Eve of a messy winter storm: Weather model live blog
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Version 0 of 1. * Winter storm warning for far northern and western suburbs | Winter weather advisory west of I-95 (map) * * Latest forecast | Storm timeline plus roads, airports, school impacts | A travel nightmare up and down East Coast | Twin storm from the past |10 snow predictions from area forecasters * 11:08 p.m final update and discussion: Here’s our revised forecast accumulation map and our final call… Simply due to forecast temperatures trending up during the course of today, we’ve shaved off about 1-2 inches of snow from each zone. Along the I-95 corridor, that doesn’t leave much, although we still think they’re will be non-accumulating “conversational” flakes in the air (most likely in the midday to early afternoon timeframe)- maybe even a little build-up mostly on grassy areas. To conclude, we’d like to stress this is very likely to be an “elevation” snow event. That is, areas above 500 and especially 1,000 feet have a much better chance to receive accumulating snow than lower spots. The I-95 corridor, where elevations rapidly drop-off – is very likely to set up as the dividing line between areas that are snowier versus areas that are rainier. Of course, the boom scenario still applies- if temperatures are just a little colder than forecast and heavy snow bands set up, several inches could still materialize, even along the I-95 corridor. Alternatively, it’s also possible the I-95 sees nary a flake of snow if it’s a little warmer than expected. Our next post will publish at 5 a.m. Wednesday morning. 11:00 p.m. update: Our new accumulation map will out very shortly (graphics team making our requested tweaks). Here’s a nice model simulation of the radar and rain snow line at noon tomorrow from the HRRR model. It’s amazing how distinct the I-95 dividing line is.
10:48 p.m. update: Stand by for about 15 minutes. We are going to tweak our snowfall forecast and post a new map. If you’ve successfully read between the lines this evening, you’ll know what kind of adjustments we’re making… 10:45 p.m. update: The GFS did come in wetter, but for D.C. proper, would only suggest perhaps 2-3 hours of mostly non-accumulating snow with temps in the mid-30s. And it shuts off the event pretty quickly… with not much precipitation after 1 p.m. and it could all end by around 3 p.m. Other models, however, keep the precipitation going a little later into the afternoon. 10:35 p.m. update: The GFS is coming in – at least at first glance – wetter than earlier runs, but temperatures still seem mighty marginal for snow – until a lot of the precipitation has exited (during the afternoon). The changeover from rain to snow in this model would probably be around noon to 1 p.m. in D.C., but there’s not much precipitation in the model after that point. 10:15 p.m. update: Temperatures, ahead of the storm, are still very mild at Reagan National – where it’s still 51 at 10 p.m. However, at Dulles, it’s down to 41 and down to 44 at BWI. It’s easy to see how these latter two locations have a better chance of some snow. Note that at all three airports the dew points are on the high side – from 35 to 37. They’re a bit higher than indicated by models and, if you’re pulling for snow, you’d like to see them closer to freezing (since the dew point indicates what temperature the air would fall to if it was saturated, i.e. precipitating). 10:05 p.m. update: Here’s another not so great indicator for snow – the HRRR model, one of our short-range models, shows temperatures above freezing pretty much everywhere at 11 a.m. Wednesday. See below: Its temperatures are above freezing more or less everywhere and in the mid-to-upper 30s in the D.C. metro area – not cold enough for snow accumulation. Even in places like Frederick and Leesburg, most accumulation would focus on grassy areas – with road accumulation confined to heavier bursts – if this model is right. The latest temperature forecast are starting to suggest more of an elevation-snow event. In other words, the hills and mountains – above 1,000 feet- may be the primary locations that cash in with significant snow. 9:55 p.m. update: CWG winter weather expert Wes Junker emailed after examining the NAM model’s snow prospects for D.C. His words: “I am not impressed.” 9:50 p.m. update: While we await the GFS, let’s take a quick look at the SREF model from this earlier this evening. This is its forecast of the rain-snow line between 7 a.m. and 5 p.m.: Notice how it inches eastward to around Dulles by 11 a.m. but then seems to stall before reaching the I-95 corridor between 1 and 3 p.m. This suggests there’s going to be some resistance in getting the rain to changeover to snow in the I-95 corridor. This is another bearish indicator for much snow accumulation inside the Beltway and points east. By the time it has snow over the District, the precipitation will be close to ending. 9:35 p.m. update: Even though the NAM shifted slightly cooler at the surface in this evening’s run compared to the afternoon run, the trend in the model’s snowfall depth is not your friend if you’re a snow lover near the Beltway or even a little to the west: Note that this product forecasts snowfall depth so it will underestimate snow accumulation some – but it’s pretty underwhelming for the immediate metro area. The decline in snow cover in the successive runs indicates less available cold air at high altitudes. 9:20 p.m. update: Here’s a visualization of the rain-snow line at 10 a.m. Wednesday morning from the high resolution NAM model – which positions it through Warrenton, just east of Leesburg and then through northern Montgomery County. Its areas roughly north and west of that line, particularly at higher elevations, that have the best chance of getting signficant snow. 9:15 p.m. update: The NAM model output shows Dulles Airport will be 1-2 degrees colder than National Airport. That may make the difference between “conversational snow” (snow that falls and doesn’t accumulate much) and snow that actually blankets the landscape some. The changeover to snow should occur a couple hours earlier at Dulles compared to National, but even there temperatures are just marginally cold enough for snow – hovering between about 33 and 35. 9:05 p.m. update: The NAM model has come in a hair colder than its afternoon run, but a hair warmer than its morning run. It’s also slightly wetter than the afternoon run, but not as wet as the morning run. Basically, it implies a changeover to from rain to snow probably around noon at Reagan National, with a few hours of wet snow, possibly mixing with rain at times, through around 3 or 4 p.m. Its temperatures are between about 33-36 while it’s snowing, implying – if the model’s right – not a lot will stick around the city, but perhaps there will be a covering on grassy areas and car tops. 9:00 p.m. update: Let’s begin with some storm key points and our latest forecast accumulation map…
About this live blog: Above we live blog the latest computer model guidance. Every evening around this time is when the U.S. models, primarily the North American Model (NAM) and the Global Forecast System (GFS), provide updated information. You shouldn’t interpret any of the updates below as an official change in our forecast (though once the all the data is in, we’ll say whether it suggests any major changes from our current forecast). Rather, this is meant to be a play-by-play of the model madness that always precedes winter storms around here.
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