It’s only December 6th and we’re already talking about the potential for a winter event across parts of the Southern Plains and the Southeast. Since we’ve already covered the forecast for the Southern Plains, I’m going to focus primarily on the Southeast. A system earlier in the week moved across the Northern Plains and has now trekked northeastward into Canada. A cold front associated with that system pushed across the eastern U.S. earlier in the week and has now made its way to the Gulf coast and along the East coast. Behind the front, a broad trough has become established over the eastern two-thirds of the nation. With this kind of setup, mid to upper-level wind flow across the southeastern U.S. is generally from the southwest; thus, the initial cold front has already slowing down and will eventually stall out near the Gulf coast and East Coast. The shortwave feature that will be responsible for bringing snow to Texas is going to interact with the broad trough and a second shortwave feature. This interaction is going to sharpen the trough, which will eventually extend into the Southeast. Given that the frontal boundary will be stalled out near the coast as all of this is unfolding, this is going to induce the development of a surface low pressure system that will ride along that boundary along the Gulf coast and up the East coast.
For those in the Southeast, most who have lived in that region for any extended period of time know that most winter storms that unfold across that region involve a low pressure system developing near the coast along the Gulf of Mexico, which then crosses over and moves off or along the East coast. Since cold air is often limited, forecasting these events can be challenging throughout the extent of the event. Since it’s still early December, that even adds additional challenges.
First and foremost, it should be noted that most of the model guidance suggests at least some wintry weather (snow!!) across the Southeast (more on specific locations in a bit). However, snowfall accumulation projections range from no accumulations to several inches of snow for parts of the Gulf coast northeastward to the East coast. For events such as these, I either like to see that a pre-existing colder air mass has become established over the region before the event, or that there is going to be sufficiently cold air provided by a certain atmospheric feature in time for the event (for example, high pressure over the Northeast). The first cold front that has now pushed through the region has brought in much colder conditions, but given the broadness of the trough and the time of the year, there really needs to be an additional source of colder air spilling into the region. Throughout the day on Thursday, the colder air to the north-northwest of the area will continue pushing southeastward, but despite that, we’re still going to be dealing with the classic borderline winter storm/event in the South.
The NAM model has generally been the most aggressive with snowfall accumulations across the Southeast, which extend those accumulations down the Gulf coast. The hefty snowfall accumulations occur for most locations later on Friday into Friday night, even though it has precipitation (mostly rain) spreading across many locations in the Southeast tomorrow. If you’re looking to get noteworthy accumulating snow, it’s best this event occur later on Friday like what the NAM is showing for many locations, and given that it would be getting dark or already dark, this would greatly increase the odds that temperatures would be sufficiently cold throughout the atmosphere to support snow or a transition from rain to wet snow. This would also allow more time for colder air to seep southeastward. The GFS is less aggressive, primarily because it has the heaviest precipitation moving out more quickly. The European model, which is generally more reliable, falls somewhere in between with accumulations. Remarkably, all of the guidance is at least showing some accumulating snowfall for some locations in the Southeast. However, it should be noted that these maps depict a 10:1 snow to liquid ratio, but given the warmer ground temperatures and the fact that rain will mix in (at least initially), these totals could be overestimated for some locations on all three models. However, higher snowfall rates across some locations could offset some of these hindrances for snowfall accumulations.
Figure 1: NAM model snowfall accumulation forecast for the next 72 hours. Source: Tropical Tidbits
Figure 2: GFS model snowfall accumulation forecast for the next 72 hours. Source: Tropical Tidbits
Figure 3: European model snowfall accumulation forecast for the next 90 hours. Source: Ryan Maue
Remarkably, residents living in southeast Louisiana, southern Mississippi and possibly extending into western parts of Alabama have the greatest chance of this event unfolding for them. Since this region will be some of the first locations to feel the effects of the colder air mass digging southeastward, precipitation developing and moving across the region will have the greatest chance of making the transition to snow either late Thursday or early Friday. Predicting snowfall accumulations from eastern parts of Alabama into Georgia and South Carolina gets quite challenging. The later this event unfolds for those locations on Friday, the better for accumulating snowfall. The more aggressive snowfall totals that span across parts of North Carolina into southeastern Virginia are warranted. That’s another region that I feel has a pretty decent chance of this event unfolding.
Due to the uncertainty that exists, this will require me to post a follow-up update tomorrow. However, residents in the general swath of accumulations that are being depicted by the model guidance should prepare for this event. I expect the majority of accumulations to occur over grassy surfaces; however, issues on the roadways could occur for localized regions where the snowfall rates are higher. That will be something that we’ll have to determine tomorrow. Nonetheless, it is still very early in the meteorological winter, and it’s remarkable that we even have an event such as this to forecast! Firsthand Weather will keep everyone updated as much as possible before and during this event and will be posting numerous updates on our Facebook page.