Even though near-record highs are possible tomorrow, the risk for an early-season winter storm looms large as we get toward the biggest travel day of the year this coming Wednesday. Much colder air begins to filter in on Tuesday after a cold front pushes through and moves offshore. At the same time, a coastal storm is likely to develop near the Southeast U.S., moving north and northeast, more or less paralleling the East Coast. Where exactly the storm passes, and how quickly it strengthens, will have significant impacts on pre-Thanksgiving travel plans locally and across the Northeast. Again, let’s note up front that the uncertainty in this forecast remains moderate to high. There are still a number of scenarios, though it’s possible the outer bounds are closing. Confidence remains low. Here are the scenarios as they stand early this afternoon: 1) A strong storm hugs the coast, spreading moderate to heavy precipitation up the urban corridor. A swath of significant snow is possible, especially in elevated areas N&W of the cities. High winds and coastal erosion concerns as well. (Euro, UKMET, GFS) — Moderate/High impact. 2) A less energetic storm travels far enough east to keep moderate and heavy precipitation closer to the shore. Significant snow risk likely lower than in above, partly due to precipitation being focused in lower elevations near the coast. (Canadian, Experimental/New GFS) – Low/Moderate impact. 3) Storm organizes slow enough to keep precipitation mostly or fully offshore. Some clouds and a breeze the main concern. Showery near coast? (NAM) — No/Low impact. Compared to yesterday , we’ve seen weather models come closer together, with a majority having some storm impacts felt across the area and through the broader Northeast during the day on Wednesday. A total miss seems less likely. CWG’s winter weather expert Wes Junker checked in with his thoughts on the scenarios above, as well their respective odds: The models seem to be slowly trying to seek a consensus but still have not arrived at one. This morning’s GFS has shifted west with its low track and has joined last night’s European and UKMET models in spreading heavy precipitation across the region. However, the 12Z NAM and parallel GFS (the new model on the block) are still keeping the low far enough offshore to miss the D.C. area. Scenario 1) A flush hit: Surface temperatures are expected to be above freezing during the bulk of the storm, complicating the forecast. Such a scenario would have a significant impact as it could produce significant snows across portions of the Northeast, especially west of I-95. Scenario 2) Heaviest precipitation closer to the coast: Temperatures with this a scenario would likely be a little above freezing during the bulk of the precipitation but some snow still would probably fall towards the evening, even east of the city. The precipitation could have a moderate impact on travel as it would at the very least produce wet roads. A number of European and GEFS ensemble members predict such a scenario. Scenario 3) Fish storm: The storm stays far enough offshore to miss us completely. My best guess is that one of of the first two scenarios is more likely than the third, but with little confidence. Given the expected timing, possibly starting in the D.C. area near sunrise on Wednesday and then quickly progressing up the coast during the day, snow that does not accumulate or heavy rain could prove nightmarish for travel. Some model forecasts indicate the equivalent of one inch or so of rain, which would be around 10 inches of snow if temperatures were cold enough (which is unlikely since most models indicate above freezing temperatures during the heaviest precipitation). Wet snow is certainly a real possibility in the immediate D.C. metro area, but early-season snow events often have certain characteristics. Namely, higher elevations – because they are colder – are more likely to get accumulation. The few extra degrees colder that often comes with altitude can make a huge difference for accumulation prospects. While we may have wet snow or a rain- snow mix and 35 degrees near the city, that won’t stick well. But, above 500-1,000 feet, where temperatures may be 32-33 degrees, it’s another story. As such, it’s hard not to currently favor north and west of D.C. — and similar for the cities up I-95 — for the highest risk of significant snow. Advertisement The wild card is the intensity of precipitation. If it snows heavily enough, it can cool the air at lower elevations allowing snow to accumulate some – a possibility even inside the Beltway. There are also indications that this storm could rapidly develop, in “bombogenesis” (24 mb pressure drop in 24 hours) style as it races from near the Carolinas Wednesday morning to near New England Wednesday night. That ups the ante when it comes to major travel disruptions rippling across the Northeast and beyond. Something worth considering now if you have any plans in that period. We’ll of course have a lot more as this unfolds.
Posted on: Mon, 24 Nov 2014 11:32:14 +0000
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