFALL RATES NEAR 1 INCH PER HR WILL STILL BE POSSIBLE...ESPECIALLY WHERE LOCALIZED CONVECTIVE ENHANCEMENT MAY OCCUR. ADDITIONALLY...13Z SFC OBS SHOW A 988 MB CENTERED JUST E OF KUIN. AS THIS SHIFTS ENEWD...TIGHTENING OF THE SFC PRESSURE GRADIENT ALONG ITS WRN/NWRN PERIPHERY WILL OCCUR AND RESULT IN INCREASING WIND SPEEDS THROUGH 18Z. 06Z NAM FORECAST MODEL SOUNDINGS SUGGEST WIND GUSTS AS HIGH AS 50 MPH WILL BE POSSIBLE...WHICH WILL LEAD TO SIGNIFICANT REDUCTIONS IN VISIBILITIES AND BLIZZARD CONDITIONS. ..ROGERS.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...LSX...DVN...DMX...EAX... LAT...LON 41429040 40669088 39749172 39509181 39249276 39449323 39799340 40829308 42119149 42239097 42049059 41429040  361 ACUS11 KWNS 201449 SWOMCD SPC MCD 201449 FLZ000-GAZ000-ALZ000-201545- MESOSCALE DISCUSSION 2189 NWS STORM PREDICTION CENTER NORMAN OK 0849 AM CST THU DEC 20 2012 AREAS AFFECTED...SERN AL...FL PANHANDLE...SWRN GA CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY VALID 201449Z - 201545Z PROBABILITY OF WATCH ISSUANCE...80 PERCENT SUMMARY...TORNADO THREAT IS INCREASING ACROSS THE FL PANHANDLE AS MARITIME TROPICAL AIRMASS BEGINS TO SPREAD INLAND. NWD EXTENT OF THIS RETURNING LOW LEVEL MOISTURE WILL BE THE GREATEST LIMITING FACTOR IN SEVERE POTENTIAL. DISCUSSION...BOUNDARY LAYER AIRMASS IS SLOWLY RECOVERING ACROSS THE FL PANHANDLE...ESPECIALLY ALONG THE COAST WHERE SFC DEW POINTS ARE NOW RISING THROUGH THE MID 60S. AS THIS AIRMASS SPREADS INLAND INCREASING BUOYANCY SHOULD ENHANCE THE PROSPECT FOR ROBUST UPDRAFTS WITHIN A STRONGLY SHEARED ENVIRONMENT...QUITE FAVORABLE FOR ROTATING UPDRAFTS AND ISOLATED SUPERCELLS. UPSTREAM...ZONE OF MULTI-BANDED PRE-FRONTAL CONVECTION WITH EMBEDDED SUPERCELLS IS PROGRESSING ACROSS AL/WRN FL PANHANDLE INTO THIS RECOVERING AIRMASS. FOR THIS REASON A NEW TORNADO WATCH WILL BE ISSUED TO ACCOUNT FOR ISOLATED TORNADO THREAT. ..DARROW/CORFIDI.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...JAX...TAE...BMX...MOB... LAT...LON 31888519 31068314 29608346 29648538 30788647 31888519  752 ACUS11 KWNS 201557 SWOMCD SPC MCD 201556 GAZ000-ALZ000-FLZ000-MSZ000-LAZ000-201700- MESOSCALE DISCUSSION 2190 NWS STORM PREDICTION CENTER NORMAN OK 0956 AM CST THU DEC 20 2012 AREAS AFFECTED...SERN MS...AL...WRN FL PANHANDLE CONCERNING...TORNADO WATCH 689... VALID 201556Z - 201700Z THE SEVERE WEATHER THREAT FOR TORNADO WATCH 689 CONTINUES. SUMMARY...SEVERE THREAT WILL SPREAD FROM WEST TO EAST ACROSS THE CENTRAL GULF STATES THIS MORNING WITH THE GREATER TORNADO THREAT SHIFTING ACROSS SRN AL/FL PANHANDLE. DISCUSSION...WRN MOST SQUALL LINE AHEAD OF THE COLD FRONT IS ADVANCING EWD AT ROUGHLY 35-40KT AND GIVEN THIS MOTION IT SHOULD APPROACH THE ERN PART OF WW689 BY 1730Z. ERN MOST PRE-FRONTAL CORRIDOR OF CONVECTION WITH EMBEDDED SUPERCELLS IS MOVING A BIT SLOWER AND WILL LIKELY BE OVERCOME BY FASTER MOVING FRONTAL SQUALL LINE ACROSS THE CNTRL FL PANHANDLE. GREATEST SEVERE RISK APPEARS TO BE WITH ROTATING STRUCTURES THAT ARE ROOTED WITHIN MORE MOIST/BUOYANT AIRMASS LOCATED WITHIN ROUGHLY 75 MI OF COAST. ..DARROW.