Weather model simulations of possible storm next week. Top left is latest European model showing possible severe impact in eastern New England. Top right is latest Canadian model with severe impact in the northern mid-Atlantic. Bottom image is latest GFS model showing storm well out to sea, potentially threatening Bermuda. (Top left Environment Canada, top right Weatherbellmodels.com, bottom StormVistaWxModels.com)

Because of the pre-storm hype resonating through social media streams, let’s clear the air by answering some basic questions and sharing some expert opinions...

How likely is a big storm? The pieces are there for a big storm, the question is whether and where they come together.

Based on current information, I’d give slightly better than 50 percent odds that a significant storm impacts some place in the mid-Atlantic and/or Northeast early next week, but just a 20 percent chance of a storm affecting Washington, D.C. directly. A landstrike north of Washington, D.C.’s latitude appears to be more likely than south. So this could end up being a bigger deal in New England than the mid-Atlantic.

As always with East Coast storms, the location where the different pieces of energy come together and at what time will play a huge role in where, if anywhere, the worst effects are. This cannot be predicted with confidence 7-9 days ahead of time (right now).

What is the worst case scenario? For the mid-Atlantic, it would probably resemble current model simulation of the Canadian model or last night’s European model. In these scenarios, what’s now tropical depression 18 intensifies into a hurricane - named Sandy - near the Bahamas and then combines with a powerful mid-lattiude cold front (upper level trough) approaching the East Coast. The result is a super-intense, slow-moving “hybrid storm”- as powerful as a major hurricane. Potential effects include:

* tremendous coastal flooding (exacerbated by a full moon and higher than normal tides)

* copious rainfall and damaging winds extending a few hundred miles inland - including the Washington, Baltimore, Philadelphia and New York City regions.

(For eastern New England, the latest European model, shown above, is a fairly dire scenario)

What about snow in this worst case scenario? The European model from last night, taken literally, suggested some heavy precipitation falling as wet snow during the storm, even at low elevations in the mid-Atlantic. I don’t buy that solution at all. Because any storm will have tropical roots, it will likely transport some of that warm, moist air inland - even if D.C. were on the cold side of the storm.

At high elevations in western Maryland , interior Pennsylvania and, perhaps, the Blue Ridge, some - or even a lot of - heavy snow would be possible depending on the exact track of the storm. Obviously, if heavy snow were to fall in late October in places where foliage remains, the tree damage could be devastating.

How likely is this worst case scenario? Probably around 5 percent but subject to change (upward or downward)

What are different meteorological voices saying about this storm scenario?

At the heart of the issue, meteorologically:

Steve Tracton, CWG: “The biggest issue is whether [tropical storm or hurricane] Sandy phases [combines] with the approaching front and upper air low such that it undergoes “extratropical transition”, that is, a tropical cyclone moving into the mid-latitudes and transforms into an extratropical cyclone.”

“Tropical and extratropical cyclones are driven by fundamentally different physical mechanisms (sources of energy). In a “perfect storm” scenario (like that seemingly in the ECMWF prediction) the hybrid can generate a system more powerful and pose a more serious threat (wind/ precipitation) to land and maritime interests than either system alone.”

“That may or may not happen, but that it is a distinct possibility warrants a loud and clear “Heads Up” to stay tuned.”


On historic precendent

Elliott Abrams, AccuWeather: “There is a significant historical precedent for a storm to form around where this is located... and then turn into a real hurricane. In 1878, a storm formed near Jamaica (near where the new storm is) then became a Category 2 hurricane and moved right up the East coast. The center passed east of Florida, then came ashore in eastern North Carolina and stayed inland until it turned almost straight east over the southern parts of Vermont and New Hampshire. There was extensive damage from the Carolinas to New England, and more than 71 people were killed. This storm came to be known as the Gale of ‘78.”

Jesse Ferrell, AccuWeather: “...remember that this has happened before, with Hurricane Wilma in 2005 when it (almost) merged into a nor’easter and moved up the East Coast causing 74-mph wind gusts at the Jersey shore and over 20 inches of snow in the mountains.”

Joe Bastardi, WeatherBell (via Twitter): “The amazing thing about all this is the PATTERN argues for it.. large scale very similar to Hazel, 1954.”

Specific forecasts

Larry Cosgrove: “The Mid-Atlantic region (including the Virginias) and lower Great Lakes are in for one hell of a time in the October 29 - 31 period. Snow may be a big problem in elevated areas west of the storm track, which I think will be from Cape Henlopen DE to near Toronto ON.” (via Mike Masco meteorologist Facebook page)

Ryan Hanrahan, Connecticut broad meteorologist: “The most likely scenario is a storm that’s no big deal here in Connecticut. To get a hurricane-like storm up the coast in October is very rare. To get a storm that interacts just perfectly with the jet stream to strengthen as it moves toward the northeast is very rare. To get a storm that is absorbed by a digging trough and is flung northwest into the northeastern U.S. is exceptionally rare.”

Matt Noyes, New England broadcast meteorologist: “So...where this leaves us is a forecast of a likely coastal creature with impact of heavy rain and gusty wind sometime Sunday-Tuesday timeframe, but just how much and just how strong remains to be seen. ... part of the reason for focus at this point is that, if everything comes together in a worst-case scenario, this would be an extremely damaging storm for much of the coastal Eastern United States, and particularly the Mid-Atlantic and Northeast corridor.”

Joe Bastardi (via Twitter): “Ecmwf/Canadian continue in weatherbell camp of possible historic weather event on east coast.”

Premature hype

CWG tropical weather expert Brian McNoldy (via Twitter): “this hype is already ridiculous. Doesn’t matter if it’s 8.5, 8.25, or 8.0 days out, it’s still too far out!”

Eric Fisher, the Weather Channel (via Twitter): “Talking about run to run model changes 7-8 days out seems a bit much. Let’s keep the obsessing at bay until D5 [day 5]”

Bottom line

There is potential for a significant to historic East Coast storm next week, but there is also potential for no storm. For the D.C. area, the most likely impacts would be wind and rain if there’s a storm, with perhaps a snow threat in the mountains. Of all locations, the beaches are most likely to be affected (possible impacts: coastal flooding, beach erosion).

Stay tuned to forecasts over the coming days. It will probably take until mid-to-late week or even longer for forecasters to have a decent handle on this complicated weather scenario.