UPPING THE HEAT ANTE...

The last week was a wet one for many parts of the central Midwest. A stubborn pattern loaded with moisture brought numerous rounds of thunderstorms with heavy rain. The warmer the temperature, the more moisture the air can contain. The chart below, from the Iowa Mesonet, shows precipitable water vapor from balloon soundings taken at the NWS office in Davenport. This particular variable summarizes the amount of water within the atmosphere if it were all converted into a liquid depth. The amount of water vapor that air can hold increases with temperature, so it's only logical that the highest precipitable water values occur when the atmosphere is at its warmest during summer. The chart presents some monthly max and min stats along with the climatological range and 2025 observations. Notice, since June 1, there have been numerous times when water vapor has spiked far above average into the range of 1.5 to 2.20 inches.

These high water vapor rates are materialized over a duration of less than an hour. So a two-inch value can support hourly precipitation rates from two to perhaps as much as four inches per hour. Not only does that bring lots of rain, it produces flash flooding, and many parts of the region have seen some of that recently.

As proof of how wet the pattern has been, here's the rainfall that's fallen since the 1st of July. Some areas near Wapello in southeast Iowa have seen over 9 inches of rain. Many spots from eastern Nebraska through central Iowa into NW Illinois have picked up 6–8 inches. That is roughly where the "ring of fire" (the northern edge of a hot, humid air mass) has been persistently situated.

For perspective, rain departures of 4, 5, and even 6 inches are fairly widespread over all but my southernmost counties the past 20 days.

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TURNING UP THE HEAT...

Going forward, attention turns to a building ridge of high pressure over the east-central U.S. Come Wednesday, the 500mb pattern has the ridge in a favorable position to finally deliver a healthy dose of steam here in the central Midwest.

While temperatures will only be about 8–10 degrees above normal, they are coming at what climatologically is the hottest time of the year. That get highs into the low 90s in most areas.

Low 90s in itself are not problematic, what is are the extreme dew points. Thanks to full on crop evapotranspiration, they are shown on the 3k NAM reaching into the upper 70s to near 80.

That's expected to drive Heat Index Values as high as 105 degrees in spots.

In advance of that, Tuesday will not be as wicked but should be noticeably warmer and more humid with heat index values near 92 north to around 103 in the south. As a result of the steamy weather, the NWS has issued an extreme heat watch for the period late Tuesday through Thursday. Wednesday has the highest potential for the worst heat and humidity, with some improvement likely as early as Thursday.

The experimental NWS heat risk product shows my area Wednesday in a level 3 of 4 risk of major heat.

Major - This level of heat affects anyone without effective cooling and/or adequate hydration. Impacts likely in some health systems, heat-sensitive industries and infrastructure.

This just in, a heat advisory has been issued Tuesday for the area south of a line from Waterloo to near Clinton and on southeast to Princeton, Illinois from noon to 7:00pm. for heat index readings up to 105. Take it slow and drink plenty of fluids if you are doing strenuous work outdoors in that part of my area.

Because of the very warm air aloft that builds into the region, our reprieve from thunderstorms is expected to last until Thursday afternoon or evening. Not to say, a stray storm or two won't pop up in the high instability atmosphere, but with the high level of capping they should be isolated at best Tuesday and Wednesday.

Thursday, especially later in the day or evening, the door opens to thunderstorm chances again with the approach of a weak cold front. Before it arrives, another sultry day is anticipated, but highs appear cooler, back around 86-91 with heat index values of 92 north to 99 south. The reason for the relief is a front that may deliver afternoon thunderstorms and enough cloud cover to temper levels to a more tolerable level.

As for storms, most guidance indicates some initiating in the NW Thursday afternoon that gradually drift south into the region overnight. From there, the front encounters the ridge and turns nearly stationary for a time in southern Iowa, indicating the threat of additional storms through Saturday (mainly across the central and south), closer to where the boundary gets hung up.

Water vapor is again shown soaring to potent levels of 2.00 to 2.50 inches.

Any decent updraft in that environment has the potential to crank out robust rainfall in a short period of time, meaning heavy rain will be back on the table later Thursday and perhaps into Friday. Here's what models are suggesting for rainfall through Thursday through Saturday.

The EURO

The GFS

While temperatures and humidity will lower some, the front won't be far away and Friday right into the weekend a rather warm and muggy brand of weather will remain in place. Typical of mid-July.

Well into next week, the area looks susceptible to occasional ridge riding thunderstorm complexes due to NW flow around the heat dome centered to the south. I'm no expert, but in my local area the crops (especially the corn), are looking sensational, reaching heights of 10 feet. With the pattern ahead, I'm expecting it to continue to flourish. Gear up for some steamy mid-week conditions, it's coming! Roll weather...TS