FRISKY STORMS A FRIDAY AFTERNOON OPTION...
Before we get to Friday's storm potential, the new drought monitor is out and the trend for drought expansion continues to be noted around the central Midwest. Here's the corn belt as a whole. Leading the way again is WC Iowa where extreme drought prevails. Roughly 40 percent of the Midwest is at the very least drier than normal.
This is the amount of change that's taken place across the region over the past week. All the areas in yellow advanced a drought class, not surprising with the hot and relatively dry weather.in those areas.
Iowa is the epicenter of the dry conditions this summer. As of August 25th, 96 percent of the state is experiencing drier than normal soil levels. Over one-third of Iowa is in severe to extreme drought.
These are the weekly changes observed in Iowa.
Below are the corn belt precipitation departures since the first week of June.
August has been especially dry around the majority of my area with departures in most spots 2 1/2 to 3 1/2 inches since the beginning of the month.
Now that I've demonstrated the need for rain, lets look at prospects through Sunday night. First and foremost, if it's going to happen the window of opportunity is restricted to Friday. The first chance is Friday morning and that should only involve the area near and north of HWY 20. Much depends on a convective system Thursday night that is taking shape over South Dakota. With a fairly stout cap in place over most of Iowa and Illinois, the main thrust of what develops should impact Minnesota, Wisconsin, and far northern Iowa. If the MCS can generate a strong enough cold pool the outflow could get some storms down around HWY 20. That's a big if and I prefer a northward play with this wave of precipitation.
That brings me to Friday afternoon and here's where the forecast is still far from clear and will deserve watching as the mesoscale dynamics unfold throughout Friday morning. Assuming my area stays dry Friday morning and we get the necessary heating, afternoon instability should be pretty robust. The EURO shows significantly less CAPE than the GFS. That indicates less potential for strong updrafts. However, the GFS is ripe showing a much more impressive environment. Here's the comparison of the two at 4:00 pm.
Again, with the necessary heating the GFS scenario is plausible, especially with steep lapse rates in place.There will be a cap to contend with and the strength of the cap will be an important factor in the eventual outcome of any severe weather. The other key is the forcing that arrives in the form of a cold front late afternoon. At some point the cap should break but when and where is still open for debate. If it breaks and instability is optimized, rapid thunderstorm growth is a real possibility as are downburst winds and hail. The Storm Prediction Center is all in as of Thursday night showing an enhanced risk of severe thunderstorms (3 out of a risk scale of 5).
I still have doubts about where the cap breaks and how much of my area gets into the storm and rain potential. To me (as of Thursday night) the area southeast of a line from Cedar Rapids to Dubuque appears to have the best chances. However SPC in their outlook is much further northwest. Since they are the experts I will defer to them and just throw my idea out as food for thought. We will have a much better idea of prospects later in the morning. Keep an eye out for an update.
As for rain totals, there could be some spots with heavy downpours but the the storms are likely to be fast movers so even in areas where they occur, rain amounts may not be that impressive for their overall strength. Both the GFS and EURO focus the heavier rains over the NE half of my region.
The EURO shows this for rainfall. The coverage is even less with it's later initiation and lower instability parameters.
Whatever happens, cooler and drier air arrives for the weekend putting this late season heat wave to bed. The weekend looks really nice after what could be a few fireworks Friday. Roll weather...TS