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...FFC...BMX...MOB...JAN...LIX...LCH... LAT...LON 33199004 33208519 30108702 30109172 33199004  080 ACUS01 KWNS 201632 SWODY1 SPC AC 201630 DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 1030 AM CST THU DEC 20 2012 VALID 201630Z - 211200Z ...THERE IS A SLGT RISK OF SVR TSTMS FROM THE ERN GULF CST NE INTO THE ERN CAROLINAS... ...SYNOPSIS... INTENSE MO UPR LOW/TROUGH WILL CONTINUE NE TO NRN IND THIS EVE...AND ENE TO LK ERIE EARLY FRI...ASSUMING A SLIGHTLY NEGATIVE TILT IN THE PROCESS. ATTENDANT SFC LOW NOW NEAR PEORIA SHOULD REACH THE SOUTH BEND AREA THIS EVE...AND SW ONTARIO EARLY FRI...AS A SECONDARY CENTER EVOLVES THIS EVE OVER UPSTATE SC ACCELERATES NEWD TO ERN PA/NJ. VIS SATELLITE LOOP SHOWS A CONFLUENCE BAND MARKING THE ERN EDGE OF RICHER LOW-LVL MOISTURE RETURN ATTM EXTENDING ROUGHLY S-N ACROSS THE MIDDLE PART OF THE FL PANHANDLE /W OF TALLAHASSEE/. THIS FEATURE SHOULD PIVOT EWD THROUGH THE TODAY TODAY...IN TANDEM WITH W-TO-E MOTION OF LARGER SCALE WEATHER SYSTEM. COUPLED WITH MODERATE SFC HEATING/ABSENCE OF WIDESPREAD CLOUD COVER TO ITS E...SETUP SHOULD SUPPORT APPRECIABLE SFC DESTABILIZATION ACROSS N FL AND SRN/CNTRL GA THIS AFTN. BY EVE AND INTO TNGT...LOW-LVL DESTABILIZATION ALSO SHOULD OCCUR FARTHER NE ALONG THE SC/NC CSTL PLN AS VORT LOBE ROUNDING BASE OF CNTRL STATES TROUGH INDUCES WEAK LEE CYCLOGENESIS/STRENGTHENING OF SSWLY LOW-LVL FLOW OVER SC. ...ERN GULF CST TO CAROLINAS TODAY/TNGT... PRE-COLD FRONTAL BAND OF DISCRETE/SEMI-DISCRETE TSTMS/SUPERCELLS SHOULD CONTINUE GENERALLY E INTO DESTABILIZING ENVIRONMENT OVER PARTS OF FL AND CNTRL/SRN GA...EXTENDING A RISK FOR DMGG WIND AND OCCASIONAL TORNADOES GIVEN CHARACTER OF WIND PROFILES. THE STORMS MAY MERGE INTO ONE SW-NE SQLN BY EARLY EVE. THE SVR THREAT MAY ABATE SOMEWHAT FOR AWHILE LATER TODAY/EARLY TNGT OVER ERN GA AND WRN SC AS THE STORMS INGEST DRIER LOW-LVL AIR STREAMING N FROM THE FL PENINSULA. BUT SOME DEGREE OF REJUVENATION/RE-DEVELOPMENT APPEARS LIKELY A BIT LATER TNGT OVER SC AND NC CSTL PLN GIVEN EXPECTED INCREASE IN WIND FIELD/LOW-LVL MOISTURE FLUX AND ASCENT ASSOCIATED WITH SECONDARY CYCLOGENESIS. RECENT INVASION OF POLAR AIR INTO THE WRN ATLANTIC SHOULD LIMIT OVERALL SVR THREAT NWD INTO THE MID ATLANTIC STATES...ALTHOUGH ISOLD SVR WIND GUSTS CANNOT ENTIRELY BE RULED OUT IN THAT AREA TOWARD 12Z FRI. ...MIDWEST/OH VLY... INTENSE FORCED ASCENT ASSOCIATED WITH JET EXIT REGION OF IL UPR LOW/TROUGH...AND LOW-LVL UPLIFT/HEATING ALONG ASSOCIATED COLD FRONTAL SEGMENT MAY SUPPORT LOW-TOPPED SHOWERS/STORMS IN SYSTEM DRY SLOT OVER WRN IND. SHOULD THIS ACTIVITY SUFFICIENTLY STEEPEN...THE CONVECTION MAY POSE A RISK FOR AN ISOLD DMGG WIND OR TWO AND/OR SMALL HAIL TIL LATE AFTN. ..CORFIDI/COHEN.. 