Conversational snow flakes may move inside beltway Saturday afternoon; tree damage and power outage risk outlying N & W areas

* 2:30 p.m.: Winter storm warnings for Loudoun and Frederick county (and points north and west, including much of northern Md. and northwest Va.) Saturday - primarily above 1,000 feet *

* 12:55 p.m.: Winter storm watch extended into Montgomery and Howard counties *

A rapidly developing, intense East Coast storm will bring unseasonably cold temperatures and heavy precipitation to the entire metro region Friday night and Saturday. For the immediate metro region, rain is likely to be the predominant precipitation type, but snow may begin to mix with the rain late Saturday morning and possibly change to all snow briefly during the afternoon. Accumulation is unlikely but cannot be ruled out.

Towards western Montgomery, Frederick and Loudoun counties, a changeover to snow is more likely to occur and occur earlier, with the potential for some accumulation, especially at higher elevations. Accumulating snow on trees with foliage presents a significant power outage threat.

Because this is a very dynamic storm system and a slight change in temperatures could mean the difference between no snow and several inches in any given location, this is a low confidence forecast. Even in Washington, D.C. there is an outside chance (15% or so) of 4” of snow.


Light rain develops. Temps 40-45.
Rain. Temps 38-43.
Rain changing to snow N &W. Rain elsewhere. Temps 36-41.
Snow N & W. Rain, possibly mixing with/changing to snow elsewhere. Heavy precipitation. Temps 34-39.
Snow N & W, gradually ending. Rain or snow ending elsewhere. 33-38.


What does Wes Junker, Capital Weather Gang’s Winter Weather Expert, think? He writes in the following:

Despite issuing a snow accumulation map for the area, there still is an incredible amount of uncertainty inherent in the forecast. The one thing we are pretty sure of is that the approaching system will produce lots of precipitation. Beyond that, the simple fact is that the outcome becomes muddled.

One model (the GFS) has really trended warmer while the other (the NAM) switched to colder last night but, now, while still forecasting plenty of snow, has waffled back to a slightly warmer look. The differences in the two models are largely due to the track of the circulation at around 5000 ft: the GFS track takes it right over us which assures the area of being too warm for snow if it is correct. The European model still remains a cold enough to produce snow especially in the western suburbs but its upper level pattern does not look quite as snow-conducive as yesterday.

Given the time of year and that it often has a little better forecast of the mid-level features than the NAM, right now I lean slightly more towards the GFS solution.

Where is the storm now?: Track it on national radar.

Has it snowed before in October in Washington, D.C.? Yes. Capital Weather Gang’s Ian Livingston reported earlier this month:

Accumulating snow has fallen on five days (out of 4,340 in the record through 2010) during October. The earliest accumulating snow of the month — and on record — fell on the 10th in 1979 (yep, at National!) when 0.3” was measured. The most October snow in one day was 2” on the 30th in 1925 (with another 0.2” the day after). Ten other days have witnessed a trace of snow in the month.

Higher amounts have occurred in outlying north and west suburbs and snow has fallen more frequently there.

Might it snow more than forecast? Yes. There is as much as 50% chance of 1” in Washington, D.C. , 40% chance of 2”and 30% chance of 4” according to NOAA. And the north and west suburbs have a 30% chance of 6”. We think these probabilities might be a little overdone, but the possibility of more than conversational snow in the immediate metro region cannot be dismissed.

What about the tree damage and power outage risk?The tree damage and power outage risk is primarily in areas which receive at least 3 or 4” of snow. Areas most vulnerable include northwest Virginia along and west of I-81 and into north central Maryland. In these areas, the weight of the wet snow on top of existing foliage is likely to cause tree limbs to snap which may down power lines. Earlier this week in Denver (from this same storm system), more than 100,000 residents lost power due to the heavy wet snow. AccuWeather’s Jesse Ferrell has a great blog which discusses the threat. An excerpt: The most likely area to take on severe damage from this storm is north-central Maryland and the Virginias, where elevation may be low enough that the leaves have already fallen, and where no damage was caused by Hurricane Irene.

Might it not snow? Yes. If temperature turn out to be slightly warmer than forecast, snow may not occur inside the beltway. And even areas farther west under winter storm watches may not see accumulation.

Will the snow stick? Where accumulating snow happens, most of the accumulation will occur on grassy areas. However, when it snows heavily enough, slushy accumulation on roads is possible.

Where it mostly rains, how much rain are we talking about? 1-2” of cold, nasty rain.

How much snow are other outlets forecasting? ABC7 snow map (version 1) | version 2; WUSA-TV9 snow map; NBC4 snow map; NWS snow map

Will the storm affect flights? Because most snow that falls won’t stick that well, this shouldn’t be a storm that cripples airports. Having said that, Dulles Airport may get accumulating snow that lowers visibility and delays flights. While National Airport is unlikely to get much snow, moderate to heavy rain will delay flights not to mention delays related to travel disruption throughout the Northeast.

What kind of weather is expected to our north towards New England: See this post.

How long will any snow last? Not long. While it will be unseasonably cold on Sunday, the day after the storm, temperatures will rise through the 40s and the relatively strong late October sun will melt snow rather readily.