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UPDATE 7:25 PM: Many reports of sleet and some snow coming in from the Roanoke and New River valleys now as cold temperatures are moving in from the north.  You can see some of those reports in the comments at the end of this post. This may portend an early start to wintry weather tonight, and potential for slick spots. This is the first wave, overrunning precipitation, not the major storm that is expected to develop Sunday. END UPDATE

UPDATE 8:35 PM: Here's a photo of snow falling in the lights at Tanglewood Mall, taken by Jordan Fifer of Roanoke.com END UPDATE

A good place to start when so much guidance is now leaning toward a significant snowfall ... possibly major snowstorm ... for Southwest Virginia is: What can go wrong?

Lots -- but none of it is likely at this point.

(1) The storm could simply slip just east of our area. All forecast guidance is now leaning against this idea, but weather doesn't happen in computers, it happens in the atmosphere, and that's at least a slight possibility.
(2) The sharp back edge of the snow shield could set up just east of our area, or even halfway across our area. It could set up between Roanoke and Blacksburg, with the Star City shoveling while the Drillfield is dry. Is that likely ... no way. But there will be someone somewhere on the outside looking in. Probably eastern West Virginia and the coalfields of far southwest Virginia at this point, but those areas aren't out of the running for snow either, especially if westward trends continue (and some of those places will get snow tonight before the bigger storm even matters).
(3) Forecasters in the weather service office at Blacksburg are a bit concerned that convection near the coast will cut off moisture flow. This is a concern to be sure, but it's usually more of a problem when our main flow from the Gulf of Mexico is cut off rather than a secondary flow from the Atlantic, as would be the case this time. This would more likely cut down rather than cut out snowfall, but some marginal western fringe areas might not get the moisture they need for snow.
(4) There's some chance that surface temperatures will be a little warm when the precipitation begins, and we might lose a little early precipitation to rain and sleet, cutting down on snow totals.

But there's also a lot that can go wrong on the other side of the forecast, in the direction of more snow. There is a chance that the upper-low gets cranking right overhead, convective bands of snow move through that dump multiple inches in an hour, and the comma-head snow area only slowly departs Monday.  If some or all of that happens, then not only will Roanoke's almost snowless winter go by the boards, but the 13-year hiatus since the last foot-deep snowstorm could end. Possible, yes. Likely ... almost never is a foot of snow likely more than 18 hours before show time. Even the mighty 2-footer of early January 1996 started with a 6-to-10-inch forecast.

You will notice that the National Weather Service-Blacksburg has eliminated counties along the Virginia-West Virginia border from the winter storm watch. In fact, those counties, Giles, Bland, and other neighboring counties, typically some of the heaviest-snow counties in our area, are islands in a sea of advisories and watches. I wouldn't be surprised if these counties get added back into a watch, warning or advisory later on Sunday, but for now, the weather service is expecting the sharp cutoff to fall about right there. (Also those counties have a slightly higher threshhold for a winter storm ... 5 inches instead of 4, as it is from the Roanoke Valley east). So, let it be noted that Roanoke County and Montgomery County are on the western fringe of the winter storm watch. Something to keep in mind, though I personally expect a slight shift west in this forecast.

The Hydrometeorological Prediction Center agrees, as judged by its 4-inch and 8-inch maps through Sunday evening, and the 4-inch and 8-inch maps  from Sunday evening through Monday evening. Taken literally, these depict what would be as much of an I-81 snowstorm in Virginia as the much vaunted I-95 snowstorm ... though it would really be both.

So inside of 24 hours when snow may begin falling (besides whatever might occur overnight, not much south of I-64, I don't think), I'm thinking this one is a GO.

If the Roanoke Valley misses this one ... it's just simply not meant to snow in 2008-09, and let's get the heck on with spring.

