UPDATE 4:15 PM
WINTER STORM WARNING ISSUED FOR THURSDAY AFTERNOON/EVENING — Potential for 4-8 inches of snow in most of Southwest Virginia.
Will issue full new update by 7 p.m.
UPDATE 9 AM
Summary: A vigorous upper-level disturbance moving across the South will trigger a deepening surface low-pressure sytem in the Carolinas on Thursday that will likely cause the freezing level to drop rapidly downward through the atmosphere, changing rain to snow, which may become heavy at times by afternoon and evening. Widespread 2+ inch accumulations are likely with a zone of 6+ possibly developing and some localized 10+ amounts.
Told you on Monday I was leery of this vigorous little system. But, contrary to a brief mention of possible light snow buried deeply in today’s Weather Journal column, it’s having delusions of grandeur now, as depicted on the forecast models. And they might not be delusions. What’s happening is that a strong piece of upper-level energy, leftover behind this week’s rain-a-thon, will zip across the South during the next 24-36 hours. A surface low-pressure system will rapidly deepen over the eastern Carolinas as this piece of energy goes “negatively tilted,” a fancy way to say its axis is pivoting into more of a southeast to northwest alignment rather than southwest to northeast, an alignment that maximizes moisture flow, lift, winds aloft, and the ability for cooling to occur in the mid and upper layers of the atmosphere. Moisture will spread over us again on Thursday, likely beginning as some light rain in most parts of Southwest Virginia. But as the low deepens and what is known as “dynamic cooling” drops the freezing layer lower and lower in connection with the falling atmospheric pressure, rain will likely change to snow very rapidly, the rain/snow line moving downward in elevation and southeastward. There is the potential for a few hours of heavy snow, 1 to 2 inch per hour rates, possibly more at times, maybe even accompanied by thunder as this storm rapidly matures late Thursday afternoon and early Thursday evening. Where, exactly, the heaviest snow occurs is subject to some conjecture. The 6Z NAM simulated radar map for early Thursday evening, inset at left, sets up heavy echoes up and down the western side of Virginia. Most early indications are that the I-81 corridor southwest of Roanoke may have the best chance of some 6+ and perhaps locally 10+ inch snowfall amounts. Widespread 2-6 inches appears likely at this point. As is typical, higher elevations will have a better chance of seeing bigger amounts, but perhaps an even bigger factor is precisely where the heavy snow bands set up, something that can’t be easily forecast with much lead time. What can go awry with this setup? The two biggies would be if the storm system moves even farther north or if some of the upper dynamics are misread by the models and the mid-upper-level cooling isn’t as great as expected. We’re getting pretty late in the process (24-36 hours from storm) for the former to occur, though it certainly can, and experience tells me that the upper dynamics are more often a little stronger, not weaker, than what is modeled with similar systems. Like last Feb. 19, when our region got 5 to 9 inches of snow, the ground is much warmer than normal, and surface air temperatures will be marginal (falling to near 32), so intensity of snowfall rate will have to overcome the melt rate to initiate accumulation. But the temperature won’t be falling from low-mid 60s the day before and from the low 50s early in the morning, as it did on Feb. 19, and the chilly rain falling now is cooling the ground a little closer to snow-sticking temperatures than the warm Saturday sun did then. The rush-hour timing of heavy snow and the potential for heavy wet snow to cause power outages are a couple of major problems that could develop. Please be aware that travel Thursday evening may become very difficult in much of Southwest Virginia, and consider alternate plans if possible.
Linked here is a list of NWS-Blacksburg area rainfall totals through 9 p.m. Tuesday. The two-plus-day rain marathon that has left widespread 1-4-inch rainfall amounts in Southwest Virginia — and locally more near the North Carolina border — will taper and end this morning. A brief period of icing or sleet may occur at some higher elevations, especially north of Interstate 64.
Wednesday’s late-night North American Model (NAM) and Global Forecast System (GFS) came in amazingly similar on a dramatic solution for this late-week, southern-stream system we’ve been batting around since last weekend. These solutions both develop a low rapidly over the Carolinas and squeeze out abundant moisture in cold air over western Virginia and North Carolina with an intense pocket of upper-level energy, much of it snow after a period of rain or wintry mix to start. The timing for this possible event now appears to Thursday afternoon and evening — not Friday, as we have earlier discussed. There could be some rather heavy amounts IF these models are onto something. We are within 36-48 hours of the possible event now, so we would hope that model accuracy, agreement and consistency would be moving toward reality now. Potential flies in the ointment: Maybe the system trends even farther north, maybe the mid-level cooling isn’t quite as great as modeled, either of which would result in more rain or mix. Bottom line: If you have travel plans in or near our region on Thursday, pay attention to what could be some rapidly changing forecasts over the next 24-48 hours. I’ll follow it as best I can (with some non-weather-related time challenges) on this blog.
Many of you will be up before I am on Wednesday morning. For an up-to-date look at current thinking on Thursday’s situation, check out the fresh Day 2 outlook for heavy snow potential and the Day 2 outlook for probabilities of various amounts of snowfall, linked here, from the Hydrometeorological Prediction Center.
The Arctic outbreak is still on for next week, likely arriving late Sunday or Monday, and peaking about midweek. More details on that once we get past the possible Thursday snow.