12/20/2012  483 ACUS11 KWNS 201653 SWOMCD SPC MCD 201652 ILZ000-IAZ000-MOZ000-202245- MESOSCALE DISCUSSION 2191 NWS STORM PREDICTION CENTER NORMAN OK 1052 AM CST THU DEC 20 2012 AREAS AFFECTED...FAR NERN MO...FAR ERN IA...CNTRL/NRN IL CONCERNING...BLIZZARD VALID 201652Z - 202245Z SUMMARY...BLIZZARD CONDITIONS WILL OVERSPREAD AREAS ALONG AND EAST OF THE MS RIVER LATE THIS MORNING INTO THE AFTERNOON...AS A STRONG SFC LOW DEEPENS AND SNOWFALL INTERACTS WITH VERY STRONG WINDS. DISCUSSION...RECENT WATER VAPOR/IR SATELLITE LOOPS DEPICT A WELL-DEFINED COMMA HEAD STRUCTURE WITH A COMPACT VORT CENTER APPROACHING THE MS RIVER N OF ST. LOUIS. AN ASSOCIATED 985-MB SFC LOW IS ANALYZED JUST NW OF PEORIA AT 16Z. ENHANCED CONVECTIVE INSTABILITY AND WEAK WAA WITHIN THE BRANCH OF THE BIFURCATED WARM CONVEYOR BELT THAT WRAPS AROUND THE CYCLONE WILL SUPPORT BANDS OF MODERATE TO LOCALLY HEAVY SNOW W/SW OF THE SFC LOW...WHICH IS FORECAST TO TRACK NEWD AND REACH CHICAGO BY MID/LATE AFTERNOON. SNOWFALL RATES OF 0.5-1 IN/HR WILL BE COMMON WITH THIS ACTIVITY. AN ISOLATED LIGHTNING STRIKE MAY OCCUR WITH THE MOST INTENSE SNOWFALL RATES. THE TIGHT SFC PRESSURE GRADIENT WITHIN THE WRN SEMICIRCLE OF THE LOW WILL FURTHER TIGHTEN AS THE LOW DEEPENS AT A RATE OF 0.5 TO 1 MB PER HOUR. THIS IS IN RESPONSE TO ITS INTERACTION WITH A SECONDARY SUBTLE VORT MAX NEAR THE APEX OF THE DRY SLOT CURLING INTO THE COMMA HEAD NEAR CHICAGO. FORECAST SOUNDINGS SUGGEST 45-60 KT OF 0-1-KM MEAN FLOW EVOLVING WITHIN THE SWRN QUADRANT OF THE LOW...WHICH WILL LIKELY BECOME MANIFESTED AT THE SFC AS WLY TO NWLY WINDS OF 35-50 MPH WITH GUSTS REACHING 60 MPH. HIGH-RESOLUTION GUIDANCE SUGGESTS THAT THE SPATIOTEMPORAL PHASING OF THESE WINDS WITH THE SNOW FOR 2-4 HOURS WILL OCCUR...CREATING BLIZZARD CONDITIONS WITH NEAR-ZERO VISIBILITIES. BLIZZARD CONDITIONS WILL LIKELY MOVE ACROSS THE MS RIVER PRIOR TO 18Z...REACHING A LINE FROM NEAR ROCKFORD TO SPRINGFIELD BY 20Z...AND A LINE EXTENDING FROM E OF ROCKFORD TO NEAR BLOOMINGTON/NORMAL BY 22Z. INITIALLY...RAIN/SLEET MAY BRIEFLY MIX WITH THE PRECIPITATION BEFORE A QUICK PHASE CHANGE TO SNOW OCCURS AS THE LOWER TROPOSPHERE COOLS. ..COHEN.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...LOT...ILX...LSX...DVN... LAT...LON 42388998 42328872 41278834 39738847 39268919 38969009 39119109 39689159 40029154 40429147 40999125 41429092 42099043 42388998  940 ACUS02 KWNS 201721 SWODY2 SPC AC 201719 DAY 2 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 1119 AM CST THU DEC 20 2012 VALID 211200Z - 221200Z ...NO SVR TSTM AREAS FORECAST... ...SYNOPSIS... AS A LARGE UPPER LOW/TROUGH PROGRESSES EWD ACROSS THE ERN U.S. THIS PERIOD...A SECOND LOW/TROUGH IS FORECAST TO LINGER OFF THE WRN NOAM COAST AS A SERIES OF LEAD WAVES EJECT NEWD CROSS THE PAC NW/NRN CA IN SWLY CYCLONIC FLOW ON THE DOWNSTREAM SIDE OF THE MAIN TROUGH. WITH THE SURFACE COLD FRONT ASSOCIATED WITH THE ERN