WINTER STORM WATCH ISSUED PRIMARILY FOR THREAT OF FREEZING RAIN, 1/4 to 1/2 INCH ICE ACCRETION, SUNDAY
Friday will be another mild day — lots of highs in the 60s — with increasing chances of showers (and some thunder). Some Friday night pre-Christmas festivities will be dodging showers. Then will come the cold front, a breezy and colder Saturday, and what we’ve been talking about for days, Sunday’s potential winter storm.
Friday night it will be time to nail down things a little more specifically, as much as that can be done, but tonight, let’s take a long last look at a series of scenarios on how Sunday could play out. I’ve detailed 7 possible scenarios, 3 listed as “favorites” and 4 as “darkhorses.” Feel free to cast your vote below on which scenario you think will happen in the Roanoke/New River valleys (will vary toward colder stuff north and less so south, generally.)
(1) Glaze-topped-sleet: This would be the National Weather Service leading scenario as of Thursday evening, in which about 1/3 of the precipitation is sleet (maybe a little snow mixed in at first) and 2/3 is freezing rain. After about a half inch of crunch, .25-.50 amounts of ice occur in much of the region, with scattered power outages and some slick roads. This would seem to be the majority “opinion” of forecast models with moderate amounts of precipitation, some early mid-level cooling that could allow sleet (and some snow) to reach the surface, and surface temperatures remaining below freezing throughout Sunday.
(2) Wintry mix/”crunchy storm”: The three types of wintry precipitation could fall in similar proportions to each other. OK, it’s doubtful that it will be 1/3 snow, 1/3 sleet and 1/3 freezing rain. But I’ll count any combination of the three in which one type accounts for no more than 50 percent or no less than 20 percent. Many forecast models are fairly close to this scenario, the biggest question mark being the snow on the front end. Predominantly snow could last til about mid-morning as cold air aloft hangs in a bit longer than current forecasts due to evaporational cooling in very dry layers and/or a bit stronger push from the cold-air damming high pressure systems, After 1-3 inches of snow, sleet falls well into the afternoon, accumulating another inch or so (squashing the snowpack before it, so it probably doesn’t really add much to the total on ground) before finally giving way to freezing rain/drizzle to crust it all over nicely with .10 to .25 inch of ice for the late afternoon and evening. As of this writing on Thursday evening, this is my slight favorite over scenario 1.
(3) Major/historic ice storm: Some forums have used these words for days. And it is POSSIBLE. I’m calling widespread half-inch of ice a “major” ice storm, for lack of any other definition. “Historic” is best left to post-storm analysis of human impact. Gulf moisture flows over Southwest Virginia more thickly than most models show, freezing rain begins from the outset or only after a brief period of sleet/snow mix, and then falls for several hours at a steady rate with temperatures mostly in the 20s. If this happens, big trouble.
(4) Sleet-a-thon: It’s been a while since we had one of these, but sleet can sometimes pile up to impressive amounts. If the early evaporational cooling is stubborn in the mid layers and a thin layer of warmth overruns it, sleet could last for more than half of the event and pile up a couple of inches by itself even with little snow to fluff it up. Roanoke’s result in the President’s Day II storm of 2003 is something like I would have in mind for this.
(5) Dryout: Moisture is thin and amounts are light of anything that falls. NAM has been playing around with this scenario — one run on Thursday actually split the moisture fields, leaving the Roanoke and New River valleys with less than a tenth of an inch of liquid, total. I think this is the LEAST likely of all the scenarios at this point, but I’m never going to say never, especially now that at least two runs of the NAM have shown something similar.
(6) Spotty ice/cold rain: A couple of days ago this looked to be in the upper tier of possibilities. Steadily colder model runs suggest now it is unlikely that morning freezing rain will give way to 32-35 temperatures during the afternoon for only spotty amounts of ice. If for some reason the cold air is not as deep as expected and the warm air rising over it is stronger, this could still possibly happen. But I doubt it.
(7) Surprise snowstorm: This is definitely NOT out of the question. A quick rush of moisture into cold, dry air, could induce rapid evaporational cooling, and moderate-heavy snow for a few hours Sunday morning. A quick 3-6 inches is dumped before significant mixing with sleet can occur — and by the time it’s warm enough aloft for freezing rain, most of the moisture has pushed north, and all we’re left with is freezing drizzle. Dec. 4-5, 2003, Jan. 16, 2008 and even April 4, 2013, offer some precedent for something like this occurring. I would rank this second most likely among the darkhorses, behind the sleetathon.