As for start time, this is still a tricky question. Still looks to be as early as around 11 a.m. or perhaps as late as around 4 p.m.. After that the story remains much the same as it has — we expect heavy snow at times through Saturday and into Saturday night — along with strong winds and the potential for power outages.
Overview: Will the models blink? They have been steady as a rock in predicting a huge, if not historic, winter storm for the D.C. area starting Friday and lasting into Saturday night or early Sunday. The consistency has been remarkable, and we’ll watch to see if it stays that way as model output rolls in this evening.
A few aspects we’ll especially be keeping our eye on;
Start time: So far this has been the trickiest part of the forecast, with various models having shown start times as early as around 10 a.m. Friday and some as late as around 6 p.m. A start time between 11 a.m. and 3 p.m. is our current thinking. We’ll see if we can narrow that window, or if we need to shift the window, based on this evening’s model data.
Accumulations: The models have been virtually unanimously consistent in forecasting double-digit snow totals across the D.C. area. Our current forecast calls for 12-20″ as the most likely amount for much of the immediate metro area, except 15-25″ north and west of the Beltway. Will any of this evening’s models throw us an unexpected curveball? We’ll let you know.
Winds: Unfortunately our current forecast calls for wind gusts in the 30-40 mph range on Saturday, which combined with heavy snow could very well cause substantial power outages. We’ll keep our eye on whether the wind forecast fluctuates in either direction.
Enjoy the running commentary. At the end of the evening we’ll post some concluding thoughts on whether we’ve seen anything that would substantially change our forecast.
11:30 p.m. update: GFS shows winds at their max on Saturday morning, sustained around 30-35 mph in D.C. and up to 40-45 mph toward the Chesapeake Bay. That would put wind gusts in the 40-50 mph range. I think this is overdone, but even if you subtract 5-10 mph off of everything, we’re still going to see power outage issues, especially from around D.C./I-95 toward points east.
11:20 p.m. update: One of the big questions with all big storms like this, is where might heavy snow bands set up and produce localized heavier accumulations. Looking at the map below, this particular GFS run puts such a band (25-30″) in west central Virginia. But models are not good at pinpointing such bands, so this will probably be a moving target.
11:10 p.m. update: Overall the GFS still shows a massive storm for the area, with generally 15-20″ across the core of the metro area.
11:05 p.m. update: Well, as with the NAM, with the GFS we are seeing that sleet/rain line cutting in closer toward the District from the southeast, which has reduced totals somewhat just south and east of D.C. and I-95. Around 12-18″ for the immediate south and east suburbs, rather than the blanket 18-24″ we saw in the afternoon model run (see 10:30 p.m. update below).
10:50 p.m. update: Similar to the NAM graphic we showed earlier (scroll down below), this is the GFS forecast for relative humidity at about 10,000 feet up. The dark green is a good approximation for the precipitation shield, which is shown below as arriving inside the Beltway from the southwest by 1 p.m. Friday. Generally I’d say this model indicates a start time for the D.C. area between 11 a.m. and 2 p.m.
10:45 p.m. update: The GFS is showing a start time of approximately noon to 2 p.m. Friday, about the same as the NAM. Will post a picture demonstrating this in just a few minutes.
10:30 p.m. update: Before we get our eyes on this evening’s GFS model run, let’s look back at what this afternoon’s run showed, so that we’ll be ready to compare. Below was the snowfall forecast for 7 a.m. Friday through 7 a.m. Sunday, showing a solid 18-24″ across the entire region.
10:20 p.m. update: Here’s a quick update on schools for tomorrow. Fairfax County and St. Mary’s County both opening 2 hours late. Might see some more those.
10:10 p.m. update: While we await the GFS to start rolling in, a reminder that we’re soliciting your help to name this storm! The suggestion box is open…
10:05 p.m. update: How about the winds? The NAM is predicting winds around 20-25 mph on Saturday with gusts around 30-35 mph. That would probably be enough to cause some power outages, though perhaps maybe not mass outages..
9:55 p.m. update: The NAM shows generally 2-2.5″ of liquid precipitation for the D.C. area, which would translate to around 20-25″ of snow. However the NAM snow depth forecast (shown below) shows more like 10-15″ from the District toward points south and east, with 18-24″ north and west of D.C. I think it may be under-doing the band immediately south and east of D.C., but let’s see if we see a similar trend in any other models.
9:40 p.m. update: Looking over the NAM model, it suggests all snow for the entire area except for sleet and rain pushing from the southeast into St. Mary’s and Calvert counties. just about to get the model’s total storm accumulation, and it’s going to be a big hit.
9:35 p.m. update: The NAM simulated radar forecast for 4 p.m. Friday shows the more significant precipitation (non-grays) just reaching the Beltway. It actually shows things starting with a bit of rain. I don’t buy that part. Should be sufficiently cold for snow.
9:20 p.m. update: Here comes the NAM model, which can be a pretty good model inside of 48 hours. Below is a forecast of relative humidity at about 10,000 feet up in the atmosphere at 1 p.m Friday. It shows the 70% threshold, which is typically a good indicator of precipitation reaching the ground, arriving in the metro area from the southwest around noon to 2p.m.
9:15 p.m. update: Before we dive into the models, a word of warning to be very careful if you must drive anywhere this evening. Tonight’s snow was no more than a half-inch to around 1″ in spots. But it turned untreated roads into skating rinks. Unfortunately treated roads were few and far between despite ample warning in the form of a Winter Weather Advisory issued by the National Weather Service this morning.