WEATHER BLOG: Why forecasting tonight's low temps is tricky

By Brian McClure, Meteorologist
Last Updated: Tuesday, January 07, 2014, 4:14 PM EST

Here is a window into our world in the Bay News 9 Weather Center:

Even on a day that is mostly clear and just chilly (appearing like an easy forecast from the outside), we are racking our brains trying to predict our overnight lows tonight.

There area a lot of things to consider:  It’s obviously cold, so as soon as the sun sets our temperatures will start dropping. If the skies stay clear and the winds go calm, they will drop very fast due to a phenomenon known as “radiational cooling."

To put it simply, the coldest air is allowed to sink down to the ground while the heat is radiated upward. This is a common phenomenon. But it’s not that simple. If the high clouds roll back in, which it appears they will, the clouds will act as an “insulator” or blanket, which will help to keep up our temperatures up and not allow them to drop to the lowest potential.

The other thing to consider is whether the winds will really go calm. Locations near the coast probably won’t, while inland “protected” areas and low-valley locations likely will go calm.

The wind tends to “mix” the atmosphere, keeping it from reaching its lowest potential. But when the winds go calm, the coldest air settles down and temperatures can really plummet. This is why temperatures can vary by huge amounts on nights like this within a short distance.

With that in mind, because it’s already cold, our daytime temps in the 40s will likely drop quickly into the 30s tonight, but then level out at some point around 2 or 3 a.m. when thicker clouds roll in.

Citrus, Hernando, and Pasco county locations will drop the lowest and fastest. Coastal locations will have a little breeze to keep the atmosphere “mixed” a bit. So expect lows generally in the 25-30 degree range in Citrus County, 26-31 in Hernando county, 28-33 in Pasco, 30-35 in Polk, 32-37 in Manatee, 30-36 in Hillsborough, and 32-39 in Pinellas county.

Watch our county-by-county forecast at 19 minutes after the hour for details within each county. No matter your exact low there likely will be lots of frost in areas that get near freezing. So keep that in mind if you want to cover a sensitive plant. If we see the trend change we will update our county-by-county forecast through the afternoon/evening.