GFS model run from tonight shows 1-2+ inches liquid equivalent precipitation falling from Wednesday’s storm. How much of that falls as accumulating snow is the outstanding question.

11 p.m. update: This evening’s model runs suggest strong odds of heavy precipitation in the region starting late Tuesday night through Wed nesday. The most difficult element of this forecast - assuming the storm remains on track (a fairly big assumption) - will be how much of this precipitation falls as snow and sticks. Right now, it would appear high elevations of western Virginia and Maryland stand a strong chance of getting a major snowstorm. As you get closer to the District, it’s too soon to say with confidence heavy snow will accumulate, but odds are better than they’ve been since Commutageddon (January 26) in 2011. We’ll provide update snow accumulation probabilities tomorrow.

10 p.m. update: Here’s an update I just posted to our Facebook page:

While we await new models this evening, a few points on the Wednesday storm:

1) Confidence that the D.C. area will see significant amounts of precipitation has increased; but it’s not 100%

2) Right now, the trickiest aspect of this storm is the distribution of snow vs. rain and how much snow accumulates

3) This may well turn into an event where there a big differences in accumulation over a small area; for example, places like Upper NW DC could get noticeably more snow than downtown, National Airport; having some elevation will help keep temperatures closer to freezing, whereas areas near sea level may stay above freezing making it tough for snow to stick.

4) Someone within 60-100 miles of DC, especially on the west side, could well get hit hard by this storm....

5) Small variations in storm track will make a big difference in how this storm plays out, so nothing is locked in at this point.

I’ll be back with one more update around 11 p.m. on our latest thinking based on the evening’s models.

- Jason Samenow

From 3:30 p.m.: Odds are increasing the Washington, D.C. area will see a period of precipitation Tuesday night into Wednesday or Wednesday night. And if the precipitation is heavy enough, then this looks to be our best chance of significant snow all winter long. That is still a big “if,” however. So while our chance of at least 1 inch of snow is up from yesterday, it remains below 50/50 for the time being:

Next chance of accumulating snow: Tuesday night-Wednesday night

Chance of at least 1 inch: 40 percent (up from 30% yesterday)

Keep reading for more details...

Today’s models continue to advertise the potential for a significant storm across the mid-Atlantic Tuesday night through Wednesday night, including in the Washington, D.C. metro area. Possible scenarios include:

40% chance: A minor to moderate storm producing ~1-4” of snow, likely mixed with rain and perhaps some sleet
35% chance: A storm that is mainly rain with little to no snow or sleet accumulation.
25% chance: A moderate to major storm producing ~4” or more of snow, possibly starting as or mixed with rain and/or sleet.

After a few days of models flip-flopping between a storm that might track too far to our south for significant snow, to one that might ride up the coast and give the big cities along I-95 including D.C. a major winter storm, models now seem to be converging on a storm that will bring a period of precipitation to much of the mid-Atlantic but then head out to sea.

Last three runs of the GFS weather model have trended heavier precipitation north into the D.C. area (NOAA)

While the odds of a major Nor’easter that would turn up the coast and wallop Philadelphia, New York and Boston are down, they are not completely out. Similarly, there’s still a chance the storm slides mainly south of the D.C. area, but likewise those chances are decreasing.

So with the overall trend pointing toward significant precipitation locally, the big question is whether it will be heavy enough to pull down cold air from the mid-levels of the atmosphere and support accumulating snow. If precipitation is too light, then temperatures here near the ground would probably stay above freezing, which combined with a strengthening March sun would limit the potential for snow and especially accumulating snow.

Right now we’re *leaning* toward at least some accumulating snow, especially for colder areas west of I-95, and the best chance we’ve had all winter of seeing more than 1 inch. Of course that’s not saying much given the winter we’ve had, and it’s not a terribly confident “lean,” either.

Here’s what our winter weather expert Wes Junker had to say after reviewing the latest information:

“The range of possibilities seems to be narrowing as today’s models are now predicting significant precipitation across the region. The most likely scenarios now are a more significant storm that produces some accumulating snow especially for folks west of I-95, or or storm that produces modest precipitation primarily in the form of rain.

The models continue to vacillate between solutions that would bring the heaviest precipitation right through the area giving us a better chance for at least some accumulating snow, and those that would keep the heaviest precipitation just to our south likely meaning mostly rain.

This is one of those cases where the precipitation gradient (how quickly it drops off from south to north) is greater than normal, so only slight shifts in the track of the storm’s low-pressure center can make significant differences in the amount of precipitation we get.”