The Weather Prediction Center is anticipating a messy 24-hours from 02Z 14 February through 02Z 15 February 2021. The map indicates a large swath of the Southeast receiving rain, mixed precipitation, freezing rain, and/or snow, as shown in Figure 1. The travel impacts will likely be vast, though this is dependent upon the type of precipitation that falls.
Figure 1: Weather Prediction Center national forecast chart, valid 02Z 14 February through 02Z 15 February 2021.
Visible satellite imagery shows thick clouds blanketing much of the United States, as shown in Figure 2. Across the country, there is a general lack of convective activity, evidenced by the flat and uniform appearance of the cloud cover. There is some minor convective activity bubbling up from the Gulf of Mexico and making landfall over the Mississippi/Alabama coast. This may provide some moisture to the winter storm system. However, the main story with this system lies with the atmospheric profile, which determines what precipitation will fall as rain, freezing rain, sleet, or snow.
Figure 2: GOES-16 visible satellite imagery, valid 12:16Z through 16:26Z 14 February 2021.
This temperature difference can be deciphered in part by examining radar imagery, illustrated in Figure 3. Off the east coast, the precipitation is falling as rain. The scattering of the radar rays by raindrops makes this precipitation appear splotchy. This can be contrasted with the precipitation that’s falling as snow over the Great Plains, as this has a smoother appearance. The trouble with using radar to forecast an event such as this winter storm is that it’s very difficult to tell areas of freezing rain and sleet on radar imagery. This is where the atmospheric temperature profile needs to be considered.
Figure 3: Radar imagery, valid 17:11Z 14 February 2021.
At the intersection of the snow/sleet/freezing rain line sits Nashville, Tennessee. A forecast skew-T sounding for 04Z 15 February shows a very small warm nose preventing the precipitation from falling as snow. The frozen precipitation will fall through the atmosphere until it reaches approximately 840mb. At this point, it will melt, as the atmospheric temperature is above freezing. Around 870 mb, the precipitation will freeze again, as the surroundings are below freezing down to the surface. The size and altitude of the warm nose determines whether the precipitation falls as either freezing rain or sleet. As a result, the impacts of this winter storm are extremely difficult to forecast.
Figure 4: Forecast skew-T sounding for Nashville, Tennessee, valid 04Z 15 February 2021.