PLAYING THE NUMBERS GAME WITH RAIN...
To rain or not to rain, or anything in between...that is the question facing forecasters going into the weekend. After pondering the dilemma much of Thursday I've come to the conclusion that the rain that falls this weekend is likely to be heaviest over the SW half of my area with most of it falling Friday. These are the amounts the Weather Prediction Center is showing but I have some concerns it will end up being this generous, especially in the NW half of my area.
The primary focus Friday is centered around a warm front and the convection it has generated overnight. Storms fired in SE South Dakota and NW Thursday night and the alignment of the jet stream and the warm front is driving them southeast. As the remnants enter the region early Friday my expectation is that the storms will likely hold together over the morning hours, especially from the Quad Cities southwest. There are suggestions my NW counties may not be impacted by this first wave of rain, remaining cool but dry (especially NW of a line from Dyersville to just north of Clinton on into Illinois). That said, the duration and extent of the storms that do occur will play a major role in determining what happens later Friday and Friday night. As it stands now SPC has a slight to enhanced risk of severe storms over the southwest half of my area where strong instability and shear is anticipated near the warm front.
However, the severe weather threat Friday afternoon or evening is contingent on instability. If the morning storms linger too long and clouds limit daytime heating the stable rain cooled air will be in place. The convectively induced outflow boundary could easily push the warm front south, perhaps just catching far southeast Iowa. That means the severe threat is further south than what SPC is showing limiting the concern for all but the far SW corner of my area. Even there (in southeast Iowa) the window would be limited and by evening it's heavy rain that becomes a bigger concern. It will be a different story just to the west in SC Iowa where that region is under the gun for potent storms with conditions far more favorable for severe weather. Here's the excessive rain outlook for my southernmost counties.
From what I am seeing Thursday night, I like the idea of a lower severe weather threat Friday that's confined to far SE Iowa. I'm also seeing a trend for more dry air over my northeast counties where there may not be much if any rain at all through Friday. That's a big change. In fact, overall models are really struggling with the evolution and placement of the upper air low after Friday. Now most models are now indicating a solution that is less wet in my northern counties. The EURO has really changed its tune the past 24 hours going from rainfall that looked like this a day ago.
To this Thursday night.
The GFS has an entirely different look dumping beneficial rains much further north and ones that are even heavier and more problematic than the EURO shows across the south.
Frankly, this is a challenging forecast with lots of mesoscale details involved. That means there's room for things to change and I think the best approach right now is to take it day by day knowing doubt is on the table. Whatever happens, the heaviest rains over the weekend are likely to be found in the southern half of my area, especially south of I-80. Undoubtable it will remain cool with highs staying mainly in the 70s through Monday. Clouds are going to be a big factor as well with limited sunshine expected the next 4 days. Anyway, I've spent more than three hours on this post and I'm hoping I've played my cards right and this all makes sense. Happy Friday and roll weather...TS