Take a look at this. It's the forecast PWAT values (available precipitable water) for rain Saturday evening just east of Cedar Rapids, Iowa. There's a max of 2.97". That is flat out off the charts for September 21st! Hard to do at any time of the year. Wow.
Just to give you an idea how high that is, here's the percent anomaly and that value is 300% above what's typical.
When there's that much moisture available what do you get? Flash flood watches! Here they are.
With this exceptional amount of moisture and a slow moving front providing sufficient forcing, excessive rainfall is is a given. The biggest question is precisely where and the experts at the Weather Prediction Center show the best chances Saturday over Kansas and Missouri. However, it wouldn't take much to push that further northeast into SE Iowa and WC Illinois. By the way, excessive rains are categorized as amounts that in 1 to 6 hours time exceed flash flood guidance at a point.
The greatest potential for rains of 1-2 per hour will be from late Friday night into Saturday evening. That's the period when the moist conveyor belt is aimed directly at the SE half of my area, roughly southeast of a line from Cedar Rapids to Dubuque. Amounts north of there should taper off rapidly and totals northwest of a line from near Waterloo to about Madison may be less than 1/2". Southeast of there 1 to 3" totals are expected with the heaviest the further south you go. If some training takes place some local spots could top 4". The Weather Prediction Center shows this for rain potential.
The EURO has this for totals.
The NWS in the Quad Cities has posted this for amounts through Sunday.
They also put this graphic out regarding the flash flood watch that covers much of my area.
While thunderstorms are likely into Saturday night warm cloud depths from the excessive moisture will limit the ability for storms to generate and sustain cold pools. That should also keep the risk of severe weather limited although SPC does show a slight risk in some areas.
The whole system gradually departs from NW to SE late Sunday leaving a pretty decent day on tap for Monday. Unfortunately, many of us will have to deal with periods of showers and storms into at least early Sunday. Many spots should see a good soaking. By the way, those types of rains have been hitting some of my northern counties up around HWY 20 much of the month. Look what's happened in Dubuque so far this month with 11 days to go. The 11 inches that's already fallen makes this the 6th wettest September on record with more on the way.
That's it for now. Stay high, dry, and roll weather...TS