The last half of the summer season often features slow moving thunderstorm complexes fed by ample amounts of atmospheric moisture and instability. Flash Flooding is a typical result with rain amounts exceeding rates that the landscape can manage. The main warning headline that the National Weather Service (NWS) has for this situation is the Flash Flood Warning. A messaging escalator that the NWS has is to tag especially dangerous flooding as a Flash Flood Emergency. Two such warnings were issued in Texas yesterday and one last week in Missouri. The IEM attempts to curate a list of these emergencies with the featured screenshot from the online tool that presents them on a map along with a tabling listing. A link to the raw data is available as a GeoJSON on the page or you can use the regular download portal to request a filtered dataset including just these events.

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A surface high pressure system and associated upper level ridge have been in firm control over Iowa's weather these past few days with hardly a cloud in the sky. Unfortunately, very hot and dry air have lead to numerous wildfires over northern Minnesota and into Canada. This "heat dome" system, as it is sometimes colloquially called, has been impressively strong. During the warm season, one measure of its strength can be found by the maximum height of the 500 hPa pressure surface. Due to the ideal gas law, a very warm lower atmosphere will vertically "push" the height of a pressure surface further away from the ground. Meteorologists like to consider a height of 6 kilometers has an impressive height for the 500 hPa pressure to be found. And that finally brings us to the subject of today's featured chart. The chart presents the 2026 observed heights and period of record ranges of the 500 hPa surface as recorded from the Minneapolis sounding. The monthly max and min values are labelled. The sounding from Monday evening (00 UTC 14 July) came in at 6,010 meters, which according to unofficial IEM archives is the highest height on record for the site. Typically, these very strong "heat domes" happen further south and particularly southwest with 6km heights happening a few times per year.

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The past few mornings have featured strong "radiational cooling" events thanks to clear skies and calm winds. Such events often highlight micro-climate differences between weather stations, which was the case when you compare temperatures at the Ames airport vs Des Moines. Temperature differences between the two stations reached 8°F during the morning hours. A daily feature last week highlighted the Des Moines heat island, which tends to buoy temperatures at night time. So with the Ames station being much less urban, it stands to reason that large temperature differences could happen when the heat island effect is most pronounced. The featured chart takes a bit of a deep dive into air temperature differences between the two sites. The top left panel shows the raw values so far this July. The top right panel presents the frequency of Ames being warmer by hour and week of the year. The bottom left panel shows yearly aggregate statistics. The bottom right panel partitions the data by month of the year. Perhaps the most interesting sub-plot is the upper right, which shows two clear areas of increased frequency of Ames being warmer than Des Moines. The reason for these two likely has to do with increased humidity levels due to nearby cropping increasing morning temperatures during the growing season and then a sandier soil allowing for increased heating during the day vs Des Moines.

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The IEM processes a near real-time feed of National Weather Service NEXRAD (RADAR) Level II and Level III products. The Level II data is then made available on a simple website built for a number of radar display software programs to easily use. The Level III data is processed into a number of products, including mosaics and single site imagery. The featured map is a Monday evening snapshot of some internal IEM monitoring to ensure the data feed is performing as expected. All of the sites are shown in the green (green is good) with low latencies. The NWS has a very nice monitoring page as well, for those of you interested in such things!

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After numerous funnel clouds were reported over Iowa on Thursday, Friday saw even more funnel clouds reported with even a few reaching the ground as brief tornadoes. This is a favorable time of year for such events as strong instability, low cloud decks, and weak wind shear allow for updrafts to stretch atmospheric spin into tighter vortices. The featured lapse is from the KCRG-TV webcam at the University of Northern Iowa (Cedar Falls/Waterloo) showing a brief funnel cloud during the afternoon hours on Friday. As a reminder, the IEM attempts to curate interesting webcam lapses of weather onto YouTube with the featured lapse found here. And for those of you that like pretty sunsets, this one from Peosta is for you!

The weekly update to the US Drought Monitor was released Thursday morning and while most of Iowa is drought free, pockets of dryness continue to fester over northcentral and northwestern Iowa. One of the driest long term reporting locations as of late has been Spencer. The featured chart presents a number of precipitation metrics for the site. The individual graphs are best interpreted by reading them right to left as they display trailing period metrics for the plotted date up until 9 July. The right side (near term) values are a bit noisy as a recent rainfall event skews metrics over a short duration of time. The bottom left panel plot is probably the most informative as it plots standardized precipitation index (SPI) with the associated drought thresholds. Departures do not get too far out of hand until you reach back into early fall 2025.

