Just a few days after record warmth (75 degrees!!) the models are pointing to a potential weekend snowstorm for the Mid-South this weekend. With the storm over 100 hrs away, there is no point trying to speculate on snow amounts or even which models (and which model runs) are the most accurate, since that often turns into wishcasting. Rather, at this point I will discuss the main players on the field and discuss the likely evolution of the synoptic features.
Starting with the current map the prominent features at 200 mb are the trough over the Southwest US which has been the primary rainmaker this week, the polar vortex over NE Canada which has supplied the fresh arctic air for the ongoing ice storm, and the SE ridge which was responsible for the record warmth. At the mid-levels, there are three important shortwaves. SW1 is over Baja California, which will bring rain to the Mid-South Wednesday night into Thursday (and snow into New England Thursday), SW2 is over British Columbia which will become a clipper that brings fresh cP air into the Mid-South Friday, and SW3 is barely visible over the Aleutians in the Gulf of Alaska. This last shortwave is the storm that will affect the Mid-South with a possible snowstorm this weekend.
By Thursday afternoon, all models are consistent (see both the GFS and NAM) with SW1 over the Midwest, SW2 over Lake Superior, and SW3 over Vancouver. By Friday afternoon, the models start to diverge in their treatment of SW3 but more importantly, they all show a fresh, cP air mass spreading over the central part of the U.S. (1028 mb high).
It is always important to remember with snowstorms in the Mid-South that the rain/snow boundary will be determined by the exact track of the surface low, the location of the 850 mb 0 degree C contour, the 540 dm thickness line, and the track of the 850 mb low. Deviations in the tracks of these features by 100 miles or so can mean the difference between a 6-10 inch snowstorm or a cold rain. Since there is no way to know which model currently has the best handle on the above features (and this won’t be known until Thursday afternoon at the earliest), I think it is best to discuss past history and what is most LIKELY to happen synoptically.
I think the key synoptic factor this weekend will be the battle between the combined forces of the near-record warmth near the Gulf Coast and the SE ridge against the cP high that will overspread the central U.S. on Friday. When SW3 ejects from the Southwest on Friday, the surface low will likely follow the baroclinic zone that separates the cold, dry air to the NW and the warm, humid air to the SE. This baroclinic zone will be just south of the Mid-South. One way to think of these baroclinic zones is like a line of scrimmage in football. As the SW energy starts to deepen, the direction of the resulting surface low will depend on which side of the line of scrimmage wins – the cold, dry air or the warm, humid air. The combination of the SE ridge and record warm air mass is somewhat like the New England Patriots offensive line. In other words, they are a juggernaut that usually wins the battle in the trenches. Meanwhile, the fresh cP air mass is nothing special and is not supported by any upstream ridging that may help anchor it in place. In other words, it is like the defensive line of a mediocre team. As has been the case for the past 12 months, I would expect the SE ridge and warm air (the Patriots) to push the baroclinic zone (line of scrimmage) north and west of what is currently shown on the models and for this storm to move directly over south-central KY and eventually west of the Appalachians with a secondary low forming off the VA coast and eventually becoming a major nor’easter for New England.
My first guess for south-central KY is that the storm may start as snow late Friday-early Saturday (possibly accumulating an inch or two) before changing over to rain for Saturday. Once the storm moves to our NE and cold advection kicks in, I can see another inch or two accumulating before ending as flurries on Sunday. I think the best place for snow from this storm will be on a Louisville-Cincinnati line (possibly 6″ plus). I must point out that the range of possibilities at this time due to the uncertainties in storm tracks for south-central KY includes anything from a storm that is all rain to a 6-10″ snowstorm. It is important to not get too excited about any particular model run and spend more time looking at the model trends (especially the 540dm line and 850 mb 0C line).
Regardless of how the storm ends up, the rest of December should warm up in a big way as we enter a split flow pattern. The subtropical jet stream should keep us in an above normal precipitation pattern for the rest of the month.