A exceptional synoptic set-up will provide the central Midwest with abundant moisture supporting a multi-day rain event that looks to be unusually heavy and widespread. Water vapor will be near record levels for early September promoting excessive rainfall as clusters of thunderstorms roam the periphery of a massive east coast high.
The amount of moisture the pattern brings over the next 7 days is remarkable and highlighted by PWAT's (precipitable water vapor) that's nearly 4 standard deviations above normal in parts of Iowa and Wisconsin.
While I can't say precisely where, I do expect some part of the Midwest will rack up at least 10" of rain by the middle of next week. As the scenario unfolds over time, river flooding and flash flooding will be a growing threat where repeated convection occurs. The Weather Prediction Center shows this for 7 day rainfall. The product is smoothed so it does not show pockets where totals could be substantially higher.
As it stands now, and this will change with time, the WPC has issued a river flood outlook centered on the heart of my local area.
WPC also shows a flash flood risk for 3 consecutive days centered on Iowa. This could extend into days 4 and 5 depending on how things evolve.
While models struggle ton pin-point the heaviest rains over such a lengthy time period, they can give us accurate trends regarding their mostly likely geographical placement, especially when there is consistency in multiple solutions. Here is an example of what the U.S. Model (the GFS) is showing for 16 day rainfall totals. The vast scope of the heavy rain is mind boggling. That much water over such a large area is bound to cause problems.
Our high resolution 3k NAM puts out these totals over just the next 60 hours. There are some 6-9" totals in the dark orange colors, especially over EC Iowa and NW Illinois.
The last thing I will say is that models are far from exact so just because it says 5" over your town it doesn't mean 5" will fall. We consider models guidance in the sense that they "guide us" to potential trouble spots. That said, the trends I'm seeing are disturbing. The writing is on the wall....we've got water and lots of it in the coming days. We'll see how it all turns out. Despite the rain, here's hoping you have a great holiday weekend. Roll weather...TS