It has been a very hot and humid start to September with high temperatures again soaring into the 80s and 90s for Iowa on Thursday. For Des Moines, the first three days of the month have had a high temperature of at least 95°F. The featured chart looks into the frequency of a given day of the year participating within a streak of at least as much days with such a warm high temperature. The earliest and latest streaks are listed off to the side. There have been later streaks into September on record, so this current one isn't exceptionally rare. The hot weather will continue into the Labor Day weekend with only slightly cooler weather expected next week, but still well above average.

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Wednesday was another too-hot-for-September day with high temperatures well into the 80s and even 90s for most of Iowa. Scattered showers did develop over northern Iowa and provided a small respite from the hot temperatures. The featured chart presents one minute interval data from the ISU Soil Moisture station west of Ames and very close to the Farm Progress Show location near Boone. The chart plots air temperature, dew point, solar radiation, wind speed, wind gust, and precipitation. A number of interesting features can be seen during the afternoon hours. It got cloudy as shown by the reduced solar radiation, but only 0.02" of an inch of precipitation was reported during this time. Wind speeds slackened due to reduced solar heating and subsequent reduced vertical mixing down of higher momentum air. Dew point temperatures creeped up during this period as well, which is likely due to some evaporation of water drops before reaching the ground and decreased vertical mixing of transpired water by vegetation. At about 4 PM, sunshine returned and the processes reversed with increased winds, slightly lowered dew point due to vertical mixing and increasing air temperatures. By 6 PM, air temperatures returned to where they started at noon at 90°F. One minute charts are always fun to diagnose!

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The first day of September felt nothing like fall with near record high temperatures and very humid air. For Sioux City, the maximum reported dew point temperature based on hourly reports was 77°F, which was an unofficial record maximum for the month of September. Yes yes, setting such a record on the first day of the month somewhat diminishes the record as it would not be a record during August. That issue aside, the featured chart presents the hourly maximum dew point reports for the site during the month of September. Dew point readings on Monday were buoyed over northern Iowa thanks to "moisture pooling" along a frontal boundary. Hot and marginally less humid conditions will continue today as summer rolls on!

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The last day of August was quite hot with high temperatures in the upper 80s and 90s with dew points well into the 70s for Iowa. Summer is certainly not ending with the calendar now turned into September with a number of hot days yet to go. Having high temperatures around 90 degrees for the first of September is nothing too out of the ordinary and in fact, is about the average last date of the year for such a temperature. While the featured chart looks like a funnel, it isn't necessarily a "funnel plot" as the feature title suggests. The plot shows the period each cold season between the average last and first day of the calendar year at a given high temperature for Ames. The chart nicely shows a general rule of thumb for fall Iowa with with the last 90 around 1 September, the last 80 around 1 October, and the last 70 around 1 November. This chart can be kind of depressing if you like the warm temperatures as it shows how long it takes for them to return during the spring season!

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The signs of fall are pretty much everywhere now with even the calendar about to turn into September tomorrow, but the first week of the new month will very much feel like mid summer. The full pull of fall climatology does not arrive for a number of weeks yet as illustrated by the featured map. The map presents the weekly change in high temperature climatology between 31 August to 7 September. Values are rather meager with Iowa shown in the 1 to 2 degrees of cooling over this period. Some of the reasons why summer can stick into September include ample sunshine, elevated humidity levels, active vegetation, and warm soils that take a while to cool off.

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One of the most popular and seemingly internet unique datasets curated by the IEM is an archive of National Weather Service Watch, Warnings, and Advisories. This archive became tenable with the implementation of VTEC within the raw text products during the mid to late 2000s. So with about a 20 year archive of data, climatologies are possible! The featured chart is from a newly added "Autoplot" that computes the monthly average number of NWS Warnings/Advisories by month for Story County (Ames, IA). These alerts are grouped into general categories. Without getting too far off in the weeds here, the NWS issues some of these products for forecast zones and some for counties. Within Iowa, these are the same geographic regions, but this is not the case for most states. So to get reliable stats, the chart combines the "county" and "forecast zone" that covers Story County. The purpose of the chart is to highlight the seasonality of various alert categories and also provide a sense of frequency. Note that this chart does not include watches, but only warnings and advisories. June is clearly the most frequent month for Severe Thunderstorm Warnings, but May is for Tornado Warnings. January is the overall "winner" for most events on average with September having the least. You can generate this chart by NWS forecast office, state, and even FEMA region. Feedback is certainly welcome!

