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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Thunderstorms erupted just before sunset Friday evening over southern Iowa. The storm environment was characterized by high instability and low shear, so the storms did not move very fast and their isolated nature allowed for an incredible viewing of a lightning show. The featured map presents lightning stroke density counts courtesy of Vaisala. The highest totals can be seen just southeast of Des Moines. Even with the marginal shear, they were likely able to produce brief tornadoes. A repeat situation is expected this evening over parts of southern Iowa.

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Thursday saw much needed rainfall over a decent chunk of Iowa. The Ames area received two to over three inches of rain, so it is a good time to check in on the ISU soil moisture sensors to see if the rain soaked in. The featured chart is from the sensor at the "Kitchen Farm", just southwest of Ames. The chart plots the soil moisture sensors at various depths down to 40 inches below the surface since the start of June. The side panels show the depth soil moisture values at the start of June and the most recent observation plotted. The soaking nature of the rain is immediately noticeable with even the shallowest depth starting to dry as the water soaks deeper into the soil. You may be wondering what is happening with the 40 inch value as a 0.7 volumetric value is very high. The belief/hope is that the water table rose to a depth shallower than 40 inches and so the sensor at that depth is submerged. The sensor is in a "prairie pot hole" region with a shallow water table.

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It has been a somewhat strange year for severe weather to date for the United States. April was very busy for severe weather in and around Iowa, but it continued to be mostly quiet over the southeastern US. May was about the exact opposite of April with much quieter weather, but also for much of the rest of the country as well. The featured chart looks into an aspect of this by totaling up the number of Severe Thunderstorm and Tornado watches issued by the Storm Prediction Center for the year to date period. The top panel shows the total number of watches with the second panel totaling the number of watch outline polygons (note they are no longer the official watch area, but a close approximation) that touched Iowa. The bottom panel plots the percentage touching Iowa. Interestingly, 2026 comes in with the highest percentage since 2004!

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The featured chart compares yearly precipitation totals between Cedar Rapids and Ames. Blue bars indicate Cedar Rapids was wetter than Ames by the given margin and red bars indicate Ames was wetter. This chart nicely shows the variability that exists on the yearly time scale between two sites that have an overall average difference of just 0.37 inches (~1% of the yearly total). There have been just under 20 years with the difference between the two sites over 10 inches (~30% of the yearly total). The overall frequencies of which site is wetter is not that much different with Ames only have six more such years. This just goes to show how variable precipitation can be over even considerable periods of time.

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Monday was yet another rather warm day with high temperatures well into the 80s for most of the state. For Des Moines, it was the 19th day so far this year with a high temperature in the 80s. The featured chart presents the climatology, 2025, and 2026 totals for year to date days with a daily high temperature within the given temperature range. The totals within the 70s and 80s are both well above average for 2026, but interestingly about average for the 60s. The forecast for the rest of the week looks to continue totaling up days with highs in the 80s.

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On average, June is Iowa's wettest month. So it seems appropriate for the start of June to look into precipitation climatology. The featured chart presents the official NCEI daily climatology for Des Moines along with the previous official version. NCEI updates the climatology every ten years to represent a recent 30 year period. A climatology of precipitation is somewhat tricky as the dataset is dominated by dry days (zeros) and heavy rainfall events that skew simple daily statistics. Average daily values become somewhat meaningless, but the annual cycle is still illustrative as shown by the featured chart. It is interesting to see the July dip that shows up in the current (1991-2020) climatology, which is likely due to a number of dry Julys during the 2010s.

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Our early summer warmth continued on Thursday with highs well into the 80s and even a few 90s across Iowa. For Sioux City, Thursday was the fourth consecutive day with a high temperature of at least 90°F. The featured chart looks into the frequency of a given day of the year participating within such a high temperature streak. The side tables list out the earliest and latest such events on record for the site. This year's event ties for sixth earliest. The high temperature for Friday won't be quite as warm, so the streak will end at four days for the site.

