Tuesday was yet another rather dreary day with more rain and plenty of overcast cloudiness. The featured chart looks into the monthly frequency of overcast conditions as reported by the Des Moines airport weather station. There are number of caveats with long term comparisons of cloud coverage as techniques and instrumentation has changed over the years. That aside, the top panel shows the monthly max and min values along with a simple mean and actual 2026 observations for percentage of hours with the sky coverage being overcast. The bottom panel plots the yearly values for September. The 2026 value is certainly above average and about the fourth highest on record so far. Thankfully, a bit more sunshine is expected today before more cloudiness returns.

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The rainfall totals for September continue to amaze with today's feature denoting the largest seven day precipitation total by year for Waterloo. The top left panel shows the seven day period with the color representing the total. The bottom left panel shows the frequency of a given day of the year participating within this maximum period each year. The top right panel shows the yearly maximum value. The bottom left panel shows the cumulative distribution. The 2026 value is currently 4.46 inches ending on 20 September. The bottom left panel interestingly shows these events being more frequent during September than during August for the site! September is even comparable to the climatologically wetter months of May, June, and July.

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The parade of heavy rainfall events this September continued over this past weekend with the highest totals found over eastern Iowa. The featured map presents the combination of NOAA MRMS 48 hour estimates ending 7 AM Sunday with available NWS and CoCoRaHS reports overlaid. There are a number of stripes of rainfall ranging between five and seven inches. Thankfully while more rain is in the forecast, intense totals like those plotted do not appear possible. Temperatures will be cooler as well and much more fall like. A side note that unfortunately an IEM plotting tool does not yet exist to generate maps like the featured one today.

The featured chart is perhaps "Exhibit A" as to why Iowa continues to see heavy rainfall events this September. The chart shows average precipitable water during the month of September based on evening soundings from the Omaha area. This value represents the amount of water within the atmosphere and is expressed by a depth (plotted in millimeters). You can sort of think of the value as the amount of rain that would fall if all of the water was squeezed out of the atmosphere. While the 2026 value still has two weeks of time to pull the average lower, the plotted value is well beyond any previous September on record for the site. Storms that develop within an atmosphere loaded with water vapor are able to efficiently produce rainfall due to limited dry air and typically warmer cloud depths. Multiple rounds of such storms are expected on Friday and into the weekend.

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After the very hot start to September, temperatures have become much more fall like and it has recently felt more like fall. The actual start of the fall season is subjective with some claiming 1 September, others claiming the Tuesday after Labor Day, and others the fall equinox date. The featured chart attempts to take a statistical approach to access the date when the trailing 91 day average temperature starts to decrease. This effectively denotes a 91 day period with the warmest average temperature for the year. The middle subplot provides the date each year for Ames that this 91 day period ended. A simple average puts the date at 6 September, which is conveniently when this period ended this year. The bottom plot extends this algorithm then to consider the duration from the end of summer to the start of the winter season (coldest 91 day period). The average number of days is just 85.5, which implies our fall season is about a week shorter than an equitable season should be! There is certainly much variability with some fall seasons shown to last less than 60 days and others over 100!

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The precipitation totals for September continue to pile up as daily rounds of storms frequent the state. For Des Moines, the September 2026 total now stands at 10.52 inches and is the second largest total on record during the month of September behind 14.81" during 1914. With half of the month to go, it would seem "easy" to break this record, but the featured chart tells another story. The chart plots the accumulated precipitation total for 2026 (red line) and 1914 (black line). The tan lines represent the appending of previous year's 16-30 September data unto the 2026 time series. This effectively provides potential scenarios for where the 2026 total could end up. Using this method and remarkably, only 7.4% of previous years would push the 2026 total to a record value. The main issue is that the 2026 total is still 4.3 inches short of 1914 and that total in itself would be extreme for the last half of September. Climatology is also working against larger precipitation totals by the day as the atmosphere continues to cool and hold less water vapor. Having said all of that, the forecast looks to bring another two inches, at least, for the remainder of the week. So stay tuned!

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Monday saw a number of rounds of rainfall to start off what looks to be a very wet week. The featured chart looks into the number of distinct hours per month with the automated weather station reporting some form of rain (RA METAR code). Note that this is not exactly a total number of hours with rain as individual reports cover a shorter duration of time than one hour. The chart is still quite informative and tells an interesting climatology story for Iowa. The blue bars are simple averages since 1973 (the modern observation era). It is interesting to see July with the lowest average among the months stretching from March through October. The reason is that storms are much less organized and more isolated than other months, so while they do produce significant rainfall totals, the overall duration decreases vs larger scale precipitation events. The 2026 observed values are shown by the red bars and the September total is well on its way to finish above average with half of the month yet to go.

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A remarkably wet week of weather is forecast over the Midwestern US. The featured map presents Weather Prediction Center forecast precipitation over the next seven days ending Sunday evening. Most of Iowa is shown at three inches and higher accumulations over this period. The regions plotted in black are over seven inches for this period! This forecast "paints" precipitation with broad brush strokes, so locally much higher amounts are anticipated over more isolated locations. This week will certainly put the breaks on any fall season harvest progress as it appears no locations within Iowa will avoid significant rainfall.

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With high temperatures on Friday expected to reach the lower 80s for much of Iowa and a handful of days over the coming week as warm, it is a good time to check in on the number of days per year with a high temperature of at least 80°F for Ames. The side tables list the top 10 years with the most and least number of days. The bars are colored by their departure from a simple period of record average (black line at 91 days). The 2026 total hasn't quite breached the top 10, but is already well above most years over the past two decades. It is also the seventh consecutive year above average.

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Yesterday's daily feature presented a 24 hour precipitation estimate with reports overlain over the Des Moines Metro showing a stripe of 5 to 6 inches over southern Polk County (Des Moines). One of the observation locations within that stripe was the Des Moines Airport. The weather station there recorded 3.77 inches on Tuesday (a daily record) and 2.12 inches on Wednesday making for a two day total of 5.89 inches. The featured chart presents the largest two day precipitation totals for Des Moines and shows the most recent event coming in fourth largest on record and the largest total since the 1970s.

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It has been quite the year for big rainfall events over the Des Moines metro area. Last night's event brought severe thunderstorms to the metro area with intense rainfall rates leading to flash flooding. The featured map combines NOAA MRMS 24 hour precipitation estimates ending at 7 AM with NWS and CoCoRaHS reports. A stripe above five inches is shown across southern Polk County (Des Moines). Previous and similar IEM feature maps include 4 July and 21 July.

While still humid, temperatures did cool off a bit on Sunday into Monday with a number of locations experiencing the first sub 70°F temperature reading of this September. For Des Moines, the low on Labor day was 67°F (hi kids), which was the coolest reading over the first seven days of September. The featured chart presents the coolest low temperature over the first seven days each year on record for Des Moines. Remarkably, four other years had experienced a warmer minimum over this period including 2015. Plenty of warm days are forecast for the shortened work week before perhaps cooler weather arrives for the following week.

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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.