http://www.washingtonpost.com/blogs/capital-weather-gang/post/monster...

A few computer models have conjured up a storm of epic proportions for the mid-Atlantic and/or Northeast next week. But before anyone presses the panic button, other models keep the storm out to sea.

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.”


By Jason Samenow / October 22, 2012