TALK ABOUT CUTTING A FINE LINE...
As expected Tuesday was a mean day around the region with heat index values that reached or exceeded 105 degrees. In the Quad Cities with a dew point of 80 and a temperature of 92 it soared to 109 for a time, the highest heat index of entire the summer!
Also as expected, all that steam boiled up in the form of thunderstorms during the late afternoon and evening. With water vapor in the 1.8" to 2.2" range, where storms formed intense downpours were found and some wind gusts of 50-60 mph produced scattered reports of damage. Platteville, Wisconsin not far from Dubuque reported 70 mph gusts. Unfortunately, Tuesday's storms missed the area around Cedar Rapids and HWY 30 down to I-80 and Iowa City. Rain system after rain system has avoided that part of my region for much of the past two months. Radar estimates from the Doppler indicate a swath of 1-4" rain totals from HWY 20 SE of Waterloo to just north of HWY 30 and Cedar Rapids all the way across northern Illinois.
The last time the Cedar Rapids airport recorded an inch of precipitation in a 24 hour period was December 29th of 2020. For the year the rainfall departure stands at -10.50". Below is a larger perspective showing little in the way of rain south of HWY 30 until you get into Illinois. A strong cap (warm air aloft) snuffed out development in many of my southern counties.
One thing that was interesting to watch was an outflow boundary that pushed out of the thunderstorm complex early Tuesday evening. Essentially, rain cooled air was forced out of the storms by way of their downdrafts...like a mini cold front. You could actually see the process on the radar. While the outflow boundary was not associated with any rain, places near I-80 noticed a significant drop in temperatures and dewpoints. In the Quad Cities, the dew point which had been at 80 earlier dropped to 66 and the temperature went from a high of 93 to 74 degrees in short order. The heat index dropped 35 degrees from its peak of 109 as the boundary passed. That's how I spell relief!
Wednesday we get to enjoy a similar set-up as heat builds instability during the day and at least scattered storms later in the afternoon and evening. Speaking of the heat, dew points should make it back into the mid to upper 70s with highs in most areas 90 to perhaps 94 far south. Heat index values should hit 100-105 south of HWY 20 and could approach 110 in the far south. Another mean day in that regard. Projected heat index values on the 3K NAM look like this.
Heat Advisories will again be flying for the entire area.
Surface cape (instability) is likely to reach 4 to 5,000 j/kg Wednesday afternoon. That's a lot of potential energy so again if a storm can go up in that environment it's likely to be heavy rain producer and a wind threat. The issue again could very well be the cap which may suppress storm development, at least through much of the day. Eventually, a weak front will enter the picture acting as a catalyst to break the cap and fire scattered convection. Models are not in great agreement when and where this will happen. The risk for a few strong to severe storms exists. SPC blankets the area in a slight risk assessment.
In general, the overall picture remains unchanged Thursday as the front waits for a kicker to send it through the Midwest. Another hot humid day is expected with moderate to high instability and some severe weather potential, especially near and south of I-80. The area south of HWY 30 is also susceptible to heat index readings of at least 102 degrees.
Finally, extended relief arrives Friday as the cold front dives southeast and NW flow aloft develops. You can see the pattern change below on the 500mb jet stream depiction.
Once the upper level change is established the weekend and well beyond looks pleasant and dry with near to slightly below normal temperatures. More important, humidity levels will be way down eliminating the steam we've been dealing with for nearly a week. Hopefully, those areas that need rain can get it Wednesday or Thursday because once the NW flow sets up it could be 5-6 days before the next opportunity comes around. Stay cool and roll weather...TS.
