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.

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.

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.

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.

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.

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.

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.

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.

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.

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!