Thursday will be a day of heat and humidity for most of the Midwest. Instability will be high and the atmosphere will be ripe for thunderstorms, especially with the arrival of a cold front towards evening. The big question is, can we pop the top of a cap that has the potential to minimize or even inhibit convection?
Let's start with the heat. As it stands now many spots have a good chance of cracking 90 degrees. Much of my area should end up in the low 90s. In fact, some locations could see the warmest temperatures of the year so far. Here in Cedar Rapids we would have to reach 93 to tie June 13th for the highest reading of 2017. It should be close.
Adding insult to injury will be the humidity levels. Tropical moisture will we readily available leading to dew points in the 70s. The GFS is the most extreme with readings in the mid to upper 70s. That's probably 2-4 degrees too warm but even the lower levels would create heat index values of 100 degrees. Here what the GFS has for dew points.
No matter what else happens Thursday, it's a given that sizzling conditions will be widespread across the Midwest.
That leads us to thunderstorm potential which is appreciable where storms can form within CAPE (convective available energy) exceeding 3,000 j/kg.
It's likely that moderate to strong destabilization will allow scattered severe storms to form in Minnesota and Wisconsin ahead of a cold front late in the day. Shear profiles are supportive of updraft rotation and supercells. Hail and damaging winds are the primary concerns but a couple tornadoes are possible. Eventually the storms should evolve into a linear complex that could clip northeast Iowa as it heads for northern Illinois late in the evening.
Further south in Iowa and much of my area the cap (a layer of warm air aloft) makes development much more questionable. Some models develop convection but others hold the cap firm and keep storms isolated at best. While it's difficult to say for sure, it does appear the cap will be formidable and could lead to only widely scattered storms in this area. I'll have a better handle on the potential later Thursday.
Come Friday much cooler and drier air will overspread the region getting the weekend off to a fine start. By that time we'll be ready for the break, at least I will. Roll weather...TS