Wednesday was yet another very warm day this week for most of Iowa, but showers and clouds kept northcentral Iowa cooler and the rest of the state was a couple degrees cooler than previous days. For Des Moines, the maximum heat index was 98°F, based on available hourly reports. The featured calendar presents the daily maximum heat index values for Des Moines since the first of April. The calendar nicely shows that prior to this week, there was not much to speak of for high index values with only two days in the 90s earlier in June. The summer time heat looks to stick around into next week.

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Last week, the daily feature denoted the 25th Anniversary of the IEM! Even though the project is only 25 years old, a number of datasets have been back filled to dates prior than 2001 based on upstream archive availability. One of those back filled datasets are NEXRAD Mosaics. The featured image is a postage stamp plot showing a snapshot of the mosaic product at 8 PM each 30 June since 1997. It is interesting to visually and quickly compare the years and how the RADAR presentation looked on each. One item to note is that the IEM archive for this product goes back to 1995, but upstream archive availability is somewhat spotty prior to 1997, so that is why 1997 was chosen as the first year for this visualization.

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Monday was another steamy hot day with heat index values again over 100°F for most of the state. The featured chart looks into the yearly maximum heat index value for Ames during the month of June. The top panel shows the frequency at or above the given threshold. The bottom panel shows the yearly maximum value for each June. The 2026 value of 106°F (so far) comes in at about a one in five year frequency. There is plenty more hot weather to go this week, but humidity levels are expected to drop a bit to make things a little less insufferable for Tuesday.

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Outside of a rain cooled area of northeastern Iowa, Sunday was steamy for the rest of Iowa with heat index values reaching the warmest values to date in the upper 90s and 100s. The featured map presents IEM computed maximum heat index values from the airport weather stations. More of the same is expected for the rest of the week, but like what happened over northeastern Iowa on Sunday, thunderstorm complexes can take a significant bite out of afternoon heat. These complexes are difficult to forecast days in advance and with ample heat and humidity around, they have the ingredients to persist over longer than expected periods of time.

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Our current stretch of very pleasant June temperatures is about to abruptly end with much warmer and more humid weather arriving on Sunday and lasting into the start of July, at least. While the IEM website does not offer much for forecast information, there are a few tool and archive resources regarding Model Output Statistics (MOS). The featured chart combines time lagged ensembles of National Blend of Models (NBM) extended (NBE) temperature output for Des Moines along with computed observations attempting to replicate the time periods that the MOS provides forecasts for. Rewording, the MOS does not produce calendar day high and low temperatures, but max and min values over specified time periods within the UTC timezone. Nothing can be easy, can it? Anyway, these values are typically also the calendar day high and low temperature, but not always! The bars represent the range of forecast values when you combine previous MOS outputs containing a forecast for the given date. The dots are the observed "verification". The last three days of June are shown to have significantly warmer high and low temperatures than what we have enjoyed for practically the entire month!

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With less than a week to go in June, it is a good time to check in how the growing season is going at about near the half way point. The featured chart presents accumulated growing degree days, precipitation, and stress degree days for the ISU Soil Moisture station just west of Ames for this year and last. Each both presents the accumulated climatology based on longer term NWS COOP data. The chart nicely tells the story for 2026 that has been mostly common for the entire state. We are in good shape with growing degree day accumulations, but have recently been loosing a bit of ground with the cooler overnight temperatures. Precipitation remains a bit below average, but nothing too dry. And stress degree days remain well below average, which helps to mitigate impacts from below average precipitation. The story of 2026 does look to change next week with much warmer temperatures and increased humidity.

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While it felt a touch more humid on Tuesday than previous days, it was still another very pleasant June day in Iowa. Maximum dew point temperatures reached the lower 60s, but not much more than that. The featured chart presents the daily range between the minimum and maximum dew point temperature for Ames based on available hourly data for 2026. Any days with a maximum of at least 65°F are highlighted in red. You can see that such days have been rather limited this year and remarkably, April has about as many such days as any following month. While the next few days look to continue the pleasant weather, full blown summer heat and humidity look to arrive soon, so enjoy it while it lasts!

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This past weekend saw another rather significant outbreak of severe weather over Illinois into Indiana. It has been quite the severe weather season over this region as shown by the featured map. The map presents unofficial IEM computed statewide ranks for total number of NWS issued Tornado Warnings. A value of 25 would indicate the most such warnings on record for the given state since at least 2002. The states of Wisconsin, Illinois, and Michigan are shown with the most on record since 2002 with Iowa and Indiana not far behind in relative tally. The map nicely shows the "see-saw" pattern with the near record totals over the eastern corn-belt and much lower values over the southeastern US and especially Texas.