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Cloud watchers over central Iowa were treated to Lenticular Clouds on Wednesday afternoon. While this type of cloud is often observed on the leeward side of mountains, it takes a special environment to produce them in the absence of significant topography. In this case, a northeasterly propagating gravity wave into flow going in the opposite direction likely combined to produce a "standing wave" environment necessary for these clouds to form. You can find a satellite lapse showing the gravity wave interaction. The clouds have a smooth appearance as the flow sculpts the cloud stuck in the crest of the standing wave. A time lapse from ISU was captured and available on YouTube that shows the stationary nature of the clouds. Again, you'll find plenty of much fancier and more dramatic examples of these clouds near mountains on the Internet, but such displays on the plains are rare! Hope you were able to see them.

USDA NASS published their weekly crop progress update yesterday and estimated 94% of Iowa's corn crop now planted. The featured map presents the statewide estimates with a departures against a simple and linear interpolated ten year average valid 24 May. All of the major corn producing states are shown in good shape. The mostly dry weather this week will likely push Iowa's value to near 100% by next week as the spring planting season draws to a close.

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For most of the state, rainfall amounts over the Memorial Day weekend were rather meager. Much of the state continues to "run on fumes", so to speak, after the wet April and heavy rains mid May. The featured chart shows the fickle rainfall story for Ames this May with the blue bars showing observed daily rainfall with the red bars indicating the month to date departure from average. The observed and climatological accumulation are plotted as well. Those aforementioned mid May rains are about all there is to speak of for Ames during May. The big rainfall on the 18th put the month-to-date departure barely above average and the departures are again grown since. The bad news is that the next week and perhaps even the next two weeks look very dry with warmer temperatures and reduced humidity, which will quickly escalate the drought situation during what should be the wettest portion of the year.

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The featured map presents May to date precipitation departures courtesy of PRISM. There is plenty of red (below average departures) to be found on the map. Outside of the rainfall events of last weekend, there has not been much to speak of for rain during May. Much of Iowa is surviving from the very wet April and the periods of reduced water demand like what was found this week with the cool and cloudy conditions. The precipitation forecast for the rest of May is not that bullish and temperatures will be increasing, which will start to materialize impacts over the driest parts of the state this May. For this time of year, it roughly takes about an inch of rain per week just to keep up with climatology, so departures can rapidly build when we get dry periods during the wettest portion of the year!

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After a very dry start to May, this past weekend saw numerous rounds of severe weather that also brought much needed rainfall. Such is the conundrum this time of year, needed and significant spring season rains often come with thunderstorms that are sometimes severe. The featured chart attempts to look into how much precipitation is associated with severe weather by totaling up one minute interval precipitation totals from the Des Moines airport when either a severe thunderstorm or tornado warning is activate for the airport. The left two panels report the precipitation partitioning percentages during, within an hour, and outside of severe warnings. The upper right panel shows the frequency of a given one minute intensity happening during a severe weather warning. The bottom right panel is a bit of a downer, but denotes how much of the one minute precipitation is likely missing within the one minute observations (long story, but NCEI's collection and distribution of one minute data has become very unreliable). So the overall percentages are not that large with precipitation totals near 10%, but it certainly accounts for a good portion of the most intense rainfall totals.

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For mid May, Tuesday was a bit chilly with high temperatures struggling to get out of the 50s. Such chilly temperatures imply a meager growing degree day accumulation, which is ever important this time of year to promote crop development. The featured chart presents daily growing degree days for Des Moines with the observed value on Tuesday plotted along with two model forecasts, a crude daily average accumulation (short daily lines) and the associated accumulated values. The NWS NDFD forecast does go out as far as the GFS deterministic model, so that is why the red bars stop on the 26th. The chart nicely shows that we'll have four more days to go with below average GDDs prior to about a week of warmer weather taking us into June.

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