PATTERN CHANGE IS UNDERWAY...
The last few days have been warm, muggy, and in select areas stormy. Rain has occurred in some part of my region for the past 6 days. Unfortunately, it's not been evenly distributed and while some locations have had 4-5 inches, others hardly enough to settle the dust. Here's the 7 day rainfall totals showing a swath from SC into EC Iowa and a small part of Illinois where amounts have been 1/10th of an inch of less.
These are the 24 hour totals ending Wednesday morning. More fell in the afternoon across the north and additional rains came late Wednesday night in my southern counties, generally south of HWY 34. Notice that area from Cedar Rapids to the Quad Cities and back to the SW where there was no rain. That's the region that has consistently been missed going back to January.
In fact, if you look at precipitation departures since January 1st, you can see below a clear trend for dryness near and north of I-80 where rainfall deficits of 8-11 inches are common in the orange areas. On the other hand, over SE Iowa its been wet with rainfall surpluses of 5-9 inches. There's a 20" rainfall difference in just 100 miles from Cedar Rapids to Burlington. That wall of plenty has been staunch!
There's one more day of heat and humidity and potential storm chances before a pattern change gets established over the weekend. The front that will bring refreshing weather this weekend will just be entering my northern counties around mid-day. Out ahead of it steamy conditions are again expected and heat advisories are are likely for at least the southern half of the region. How sticky we get will be determined by the amount of clouds we encounter. There's even a chance a few storms could roam the area early but I'm not seeing much support for that idea in the short term models. Overall, there should be enough sunshine, heat and dew points in the 70s to create heat index values of 95 to 103 north to south. The HRRR shows these values at mid-afternoon.
That will also fuel instability with CAPE ahead of the front of 3,000 J/KG That's down from the past couple days but still plenty of energy for storms.
The problem, as it has been the few days, is the warm air aloft that caps the atmosphere restricting thunderstorm development to a small area just ahead of the front. I do think some storms will fire but the timing of the front will be critical to get any rain in my central counties that really need it. If the cap holds too long the storms may not form until after the boundary gets near or past I-80. That would indicate the best rain chances confined to the southern half to one third of my region. There is just no way to know with all the mesoscale details involved until we closer to storm initiation.
Whatever happens with the available instability, where storms form they will have the potential to produce localized heavy rain and perhaps gusty winds. SPC has a slight risk of severe storms over the southeast half.
Finally the front passes leading us into a weekend of cooler temperatures and lower humidity. That holds until temperatures warm towards Tuesday of next week. I did want to stress the change in dew points that are on the way. Many spots Tuesday were experiencing numbers up around 78-80. By Sunday the EURO is showing dew points of 50-55, a drop of 25-30 degrees.
That's going to bring us a couple nights in the 50s and highs through the weekend will be in the lower 80s. Toss in some sun and you have yourself some outstanding mid August weather. I'm in. Roll weather...TS
