Is it really that time of year again already? For most of us, we skipped fall and went straight into winter last month. Although wintry precipitation has already impacted parts of the United States, the first legitimate chance for a winter event farther to the south will come in about 6 or 7 days. The goal is never to address local-scale specifics in the long-range, but we can begin discussing the pattern that could support a winter storm. This allows us to establish an initial framework by looking at the big picture first, and then we can build upon that foundation with specific details in the coming days.
Will the mid and upper-level atmosphere support an early-season winter storm across portions of the South?
The first step, especially at this point, is to look at what’s currently going on well-above the surface and attempt to determine how that pattern will evolve over time. From this, it’s possible to infer what could occur at the surface without it being necessary to look at modeled surface output at this point. A closed mid-to-upper level low pressure system was located over the central U.S. yesterday (Saturday) and has now moved northeastward over the Great Lakes. A cold front, associated with a surface low that developed in response, will push all the way through Florida by mid-week. Broad troughing will remain established over the eastern U.S., keeping an anomalously cold air mass in place.
Now, here’s the main reason I made a post on November 29th about the possibility of a winter storm. A split-flow regime is expected to become established over the far western U.S. Let me explain what that means. With this setup, the jet stream splits into northern and southern components. The northern component (the polar jet) will extend well into western Canada and Alaska, while the southern component (the southern jet) will eventually dip into Baja California. Now, check out the map I posted under this paragraph. You can see the broad trough over the eastern U.S., ridging over western Canada and Alaska, and a shortwave extending into southern California and Baja California. I drew arrows to indicate mid and upper-level flow. Do you see how the flow begins to merge back together over the central U.S.? When this occurs, this is called confluence. As this confluence occurs, this will result in sinking motion over the Great Plains and will support the development/maintenance of strong high pressure over that area. Winds flow clockwise around a high pressure system in the Northern Hemisphere, and cold, Canadian air, will wrap around on the east side of this high. There’s your cold air source.
This will allow another cold front to push southward, and a surface low will develop in response to favorable dynamics just to the east of the shortwave over California. As this shortwave treks eastward late week into early weekend, so will the surface low, which will probably ride somewhere along the frontal boundary. This will result in rainfall across drought-stricken southern California and the Southwest and a swath of wintry precipitation that will extend somewhere from the Southwest/Southern Plains to the East Coast.
If you were to ask me how far to the south I believe frozen precipitation will occur, in short, I’ll tell you I don’t know. But, I’ll give you some insight on this. With conditions favoring high pressure over the central U.S., I’m comfortable saying that it wouldn’t be too difficult to get frozen precipitation (snow and/or ice) as far south as parts of Oklahoma, Arkansas, and Texas. Farther eastward into Tennessee and northern Mississippi/Alabama/Georgia, it becomes a bit more of a tough call. I’m especially paying close attention to northern South Carolina, most of North Carolina, and parts of Virginia, due to the potential for cold air damming to establish itself east of the Appalachians as surface high pressure moves eastward. That’s why in our ‘best chance for wintry precipitation’ map (shown below), we currently depict higher probabilities across those locations. Of course, it should go without saying that we will have to modify this map between now and next weekend, and keep in mind that wintry precipitation is not exclusively snow. We can iron out all of those details later.
Again, we’re simply trying to determine how this pattern will evolve. Any unforeseen changes in that would completely throw off my current expectations for next weekend. Have fun looking at all of the snowfall projection maps, but it’s important to understand the major limitations of accumulation projections this early in the game. And no, I’m not expecting nearly 3 feet of snow in parts of South Carolina like what the European model is showing.
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A special thank you to Kimberly Gnat for sharing a picture with us of the snowstorm near Chicago late last month, which we used as the featured photo.