As has been the case since about the middle of December, it is model madness today in regards to the major winter storm that will affect the Southeast next Sunday night through Wednesday.
The GFS is trending faster and weaker with the system. If this model were to verify then most areas between I-40 and I-20/85 would see a minor to modest snow event, with the southern Appalachian mountains making out pretty good, and areas north of I-40 doing ok. Nothing rip snortin’ as my friends in Texas would say, but still a nice little winter event.
Meanwhile, the 0z run of the ECMWF (European model) shows a classic Southeast snowstorm track with plenty of cold air in place to create a heavy swath of snow from Birmingham to just north of Atlanta to Raleigh. With a heavy dumping of snow in the southern Appalachian mountains.
Then you have the NAM, our high-resolution short range model that is showing the low pressure system also taking a favorable route for significant Southeast snow, but without the cold air that the GFS and ECMWF show. Thus, you end up with rain for much of the duration of the storm south of I-40, with a changeover to flurries and snow showers on the back side as colder air dumps in. Again, the southern Appalachians get a heavy dump of snow.
So, based on what the “Big Three” are showing today, one can conclude that there is a pretty good chance for a nice dumping of snow in the mountains of TN and NC, with potentially a foot or more adding up between Monday night and Wednesday evening. Monday night and Tuesday being synoptic snow, meaning snow with the system itself, then Tuesday afternoon through Wednesday being a prolonged period of northwest flow snow and snow showers behind the system that adds yet another 3 to 6 inches on top of the 4 to 8 inches that has already fallen. The ski resorts will be lovin’ it! This part of the forecast one can feel somewhat confident about. However….
For most of us in the Southeast who do not live in them thar hills, the snow forecast for Monday and Tuesday is still a very difficult one. Now it would be a lot easier if the forecast models had been performing consistently well this winter, but that is not the case. All of the big three, the GFS, NAM and even the ECMWF (statistically the most reliable medium range model) have been performing consistently poorly since about the middle of December. So, what we have to do is look at the performance of each one over the past 50 days or so, and try to draw some conclusions on which one is going to do the best based on past performance.
The GFS has tended to be too fast with moving systems in the 1 to 5 day range. This was very much the case with the last system – the ice storm. I feel that it is again too fast with the movement of the current system.
The ECMWF has been inconsistent this winter in the 6 and 7 day period, but it has performed much better than the GFS in the 1 to 5 day range. Therefore, I would tend to lean more toward following the ECMWF solution with this particular storm.
Finally, the NAM had not performed well at all as a short range forecast model up until this past storm. It did pretty well with the ice storm west of the Appalachians, and I regained some faith in this model again. However, with the system that will be developing this Sunday I feel it is under-forecasting the cold air. There is a decent snow and ice pack north of the Southeast region, and this is going to help with the southward penetration of cold air into the Southeast. Also, the NAM does not show much of a surface high pressure reflection across the northern Plains, the source of the cold air for our early next week storm. It has a 1034mb high over Idaho. Meanwhile, the GFS and ECMWF both snow a nice over 1030mb high pressure system seeding the cold air for the Southeast. I’d tend to buy the colder models over the warmer NAM at this point.
So, with all this said, my forecast for this system at this time follows closely what the ECMWF is showing in terms of surface low track and cold air, although I do feel the track of the low will be a bit more north and west of where the 0z ECMWF is forecasting it. I am also using the NAM as my forecast model of choice in regards to the speed of the system. This all translates into the potential for a swath of significant snow stretching from northwest MS middle TN, northern AL and north GA, and into western and central NC. The mountains of NC and TN will also see a heavy snowfall of a foot or more in some locations. By “northern AL and north GA” I’m talking about a line from Tupelo, MS to Cartersville, GA to Taccoa, GA. Still too early to pin down amounts in the potential swath of significant snow, but amounts could be over three inches. South of that more significant snow, I’d say accumulating snow will be possible along I-20/85 for cities like Birmingham, Atlanta and Raleigh. Again, it is too early to pin down a specific amount forecast so I’ll give a range of anywhere from a trace to 3 inches.
Finally, we cannot forget there will be a severe thunderstorm component to this storm. First for southeast TX and southern LA on Sunday, then along the Gulf coast of MS, AL and the Florida panhandle on Monday. Parts of extreme south GA and the Florida peninsula on Monday night. Then the eastern Carolina’s on Tuesday. Main threats will be damaging winds and some hail.
I just took a look at the 12z NAM, and here are some quick thoughts on that. First off, the NAM continues to show a slower speed with this system. I am even more confident now that the faster solutions shown by the GFS and ECMWF are less likely to verify than the slower NAM. So, I think the NAM is definitely onto something in regards to speed. Next, the 12z NAM is snowing the surface low tracking across the Gulf of Mexico, well south of the coast. This has resulted in a deeper southward penetration of the cold air, so the 12z NAM is definitely colder than its predecessor. Based on what I am seeing, I’d still say a swath of significant snow is possible for much of central and southern AR, northern MS, northern AL, much of TN and western and central NC. Maybe not so much of a dumping for the mountains of TN and NC if the low tracks that far south. I still buy the ECMWF track of the surface low more than I buy into the southern track depicted by the NAM. It’ll be interesting to see what the 12z ECMWF shows.
Bottom line: Still more questions than answers, but we are definitely starting to get some answers now, and more will come into focus later today and especially tomorrow, so stay tuned!!