* Winter weather advisory for Fairfax, Montgomery, Prince William, Howard and southern Fauquier counties. *
* Winter storm warning for northern Fauquier, Loudoun and Frederick counties (and points north and west, including much of northern Md. and northwest Va.) Saturday - primarily above 1,000 feet *
12:45 p.m. update: The transition from rain to mixed precipitation and snow appears to be increasing its movement to the south and east of late, and reports of sleet or a change to snow are encroaching on I-95. Any period of sleet -- perhaps temporary most spots -- may help “prime” the surface for extra snowfall accumulation if and when it begins. Temperatures are beginning to approach freezing in some locations over western and northern parts of the area, which might help roads pick up slush. Cautious is advised in places with snow accumulation occurring.
11:40 a.m. update: Watching radar and obs, the storm is now mainly transitioning to its cold phase across the area. Some drier air has worked into chunks (mainly south and east portions) of the region and to the south and east of there. Elsewhere, radar continues to show bands of mixed precipitation and snow to the northwest of I-95. Over the next few hours, I-95 should become more and more the dividing line as rain tends to want to mix with or change to snow. Though there may be relative pauses, banding in this period can produce moderate to heavy rain and snow.
10:15 a.m. post: Talk about a raw October morning. Rain that mixed with and/or changed to snow in parts of Loudoun, Fairfax and Montgomery counties (as well as north and west) continues mostly unabated. The wintry mix is also on the move, though spots further south and east are still enjoying a really cold rain. Thus far, outside elevations around 800-1,000+ feet or so, many with snow have seen only light accumulation, mainly on grass or elevated objects.
We still generally like our forecast, but let’s take a quick look at some additional ideas this morning....
* Elevation is important. We’ve highlighted that all along, but it’s worth keeping in mind. Places particularly above 1000’ switched to all snow fastest and are having an easier time accumulating thanks to colder temperatures. In lower elevations, the snow mix and change to all snow will continue to progress south and east through the midday. Unless very heavy rates impact lower elevations, accumulations should tend to be on elevated objects, though roads may become at least temporarily impacted here and there.
* It’s cold, but temperatures are marginal. Readings have been steady or dropping at the surface all morning, and they’ll start to do so up above us as well during the next few hours while the low pressure off the Carolina coast further develops. This means snow is increasingly likely to the west and north and that places toward D.C. or even southeast are still in the game for a little later in the day. All the rain that’s fallen should aid melting though, and a bit of warmth aloft has promoted some sleet.
* The storm won’t rage into the night. We’re probably in or heading into the peak period the next few hours, particularly in the far north and west suburbs where snowfall rates could be occasionally intense through the midday. That said, our timeline from earlier should hold, and precipitation may even try to wane or cut off by afternoon (particularly further south and east) as a dry slot rotates near the area. However, the latest NAM computer model simulation shuts everything down between 5 and 8 p.m., which seems a little late at this point.