While much uncertainty still exists on specifics, confidence continues to increase that a winter storm (potentially significant) will be impacting the Southeast, among other regions, this weekend. In this article, I will primarily be focusing on the southeastern U.S. (including the Tennessee Valley), while Christopher Nunley will be posting an article tomorrow, which will address potential impacts across the Southern Plains. This system could produce impacts from coast to coast, including over the Rockies, which will also need to be addressed.
The Pattern Leading Up To The Potential Winter Storm:
When attempting to determine how big of a punch a winter storm might bring, I always take a look at the overall pattern that will precede the event. Currently, a low pressure system is located over the lower Mississippi Valley, which will trek northeastward tomorrow across the Tennessee Valley and Ohio Valley. While this will act to transport warm, moist air into the southeastern U.S. initially, cooler air will rush in behind the low as it moves into Canada.
A series of shortwave troughs, one that will be particularly robust mid-week, will move across the Great Lakes and Ohio Valley and eventually eastward into the Northeast. This will act to maintain the general placement of a longwave trough over the eastern third of the nation, and a strong cold front will eventually sweep all the way through the Gulf Coast states to the East Coast by Wednesday into Thursday.
Figure 1: On Friday, the latest GFS has a longwave trough over the eastern third of the U.S. with the potential winter storm located over the Rockies.
As far as having the necessary cold air in place for a winter storm across the southern states, this kind of setup is typically what needs to occur. As it seems right now, the cold air will be in place before the system of interest moves across the Southeast. While some regions could start with a mixed bag of precipitation or even just rain, this is shaping up to be an all-snow event for many locations with a rain/snow line setting up somewhere along the Gulf Coast states.
Potential Impacts, Locations To Watch, and Uncertainties:
A vigorous shortwave (the feature that will trigger the development of the winter storm) will move into northern California/ Oregon after midweek, which will dig into the Southern Plains early weekend and trek eastward across the Gulf coast states through the weekend. This will keep the colder air reinforced over the eastern U.S., and this feature will trigger the development of a surface low pressure system that will eventually move along the Gulf Coast states. The low pressure will likely be placed in the right-entrance region of a jet streak (very strong winds aloft) located along the East Coast that could aid in the further strengthening of this low pressure system once it moves just off the Southeast coast.
There are a couple of uncertainties that I have at this point that should somewhat clear up by midweek. First, it is uncertain how far north the region of precipitation will reach. Sometimes the issue with pre-existing cold air masses located over a region is that the moisture can get suppressed quite a bit to the south. Another factor to consider is deep convection (thunderstorms) along the Gulf coast and Florida that could rob moisture from regions farther north. These are some factors that always must be considered with such a setup. This uncertainty is definitely being demonstrated by the forecast models. . .the operational GFS model has a monster of a winter storm for a large portion of the Southeast while the operational European model has generally kept precipitation amounts lighter the farther north one is located. With that said, almost all model guidance has some amount of moisture making it fairly far north, however, this puts locations like Tennessee and southern Kentucky in a “wildcard zone.”
Figure 2: The latest GFS (top two images) has the precipitation field farther north than the latest European model (bottom two images). Ensembles are likely more useful in this range.
While this is subject to change, locations that need to watch the potential of being impacted by a winter event/storm (potentially significant in some of these locations) include the northern half of Mississippi, the northern half of Alabama, the northern half of Georgia, the northern half of South Carolina (including Midlands), much of North Carolina, far southern Tennessee, and southern Virginia.
Additional regions that could be impacted (which will be addressed tomorrow in Christopher’s article) include parts of the Southern Plains, Arkansas, and northern Louisiana. He will be covering those locations during the extent of this event.
Regions that are currently on the line include the rest of Tennessee, southern Kentucky, and the rest of Virginia due to uncertainties on the northern extent of the moisture.
Right now, the focus is currently on the overall pattern and the feature that will be responsible for this potential event. Given the blocking signature currently located over the eastern Pacific, forecast models are especially unreliable at this time. I encourage you to disregard forecasts, such as those produced by weather apps, that are solely based on model data. Over the next two days, we will be able to better specify locations and will make the necessary modifications to the possible impact zones. Throughout the week, we will begin to nail down exact timelines, precipitation-type for your location, and amounts.
This only marks the beginning of the forecasts and coverage that will be doing on this potential event. Be sure to follow Firsthand Weather on Facebook and Twitter, as we will be posting several updates per day on there.