For Iowa State students, this is Finals week. For K-12 students, just a few weeks of school remain until summer break. So it is a good time to think about summer and present a metric to compute when the summer season begins. If you consider the summer season as the warmest consecutive 91 day period (1/4th of the year) each year, the featured chart plots the start date each year for this period for Ames. The dots are colored by the average temperature departure for this period over all years. A simple linear trend line is plotted as well. The overall average is around 7 June, which just over three weeks away, but plenty of years got started in just over week from now.

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Our mostly dry and lack of humidity start to May continued on Tuesday with dew point temperatures in the 30s and lower 40s. The featured chart presents a climatology of daily maximum dew point as calculated by available hourly observations from the Des Moines airport. The 2026 observations are shown as blue dots on the top panel. Each of the daily dots for May is found below a simple long term average for the day. This is in stark contrast to the elevated humidity levels that were common for much of April.

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It has been a rather strange start to the growing season. The month of April was quite warm and wet, leading to an early start to the growing season with plenty of growing degree days accumulated prior to the start of May. May has now started off rather dry with low humidity levels allowing overnight temperatures to cool below freezing over a number of days so far this May. USDA NASS released their weekly crop progress report yesterday and estimated 72% of Iowa's corn crop having already been planted, a 30 percentage point increase over the estimate last week thanks to the mostly dry start to May. The featured chart looks into the weekly percentage point change in crop planting progress as estimated by NASS. The left hand labels show the year and the maximum weekly change value for each year. A 30 percentage point change is about at the middle of possible yearly maximum values.

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It is difficult to imagine the weather on Mothers Day being any better with ample sunshine, a cool breeze, low humidity, and high temperatures around 70°F. The featured chart presents the yearly Mothers Day high temperature as recorded at Des Moines. The previous two years had quite warm temperatures with highs above 80°F, but this year's high of 72°F was just a degree or so above a simple long term average value. While most mothers probably would have preferred it not to rain yesterday, much of the state is starting to need rain again as the growing season is now fully underway and it has been about two weeks since Iowa's most recent widespread and significant rainfall. The near term forecast is not very optimistic about upcoming rainfall events.

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Temperatures were rather pleasant on Thursday with mid 60s common over much of Iowa during the afternoon hours. Ames reported a temperature of 66°F at 3 PM. The featured chart looks into the frequency of having such a temperature by month for Ames. The bars present the frequency of having a temperature within the inclusive range of 63 to 67°F at 2 PM LST (3 PM CDT) with the blue dots showing the frequency below that range and red dots above. May does have the highest frequency of such temperatures, but only by a slim margin to April. It is kind of interesting to denote the near identical frequencies between February and July!

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The start to May has seen a mix of warm and chilly days, but the lack of humidity has been consistent so far. The featured chart presents a daily climatology of humidity for Ames as expressed by mixing ratio (ratio of water to air). The top panel shows the daily range of observed values with the red line showing the 2026 values to date. The bottom panel shows the daily departure from the simple average. The start of May has seen values near the bottom of the observed range with the departure from average being the largest so far this year. Such abundant dry air is not conducive to precipitation, but the very wet April and cooler air temperatures have limited any substantial impacts so far. While the near term forecast does have precipitation chances, totals do not appear to be all that substantial.

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For Des Moines, the high temperature on Tuesday was just 59°F. The rest of the state did not fare much better with most below 60°F for the day. The featured chart presents the day of May frequency of having a high temperature below 60°F for Des Moines. The overall frequency for a single May is about 3.5 days, so these events are not terribly rare. The chart does nicely show the near linear decline in frequency as the month progresses, but another repeat looks to be in store today before warmer weather returns starting Thursday.

