Forecast guidance has been very consistent with the threat for freezing rain Friday. What has not been as consistent is how much icing will occur. Confidence is high that we will see some icing, especially on elevated surfaces. Ground temperatures are above freezing for the most part with the exception being the plateau. The plateau is likely where the worst driving conditions will be; however, even some untreated valley streets may become icy. Regardless, travel is not advised Friday morning.
Timing of the precipitation varies a bit from forecast model to forecast model. I have decided to blend all available guidance and place a range of times that impacts will be felt across the valley. It looks like light freezing drizzle will start between 5am and 7am CST. Travel problems will likely initiate shortly after precipitation initializes in the higher elevations. The greatest threat for icing in the Sequatchie Valley will be as relatively heavier precipitation moves in around mid to late morning or so just before temperatures warm above freezing.
Expect bridges and overpasses to become icy for your morning commute. I am VERY confident that no one will see a single flake of snow from this system. Max surface to H500 temperatures will be just shy of 10C allowing for complete melting of ice nuclei. Given this, sleet is even doubtful, especially as time progresses.
It looks as if temperatures in the valley will warm above that critical 32F mark around noon. Being consistent with ranges, I’d say the transition will occur between 11am and 1pm. Chattanooga and points east will change over a bit later than Marion Co as the wedge of high pressure (responsible for the shallow pool of cold air leading to freezing rain) breaks down. As expected, higher elevations will take a bit longer to change over as a sharp temperature contrasts will be recognized between the surface and 3Kft. I expect a change over in these locations between 1pm and 3pm.
Ice accumulations look light for the most part. QPF (precipitation amount forecast) only indicate .05”-.15″ during the duration of the freezing temperatures for valley locations. Heavy amounts will be located north and west of the area and will not arrive until after the temperatures increase. For this reason, the National Weather Service decided to go with a lower category headline (Freezing Rain Advisory) rather than the more significant Winter Storm or Ice Storm alerts. Rain will end rather quickly at sunset or a little before.
*Freezing Rain Advisory in effect through Friday Evening*
Friday- Freezing drizzle early then light freezing rain before 1pm with rain likely after 1pm. High Temperatures will be around 38F in the valleys. Higher elevations will reach around 34F degrees. Valley ice accumulations look to be between 0.05” and 0.15”. Higher elevations will receive between .1” and .2”. The highest elevations may experience “some” power issues; however, those should remain only isolated at best in those location.
Meteorologist: Robert Frye
Issued: 7:00pm CST 1/24/13