Thoughts about frosts and freezes always come around with the first few big “cool” snaps of the Fall. Low temperatures by Tuesday morning should drop to the lower and middle 40s around almost all of the Tennessee Valley; the exceptions will be some low-50s on Sand and Lookout Mountains.
With temperatures only getting down into the 40s, it will be chilly; however, it is not quite cold enough to be worried about frost.
So when can we expect frosty weather? We typically don’t see the first frost until at least October 25th in this part of Alabama and Tennessee. From some of today’s model runs, it looks like some light, patchy frost is fair game after a cold front passes late next week.
This is the 18Z (midday) run of the GFS model showing minimum temperatures as cold as 35 to 40 degrees on October 21:
The early morning run of the GFS ensembles gives us a similar conclusion showing the possibility of temperatures dropping below 40 degrees. The box & whisker plot shows that the most likely solution is for lows in the 40s and 50s; however, the plot does indicate some small potential for some 30s around October 20-22nd.
Another model is showing some cold weather around the same time: the CFS (climate forecast system). Note the big bright green area indicating temperatures that could be as much as 5ºC (10ºF) below average. Yes, that would be very significant cold air for late October:
Here’s the graphical explanation of our average first front dates in North and Central Alabama as well as a few statistics on the first and last freezes of the season:
So, the bottom line is that there is no real danger of a widespread, killing frost or a freeze before October 20th. After that, there is at least a chance for cold enough weather for frost before the month is over.