Hello and good evening to everyone. I know this forecast update is late but I wanted to get some info out on the upcoming potential for winter weather.
First I have to start by saying that I believe this will be a low impact winter weather event outside of the mountains. It is important too that you know that this forecast will be evolving through Saturday.
Lets put this together. Our first piece of the puzzle is a piece of energy that is moving through the Pacific Northwest. This upper level low pressure system moved onshore yesterday over Oregon. Here it is now:
The second piece of energy is a low pressure system back over New Mexico. This piece will be the main storm system. As this low ejects out of New Mexico and into the Arklatx it will tap into gulf moisture. Here is the low pressure as of tonight:
The rain you see from Oklahoma to Kentucky is in response to an upper level low that is moving Southeast tonight. That will bring some more snow showers to the mountains overnight tonight and maybe a few sprinkles/flurries mix to the foothills predawn Saturday Morning.
So we have the first two pieces but where is our cold air going to come from? That is our third piece for the puzzle. That is high pressure that is very weak and is transient instead of stationary. To get any winter weather in our area that clockwise flow around the high is going to have to act in concert with low pressure that will develop off the Coast of South Carolina. Another big question, how quickly can that low pressure deepen and the counter clockwise flow around it drag in more cold air down from the North. That’s where the high up over Canada and the low will have to play together perfectly to get the rain to change to snow.
This is a look at that high and low and what it will have to produce to develop snow. You can see the model indicating the weak wedge east of the blue ridge Monday Morning at 2am. That is the GFS Model. I’m not too excited about this setup producing snow outside of the mountains.
Wait though, there is more. Remember that first piece of the puzzle from above? That is the upper level low. The upper level low’s eventual timing and track is also key. Above we see the upper low advancing quicker and phasing together with the Southern stream system according to the GFS. That upper level low has its own cold air to work with and hence the reason you are seeing the changeover to snow across portions of Tennessee and Kentucky in the graphic. The GFS has held this solution to some degree over the last 10 runs. Last night and then this afternoon the European Model came on to support the GFS solution. The European though brings the core of the cold from the upper level low east of the mountains. Then out near term model trended away from both the GFS and Euro solutions, keeping the upper level low to the west and warmer as it drives it South on Monday Morning. The NAM solution produces minimal if any impacts across our forecast area. For now it is the outlying model and we have taken it out of our forecast for the time being, alwhile not completely ruling it out for future forecast updates Saturday if the trends go that way.
Before we get to our first call snow accumulations I have to note that none of the guidance suggests temperatures below freezing at the surface. Just above 3000 ft agl temp profiles quickly fall below freezing. That means above 3000ft confidence is higher that you will see accumulating snow. Below 3000 ft agl (foothills/NW Piedmont) it means rain could mix with snow and briefly turn over to snow along and North Of I40, while South Of I-40 would likely only see a rain/snow mix. In response to that I feel The National Weather Service Weather Predictions Center lays it out best with their probability forecasts. Remember, the surface temps (at least for this forecast package) never fall below freezing until Monday Night after the precipitation is gone and skies are clearing. So even though we see may see snow, roads should remain fine below 3000ft. Here are the probability maps for the upcoming event (See top of each slide for what probability you are viewing and control it at your own pace by navigating left or right)
All of that being said here is what our first call is on any possible accumulations. Remember though. This is all rain outside of the mountains for the better part of the day Sunday. Any change over to winter weather in the following forecast, again outside of the mountains, occurs after sunset Sunday and into Monday Morning. Roads likely will just be wet outside of the mountains. More updates to the forecast during the day on Saturday.
Foothills Weather Network