* Winter storm watch for entire area Sunday afternoon through Monday afternoon *
12:05 a.m., conclusions: Considering all of the model output together, we’re sticking with our general outlook for 1-4 inches in the immediate metro area, and 2-6 inches for our western and southwest suburbs. Yes, the GFS, NAM, and SREF certainly suggest the high end of those ranges but we’re going to see a lot of melting snow on contact with the ground due to above freezing temperatures as the snow gets going, especially around the city. The NAM model predicts Reagan National’s temperature to be 36 at 11 p.m. – that’s a warning sign that snow could have a tough time accumulating around the city unless the model is too warm and not sufficiently taking into account evaporational cooling that will occur as the snow falls. Bottom line: we continue to prefer conservative snow totals for this storm but it’s possible we’ll make some final adjustments during the day Sunday.
11:55 p.m. update: Here’s the GFS snow total map – assuming it was cold enough for all of the snow that falls to stick, which it’s not. In reality, totals are probably 50-70 percent of what you see here, at best:
11:45 p.m. update: The GFS is now fully out for the relevant time periods Sunday night into Monday morning. It’s actually a bit drier than this morning’s run overnight, putting out about 0.59″ of liquid equivalent precipitation compared to 0.66″ this morning. About 0.1″ of the 0.59″ of precipitation falls before 8 p.m., which might be in the form of rain or mixed precipitation, certainly not accumulating snow. Of the 0.35″ liquid equivalent that falls between 8 p.m. and 2 a.m., maybe about half of that converts to accumulating snow -or around 1-2″. From 2 a.m. to 8 a.m. Monday morning, just 0.13″ liquid equivalent falls – roughly another 1″ of snow. It does forecast another 0.07″ liquid equivalent precipitation after 8 a.m. Monday, which could lead to another 0.5-1″ of snow – although snow falling during the day Monday will have a hard time sticking on paved surfaces (though may continue to accumulate on the grass). That would suggest a total in the 2-4″ range from the GFS for the city, and probably 3-6″ for our colder suburbs west of town.
11:35 p.m. update: The GFS is out through 2 a.m. Monday morning. It shows a solid area of snow developing over the region between 8 p.m. Sunday and 2 a.m. Monday morning, although some of the snow falling in this period probably won’t accumulate due to possible above freezing temperatures and warm ground, especially around the city.
11:25 p.m. update: The GFS is starting to roll. This is an important run.
11:15 p.m. update: The regional version of the Canadian model, known as the RGEM, is underwhelming to say the least for snow lovers. It forecasts about 0.2-0.4″ of liquid equivalent precipitation for the immediate D.C. area, which would probably be around 1-2″ or maybe 3″ of snow. It has a very sharp northern cutoff with the precipitation field.
11:00 p.m. update: Towards the end of its forecast period, late Tuesday, the NAM spins up a major storm which produces copious amounts of precipitation. I’d view its forecast skeptically as this model’s accuracy is highly suspect after about 60 hours. Having said that, it’s a feature to keep an eye on and has the potential to bring rain to the region and perhaps even some mixed precipitation.
10:45 p.m. update: Here’s the high resolution NAM total snowfall output graphic (through 1 p.m. Monday). We think this is overdone because it doesn’t take into account “wasted flakes” or melting in the early hours of the storm when the ground is warm and temperatures, especially around the city, are likely above freezing. So I’d shave 1-3 inches off these totals – trimming 1″ off in our normally colder areas west of the city and about probably around 2-3″ around the city. What’s important to note here is the sharp northern cutoff. If the NAM is right, northern Maryland doesn’t get much snow from this event.
10:35 p.m. update: One of the reasons we’re being conservative with snow totals for this storm is the relative warmth preceding the storm. We see highs potentially into the mid-to-upper 40s on Sunday around the city so an awful lot of cooling is going to be required before any falling snow begins to accumulate. Not to mention the ground is relatively warm after today’s 60s.
10:25 p.m. update: The NAM is out far enough to give a reasonable estimate for how much snow it’s simulating for Reagan National Airport (and around downtown). It spits out around 0.7″ liquid equivalent precipitation for the storm. If it was below freezing the entire the storm, that would convert to around 7 inches of snow. But it won’t be below freezing during the early phase of the storm. It may take until around midnight or so for the temperature to reach freezing around the city, so there are going to be a few hours of “wasted flakes” so to speak (non-accumulating snow or perhaps some mixed precipitation). Having said that, it totally plausible about 0.4-0.5″ of that 0.7″ falls as accumulating snow, which would suggest 4″ of snow would fall even downtown if the NAM is right. In our colder suburbs west of the city, there will be fewer wasted flakes, so the NAM would suggest 4-6″ in these areas.
10:15 p.m. update: When we compare the last two runs of the NAM, during the pivotal 11 p.m. to 5 a.m. time window – when we expect a good deal of the snow to fall – there aren’t a lot of significant changes. The northern edge of measurable snow may have shifted a hair south in this latest run. The heaviest precipitation remains south of the District, but shifted east some.
10:05 p.m. update: The NAM is out through 2 a.m. Monday morning and shows temperatures quickly falling below freezing between 8 p.m. and 2 a.m., from northwest to southeast, and with snow starting to accumulate. In the map below, look how far south the freezing line (in purple) is at 2 a.m., dipping into the Va. Tidewater.
9:55 p.m. update: The SREF model run at 5 p.m. this evening supports our idea for the heaviest snow southwest of the District. It puts out a healthy amount of precipitation throughout the region, generally between about 0.75-1″ liquid equivalent. A reasonable assumption might be that about half of that falls as accumulating snow, which would imply at least 3 or 4 inches if it’s right.
Overview: Accumulating snow is likely to develop Sunday night after a preliminary period of rain and mixed precipitation Sunday afternoon. The wild card remains: how much? We’ve played forecast snow totals (1-4″ for the immediate metro area) conservatively compared to the National Weather Service (4-8″ for the immediate metro area) , for example. But the afternoon models came in a bit snowier than the morning ones – the primary basis for our first call. Should we, will we increase our snow forecast? Tonight’s model runs may nudge us in that direction, or not. Follow along as we analyze the latest model information over the next couple hours.