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The IEM project/website turns 25 years old this summer! Last week, a quasi-surprise celebration was organized by the ISU Agronomy Department (thank you Dr Brian Hornbuckle, Madelynn Wuestenberg, Rita Brueland, and others!) with many long term supporters of the IEM there to share stories and yummy food. The featured image presents yours truly attempting to hold the ice cream cake adorned with the IEM logo toward the camera without it spilling. Collaborators who helped develop the original project along with some of those who find funding support today are shown in the image. The Internet Archive captured how this website looked back on 19 July 2001. Here is to another 25 years

There continues to be no shortage of interesting things to denote from the storms of last Wednesday (17 June 2026) including low June surface pressure and a rapid change in dew point. Today's featured chart looks into the wind reports from the Marshalltown Airport. A gust of 107 MPH is shown at 5:48 AM with a sustained peak wind speed of 68 MPH just a few minutes prior. NWS Des Moines has an excellent event summary page with more details on this damaging wind event.

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While near surface air temperature can fluctuate significantly within a day or even day to day, the fluctuations in dew point are typically much lower as processes changing atmospheric water content are slower acting. Dew point changes during the summer season are even more muted as vegetation and available surface/soil water elevate lower atmospheric moisture content. A large change in dew point happened over much of Iowa on Wednesday with seasonally dry air on Tuesday replaced with more typical humid air just 24 hours later. For Cedar Rapids, the dew point temperature increased by 29 degrees Fahrenheit over a 24 hour period. The featured chart presents the largest such dew point increases during the summer months for the site. The recent event comes in first place and joins another Jun 2026 event within the top 10 largest increases! This has been a somewhat frequent pattern this warm season with mostly drier than average air being briefly replaced by more typical humid conditions. Dew point temperatures on Thursday were back again to below average values as drier air from the northwest visited the state.

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It was an energetic day of weather across Iowa and beyond on Wednesday. The day started with a strong complex of storms diving south during the early morning hours and producing damaging winds along with significant rainfall. This complex greatly altered the placement of the afternoon round of severe storms with the most damaging staying south of Iowa. A few storms were able to develop over far northeastern Iowa during the afternoon hours and even likely produced a significant tornado. The storm complex was associated with a low pressure system that was more typical of April than mid June for Iowa. The pressure was very low for June as shown by the featured chart of top 10 lowest pressure altimeter values for Mason City (NC Iowa). Yesterday comes in as the fifth lowest date since 1940 during June!

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Overnight Monday into Tuesday morning was interesting as you may have noticed clear skies when the sun set and then clear skies again when the sun rose, but then you realized that it rained overnight! Such was the case for Ottumwa. The airport weather station reported clear skies at 1 AM and at 7 AM with 0.09 inches falling during the 5 AM hour along with thunder reported. So the subject of today's daily feature attempts to look into how frequently total precipitation of at least 0.05 inches prior to 7 AM is observed by a given 1 and 7 AM cloud coverage observation for Ottumwa. There are caveats galore with such an analyses including many complexities with how automated stations report cloud coverage and changes with observation equipment and techniques over the years. The plot was limited to dates after the year 2000 as an attempt to limit some of these changes. All of that aside, the frequencies shown are the number of days with at least 0.05 inches of precipitation for a given cloud coverage combination. The 1 AM Clear + 7 AM Clear combination comes in at 2.5% frequency, so it is somewhat rare but perhaps not exceptionally rare situation. There are some interesting relationships to tease out from the plot including the increased precipitation frequencies when cloud cover amount increases between 1 and 7 AM, which intuitively makes sense.

For mid June, the past few days have seen rather crisp temperatures with overnight lows dropping into the 40s over some Iowans and comfortable highs in the 70s. For Des Moines, the coolest daily high temperature for June happened on Sunday (72°F) and coolest low (53°F) on Monday. The featured chart looks into the frequency of a given day of June having either the coolest high or low temperature for the month of June during a given year. While there is certainly a decreasing trend in frequency as the month progresses, such dates can happen any date of the month. The near term forecast does not have much in the way of significant heat.