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The start of the school year for most Iowans this week has brought rather comfortable weather for August. The featured chart takes a bit of a deep dive into dew point temperatures during August to show how much better the second half of August has been versus the first. The upper left panel plots hourly dew point percentiles during August using period of record data for Des Moines. The bottom left panel plots the observed hourly dew point percentiles as compared against the August period of record data at the observation hour. The second week of August shows up nicely with high end dew point percentiles throughout the period. Percentile values have decreased since then with some of the higher values only associated with early morning readings. The right hand table lists out the raw percentile values for the month over the period of record.

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The 2026 Iowa State Fair wrapped up this past Sunday with a new record attendance set for the ten day event. After a very soggy start to the fair over the first two days, the weather very much cooperated for the remaining eight days with five of those days setting individual daily attendance records. The featured chart presents an accounting of the number of hourly reports at the Des Moines Airport with an air temperature of at least 80°F during the ten days of the fair each year. The 2026 total of 54 hours comes in about ten hours below a simple long term average. It was also much below the previous year total around 100 hours. The 2026 attendance total topped the previous record set in 2024, which also had not much in the way of hot weather.

Yesterday's daily feature speculated about the upcoming crop yield impact of low solar radiation days during much of this August. Today's featured chart takes a more direct look into this by comparing various weather summary values over the 1-24 August period for Ames to de-trended corn yield departures for Boone County (the county the weather station resides). The top left panel shows the yearly NASS county corn yield estimates with a number of fits plotted. The red line denotes the simple linear trend and is used to compute a yearly departure value used within all other sub-plots. Eight different yearly weather summary values are plotted against this de-trended yield value. Of note for today is the upper left most plot showing daily average solar radiation as estimated by ERA5-Land. Since long term observations of solar radiation generally do not exist, a model is used to estimate the daily values. At least for this plotted metric, the relationship is not all that exciting as a simple explainer for yield outcomes. As is the general comment for plots in this space, if the relationship was simple/easy, folks would be making a fortune in the futures market!

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When one considers the weather variables for a growing season, tangible summaries of temperature, precipitation, and growing degree days along with their associated departures against average are often only considered. The primary reason is that they are well known and observed data is plentiful with long term archives for robust comparisons. Solar radiation, the energy received from the sun, is a critical weather variable for plant growth and development, but observations are not common and climatologies are difficult to produce. The featured chart presents daily solar radiation totals (tan bars) from the ISU Soil Moisture station near Ames for this August. The black line represents a theoretical near-perfectly clear sky maximum value for the given day of the year. A number of days this month have had rather meager radiation totals with five days indicated at 10 megajoules or less. The chart also shows about a two week stretch starting on 4 August with maximum values not reaching the high end of what is possible. So the question is how important this period will be to corn and soybean yields found in the field this fall? It is certainly likely not to have been positive toward maximizing yield potential.

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The featured map presents an unofficial IEM accounting of the 2026 rank for issuance of Flash Flood Warnings by National Weather Service Forecast Office. A value of "1" would indicate the fewest on record for the year to date period since at least 2002. A value of "25" would indicate the most over this same period. The map shows some interesting juxtapositions of a quiet vs a very active year. Much of the eastern half of the corn belt has seen plenty of heavy rainfall events and thus Flash Flood Warnings. The drier areas over the upper Midwestern states is readily seen. The near term forecast does not show much for heavy precipitation events over the coming week, which would be a welcome break for areas current experiencing flooding, but problematic for those needing rain to help maturing crops reach yield potential.

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With the seemingly endless parade of recent days with thunderstorms somewhere over Iowa, it is difficult to imagine that there are still dry spots in the state. The US Drought Monitor is due to be released later this morning and will likely continue to analyzed drought over Iowa. The featured map highlights some of these dry regions by plotting percent of average precipitation since 1 July based on PRISM estimates. Most of the major Iowa urban areas have been quite wet over this period, which is likely one of the reasons why ongoing drought seems impossible. Northwestern Iowa continues to be the area that could use rain the most with some of the other regions shown dry since 1 July having received recent rainfall, but not enough to make up a deficit over this chosen period plotted.

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Severe thunderstorms again frequented the state on Tuesday with the Storm Prediction Center issuing two Severe Thunderstorm Watches to cover the activity. The second watch was issued just before 7 PM Tuesday evening. The watch was effective to kill off the thunderstorm activity and the watch was subsequently cancelled around 8:40 PM. The watch duration of just under two hours is the subject of today's daily feature. The chart presents the duration of severe thunderstorm watches for counties within NWS Des Moines's county warning area. The orange line is the accumulated frequency by duration at issuance. The blue line is the accumulated frequency by final watch duration. Watches can be extended in time, so that is why there is a small overlap near the longest durations. The side panel lists out the shortest and longest watches. The two hour duration comes in at about 5%, so about 1 in 20.

