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Friday AM briefing: Winter storm watch up Sunday for heavy snow or snow-sleet mix

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Weather service snow map

This is a rarity: Roanoke actually has the highest number -- 10 inches -- among Virginia locations specified on this National Weather Service snow forecast map for Sunday. A broad brush of 6-12 inches appears likely across our region, though earlier and heavier sleet mixing than currently expected could shave a little bit off totals while still creating messy conditions.

Winter storm watches for Sunday have been issued on this Friday morning across our entire region, plus well to the west, north and south. Those going out 48 hours ahead of time are a testament to how serious the National Weather Service is taking Sunday’s winter storm, and how certain forecasters are that it will occur.

Today, the beginning phases of this winter storm will be occurring in the Upper Midwest, as a strong upper-level low dives southeastward with heavy snow in parts of the Dakotas, Minnesota and Iowa. On Saturday we’ll see it crank up near the Mississippi River to our west, as a surface low forms, and that will march eastward across the South overnight and arrive on our doorstep Sunday morning. Strong high pressure in southern Canada banking Arctic air against the mountains will ensure our precipitation is wintry, not wet. The first surface low will transfer much of its energy to a new coastal low by late Sunday, with the upper-level low moving pretty much right over us Sunday night.

Here is Friday morning breakdown of the expected winter storm in The Roanoke Times circulation area (generally, Roanoke and Montgomery counties and every adjacent locality, north to Interstate 64, south to the Virginia border and west to Interstate 77).

A high-impact winter storm is highly likely for Sunday. That should be the major takeaway from any discussion at this point. We all love to get caught up in snowfall ranges and figuring out what type of precipitation falls exactly when, but any reasonable outcome from Sunday’s storm, even those that would be somewhat less than forecasts, will yield treacherous travel and possibly scattered power outages. This is not going to miss us, this is not going to warm above freezing and rain, not in a 50-mile radius of Roanoke and points westward.

6-12 inches of snow looking likely; some places could get more, but even a little less would be troublesome. You’re probably noticing various media outlets and now the National Weather Service with overlapping, very wide ranges of forecast snowfall. Normally weather people like narrower ranges of no more than 4 inches from bottom to top number. The wider ranges are justified in this case by the inherent complexities of a storm that is obviously potent but still 2 days out. Six inches looks like the low side of current snow expectations, so even something a little less than forecast, say 3-6 inches of snow heavily mixed with sleet, would still be problematic. A widespread foot-plus storm, where almost every location in our region gets at least 12 inches, is probably not going to happen because of the speed of the heaviest precipitation moving through, not even considering chances of sleet mix. But a few places might get 12-15 inches or so.

Timing: Sunday. Probably starting near or shortly after sunrise in Roanoke, minus an hour for every 30 miles south, add an hour for every 30 miles north. Almost all meaningful precipitation is over by midnight, if not sooner. (See dry slot-wraparound snow bold-face header below). There will probably be a 6-8-hour period of the heaviest precipitation, from mid-morning Sunday to late afternoon/early evening, 10 a.m to 5 p.m. being a reasonable estimation. One slight modification is that some forecast models show a brief period of light snow early Saturday evening that could whiten things up, so beware if you’re out and about and don’t be surprised if you see flakes or some roads take on a bit of accumulation.

Level of sleet mixing is the biggest question about snowfall amounts locally. Sleet refers to ice pellets, bouncy bits of ice that are neither the tender crystallized flakes of snow nor the raindrops that freeze into glaze on surfaces that we call freezing rain. As the storm’s processes pull a layer of somewhat milder air northward a mile or two about the surface, snow falling through that layer will melt into raindrops and then refreeze into ice pellets before hitting the ground. Exactly where that sets up, the timing of it, and how much sleet there is has the biggest impact on snow totals locally, because there will be less total snow/sleet accumulation (it all counts as snowfall officially)  the more of it is sleet. It looks almost certain that sleet will make it everywhere east of the Blue Ridge at some point. From Roanoke and the Blue Ridge westward, it’s a big question mark. Sleet chances increase later in the day into the early evening.

If there is a lot of sleet, it is a unique problem in itself. It has been a long time since we have had a heavy sleet storm locally, February 2003 I would offer as probably the last such event. This probably will not be that event either, because if it sleets at your location, it most likely follows inches of snow and could be followed by more snow. Snow lovers tend to hate sleet because it inevitably reduces accumulation – you get only 2 or 3 inches of sleet for every inch of rain compared to about 10 inches of snow for that same inch of rain. But nonetheless, it can pile up, and it is a denser form ice that melts extremely slowly, inch for inch, compared to snow, and in great quantities it can be harder to remove by shovel or plow than snow. It doesn’t collect on trees and power lines like freezing rain, because it bounces off, but it is heavier on flat roofs than the same depth of snow would be. In this case it would probably add a crunchy layer or topping to the snow, somewhat compacting the snow underneath.

Freezing rain doesn’t look to be much of an issue locally. We can’t say there will be absolutely no drops of supercooled liquid that freeze on surfaces in the Roanoke-New River valleys, but it doesn’t appear that glazed trees and powerlines are our issue this time, as they will be to our south and east. Significant accumulating freezing rain probably makes it no farther west than about a Martinsville to Lynchburg line.

Do we get the dry slot or the wraparound snow Sunday night? Another huge question for snow totals. As the upper-level low propelling this winter storm lifts north-northeast, it will push a zone of drier air northward, causing the precipitation to form an arc of sorts around an area of little or no precipitation. Depending on how the system tracks, that dry slot may go right over us on Sunday night or it may be more to the east, allowing additional snow to wrap around the back side of the storm. This could add another 1-3, maybe 2-4 inches Sunday night to whatever we get in the bulk of the storm during the daytime. To get a foot, your location will probably need at least a couple inches out of this wraparound snow. Chances are better to the north and west, less likely to the south and east, with Roanoke about the 50-50 point based on varying forecast models.

I’ll be back on Saturday morning for another winter storm briefing, and then by the next morning we’ll be in “nowcasting” mode.

Contact Kevin Myatt at kevin.myatt@roanoke.com. Follow him on Twitter @kevinmyattwx.

 

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Since 2003, Kevin Myatt has penned the weekly Weather Journal column, and since 2006, the Weather Journal blog, which becomes particularly busy with snow. Kevin has edited a book on hurricanes and has helped lead Virginia Tech students on storm chases.

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