Some revised probabilities for snowfall in Roanoke:

Less than 1 inch: 20 percent
1 to 3 inches: 10 percent
3 to 6 inches: 30 percent
6 to 12 inches: 25 percent
12-plus inches: 15 percent

UPDATE 8:20 AM: The National Weather Service in Blacksburg has issued a winter storm watch extending through Monday morning (starting tonight generally along and west of the Blue Ridge, starting Sunday afternoon for points to the east).  At this point, the weather service isn't being very specific with forecasted amounts because of remaining uncertainties. But a mix of precipitation is expected overnight, shifting more to snow Sunday and Sunday night as the strong low takes shape to our south and then moves up the coast. Still much to be deciphered about track, strength, and where a possibly very hard back edge of the precipitation will develop with the main low.  END UPDATE

A season-maker ... or a heartbreaker? For snow fans in the Roanoke Valley and Southwest Virginia, it's down to that with a strong winter storm system that is very likely to develop. Some effects could be felt tonight as light rain possibly changes to sleet and snow, but the big show is Sunday into early Monday. There seems to be significant support for the possibility that this big storm will turn up the coast and fling a wall of snow westward all the way to the Appalachians and perhaps beyond. It's not signed, sealed and delivered yet, and locations east of us at this point have a more sure bet than we do, but I would just be cynical if I told you there wasn't a real possibility of a significant snowfall, or even a snow STORM over the next 2-3 days.

But let's get serious for a moment.

If you are planning travel ANYWHERE in the Eastern US over the next 3 days or so, please monitor the latest forecasts, and seriously consider postponing travel into areas that are flagged with winter storm watches or warnings. This will likely be a major, possibly historic, winter storm for some locations, though that can't be pinpointed perfeclty yet. I am convinced that someone east of the Appalachians, somewhere between South Carolina and Maine, will get snow topping a foot ... and maybe a lot of folks.

Will that be us, with the foot?

How about some updated  best-guess probabilities, total snow through Monday:

Less than 1 inch: 30 percent
1-3 inches: 25 percent
3-6 inches: 25 percent
6-12 inches: 10 percent
12-plus inches: 10 percent

We might be getting a little ahead of ourselves talking about snow. First will come quite a bit of rain ... on Friday with a front, on Saturday with a wave of overrunning moisture in the early stages of the possible winter storm for somebody in the Southeast, Appalachians or Middle Atlantic. The Hydrometoerological Prediction Center is forecasting widespread amounts of an inch or more across Southwest Virginia over the next 3 days. It is possible that some of this precipitation toward late Saturday into Sunday could be sleet or snow, but much more of it is likely to be rain Friday and Saturday before the colder air seeps back in. All of the moisture is needed because, according to the latest U.S. Drought Monitor map, almost all of Virginia is abnormally dry and most of the Roanoke and New River valleys are suffering moderate drought.

As for the snow ... it all depends on what happens with the trailing upper-level low, and the models are still chasing it around like a greased pig trying to get a handle on what it will do. Some of Thursday night's late model runs were again cutting out a strong coastal low, with at least one showing a sweep of snow well inland. This would probably not be an issue for us until Sunday, though some sleet or snow may fall Saturday evening as cold air thickens in the area. I would only say this at this point: As tired as all of us may be with this winter and its persistent weird misfires, this situation bears some vigilance. I will be watching closely with a great deal of interest to see how the pieces come together for whatever happens.

UPDATE, 4:30 PM: There are theories by the dozens bouncing around about this weekend's winter storm threat. But the Hydrometeorological Prediction Center is, at least for now, a believer in our snow drought coming to a screeching halt late Saturday into Sunday.  The HPC, in its latest forecast update, has all of Southwest Virginia not only in a slight to moderate risk of 4-plus inches, but even a slight risk of 8-plus inches with a narrow swath of moderate risk. HPC has updated its maps but not yet updated its discussion. I will be interested to see their reasoning for this forecast. (The discussion is briefly noted in a comment below.) END UPDATE

 It looks like the batters are coming to the plate for the bottom of the ninth inning of our winter without a significant winter storm. A complicated storm system is taking shape for Saturday and Sunday that will first likely send Gulf of Mexico moisture northward as light rain but gradually change to sleet and snow as colder air moves in from the north. The Hydrometeorological Prediction Center is swinging for the fence right now, with moderate to high risks of 4-plus inches of snow for everyone from Blacksburg and Roanoke northwestward and slight risks even into the Piedmont region. These risk areas have a tendency to shift 3 days out, so take it with some skepticism (OK, a lot of skepticism in this winter).