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There continues to be great interest and speculation with what will happen this fall with the rapidly strengthening El Niño. The US government forecast and most every other meteorological organization forecast has the El Niño strengthening to record levels by early winter. There are a number of practical and unknown aspects to this situation that forecasting the impact on Iowa difficult. While the current forecast has rapid strengthening, it is also forecast to rapidly decrease, which can lessen impacts. It is also strengthening during an atypical time of the year, which makes analogs more difficult to find over the historical record. The featured chart compares the 3 month averaged SOI value for August vs a three month average temperature for Iowa. For this metric, larger negative numbers are stronger El Niño. The left panel is colored by SOI value and compares the 3 month average temperature with precipitation. The right two panels isolate the temperature and precip values with SOI. Visually, more of the reddest dots appear within the bottom right quadrant of the left plot (wetter and cooler than average). The current longer term outputs from the CPC are generally neutral over the fall season. It will certainly be interesting to see what happens!

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While there were storms again on Sunday over NWS Davenport/Quad Cities's area, they did not warrant issuance of a warning and so ended a streak of seven consecutive calendar days with the issuance of at least one Tornado or Severe Thunderstorm Warning. The featured calendar chart presents the number of such combined warnings per day for the office during 2026. It has been a busy year for such warnings as denoted by previous features regarding year to date tornado warnings and WFO ranks. A manual query of the IEM database indicates this seven day streak is tied for longest for the DVN office with another seven day stretch from 2 through 8 June 2008. After a bit of a break from the severe weather on Sunday and today, rough weather looks to return to Iowa on Tuesday.

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The parade of thunderstorm complexes looks to continue on Friday and into the weekend. On Thursday, the Weather Prediction Center (WPC) issued a rare, for Iowa, Day 2 "moderate risk" of excessive rainfall for Friday into Saturday morning. The featured map presents the number of such issued risks since the start of the available archive during late 2015. Most of Iowa is shown having less than five such events over this period. Today's moderate risk was issued for southeastern Iowa, which has only slightly few more events on record. The IEM attempts to curate a downloadable archive of these outlooks and also has a simple search tool to find outlook events by a point location.

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This week has featured a parade of early morning thunderstorm complexes. The one on Tuesday morning has been preliminary classified as a "derecho" by the Storm Prediction Center. The featured movie combines a NWS RADAR mosaic with Severe Thunderstorm and Tornado warnings along with NWS Local Storm Reports. The bottom panel plots the report times of the severe wind gusts between 7 AM and 12:30 PM on 11 August 2026. Of course, it is unnerving to have such events occur so close to the anniversary of the 10 August 2020 derecho. The Tuesday event also produced a few tornadoes in Iowa and near Chicago. The Storm Prediction Center has a webpage with more information about derechos.

For the National Weather Service office at Davenport (Quad Cities), it has been a very active year for severe weather. Severe storms once again visited eastern Iowa early Tuesday morning with three EF-1 tornadoes surveyed over Delaware County. Based on unofficial IEM archives, the featured chart presents the number of year to date Tornado Warnings issued by the office since 1986. The 2026 total of 86 warnings easily surpasses any previous year. Unfortunately, more chances of severe weather are in the forecast for much of the remainder of this week.

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Atmospheric soundings are an important observational dataset useful for the forecasting the weather. The often twice-daily balloon launches by the National Weather Service capture vertical profiles of temperature, humidity, pressure, and wind. These profiles can be used to compute many useful indices to access the state of the troposphere. Once such index is Convective Available Potential Energy (CAPE), which is useful for understanding how much potential energy is available for near surface air if it were to be lifted vertically and become buoyant. The CAPE index is not necessarily easy to compute and subject to many interpretations and assumptions. The featured chart presents the CAPE index for surface based "parcels" based on soundings from NWS Davenport. The balloon launch Monday evening (00 UTC 11 August 2026) captured a very unstable environment with a CAPE index around 7,700 J/kg. Based on unofficial archives, this was the largest CAPE value for August and one of the largest on record for the site. Large CAPE values are often associated with a "cap" of warm air aloft that prevents storm development, so these environments do not automatically produce severe storms. Generally speaking, CAPE is not a lifting mechanism but an energy reservoir available to support explosive development should air be lifted to levels were it can "freely convect".

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Severe storms impacted eastern Iowa Friday afternoon producing an EF2 Tornado near Fairbank and damaging winds elsewhere. When the National Weather Services issues Severe Thunderstorm and Tornado Warnings for such storms, the warnings contain an estimate of storm speed and direction of travel. The featured chart presents such storm motion estimates from NWS Davenport during the summer months of June, July, and August (JJA). The warnings from Friday are highlighted with the red crosses. The blue lines attempt to show where the most frequent combinations of storm speed and motion reside. The bullseye appears to be somewhere near a storm motion from the west around 40 MPH. You can generate this chart for other months of the year and compare how the storm motion changes during the year.

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