When a massive cold front arrives, the cold air comes to the surface first (cold air sinks). It takes a while for the air aloft to cool. The result: it’s colder on the ground than it is overhead.
That’s what happened last night. So, it was actually warmer overhead than on the ground. That makes freezing rain and sleet.
Here’s a good illustration:
Despite being under two consecutive Tornado Watches last night, the tornado threat did not materialize. Why? That cold air came in before the storms, and undercut any chance of tornadogenesis.
The winds were strong, and sustained. Twice Saturday winds gusted to 40+ MPH, most dramatically to 48 MPH when the cold front arrived at BNA at 11:15 – 11:20 PM.
Wind damage reports in Middle Tennessee were widespread. Several large trees were dropped. For example:
— Julie Crowe (@juliecrowe333) December 18, 2016
These #tSpotter reports are invaluable to our local NWS forecasters. Reports help verify (or not) the warnings and advisories they issue. They care about getting things right, so let them know what you see.
Will The Roads Ice Over Tonight?
There is never one answer to this question because not all roads are the same. Ice formation depends on several factors, such as elevation, amount of precipitation, runoff, road traffic, asphalt conditions, and wind exposure. These vary. That said, consider a few things.
- We’ve been below freezing all day. If your road was going to freeze, it probably would have frozen already.
- The wind has been blowing. Winds dry roads.
- Bridges, overpasses, and roads at elevation are more susceptible to ice. Usually, the ice isn’t visible.
Travel may still be hazardous. Road conditions can be monitored on the TDOT Smartway page. That site has cameras and all sorts of good stuff. They also have a pretty good app
Clear Skies Tonight = A Very Cold Monday Morning
Clouds trap daytime heat. Without them, the heat escapes through the top of the atmosphere and it gets really cold.
We will wake up to 19º Monday morning, but we should peek above freezing in the afternoon.
Despite another very cold Tuesday morning (20°), we’ll be warmer Tuesday afternoon (44º).
Quiet Week Ahead
Most of the week appears quiet. That rain chance Wednesday/Thursday is only showing up on the GFS model; the Euro doesn’t have it. I would not put any money on rain then.
Christmas Eve & Christmas Guess
Remember, model/forecast accuracy this far away is low. That’s why we call this a “guess.”
The Euro model predicts a very wet Christmas Eve, with rain lingering into Christmas morning. The Euro even has a high of 67° (!) Christmas Day. This does not seem realistic. The GFS model totally disagrees; it keeps us cooler and dry both days, with rain the day after Christmas. This wild disagreement between our two major weather models leaves no confidence in the forecast.
Our NWS went with 38°/52° on Christmas Day, with a chance of rain. This is reasonable. It does not discount either model solution. Usually, the truth is in between.
Categories: Forecast Blogs