TROUGH PROGGED TO CONTINUE SHIFTING FARTHER EWD ACROSS THE WRN ATLANTIC AND SWD ACROSS THE GULF OF MEXICO...THUNDER -- ASIDE FROM AN EARLY STRIKE OR TWO INVOF THE FL KEYS -- IS NOT EXPECTED E OF THE ROCKIES. FARTHER W HOWEVER...AS SHOWERS AFFECT THE NWRN CONUS IN CONJUNCTION WITH THE PASSING LEAD WAVES...OCCASIONAL LIGHTNING CANNOT BE RULED OUT -- PARTICULARLY OVER THE HIGHER TERRAIN AREAS WHERE UPSLOPE FLOW PROVIDES ENHANCED ASCENT. ..GOSS.. 12/20/2012  483 ACUS11 KWNS 201725 SWOMCD SPC MCD 201724 GAZ000-ALZ000-201830- MESOSCALE DISCUSSION 2192 NWS STORM PREDICTION CENTER NORMAN OK 1124 AM CST THU DEC 20 2012 AREAS AFFECTED...CENTRAL GA CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE VALID 201724Z - 201830Z PROBABILITY OF WATCH ISSUANCE...60 PERCENT SUMMARY...TORNADO WATCH WILL LIKELY BE REQUIRED ACROSS PARTS OF CNTRL GA IN RESPONSE TO APPROACHING CORRIDOR OF EMBEDDED SUPERCELLS AND PRE-FRONTAL SQUALL LINE. DISCUSSION...PRE-FRONTAL SQUALL LINE OVER AL CONTINUES ITS PROGRESSION EWD AT ROUGHLY 35-40KT. PORTIONS OF THIS LINE HAVE GRADUALLY INCREASED IN INTENSITY/ORGANIZATION OVER THE LAST HOUR...ESPECIALLY ACROSS ELMORE/MONTGOMERY/BUTLER COUNTIES IN AL. THIS ACTIVITY SHOULD SPREAD INTO DOWNSTREAM PORTIONS OF GA BY 19Z WHERE BOUNDARY LAYER AIRMASS IS SLOWLY MOISTENING/WARMING. THERE IS SOME CONCERN THAT 30-40 MI WIDE BAND OF WEAKER CONVECTION MAY IMPEDE HEATING AS IT PRECEDES PRIMARY SQUALL LINE OF INTEREST. WHILE DAMAGING WINDS MAY BE THE PRIMARY SEVERE THREAT WITH CONVECTIVE LINE...ENVIRONMENTAL SHEAR IS VERY STRONG AND ISOLATED TORNADOES CAN NOT BE RULED OUT. ..DARROW/CORFIDI.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...FFC...BMX... LAT...LON 33258520 33738389 33528290 32818263 31858346 32028506 33258520  606 ACUS11 KWNS 201728 SWOMCD SPC MCD 201728 WIZ000-MIZ000-ILZ000-IAZ000-202330- MESOSCALE DISCUSSION 2193 NWS STORM PREDICTION CENTER NORMAN OK 1128 AM CST THU DEC 20 2012 AREAS AFFECTED...CNTRL/ERN/SRN WI...FAR NERN IA...PARTS OF FAR NRN IL...FAR SRN PART OF THE UPPER PENINSULA OF MI CONCERNING...BLIZZARD VALID 201728Z - 202330Z SUMMARY...BLIZZARD CONDITIONS WILL OVERSPREAD THE AREA THIS AFTERNOON. DISCUSSION...FRONTOGENESIS WITHIN THE DEFORMATION ZONE OF THE LARGE EXTRATROPICAL CYCLONE OVER THE SWRN GREAT LAKES REGION IS BEING LOCALLY ENHANCED INVOF A SECONDARY VORT MAX NEARING THE W END OF LAKE MICHIGAN N OF CHICAGO PER WATER VAPOR LOOPS. MODERATE TO LOCALLY HEAVY SNOW IS ONGOING ACROSS PARTS OF CNTRL/SRN WI...AND SNOWFALL INTENSITY WILL INCREASE AS A 985-MB SFC LOW N OF PEORIA IL DEEPENS AND TRACKS TOWARD CHICAGO. SNOWFALL RATES OF 1-1.5 IN/HR WILL BECOME LIKELY BY 1830-1900Z...HIGHEST SE OF A LINE FROM AROUND GREEN BAY WI TO FAR NERN IA. CONVECTIVELY ENHANCED SNOW RATES MAY OCCUR IN ASSOCIATION WITH NARROW ELEVATED CAPE LAYERS PER FORECAST SOUNDINGS...WHICH MAY SUPPORT AN ISOLATED LIGHTNING STRIKE OR TWO. NNELY TO NWLY WINDS WILL CONTINUE TO STRENGTHEN OWING TO THE TIGHTENING SFC PRESSURE GRADIENT OVER THE WRN/NRN QUADRANTS OF THE SFC LOW...WITH SPEEDS OF 25-35 MPH AND GUSTS REACHING 40-50 MPH BECOMING COMMON BY 20Z. THE STRONGEST WINDS MAY...AT TIMES...LAG SLIGHTLY TO THE W OF THE HIGHEST SNOWFALL RATES. HOWEVER...EVEN LIGHT SNOW AND FRESHLY FALLEN SNOW WILL COMBINE WITH THE STRONG WINDS TO CREATE BLIZZARD CONDITIONS WITH VISIBILITIES BELOW ONE-QUARTER MILE. ..COHEN.