We have an incredibly challenging and complicated forecast coming up with the potential for a severe winter storm late Christmas Day into Sunday but also the potential of a light to non-event.
First, the weather will be rather tranquil through Christmas Eve with temperatures running about 5 degrees below normal which will equate to highs in the mid to upper 40s and lows in the 20s at night. Today will be the warmest day and we could make a run at 50.
Things get complicated as we head into Christmas Day as we see several different weather systems aloft approach our area and their exact interaction and timing will determine our sensible weather. The ECMWF model continues to show a historic snowstorm around the southeast with as much as 2 feet of snow across the eastern piedmont and parts of the coastal plain of NC and eastern Virginia. It also shows heavy snow of probably up to a foot in areas of central and western Virginia and North Carolina, South Carolina and eastern Tennessee and also significant snow of at least several inches if not more over the rest of Tennessee and parts of northern Georgia, Alabama, and Mississippi. The model shows perfect timing for the phasing of 3 separate s/w’s 2 in the northern branch and one in the southern branch that would produce a slow-moving very intense storm near the southeast coast that would climb off the mid-Atlantic coast and then out to sea eventually. This is the extreme scenario. The ECMWF has been showing a significant snowstorm for several runs now and this model has made a name for itself of being the best medium range forecast model with it’s superior resolution and data assimilation methods. The ECMWF ensemble shows similar ideas to the ECMWF track, but it is less intense and a little further east.
On the other side of the envelope, you have the American models, specifically the GFS which while being very inconsistent with the track of upper level features, it has never wavered from showing either a light or non-event for most of the southeast. Now keep in mind it has shown light accumulating snowfall for the southeast, northern Ga, AL, MS, TN, upstate SC, and NC for several runs but not the type of epic snow the ECMWF has been showing. In looking at the GFS Ensemble members, or at least 11 of them, I see 2 showing an ECMWF type solution, 6 showing an light to non-event similar to the operational GFS, and 3 showing a more intermediate type of event.
Other models are not as extreme as the ECMWF as well. The 00z UKMET remains rather suppressed showing a weak low near the western Florida coast by Christmas Evening, it does eventually phase the two streams on the 26thwith deepening low about 200-300 miles east of the Carolina coast. I don’t have precip maps for the UKMET at this time range but it would imply a nice hit for coastal sections but perhaps not inland so much. The 00z GGEM also tries to phase the two streams but also seems to do so a little too late for a dramatic impact in the southeast. The QPF would imply a light to moderate event for many areas with most QPF amounts in the 0.1 to 0.5 inch range. The 00z GGEM Ensemble agrees with this as well. The new 12z Nam is in and looks closer to the light event scenario than to the big event scenario. We have to keep in mind though that the NAM is a regional model and it’s boundary conditions are supplied by the previous run of the GFS. SO often times the NAM in the 72-84 hour range will be heavily influenced by the GFS.
With all that said, you are probably asking so what the heck should we think? Well, while I do value the ECMWF and it’s superior science, the solution it offers is so extreme that this far out it is almost foolish to side with it. I do think the higher resolution will allow the model to probably do a better job capturing the exact nature and details of the phasing of the two streams in a better way than the more coarse GFS and Canadian models. What I am not sure of is the model’s depiction of the timing of the southern stream s/w and it’s intensity. The ECMWF has been constantly slower and more intense with this and the slower speed allows the phasing to occur in time. If the s/w is faster like the other models, then the phase occurs too late. Perhaps the ECMWF is right but you would think at some point the other models will trend towards this. Keep in mind this s/w will not be ashore into southern California until this afternoon, so we may continue to see dramatic spread with today’s 12z runs but perhaps convergence starts with the 00z runs tonight.
The most prudent course to take right now is to not buy the ECMWF lock stock and barrel yet, but use it to impose a westward adjustment and a slight decrease in speed and increase in intensity to the 00z UKMET, 00z GGEM, and 06z GFS tracks. This would mean the ECMWF is still too extreme, but the phasing could occur early enough to bring a moderate snow event to the Carolinas and southeast Virginia, and a lighter event to areas of TN, Ga, and AL. This is very subject to change and if the 00z ECMWF is right, we will see a paralyzing storm of historic proportions across areas of the southeast and lower mid-Atlantic.
Very cold air will pour in behind this system on Monday and into early next week and if there is snow cover on the ground record lows could be threatened. However, the models are showing a relaxation to the pattern late next week as the –NAO temporarily weakens and the Gulf of Alaska vortex influences the pattern with mild Pacific air. However the models show the west based –NAO signature sticking around and re-intensifying in early January and I think after a few days of milder temperatures cold air could once again take up residence in the southeast.