Wind is main impact expected from SandySuperstorm Mon.-Tues.; most rain to our northeast, most snow to our west
First off, here is the current position and projected track of Hurricane Sandy. Not much has changed over the last 24 hours. Because Sandy is represented by one of the white circles meaning “extratropical” long before landfall, the National Hurricane Center is electing not to issue any further tropical advisories north of the current tropical storm warning that ends at the North Carolina-Virginia border. This is causing some controversy in the weather online world, as many think people will pay much closer attention to hurricane warning than they will gale or high wind warnings. But it is what is.
Locally, we will see effects from Sandy gradually pick up on Sunday — some northerly breezes getting a bit stronger, perhaps a few showers pushed west from her toward the approaching cold front. With Sandy curving away from the coast before turning back, though, don’t be surprised if Sunday is a pretty nice day, maybe peeks of sun and highs in the 60s (yes, I think they’ll probably get in the race at Martinsville). The effects will increase when the trough catches the extratropical-former-Sandy and turns her into SandySuperstorm near the coast Monday, moving inland by Tuesday. High wind, big waves and heavy rain will be occurring near the coast.
For Southwest Virginia, here are potential impacts, especially in Monday-Tuesday timeframe:
WIND: Gusty winds are a given with this setup. How strong and how long are the questions. The National Weather Service issued high wind warnings for the high peaks of Grayson County and adjacent northwest North Carolina for Sunday night, but suggest in discussions that additional high wind watches, warnings or wind advisories are likely to be issued elsewhere. The 12 European model showed tightly packed isobars — lines of equal pressure — over our region (and much of the Eastern U.S.) on Tuesday morning. (It also shows cold air in blue streaming around the low — which might help snow spread farther east — more on that below). The tighter these isobars are packed, the stronger the winds. Right now, based simply on model estimates, it would appear winds may peak in the 20-30 mph sustained with gusts in the 40-50 range over most of the area, but it’s certainly possible that, Tuesday in particular, there could be a bit stronger winds than that. A long-duration gusty wind event like this would cause trees and limbs to break in many places, and likely cause sporadic power outages. Those could increase to more widespread outages if some of the higher wind speeds are realized. Northwest winds blowing over our mountains tend to be chopped into waves which can crash downward and cause local high wind speeds, even into valleys. There are higher winds just above the ridgetops that could be forced downward through some dynamic processes, but those are NOT expected by the weather service at this time. It is a situation that will have to be monitored for extremes, but 2 or 3 days of gusty winds will be likely across all of Southwest Virginia.
SNOW: You can think of the snow aspect of the SandySuperstorm as a winter upslope snow event on steroids. Upslope wind flow — northwest winds circulating around the storm center’s as it tracks westward through Pennsylvania or Maryland — will be the primary mechanism for heavy snow in the mountains of West Virginia and far western and southwestern Virginia. But instead of tapping Great Lakes moisture, rich moisture of tropical and Atlantic Ocean origin will be wrapped around the center, and as it is squeezed out in the lift against the mountains, snowfall could very well be measured in FEET in the 3,000-plus elevations near Snowshoe Mountain. Winter storm watches are out for several counties in that area — NWS-Blacksburg added Greenbrier County, W.Va. to the list today. The ridges west of Interstate 77 in Virginia may also get a heaping helping of snow, and lower elevations through there may at least be turned white. The high mountains along the Virginia-West Virginia border in Giles, Craig, Alleghany, and Bath counties may also be in line to get rounds of snow squalls that can collect. The big question mark is farther east, through the New River Valley, the Blue Ridge, even the Roanoke Valley and a bit farther east — those areas do fall in the HPC’s 10-39 percent “slight risk” zone for 4-plus inches. Even “normal” upslope events sometimes have squalls with enough lift and upper-level support to keep going farther eastward — Blacksburg/Christiansburg area is often a recipient of these, sometimes even the Roanoke area. Also, with this low, it is possible that an arm or two of much thicker moisture will be wrapped around the storm well to the south — some models have been showing this. If that happens and can encounter enough cold air getting pulled into the storm, measurable snow COULD extend farther eastward in Virginia. The Hydrometeorological Prediction Center places a 30 percent chance of 1 inch or more of snow in the 24 hours before 8 p.m. Tuesday well into central Virgina. So there remains quite a bit of uncertainty on snow, especially considering that temperatures just above the surface may be a few degrees above freezing in much of the area, and that ground is warm and will be hard for snow to stick to UNLESS it comes down hard and/or at night when direct solar radiation isn’t present. For our region, I would say solid snows of at least 2 inches in the higher mountains near the West Virginia line, and a good chance most other locations see at least some flakes on Tuesday, possibly mixed with rain the lower you are in elevation. Accumulations at least on grassy areas are possible IF heavier snow can develop in squalls spilling over from the typical upslope areas and/or larger areas of precipitation swinging around the storm system. For the Roanoke Valley, if there isn’t white on the ground by mid-Tuesday morning, it’s not going to happen. Highs will creep into the 40s from here eastward.
RAIN: Here is HPC’s map for totals related to Sandy, and I’ll be honest, it seems a little overdone in the Ronaoke area southward. The bulk of the rain is going to rotate north and east of us, and we’ll be in a noticeable rain shadow (snow shadow, at times) east of the spine of the Appalachians as some moisture rotates behind the storm. Maybe the HPC map makers just had a hard time depicting the rather rapid dropoff of amounts from north to south through our area. Most of our showers with Sandy may rotate through on the backside, possibly mixed with snow, Monday night and Tuesday. Rain is probably not going to a big deal with this storm here — unfortunate in some ways, as we’re a little dry. To our west, some of that thicker moisture will fall as wet snow, not rain. You can see why West Virginia may get some really big totals.