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...MQT...GRB...LOT...MKX...DVN...ARX... LAT...LON 43049136 43299129 44129082 45188947 45558791 45468697 44948695 44118757 42778851 42478929 42409016 42439070 42599108 43049136  995 ACUS11 KWNS 201810 SWOMCD SPC MCD 201810 INZ000-202015- MESOSCALE DISCUSSION 2194 NWS STORM PREDICTION CENTER NORMAN OK 1210 PM CST THU DEC 20 2012 AREAS AFFECTED...CNTRL IND CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY VALID 201810Z - 202015Z PROBABILITY OF WATCH ISSUANCE...20 PERCENT SUMMARY...AN ISOLATED SVR WIND THREAT MAY OCCUR THIS AFTERNOON. THE ISSUANCE OF A WW IS UNLIKELY. DISCUSSION...A STRONG COLD FRONT WILL TRACK ENEWD ACROSS PARTS OF CNTRL IND THIS AFTERNOON WHILE A SFC LOW SHIFTS NEWD TOWARD CHICAGO. A NARROW CORRIDOR OF SFC HEATING OWING TO POCKETS OF INSOLATION AMIDST UPPER 40S SFC DEWPOINTS IN ADVANCE OF THE FRONT IS SUPPORTING VERY MARGINAL INSTABILITY WITH MLCAPE VALUES OF 50-100 J/KG. HOWEVER...GIVEN THE STRONG SFC-LAYER CONFLUENCE ALONG THE FRONT...A LINE OF LOW-TOPPED CONVECTION WILL TRACK EWD/ENEWD ACROSS CNTRL IND...REACHING THE INDIANAPOLIS AREA BETWEEN 1830Z AND 1930Z. DOWNWARD MOMENTUM TRANSPORT IN THE STRONGEST CONVECTION MAY SUPPORT AN ISOLATED SVR WIND GUST OR TWO GIVEN 50 KT OF 1-KM-AGL SLY FLOW PER INDIANAPOLIS VWP DATA. THESE DATA ALSO INDICATE 35-40 KT OF 0-1-KM BULK SHEAR WITH MODEST DIRECTIONAL SHEAR IN THE LOW LEVELS...AND THE DEVELOPMENT OF A TORNADO CANNOT BE RULED OUT. REGARDLESS...LIMITED BUOYANCY WILL PRECLUDE THE DEVELOPMENT OF A MORE SUBSTANTIAL SVR THREAT OWING TO COLDER MID-LEVEL TEMPERATURES BEING REMOVED WELL TO THE W OF THE REGION. ..COHEN/CORFIDI.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...IWX...IND... LAT...LON 38868660 39198684 39788693 40458704 40758684 40798630 40638572 40248543 39378552 39068588 38918609 38868660  803 ACUS11 KWNS 201931 SWOMCD SPC MCD 201931 GAZ000-FLZ000-ALZ000-202030- MESOSCALE DISCUSSION 2195 NWS STORM PREDICTION CENTER NORMAN OK 0131 PM CST THU DEC 20 2012 AREAS AFFECTED...FL PANHANDLE...SERN AL...SWRN GA CONCERNING...TORNADO WATCH 690... VALID 201931Z - 202030Z THE SEVERE WEATHER THREAT FOR TORNADO WATCH 690 CONTINUES. SUMMARY...CONVECTION HAS GRADUALLY WEAKENED ACROSS MUCH OF NRN/CNTRL PORTIONS OF TORNADO WATCH. SEVERE THREAT APPEARS TO BE FOCUSING NEAR THE COAST. DISCUSSION...MAJORITY OF UPDRAFTS WITHIN CONVECTIVE CORRIDOR THAT EXTENDS INLAND FROM THE NRN FL PANHANDLE INTO CNTRL GA APPEAR TO BE GRADUALLY