June 17th-18th Event
In what has been a rapidly developing winter weather threat, all things look on track as of this evening for a nice snow burst tomorrow afternoon through Friday morning from MS/Al through eastern and central TN, northern Georgia, upstate SC, and NC/VA.
The culprit is a cut-off upper level low pressure system which will track eastward across the Southeast US over the next 36-48 hours. This upper low will spark the development of a low pressure center across the southeast which will track east across central Ga, southern SC and off the Carolina coast before taking an ENE track out to sea. What will occur is a situation where widespread rain during the day tomorrow begins changing to snow from west to east and north to southeast across the Southeast as the storm strengthens and cold air aloft is brought down to the lower levels (dynamical cooling). So this will not be your typical southeast winter storm, but enough appears to be coming together for an impactful event.
For now, I am going to leave my snowfall forecast unchanged. The data today has confirmed in my opinion my earlier forecast, and I want to watch the 00z and 06z cycles tonight and then make an update tomorrow morning. It certainly looks like the northern NC mountains into the southwest Virginia mountains may be the sweet spot in this event. I have 4-8 inches in those regions, but model data suggests amounts could be higher and an adjustment could be made in the morning there. East of the mountains across NC/Va, as the storm takes shape, a rapid change over from rain to snow will occurs a deformation band sets up across Va/NC tomorrow afternoon through the wee hours of Friday morning. This deformation band characterized by very strong mid-level upward motion (500-700mb) will consolidate into a comma-head type shape by Friday afternoon/evening across Va/NC and within this comma-head, moderate to heavy rain will turn to moderate to heavy snow by late afternoon across the NC/Va foothills and western piedmont and spread east through the rest of the piedmont and the coastal plain through the evening and overnight hours. The length of snow will likely be on the order of 3-6 hours with a longer time likely in parts of southern Virginia/northern NC. This is where outside of the mountains the secondary jackpot could occur depending on exactly how things play out. It will even be possible that thunder and lightning may be observed as the rain transitions to snow due to steep mid-level lapse rates and strong vertical motion. There will likely be lollipops of 6 inches of snow east of the mountains across southern Va/northern NC, but again I will address that in the morning update.
So I will leave it at that this evening. The areas of N Ga, upstate SC, and southern NC are the areas right now with some significant uncertainty. There will be a very sharp gradient from the moderate to heavy snow area over northern/central NC and no snow just to the south of this. Cities such as Greenville, SC, Charlotte and Fayetteville will be near the edge. The Atlanta metro area is also in a somewhat difficult place to predict, but I do think the city could see some snow, and will attempt to narrow that down tomorrow.