A SUPER SOAKER...
While the majority of my area has managed to escape drought conditions this summer, that can't be said for my far southern counties. Most of the rains the past two months have missed that region allowing moderate to severe drought to develop and expand. My southern counties are on the eastern fringe of a large region of drought centered on the Plains that's slowly creeping into the SW half of Iowa. Notice much of the Midwest's dryness is focused on the NW half of Iowa and central Illinois.
Currently within Iowa, 62.35% of the state has either abnormally dry, moderate, or severe drought conditions.
Summer soil moisture anomalies in some of my counties in SE Iowa are now 80-100 percent less than what's typical.
Over the past 9 days conditions have grown even drier in many parts of the Midwest. For me in Dubuque, we are entering Saturday with 13 consecutive days with no measurable rain.
If you haven't figured it out yet, I am setting up the fact that there's a significant need for rain in my southern counties and points west in Iowa. That is why the coming event has the potential to be very beneficial for parts of that area. On the other hand, in my northern counties, especially NW Illinois summer rain surpluses are as great as 6-8 inches. No big rains currently needed there.
That brings us to the weekend event and the impacts its rains will have on the region. As I have stated for days this is the type of storm that's ideal for putting a dent in a drought if it tracks in the right spot. In a nutshell, there will be two periods of enhanced forcing that will have the potential to generate heavy rains. The first comes Saturday night and early Sunday. At that time a narrow but intense band of frontogenetic forcing lays out from SW to NE through the region. Thanks to this models develop a heavy rainfall band along the front as it slowly inches east into Illinois overnight.
Sunday a closed 500mb circulation pivots into central Illinois. After a lull in the rains Sunday afternoon a spoke of energy rotates around the upper low and spins deformation rains back in from the northeast. That process generates another round of lift and heavy rain potential lasting into Monday. You can see in this animation the evolution of the forcing and resulting rains starting Saturday morning and ending Monday evening. It's interesting to see the 500mb circulation spin up, reach peak intensity wrapping in precipitation, then occluding and weakening as it drifts northeast.
Again, subtle differences in the circulation track and speed do result in differences in where the heaviest rains occur. Despite being 24 hours out from the event occurring, models still show enough wiggle and wobble to make this part of the forecast challenging. I'll show you what I mean by putting up the various rainfall forecasts the latest models are suggesting. I'll comment after that.
The NBM (national model blend)
The Weather Prediction Center
The 3k NAM
The 12k NAM
SO, WHAT'S THE VERDICT?
From what I have seen, the latest trends are for the upper low to develop and mature a little bitter further east into EC Illinois. If indeed that's the case, the heavy rain band would shift further southeast as well, involving more of my counties in Illinois. To give you an idea of recent trends, here's the 18z run of the rain totals on the 3k NAM (Friday afternoon). Note the bulk of the heaviest rains near and north of I-80...the max in Jo Daviess County is 6 inches.
The newer 3k NAM (6 hours later Friday night), now shows lighter amounts in eastern Iowa and very significant and likely problematic totals just east of the Mississippi in Illinois. There's a broad area of 3-7 inch rains shown there with a 9.36 max showing up in southeast Whiteside County. I suspect those amounts are highly inflated and unlikely but that's a disturbing trend. On the positive side of the ledger, some good and much needed rains would fall in SE Iowa and WC Illinois. Take a look!
Needless to say, I don't have all the pieces of the puzzle but what it do have at this point leads me to believe the GFS may have the best overall solution. For sure it won't be perfect but as a guide I like what it is showing for broad based amounts. I do think a few places could exceed 4 inches in NW Illinois but hopefully we avoid those 6 inch plus amounts the 3K NAM is depicting. Once again, here's what the GFS shows.
Another aspect of our weather that will be very evident, especially Sunday and Monday is the sharply cooler temperatures that are on the way. Saturday is a transitional day and highs in my northwestern counties may not get out of the mid to upper 60s with rain developing in the afternoon. However, from about the Quad Cities southeast highs should hit and exceed 80 before the front and rains arrive later on. Sunday and Monday look much cooler across the board with highs mainly in the 60s, perhaps not getting out of the 50s depending on the position of the upper low and the rains falling around it.
One thing is for sure, our latest run of beautiful weather is on the way out. Big changes are coming in terms of rain and much cooler conditions. Signs of fall are on the charts today. Roll weather...TS