EXPECT THE UNEXPECTED....

August 24, 2018

Expect the unexpected Friday. What did I just say? Basically that I'm not sure myself how the day will turn out. Best case scenario, scattered showers and storms fade away early and don't return. Worst case, scattered storms redevelop towards evening and blossom into severe weather producers spitting out wind, some hail, and perhaps even a couple of tornadoes. Such is the dilemma facing Midwest meteorologists.

Because of the existing potential, the Storm Prediction Center currently has a risk outlook focused on central and eastern Iowa for the late afternoon and evening hours. The storm has an impressive look on the GOES satellite imagery.

As I mentioned in my post yesterday there are a couple key factors that need to be overcome for strong storms towards evening. One will be the departure of rain and clouds from early day convection. The lack of any breaks for sunshine means less instability and a decreased potential for strong storms. 

 

Another significant factor will be warm temperatures aloft (a cap) that could suppress development until forcing (a weak cool front) arrives on the scene. The timing of the front and the amount of instability available will make or break the event.

 

What's troubling to me is that so far models have shown a wide range of options ranging from no storms to active and severe ones. Unfortunately, I don't have a great deal of faith in any one solution and I'm still on the fence.

 

However, one trend that I've seen more and more of Thursday night from the U.S. models is the placement of the best instability further south from central into s/east Iowa. Here's an example of the CAPE (convective available potential energy) the HRRR shows Friday evening. 

The EURO on the other hand is further north and east for the same time period. That fits nicely with SPC's outlook above. More contradiction!

Personally, I think the best CAPE and instability will end up a little south of what the EURO shows. That would limit the development and coverage of any storms north. Further south the CAP may break and strong storms could erupt in SC and SE Iowa and then track southeast. This is the solution I think is most likely. You can see below the 3k NAM depicting that line of thought. To sum it up I believe southern Iowa, northern Missouri, and WC Illinois stand the best chance of severe weather Friday evening. We'll keep a close eye on it.

Whatever happens, I think I've made my point that the jury is still out on the set-up and probably will be until late Friday morning.

 

Eventually this all shifts east of the area by Saturday and the focus turns to several days of heat and humidity. As dew points climb into the 70s and highs reach 85-90, the weekend will be a summery one. With kids back in school I guess it was bound to happen. Roll weather and enjoy the last weekend of meteorological summer...TS

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© 2019 Terry Swails