BUILDING UP A HEAD OF STEAM...
A little back door cold quietly passed the area Wednesday. (It's labelled "back door" because it approaches from the northeast as compared to the usual fronts which arrive from the northwest). There was just enough convergence along the wind shift line for a handful of showers but they were few and far between and for most it was another hazy dry smoke filled day due to distant forest fires. Temperatures were noticeably cooler in NW Illinois where readings actually stayed below 80. The rest of us experienced mid 80s with dew points into the 70s.
You can see the impact the front had on early afternoon temperatures Wednesday with readings over the NE half of my area 5-10 degrees cooler than 24 hours earlier. That was expected. No such luck in the SW half.
Things are going to get cooking over the weekend as the intense heat wave that has baked the Plains expands a bit sending a tentacle of steamy air through the central Midwest. The warmth will combine with a moist southerly air flow off the Gulf and the evapotranspiration from the maturing crops to generate a very oppressive atmosphere.
All the ingredients are there for temperatures Friday through Sunday to poke into the 90s, especially with 850 temperatures that rise into the range of 21-23 C. (that's at 5,000 ft.) Deep mixing due to the SW flow should allow that very warm air just off the deck to mix down to the surface. These are 850 temperatures forecast by the EURO late Saturday.
Here's the 500mb jet stream flow showing the westerlies concentrated along the Canadian border allowing some of that searing heat to spread east.
Saturday may be the most uncomfortable day in terms of how hot it feels due to the combination of heat and humidity. Below you can see the highs the EURO is projecting Saturday.
If dew points reach the levels the 12k NAM indicates (generally mid 70s) we are really in the soup.
These are the heat index values that would result. 103-106 degree values are quite common. That may be a tad high but the sweat from the corn is at its peak and it will be a key factor in how high dew points get.
As I showed you above, 850 temperatures in the 20s mean lots of warm air aloft and a stout CAP. That keeps a lid on the atmosphere and makes it hard for thunderstorms to blossom without some sort of strong forcing. All that heat and humidity Saturday leads to a very unstable atmosphere with CAPE values pushing 5,000 j/kg. That's loaded for bear.
However, the CAP is likely to hold and that's reflected in the surface depiction Saturday evening with very quiet conditions due to the lack of any decent forcing.
The EURO keeps things bottled up well into next week showing this for total precipitation through noon Monday.
The GFS on the other hand kicks up a thunderstorm complex Sunday evening that rolls through the area. I'm hesitant to put much faith in that solution but a weak short wave is shown rounding the edge of the heat. It's conceivable that it could be just strong enough to broach the CAP and fire storms. If it happens, not only could they be strong but they would no doubt dump some generous rains as shown below. Right now I tend to favor the strength of the cap and the drier EURO but that is no sure thing and something to keep an eye on.
The big question on the table now is what becomes of the heat next week. That short wave the GFS brings through knocks the ridge back to the west amplifying the NW flow allowing some cooling. The EURO is having none of that and keeps the heat on high all of next week and into the weekend. In fact, (Saturday August 2nd) the EURO produces highs across most of Iowa in the range of 100-106 degrees. It even has Dubuque at 100 which is a tough feat to accomplish up that way.
What's hard for me to buy about the EURO solution is that both it and the GFS have the MJO pushing through phase 6 in the lead up to those 100 degree highs.
As you can see in the phase analogs for July, that actually argues for cooler temperatures next week which is what the GFS is showing. What I'm saying is that the EURO is not consistent with its MJO depiction while the GFS is. The GFS may be onto something by breaking the heat next week while the EURO intensifies it. I will not be surprised if the EURO eventually comes in cooler, either that or its MJO swing into 6 is very brief or incorrect. We'll see where models are trending tomorrow.