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A front pushed through the state on Monday and while some portions of southeastern Iowa received rainfall, near surface dry air won out for the majority of the state. The featured chart presents dew point temperature climatology by wind direction based on period of record observations from Des Moines. The statistics are produced by computing vapor pressures first and then back-computing the dew point from those values so to better account for the non-linearity of dew point. The observed values for Monday are plotted as well for comparison. All of the values plotted are below the mean value with a few approaching the 5th percentile.

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After a chilly start to Saturday morning, temperatures quickly recovered and made for an excellent weekend to start off May. How cold it got Saturday morning is the subject of today's featured map displaying computed NCEP RTMA minimum temperature between 3 and 8 AM. So-called local micro-climates can introduce a fair bit of uncertainty into a map like this as subtle topographic and soil type differences can cause major differences in temperature over short distances. Normally, such an event on the second day of May would not be of much concern, but as was mentioned by the daily feature a few days ago, the significant early season warmth has gotten vegetation off on an early start and early May freeze events will continue to be of concern.

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The start of a new month is always a good time to review what happened for the previous month. The featured chart reviews hourly aviation flight rules computed from the Des Moines airport weather station. These rules consider cloud ceilings and horizontal visibility among other things to restrict aviation traffic. The featured chart only considers the weather station horizontal visibility distance and vertical distance to cloud coverage either in "broken" or "overcast" state. The chart is a nice first guess to indicate periods where there was likely poor flying weather conditions due to visibility reductions. The stormy start to April is most noticeable with reductions more isolated after the first few days.

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The forecast has chilly and even freezing low temperatures possible over the next few days. Typically, this would not necessarily be a big deal as the calendar is only at the end of April and climatology says we have a week or two to go before freeze risks substantially subside. This spring has been a bit different with plenty of very warm days and the growing season has gotten off to a very fast start. When this happens, the NWS will issue such alerts like Frost Advisories and Freeze Warnings even though climatology says freezing temperatures are more common than not. The featured chart presents an IEM estimate of the number of growing degree days since March for an Iowa statewide estimate. The 2026 total certainly stands out and comes in fifth largest since 1893.

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The weather this April has certainly been interesting with plenty of warmth and precipitation. The featured chart compares April growing degree day departures with precipitation departures for April over a statewide areal average computed by the IEM. The departures are expressed in units of standard deviation to allow a normalized comparison between the two variables. The 2026 combination stands out for being both anomalously warm and wet, while also being counter trend which favors reduced precipitation with increasing temperature. The last two days of April will be on the cooler and drier side of average, so the 2026 will value will be pulled marginally back to the middle if you generate this plot once April completes!

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While the severe weather threat on Monday mostly missed Iowa, nearby states were not as fortunate. NWS St Louis had an extremely busy day with severe weather persisting throughout much of the day. Based on unofficial IEM data, the office issued the most combined Severe Thunderstorm + Tornado + Flash Flood Warnings for a calendar day since at least 1986. The 95 warnings were 15 more than the previous day max on 25 May 2011. Thankfully, a break from the severe weather and heavy rainfall events is currently forecast for the remainder of the week. The only downside will be cool temperatures, which will not be of much help for crops already planted this growing season.

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The weather on Saturday (April 25th) was nothing short of amazing with hazy sunshine, pleasant temperatures, low humidity, and a gentle breeze. The weather and date matched the description by Ms Rhode Island in the movie "Miss Congeniality" of a "perfect date" being "April 25th, because it's not too hot, not too cold. All you need is a light jacket." So the daily feature chart plots the frequency by day of the year of having a high temperature in the low 70s, a daily average wind speed below 10 MPH, and a maximum dew point below 50°F. The black line plots the combined combination of those three. The spring and fall months of April, May, September, and October are generally the preferred time of year for such a combination to occur. The weather today will much different than Saturday with chances of severe weather along with more windy and humid conditions.