My early take on it is that I think there's a good chance we will see sleet and snow plummeting from the sky over much of the area by Saturday evening, but I'm not sure of whether it will be heavy enough and long enough in duration to accumulate too much from Roanoke south and east. The idea that previous days' warm temperatures can entirely prevent snow accumulation is somewhat overrated (it was 76 the day before the 6-to-10 inch snow on March 30, 2003, and had been above normal for most of the month), but it does usually take a heavy fall of wet snow to build a slush layer underneath to allow widespread snow accumulation, beyond grass and trees, following mild days. Today and Friday will be quite mild, as highs around 60 are possible.

These few days of cold coming up appear to be a blip in the overall trend toward milder weather that will likely resume by the middle of next week. So indeed, this may be the last chance to keep Roanoke out of the record books for least snowy winter on record.

Milder days are on the way ... at least Wednesday and Thursday, when highs could reach the 50s to near 60 across Southwest Virginia. But a new storm system moving to our northwest will bring a cold front through, possibly kicking off some rain Thursday night and Friday. It doesn't look like a soaker, though, with the storm's strongest energy up near the Great Lakes.

Even though the long-range pattern is trending milder overall, there will likely be a brief shot of cold air over the weekend into early next week. There is also some upper-level energy that may move through the Southeast early next week, which could ... well ... there's no way I'm even going to go there right now.

Remember last week when we were talking about the scenario that, if the pieces came together right, there could be a big snowstorm for the Eastern U.S. Well, the pieces did come together for a massive snowstorm over part of the Eastern U.S. ... the far northern part of it. Much of Maine was walloped by 1 to 2 feet of snow, much of it heavy, wet snow that sagged trees and broke power lines. Click here for the story.

Today's chilly winds will gradually give way to milder weather most of this week.

Another blast of Arctic air will be moving in today on stiff northwest winds, and there might be a few snow flurries or even briefly heavier snow showers around. Lows will likely be in the teens across much of the area by Monday morning. But, for those of you tired of the windy cold this year (is someone out there not tired of it?), give it a couple of days and a run of milder weather will move in.

Winter weather advisories are now in effect for several counties in West Virginia, a few in far Southwest Virginia and in the high country of northwest North Carolina, all for the forthcoming upslope snow shower event. But the winter storm watch was not upgraded to a warning, at least not yet. Latest warnings/advisories: http://www.erh.noaa.gov/rnk/emer/emer_winter.php

National/regional radar shows a pretty potent band of rain/snow moving east with the cold front across Kentucky and Tennessee. Several stations in Kentucky are in the low to mid 30s with snow falling. We'll see how well that sticks together as it approaches our area. The downslope effect on this side of the mountains and the low pulling away to the northeast are likely to dry the moisture up significantly,  but there could be a brief burst of light-moderate rain or snow with this moisture band. After the cold front goes by, the low to the north will pull in the familiar chilly northwest winds that have defined this winter, and the upslope snows will commence over West Virginia. A few snow showers will likely bleed over into the New River Valley on Sunday, and a few flakes may even fly by in the Roanoke Valley.  Considering that Roanoke has had a trace of snow on 19 days this winter, this has very much been the norm for '08-'09.

In writing about any winter in this region, one is bound to write "upslope snow showers" on at least dozen different days. But this winter has seen a seemingly infinite series of upslope snow shower events, when northwest winds blowing up the western slopes of the Appalachians triggers snow squalls, mostly in West Virginia but bleeding into Virginia. That's happening again starting overnight and continuing into Monday as a low to the north pulls another Arctic cold front through. There will be some upper-level energy to help trigger some of the snow, but most of it will be typical northwest-flow stuff. Winter storm watches are up for the usual counties of eastern West Virginia, where 6 or more inches of snow could fall, especially above 2,500 feet and on west-facing slope areas.

That sliver of high-elevation counties is wondering: What snow drought?  Snowshoe Mountain has collected, to date, 171 inches of snow ... and counting. That's a bit over 14 feet ... running well ahead of the 10-foot-per-season average, (Correction: Snowshoe Mountain averages about 180 inches of snow each year) likely on the way to 200 inches by the time the last flakes fall in May. What several days of northwest wind, 4,800 feet in elevation and some Great Lakes moisture can do.