WEAKENING. LIGHTNING OVER LAND HAS ALL BUT DISSIPATED THOUGH PRE-FRONTAL SQUALL LINE REMAINS FAIRLY ORGANIZED AS IT PROGRESSES INTO WCNTRL GA. MORE BUOYANT AIRMASS CURRENTLY RESIDES OVER THE NERN GOM AND 18Z SOUNDING FROM TLH EXHIBITS MEAGER INSTABILITY AND POOR LAPSE RATES. FOR THIS REASON IT APPEARS THE GREATEST SEVERE RISK WILL BE NOTED WITH NEAR COASTAL SUPERCELL STRUCTURES THAT MOVE OFF THE WARMER WATERS BEFORE WEAKENING. WITH TIME ADDITIONAL WEAK DESTABILIZATION IS EXPECTED AHEAD OF CLOUD CANOPY OVER NERN FL. LOW LEVEL LAPSE RATES ARE SLOWLY STEEPENING BUT UNTIL BOUNDARY LAYER MOISTURE INCREASES ACROSS THIS REGION THE PROSPECT FOR ROBUST ORGANIZED SEVERE UPDRAFTS WILL BE DELAYED. ..DARROW.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...FFC...TAE...BMX...MOB... LAT...LON 31878598 31888326 29618462 29598730 31878598  516 ACUS01 KWNS 201956 SWODY1 SPC AC 201954 DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0154 PM CST THU DEC 20 2012 VALID 202000Z - 211200Z ...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PORTIONS OF THE SOUTHEAST AND INTO THE COASTAL CAROLINAS AREA... ...DISCUSSION... THE GENERAL FORECAST SCENARIO REMAINS VALID...WITH MAINLY TWEAKS TO THE AREAL OUTLINES TO REFLECT LATEST CONVECTIVE TRENDS. WILL REMOVE THE 30% WIND AREA...AS MIDDAY TLH /TALLAHASSEE FL/ RAOB REVEALS VERY LITTLE INSTABILITY -- SUGGESTING SOMEWHAT LESS INTENSE/WIDESPREAD THUNDERSTORMS THAN EXPECTED. OTHERWISE...THREAT FOR LOCALLY DAMAGING WIND GUSTS AND POSSIBLY A TORNADO OR TWO WILL CONTINUE ACROSS THE ERN GULF COAST AREA THIS AFTERNOON...WITH SOME THREAT EXPANDING NEWD INTO THE ATLANTIC COAST STATES THIS EVENING/OVERNIGHT. ..GOSS.. 12/20/2012 .PREV DISCUSSION... /ISSUED 1030 AM CST THU DEC 20 2012/ ...SYNOPSIS... INTENSE MO UPR LOW/TROUGH WILL CONTINUE NE TO NRN IND THIS EVE...AND ENE TO LK ERIE EARLY FRI...ASSUMING A SLIGHTLY NEGATIVE TILT IN THE PROCESS. ATTENDANT SFC LOW NOW NEAR PEORIA SHOULD REACH THE SOUTH BEND AREA THIS EVE...AND SW ONTARIO EARLY FRI...AS A SECONDARY CENTER EVOLVES THIS EVE OVER UPSTATE SC ACCELERATES NEWD TO ERN PA/NJ. VIS SATELLITE LOOP SHOWS A CONFLUENCE BAND MARKING THE ERN EDGE OF RICHER LOW-LVL MOISTURE RETURN ATTM EXTENDING ROUGHLY S-N ACROSS THE MIDDLE PART OF THE FL PANHANDLE /W OF TALLAHASSEE/. THIS FEATURE SHOULD PIVOT EWD THROUGH THE TODAY TODAY...IN TANDEM WITH W-TO-E MOTION OF LARGER SCALE WEATHER SYSTEM. COUPLED WITH MODERATE SFC HEATING/ABSENCE OF WIDESPREAD CLOUD COVER TO ITS E...SETUP SHOULD SUPPORT APPRECIABLE SFC DESTABILIZATION ACROSS N FL AND SRN/CNTRL GA THIS AFTN. BY EVE AND INTO TNGT...LOW-LVL DESTABILIZATION ALSO SHOULD OCCUR FARTHER NE ALONG THE SC/NC CSTL PLN AS VORT LOBE ROUNDING BASE OF CNTRL STATES TROUGH INDUCES WEAK LEE CYCLOGENESIS/STRENGTHENING OF SSWLY LOW-LVL FLOW OVER SC. ...ERN GULF CST TO CAROLINAS TODAY/TNGT... PRE-COLD FRONTAL BAND OF DISCRETE/SEMI-DISCRETE TSTMS/SUPERCELLS SHOULD CONTINUE