JACK BE NIMBLE, JACK BE QUICK AND COLD
As I indicated a week ago all the ingredients for frost will be on the table Wednesday morning. That includes:
A large 1036mb Canadian high
Dry air
Light winds
Clear skies
Maximum radiational cooling
With all those ingredients in play, it looks like old "Jack" will be nimble quick and cold Tuesday night. For now, the biggest question is how cold? Some of the high-res CAM models are indicating lows at or below freezing near and north of I-80. The coldest scenarios are highly dependent on winds decoupling early and the ridge being directly overhead. Assuming that happens, it would not be at all out of the question for some of the valley locations (where cold air drainage is a thing), to hit 30-32. In general the most common range should be 32-37 with the coldest readings just NW of the Quad Cities. I expect frost to be widespread but the freeze potential is far less certain. I do expect frost advisories will be issued by the NWS for at least the NW half of the region Tuesday.
The GFS shows a 70-80 percent chance of a temperature less than 32 in my northern counties up near HWY 20.
If by chance a 32 degree reading makes it to the Quad Cities (which is unlikely), it would be the 9th earliest on record. In other words, an an event that is in the top 6.8 percent of all fall freezes going back to 1874. The earliest was on September 20th, 1991. The latest November 12, 1946.
Here are the earliest fall freezes from around the area from the NWS in the Quad Cities.
Still 24 hours out, here are what various models are suggesting for lows Wednesday morning.
This is the official NWS forecast at this time.
Now the various models starting with the coldest solutions.
The 3K NAM
The 12K NAM
The GFS
The national model blend
The EURO, I think at this time its warmer solution is an outlier. We shall see.