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While the numerous rounds of severe weather last week may be a bit of blur, folks in Ames likely remember the damaging winds from Thursday morning. It was an interesting storm as most of the damage happened after the initial gust front blew through and was associated with a "wake low". The featured chart presents one minute interval observations from the Ames Airport and nicely shows the wake low signature. The top panel shows air and dew point temperature. The second panel combines wind speed, gust, and direction. The bottom panel shows pressure altimeter. A wake low is a transient area of reduced pressure in response to processes happening within organized complexes of storms. These are paired with "meso highs" associated with the gust front and so create strong pressure gradients that generate strong winds. The three panels nicely tell the story of the initial gust front passage (rapid drop in temperature, increase in winds and pressure) followed by a brief lull and subsequent drop in pressure, temperature, and increase in wind speed. The period between about 8 AM and 8:45 AM saw sustained winds near 30-40 MPH with frequent gusts between 50 and 60 MPH. The white dots show the wind direction from the NE during this time, which is likely a strong reason why these winds were damaging. The vast majority of Iowa's strong winds are from the westerly direction, so vegetation and structures "get used to this" situation. When strong winds come from an unique direction, this creates damage possibilities perhaps not possible with strong winds from the west. There is a lot to nerd-out with, but will denote one more interesting item. The minimum plotted temperature reached 59°F just before the wake low departed. Environmental dew points were roughly in the mid 60s, so such a relatively cold temperature was another sign of the strength of the storm complex as thermal and dynamical processes chill the air.

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Thursday picked up where Wednesday left off with two rounds of severe weather impacting Iowa. The first round plowed through the state early Thursday morning bringing strong winds with the storm and even strong winds after the storm passed associated with what is known as a "wake low", more on that with a future IEM feature. The second round developed over southeastern Iowa and pushed off into Illinois by early evening with the most significant severe weather happening outside of the state. The featured screenshot is from the IEM Local Storm Report App showing the combination of storm reports and issued storm warnings by the NWS over Wednesday and Thursday. A new tool was recently added to this app allowing the bean counting of both LSRs and warnings over a rectangle of your choice. Individual warning and report types are counted within this box and shows a large number of events over the past two days. One important item to denote is that LSRs are preliminary, unfiltered, and sometimes contain duplicated reports. So while the LSR total of Tornadoes is shown at 13, this is not an official total number of observed tornadoes over this period!

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Multiple rounds of severe weather impacted Iowa and beyond on Wednesday with more severe weather expected today. The featured chart presents the number active Severe Thunderstorm, Tornado, and Flash Flood Warnings issued by the National Weather Service by minute between 5 AM yesterday morning till this morning. The data is presented as a stacked bar chart. The chart shows a typical severe weather life cycle with warnings first dominated by Severe Thunderstorm Warnings with a few Tornado warnings coming a bit later and then a transition to mostly Flash Flood Warnings as the accumulated effect of heavy rainfall associated with the storms produces flash flooding. Two rounds of rough weather are expected for Iowa today with the first already ongoing over southeastern Nebraska and southwestern Iowa, which will lift northeast. A second round is expected to fire early afternoon along a front that will sweep the state.

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While it was plenty warm on Tuesday, the high end index values failed to materialize as humidity levels came in a bit below forecast levels and thus kept heat index values lower as well. It was still the highest heat index of the year so far for the state. The featured chart looks into heat index metrics for Des Moines with the top panel showing daily climatology along the the 2026 observations. The bottom panel shows the daily frequency of having a 100+°F maximum heat index. Frequencies steadily increase during June before topping out mid July. You can see the first week of September has higher frequencies than the first week of June! Today will be another very warm day with chances of severe thunderstorms over eastern Iowa.

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So far this warm season, Iowa has mostly avoided oppressive levels of humidity coincident with the numerous days of abnormally warm temperatures. The lack of humidity has been the subject of recent IEM features regarding low dew points during much of May. This lack of humidity has kept heat index values in check with the year to date maximum for Des Moines being about 90°F this past Saturday. The forecast for today has the first real muggy heat of the season arriving with heat index values expected to exceed 100°F for much of the state. This would represent a ten degree jump in year to date maximum heat index value and that jump is the subject of today's IEM feature info-graphic. Based on unofficial hourly archives, the table presents the largest jump in heat index for the first 100+°F heat index of the year. If the forecast for Tuesday works out, it would be the third largest jump on record since about the early 1930s.

Update: Based on the hourly reports, Des Moines topped out at a heat index of 96°F. So this featured jump did not materialize!

The last four days have seen heavy rainfall events impact portions of Iowa with eastern Iowa receiving the heaviest totals on Sunday. These events are not necessarily widespread and so there can be "unlucky" pockets of the state that miss out on the most significant rainfall totals. One way to compute where these relatively drier pockets are located is to step backwards in time and see how many days it takes to accumulate some threshold of precipitation. For the featured map, the threshold is three inches with precipitation estimates coming from NOAA's MRMS project. This map nicely shows these small areas that have accumulation day totals in the 30 to just over 45 day range, which puts them back into the end of April. There are more chances of significant rainfalls this week, so hopefully some of these isolated areas can have better "luck" with receiving needed rainfall.

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