GENERALLY E INTO DESTABILIZING ENVIRONMENT OVER PARTS OF FL AND CNTRL/SRN GA...EXTENDING A RISK FOR DMGG WIND AND OCCASIONAL TORNADOES GIVEN CHARACTER OF WIND PROFILES. THE STORMS MAY MERGE INTO ONE SW-NE SQLN BY EARLY EVE. THE SVR THREAT MAY ABATE SOMEWHAT FOR AWHILE LATER TODAY/EARLY TNGT OVER ERN GA AND WRN SC AS THE STORMS INGEST DRIER LOW-LVL AIR STREAMING N FROM THE FL PENINSULA. BUT SOME DEGREE OF REJUVENATION/RE-DEVELOPMENT APPEARS LIKELY A BIT LATER TNGT OVER SC AND NC CSTL PLN GIVEN EXPECTED INCREASE IN WIND FIELD/LOW-LVL MOISTURE FLUX AND ASCENT ASSOCIATED WITH SECONDARY CYCLOGENESIS. RECENT INVASION OF POLAR AIR INTO THE WRN ATLANTIC SHOULD LIMIT OVERALL SVR THREAT NWD INTO THE MID ATLANTIC STATES...ALTHOUGH ISOLD SVR WIND GUSTS CANNOT ENTIRELY BE RULED OUT IN THAT AREA TOWARD 12Z FRI. ...MIDWEST/OH VLY... INTENSE FORCED ASCENT ASSOCIATED WITH JET EXIT REGION OF IL UPR LOW/TROUGH...AND LOW-LVL UPLIFT/HEATING ALONG ASSOCIATED COLD FRONTAL SEGMENT MAY SUPPORT LOW-TOPPED SHOWERS/STORMS IN SYSTEM DRY SLOT OVER WRN IND. SHOULD THIS ACTIVITY SUFFICIENTLY STEEPEN...THE CONVECTION MAY POSE A RISK FOR AN ISOLD DMGG WIND OR TWO AND/OR SMALL HAIL TIL LATE AFTN.  746 ACUS11 KWNS 202034 SWOMCD SPC MCD 202034 MIZ000-210130- MESOSCALE DISCUSSION 2196 NWS STORM PREDICTION CENTER NORMAN OK 0234 PM CST THU DEC 20 2012 AREAS AFFECTED...NRN LOWER MI...CNTRL/ERN UPPER MI CONCERNING...HEAVY SNOW VALID 202034Z - 210130Z SUMMARY...HEAVY SNOW WITH RATES OF 1-1.5 IN/HR ALONG WITH LOCALIZED BLIZZARD CONDITIONS ARE EXPECTED TO DEVELOP BY 2200Z AND CONTINUE INTO THE EVENING HOURS. DISCUSSION...WATER VAPOR LOOPS DEPICT A WELL-DEFINED BAROCLINIC LEAF STRUCTURE EVOLVING DOWNSTREAM OF A DEEP CYCLONE APPROACHING THE CHICAGO AREA FROM THE SW. ACCOMPANYING STRONG DIVERGENCE WITHIN A COUPLED UPPER JET STRUCTURE ATOP LOW/MID-LEVEL FRONTOGENESIS WILL SUPPORT AN INCREASE IN SNOW RATES DURING THE NEXT FEW HOURS. DEEP ASCENT THROUGH DENDRITIC GROWTH ZONES WITHIN THE 700-500-MB LAYER WILL SUPPORT EFFICIENT SNOW GROWTH...WITH ISOTHERMAL PROFILES IN THE 850-700-MB LAYER YIELDING EFFICIENT SNOW AGGREGATION. SNOWFALL RATES OF 1-1.5 INCHES WILL LIKELY BECOME COMMON BY 2200Z...AS ONE OR MORE NE/SW-ORIENTED SNOW BANDS DEVELOP WHILE THE SFC LOW APPROACHES. THE ENHANCED SFC PRESSURE GRADIENT WILL SUPPORT NNELY TO ENELY WINDS OF 20-25 MPH WITH GUSTS REACHING 30 TO OCCASIONALLY 35 MPH. THIS WILL SUPPORT REDUCED VISIBILITIES TO ONE-HALF MILE IN BLOWING SNOW...AND LOCALIZED BLIZZARD CONDITIONS MAY OCCUR. THE AREAS OF GREATEST CONCERN FOR BLIZZARD CONDITIONS WILL BE LAKESHORE LOCATIONS IN THE IMMEDIATE LEE OF THE GREAT LAKES...WHERE REDUCED FRICTIONAL EFFECTS OVER THE WATER WILL BE MANIFESTED. HOWEVER...THE ABSENCE OF A TIGHTER SFC PRESSURE GRADIENT WILL LIKELY PREVENT THE DEVELOPMENT OF MORE WIDESPREAD BLIZZARD CONDITIONS. ..COHEN.