Wednesday, January 13, 2016 7:57 PM

Southern Winter Storm Possible Late This Weekend But Uncertainty Exists

Matthew Holliday

I’m going to start this article by making two brutally honest remarks. First, there are a few uncertainties with this forecast, and if there is ever going to be a forecast that I’ll have to make some changes to, it’ll be this one. Secondly, I’ve seen statements from several meteorologists saying things like, “the European model almost always performs better than the GFS so the GFS model shouldn’t be taken seriously with the southern winter storm prediction.” That’s just awful meteorology. Yes, saying that the European model often does better than the American models is true. That’s fact. However, it isn’t an excuse for not analyzing the entire scenario fully, which I intend to do. Just assuming that a model will be right, regardless of how good it is, is never a safe position to be in.

We have a tricky forecast period on our hands. I’ve stated countless times that just after January 15th would be the timeframe to watch for a potential winter storm across parts of the South. The January pattern so far has fit the description very well of this being a transition month, and as predicted, there have been intermittent warmups in between. A similar pattern should continue throughout the rest of this month before more sustained cold may try to establish itself across the eastern U.S. in February. The first few initial surges of Arctic air have been more potent each time, and while these air masses have been cold, they haven’t been hugely impressive very far south. This is not too big of a surprise, but still, it’s been much colder for many than last month was.

A shortwave (a disturbance) will be moving in along the Pacific jet stream into the United States this week. This will aid in the deepening of a surface low pressure system that will move across the northern U.S., eventually reaching the Great Lakes towards the end of the week into the weekend. A separate wave will move northeast from Mexico and likely trigger the development of a coastal low early this weekend. The energy from the northern system will probably eventually transfer to the coastal low. The Northern Plains and Great Lakes region will initially get some snow from the northern system and parts of the Northeast will also get some snow from this. Parts of the Ohio Valley and surrounding regions could see some snow on the backend of this system as Arctic air rushes southward into the U.S. from Canada this weekend.

That’s only the first event to watch!! There’s another system that could develop later in the weekend into early next week, and that’s the one that’s causing all of the rumors about possible wintry weather across the southern U.S.

Southern Winter Storm Potential – Discussion:

This is a complicated forecast. I’d be lying to you if I acted like it wasn’t. After the system that I discussed above pushes northeast, Arctic air is going to rush in behind it. We’ve seen a similar scenario occur a couple of times this month, and the models have varied on the intensity of the cold each time.

Now, some people seem to think that there has to be really cold air in the South to have a shot at getting snow, and that’s not entirely true. What some individuals fail to understand is that when an Arctic air mass drops south, it can often suppress the southern jet stream too far south for any of the southern regions to get precipitation with their cold. This late weekend/early next week scenario is going to be a good case to show you my point, whether it ends up snowing across parts of the South or not. On the heels of the first piece of energy (shortwave) that I discussed earlier, another one is going to follow right behind, digging farther to the South. As I mentioned, Arctic air is going to rush in behind the first system, setting the stages for a potential winter storm across parts of the South.

Here’s the problem and the uncertainty: timing of the southern stream system and uncertainty on how far south the cold air mass will dig. The GFS model brings in the system faster, therefore it doesn’t get suppressed nearly as far south as what the European model shows. That’s why it has been consistently showing a southern winter storm. The European model has the southern system coming in much slower; therefore, the long-wave trough would already be well established across the eastern U.S. with surface high pressure much farther south. As a result, the system would get suppressed too far south to be a threat to the southern U.S. Now, some of you may be thinking, “Matthew, why not go with the European model since it often performs better than the American models do?”

I think that’s a legitimate question. This is what everyone needs to keep in mind. The GFS model actually has done better with modeling the potency of these Arctic air masses so far this winter. In the medium and long-range, the European model has tended to be too aggressive, and there are a handful of reasons these air masses haven’t been AS cold, although still cold. Also, the European model often ends up being too slow with southern stream systems and holds them back too long. Since the timing of this southern system is so crucial, it’s going to be the difference between a southern winter storm and not. During El Nino winters, southern stream disturbances often race more quickly across the U.S. along an active sub-tropical jet stream. We’ve already seen this happen so far this winter, particularly this month. In fact, the strong Pacific jet stream is likely one of the culprits behind why colder air hasn’t been able to establish itself over the eastern U.S. over a long period of time, due to the western ridge getting knocked down, which again, isn’t too surprising at this point in January given the current El Nino strength.

This GFS model brings the system much farther north as you can see. Keep in mind that each image depicts only one point in time. There would likely be a rain to snow changeover for some of these regions, and the specifics on location will change A LOT.

gfs snow map 1

GFS snow model

On the other hand, the European model has the southern wave coming in slower, keeping it suppressed south. Notice the feature in the western Gulf:

european model snow

So honestly, it’s the battle of the models. I hope that I gave you a compelling reason to at least be skeptical of the European model, and while it could be right once again, there’s enough reason for me to side with the system coming farther north right now, potentially bringing a winter storm across the southern U.S. into parts of the Mid-Atlantic states late this weekend into Monday. It’s kind of ironic that southern stream systems have been coming in too quickly for a southern winter storm this month, but in this case, it actually needs to move in quickly this time. Timing is crucial in meteorology, and models often have issues forecasting individual systems with such a volatile pattern as we have now. It would be okay for the air mass to be as cold as the European model is depicting, but the southern stream system needs to move in more quickly before the air mass can suppress it too far south.

Regions that I’m watching most closely are parts of Oklahoma, northern Texas, northern Louisiana, parts of Arkansas, northern half of Mississippi, northern half of Alabama, northern half of Georgia, northern half of South Carolina, parts of North Carolina and Virginia, and parts of the Tennessee Valley. Parts of the Mid-Atlantic could be impacted, too. I know this is very general, but due to higher than average uncertainty, I can’t get more specific than that.
PLEASE understand the limitations of forecasting an event like this that is going to be highly dependent on timing. Because of the uncertainty, I will likely have to do a follow-up forecast later in the week or early this weekend.

Folks, this is a tricky forecast. I will continue to monitor this very closely and keep a close watch on the latest trends.