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Severe weather returned to Iowa on Thursday with afternoon thunderstorms firing along a cold front over western Iowa and slowly pushed east through the evening hours. The NWS issued a number of severe warnings for damaging wind, hail, and a few brief tornadoes. Polk County (Des Moines) received a few Tornado warnings during the early evening hours with the first being issued just after 7 PM. The timing of Tornado Warnings for Polk County is the subject of today's daily feature. The plot presents the frequency of the time of day when tornado warnings are active. Note that this is not the overall frequency, but plots the relative frequency to the total number of warnings issued for Polk County. Around 5 PM is the frequency winner with just over 10% of the total 98 warnings active about at that time. It is interesting to see the low frequencies during the early afternoon hours with a dramatic ramp after about 3:30 PM. Storms occurring later in the afternoon typically have more potential energy to tap into due to the additional daytime heating that adds instability to the atmosphere. Storms occurring earlier typically have less potent environments to work with to produce severe weather and even tornadoes.

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While the recent rains have significantly helped Iowa's drought situation, portions of extreme northwestern Iowa have a bit to go yet before erasing analyzed drought over that portion of the state. The featured chart presents IEM estimated precipitation over the northwestern Iowa climate district during the months of February through April. Of course, the 2026 total still has eight more days to go, but the current total comes in about second driest over the past 25 some years. The 2026 total is above the top ten driest though as shown by the right hand side tables. A number of significant rainfall chances are in the forecast to close out April, but it remains to be seen if this driest area of the state will receive any of the largest forecast totals over Iowa.

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Temperatures started off warm Tuesday morning and warmed into the 80s for almost all of Iowa. For Des Moines, the warm-up was 21 degrees between a 9 AM temperature of 63°F to a 5 PM temperature of 84°F (hourly obs listing. The featured chart takes a bit of a deep dive into air temperature changes between 9 AM and 5 PM based on hourly Des Moines airport data. The chart partitions the data by week of the year and plots the frequency of a given temperature change. Negative values indicate a cooler 5 PM temperature than 9 AM of the same day. Values plotted in tan indicate no observations at the given temperature change and week combination. The dashed line is a 14 day smoothed climatology and blue line are 2026 observations, also smoothed to reduce daily noise. The highest frequencies are shown during the summer months when elevated humidity levels reduce temperature variability. The fall season has the largest heating values on average as residual soil warmth from summer and decreased humidity allow for stronger daytime heating.

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With the quiet weather to start this week, the National Weather Service continues to work through the backlog of severe weather reports and completing storm surveys from the activate previous week. One of those reports was a 88 MPH wind gust from the Dubuque Airport last Tuesday. The one minute interval data from NCEI is now available from this event and the subject of today's featured chart. The chart the one minute interval data between 6 and 7 PM on 14 April 2026 for Dubuque. Sadly, there is a brief drop out of the wind data prior to the 88 MPH gust, but the peak is shown with three minutes of gusts over 80 MPH and sustained winds over 55 MPH. You can find more details along with RADAR for the NWS Severe Thunderstorm Warning covering this event.

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With the numerous rounds of showers and thunderstorms last week, it is a good time to see how the precipitation totals fared. The featured map presents NOAA MRMS estimates for the inclusive period between 12 and 18 April 2026. The heaviest totals are shown over east central Iowa with isolated areas indicated over five inches. Much of the western half of the state missed out though with a few pockets of no-measurable precipitation shown. The stormy weather of last week gave way to a chilly weekend with Monday starting off below freezing over much of the state. Temperatures will warm as this week progresses with rain chances again returning by Thursday, but the heaviest totals again look to miss the same areas that missed out last week.

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It has been a very active week for severe weather over Iowa. While Thursday was a very welcome break from the rough weather this week, strong thunderstorms are very likely Friday with eastern Iowa that primary target for the worst of the storms. The featured chart presents the daily number of combined Tornado and Severe Thunderstorm Warnings issued by the National Weather Service for counties within Iowa. It is likely that four days for this week will have a higher total of such warnings than any previous day so far this year!

Updated: The featured chart was regenerated to show the Friday total of 74 warnings.

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