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...APX...MQT... LAT...LON 45958396 45778427 45598412 45508384 45348345 45058363 44748436 44568524 44428608 44918604 45228572 45498561 45738592 45548687 45728770 46128756 46448690 46688616 46748494 46348411 45958396  610 ACUS11 KWNS 202117 SWOMCD SPC MCD 202117 CAZ000-210215- MESOSCALE DISCUSSION 2197 NWS STORM PREDICTION CENTER NORMAN OK 0317 PM CST THU DEC 20 2012 AREAS AFFECTED...PARTS OF NRN CA CONCERNING...HEAVY SNOW VALID 202117Z - 210215Z SUMMARY...HEAVY SNOW WITH RATES INCREASING TO 1-2 IN/HR IS EXPECTED TO OCCUR AFTER 2200Z AND CONTINUE INTO THIS EVENING. DISCUSSION...RECENT WATER VAPOR LOOPS DEPICT A RIBBON OF ASCENT LEADING A DEEP TROUGH THAT IS AMPLIFYING OVER THE E PACIFIC. THE ASCENT WILL REMAIN JUXTAPOSED WITH A PLUME OF ENHANCED PACIFIC MOISTURE FEATURING PW VALUES AROUND 0.6-0.75 INCH THAT EXTENDS NEWD ACROSS THE E PACIFIC. THIS WILL SUPPORT AN INCREASE IN PRECIPITATION RATES DURING THE NEXT COUPLE OF HOURS. SNOW WILL BE THE PRIMARY PRECIPITATION TYPE FOR ELEVATIONS ABOVE 2000-3000 FT. THE ASCENT WILL BE AUGMENTED BY UPSLOPE FLOW OVER S-FACING MOUNTAIN SLOPES...WHILE MID-LEVEL COOLING ACCOMPANYING THE AMPLIFYING TROUGH MAY PROVIDE A WEAK CONVECTIVE ENHANCEMENT TO PRECIPITATION. AS SUCH...SNOWFALL RATES WILL LIKELY INCREASE TO 1-2 IN/HR OVER THE MOUNTAINS OF NRN CA AFTER 2200Z...INCLUDING THE KLAMATH AND SISKYOU MOUNTAINS. THESE SNOWFALL RATES WILL LIKELY CONTINUE INTO THE EVENING HOURS. ..COHEN.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...MFR...STO...EKA... LAT...LON 41512182 41132194 40892225 40572250 40392281 40042272 39932289 39962319 40342356 41212374 41822361 41972309 41912211 41512182  306 ACUS11 KWNS 202322 SWOMCD SPC MCD 202322 FLZ000-GAZ000-210015- MESOSCALE DISCUSSION 2198 NWS STORM PREDICTION CENTER NORMAN OK 0522 PM CST THU DEC 20 2012 AREAS AFFECTED...NORTH FL/SOUTHEAST GA CONCERNING...TORNADO WATCH 690... VALID 202322Z - 210015Z THE SEVERE WEATHER THREAT FOR TORNADO WATCH 690 CONTINUES. SUMMARY...TORNADO WATCH 690 WILL BE ALLOWED TO EXPIRE AT/BY 00Z...BUT A NEW WATCH MAY BE NEEDED TO THE EAST OF WW 690 ACROSS NORTHEAST FL/SOUTHEAST GA. DAMAGING WINDS WOULD BE THE PRIMARY THREAT...BUT AN ISOLATED TORNADO MAY BE POSSIBLE AS WELL. DISCUSSION...A QUASI-LINEAR CONVECTIVE COMPLEX WITH LEADING STRATIFORM AND TRAILING/EMBEDDED SUPERCELLULAR STRUCTURE CONTINUES TO STEADILY SHIFT EASTWARD ACROSS NORTH FL/FAR SOUTHERN GA EARLY THIS EVENING. THE AIRMASS CONTINUES TO COOL IN VICINITY OF THE STORMS...AND A WARMING OF CLOUD TOPS PER IR SATELLITE IMAGERY HAS BEEN NOTED ALONG WITH AN ABSENCE OF CLOUD-GROUND LIGHTNING. EVEN SO...A HIGHLY SHEARED ENVIRONMENT /300 0-1 KM SRH PER JAX WSR-88D VWP/ WILL HELP TO MAINTAIN EMBEDDED ROTATIONAL STRUCTURES WITH EPISODIC POTENTIAL FOR DAMAGING WINDS/PERHAPS A TORNADO. ..GUYER/CARBIN.. 12/20/2012 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...TBW...JAX...TAE... LAT...LON 29628342 31178268 31158138 30418123 29098246 29628342