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The featured chart presents the daily high and low temperature climatology, 2026 observations, and the departure between the two for Ames. The chart nicely shows the cooler than average weather that dominated the last half of June has been replaced by warmer weather to start off July. The bottom panel, which plots difference between observations and the climatology average, is interesting as while highs and lows generally directionally match, the magnitudes are different with high temperature departures typically having larger absolute values than lows.

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With a low temperature of 70°F on Tuesday, the Des Moines airport weather station was the warmest within Iowa among airport based weather stations. This situation happens frequently for the site due to the urban heat island effect being the strongest for the most urban airport weather sensor within Iowa. The featured chart illustrates this by computing the average low temperature percentile for Des Moines vs the rest of the long term climate stations within Iowa. Please note that there was no spatial weighting done, it is just a simple comparison. The bottom panel shows the bias between the Des Moines average value vs the rest of the state. The increasing impact of the heat island is readily apparent, but you may wonder what is happening during the early period prior to ~1940. This was approximately when the station moved from downtown to the present location, which was more rural at the time. Additionally, observation techniques changed with most of the rest of the state reporting 24 hour values during the afternoon hours, which tends to bias low temperatures warmer.

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For the Ames NWS COOP site at the ISU Ag Farm southwest of town, the two day precipitation total from the deluge last week came in third largest on record for the site. The site reported a 24 hour total of 2.17" for the afternoon report on Friday and then 4.20" on Saturday. The heavy rainfall events took a break on Monday for Iowa with Tuesday expected to be mostly dry as well. Storms will return Wednesday and likely produce locally heavy rainfall totals again with ample moisture around and slow storm motions.

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Multiple rounds of slow moving thunderstorms from Thursday into Saturday created for some very large precipitation totals over mostly central Iowa. The previous IEM daily feature highlighted the overnight rainfall totals into Friday morning. Today's featured map presents MRMS precipitation estimates for the three day period including 2 through 4 July. A considerable portion of the state is shown over four inches for just these three days. Remarkably, even with all of these rounds of thunderstorms, there are a couple of small areas shown with zero precipitation over far southern Iowa.

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The abundant heat and humidity this week continues to fuel daily thunderstorm complexes that are prolific heavy rainfall producers. One such complex produced incredible rainfall totals over northern Polk (northern Des Moines suburbs) and southern Story counties. The featured map combines NOAA MRMS 24 hour rainfall total estimates derived mostly from RADAR with observed values from CoCoRaHS, NWS COOP, and NWS Local Storm Reports. There is some timing ambiguity between these datasets with the MRMS totals ending at 10 AM 3 July and some of the morning CoCoRaHS/COOP reports not including a bit of additional rainfall after around approximately 7 AM. Additionally, more precipitation fell during the Friday afternoon hours and into Saturday morning, so it is not easy to produce an isolated storm total from these CoCoRaHS reports. NWS Des Moines got a public report of 12.60" near Elkhart (NE of Ankeny/Des Moines), which is not much lower than the official Iowa 24 hour precipitation record of 13.18" back on 13-14 Jun 1998 at Atlantic. This featured deluge is also only a few days removed from a historic rainfall back on 30 Jun 2018 that occurred over approximately the same area.

Slow moving storms and associated clouds on Thursday helped to isolate the most extreme heat index values to southeastern Iowa with places like Cedar Rapids briefly reaching the triple digits. The featured chart looks into the dew point climatology by air temperature during the month of July for Cedar Rapids. The chart nicely shows a fortunate feature of Iowa's summer time dew point temperatures with decreasing values found with the most extreme air temperatures. The reason why has to do with surface energy budgets and the requirement of more input energy to heat a higher water content air. It is also a sort of self fulfilling prophesy that drier air is able to more quickly heat and mix down warmer air aloft, so air temperatures can reach higher temperatures. The climatology plotted shows a bit of an inflection point near 95°F as the average dew point values start to decrease.

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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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