9:40 p.m. update: The model data this afternoon and evening suggest temperatures are going to have a hard time cooling sufficiently for significant snow in the immediate metro area, but that our colder areas out towards Leesburg and Frederick and to the north and west have a good chance of seeing substantial snow. Here’s our first snowfall accumulation forecast for the region…

Note that we have just low to medium confidence in these numbers and they will likely need to be refined. Confidence is particularly low in the immediate metro area because we are right on the rain/snow line. If it’s a little colder than models are predicting, some heavy wet snow could still occur. But more likely, we’re looking at more rain, eventually mixing with and changing to some wet snow with minor accumulations at best.

We’ll have more tomorrow.

From 3:25 p.m. Saturday…

* Winter storm watch early Monday morning into Monday afternoon for Loudoun, Frederick, Montgomery and Howard counties for the potential* for 5″ or more of snow *

A compact, potent storm system will punch through the D.C. region early Monday morning into the afternoon bringing several hours of moderate to heavy precipitation.  In most areas, the precipitation should begin as rain but transition to snow from northwest to southeast.

Especially in our colder north and west suburbs, heavy,  accumulating snow is possible during the morning rush hour into midday Monday. Along the I-95 corridor and to the east, it is likely to take longer for rain to change to snow, temperatures may be challenged to fall below freezing, and so accumulation prospects are more iffy.

“It should be cold enough to support rain changing to some snow, but warm enough to make predicting snowfall amounts a big problem everywhere within the forecast area,” says Capital Weather Gang winter weather expert, Wes Junker.

That said, here are our initial thoughts:

Inside the Beltway and south and east

Chance of 1″ or more: 50 percent
Chance of 4″ or more: 30 percent

North and west of the Beltway

Chance of 1″ or more: 70 percent chance
Chance of 4″ or more: 50 percent chance

Those probabilities in mind, consider these multiple factors working for and against significant snowfall.

In favor of heavy snow:

* Track of low pressure passing well to the south (see below), typically a favorable one for snow

* Lots of moisture: models – universally – simulate fairly heavy precipitation; the storm dynamics are very impressive in model simulations. “The upper level pattern is very favorable for the development of a strong banding feature that could enhance precipitation rates wherever it sets up,” says Junker.

* Onset of snow should be prior to sunrise – which gives a snow a better chance of accumulating compared to if the snow began in the middle of the day

Working against heavy snow:

* Temperatures on Sunday, the day before, will rise well into the 50s, even challenging 60.

* The cold front coming through Sunday night is not that strong, temperatures will fall only slowly.

* Models generally simulate above freezing temperatures during the storm

* The heaviest precipitation is forecast to fall a relatively narrow band, which includes the D.C. area in current simulations.  A shift in this forecast or, if the models currently are too aggressive in their simulations, would mean less precipitation and less snow potential.

The bottom line is that in order for large parts of the metro region to get impactful, accumulating snow, heavy precipitation rates are going to have to overcome marginal temperatures (through evaporational  and dynamic cooling processes) for snow and the warm ground.  Yes, under certain circumstances – we can get accumulating, heavy, wet snow even at 33 or 34 degrees.  Monday morning might be one of those cases. But if anything goes wrong – we could just end up with rain and snow and little accumulation, especially around the city and points east.

“In the past we’ve been burned when relying on snowfall rates to overcome iffy surface temperatures and frankly have busted a couple of forecasts badly. So in and around the city, we’re playing the snowfall forecast conservatively,” says Junker.

* For our colder areas, a winter storm watch means the National Weather Service believes there is a 50 percent chance of at least 5 inches of snow in the watch area; this also mean it believes there is a 50 percent chance snowfall will be less than that.