COOKING UP A STORM...
It was a hot time in the old town Tuesday. The heat pump was on and most spots reached the mid to upper 90s. Vinton, Iowa reported a maximum of 101 which is the hottest reading I saw come in over the Iowa Mesonet ASOS network. Washington and Muscatine both registered 99 and Iowa City hit 98. Of course the humidity was massive and heat index values of 103 to 107 were common. The one positive was the brisk SW winds which provided a slight but welcome cooling effect.
With a relief providing cold front still to the west Wednesday, another blistering day is anticipated with highs generally in the low to mid 90s (a few upper 90s are possible in SE Iowa and WC Illinois). Dew points will reach the mid 70s creating heat index values of 103 to 108. Excessive heat warnings are in effect for the entire region. Keep the fluids coming!
Another factor to keep an eye on Wednesday is the potential for severe weather. The heat and humidity will generate plenty of instability as shown in the CAPE values which the GFS estimates greater than 4,000 j/kg by late day.
The 3k NAM paints the same picture of a very explosive atmosphere.
The supercell composite is also high indicated the potential for organized surface based convection involving rotating updrafts. All modes of severe weather would be possible but damaging winds would be the primary concern.
The Storm Prediction Center currently has my northern counties in an enhanced outlook for severe weather with the rest of the region covered in a slight risk assessment.
Models due vary some on the timing of the front but are of the basic idea that storms would fire in the northwest late in the day and build southeast as the evening progresses. One issue that could spare or keep storms isolated in the south would be the cap aloft that could be tough to break until later in the evening. By that time the front and its forcing could have reached or just passed the Mississippi. Again, a slower solution favors a more widespread coverage. Outflow boundaries( or a cold pool from Tuesday night's storms) will have a say in the fronts progression and will be a factor we just can't determine until Wednesday morning. We'll know much more on coverage, timing, and intensity of late day storms as the day unfolds.
Models are not all that excited about precipitation totals and have quite a bit of variance in the amounts they produce. I would say the north is more favored for the more widespread rains but with very high water vapor levels, anyplace that can get a robust updraft going could see some intense downpours and a quick inch or more of rain. I would not put a great deal of stock in what models are showing for amounts in this situation but I'll but up what they are suggesting just because I can.
The 3k NAM
THE NBM, national model blend.
By Thursday morning winds will be switching to the west and a gradual decrease in temperatures and humidity will occur allowing the week to end on a dry and far more pleasant note. The improved weather should last through at least Saturday before the heat dome begins to expand once again. At some point, late weekend or early next week, another round of steam is expected to build over the Midwest. Additional heat headlines are possible as highs head for the 90s and humidity builds. There may be some low chances for nocturnal thunderstorms near the rim of the heat at some time but for now chances are generally 30 percent or less.
The Weather Prediction Center does show much above normal temperatures for the region in its hazards outlook for June 17th-21st.