Joe Lundberg

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A Day Closer to the Convergence of Two Weather Systems

October 24, 2012; 9:35 AM

Wednesday, 11:30 a.m.

The story continues to grow with each passing hour, and more and more people are jumping on the bandwagon for the major weather story of the fall thus far, and that is the potential for a blockbuster storm late this weekend and early next week in the mid-Atlantic and Northeast. The players have not changed much, so it remains more or less of a waiting game at this point.

The storm in question is Sandy, a system that has been steadily gaining strength and organization over the past 24 hours, while slowly but surely advancing northward. It's now bearing down on Jamaica, and it will cross the island this afternoon before taking aim at eastern Cuba late tonight. The reconnaissance observations have been reporting a slow deepening of the storm, now down to 973mb, and while it may weaken back to a tropical storm upon crossing Cuba, in no way should we interpret that as a sign we can breathe somewhat of a sigh of relief along the entire Eastern Seaboard.

Meanwhile, back upstream, a series of upper-level disturbances embedded in the Pacific jet stream will help to carve out a fairly deep upper-level trough that will migrate from the West Coast this morning to the Rockies tonight and tomorrow, then to the Plains tomorrow night and Friday. Part of the problem in determining the eventual outcome of these two features is that some of these upstream features are back out over the Pacific, making it harder to ascertain with any degree of certainty just how strong they are or are not.

If I have seen anything 'change' from yesterday, it may be in the GFS ensembles. While the 0z and 6z operational runs of the models still push Sandy out into the Atlantic and away from the East Coast early next week, the 0z run actually brings it back toward Nova Scotia Thursday into Friday of next week. The 6z seems to take a piece of the storm and ends up forming a new one almost out of nothing in the middle of an inverted trough stretching from Cape Cod to the Adirondacks. Here's what that looks like Tuesday morning:

By early Wednesday morning, there's a clear low tracking westward across upstate New York:

The GFS ensembles, at the point where it looks like Sandy is being sent packing, seem to try to pull it back in at the last possible moment. Look at the ensemble package on Sunday afternoon:

Now look at the forecast for Monday night:

I am still leaning in the direction of this system being pulled or driven back toward the upper mid-Atlantic coast or the southern New England coast when all is said and done. I base that on the collection of operational and ensemble forecasts that have this system track back toward New England or New Jersey and into New York or Pennsylvania in the end, and the fact that the atmosphere has been given to extremes in recent weeks. There's just so much cold air diving into the back side of the trough, and so much tropical heat and humidity coming northward out of the tropics with and ahead of Sandy, that it seems hard for this system to get brush aside. It can still happen, don't get me wrong. But the evidence seems to be mounting toward the convergence of these two weather systems into an historic storm early next week.

What would happen if the ultimate 'Perfect Storm' scenario unfolds?

1) Destructive storm surge along and east of the point of landfall. Remember Monday is the full moon, too, so we're already going to be at an astronomical high tide to begin with. If the storm hits the coast, that much more water will be piled up along the southern New England coast, and probably Long Island, if not the Jersey Shore.

1a) Extensive coastal flooding and beach erosion. Wave action will build regardless as the storm comes north all up and down the East Coast. Bring this in across the shallow continental shelf, and the waves will build. It could devastate the coastline in a way not unlike Katrina did in Mississippi and Alabama.

2) Destructive winds. This storm will undoubtedly undergo a metamorphosis from a pure tropical system now to something extratropical by the time it would make landfall. What that really means is that the strongest winds will become spread out over a larger area, rather than concentrated near the center. That means a wider swath with gale-force winds, as well as hurricane-force wind gusts, especially if the pressure ends up lower than we see now, which some models have been suggesting for days.

3) Inland flooding. A storm of this nature will still have a tropical imprint all over it. Throw that moisture inland over a stalled front, and squeeze the daylights out of it from the whole column cooling and the process of that tropical air getting thrust up into the higher elevations and over the colder air drilling underneath the storm from the southwest, and you have the potential for 6-10 inches of rain in a 24- to 36-hour period.

4) Heavy wet snow. This may be the least of the concerns in the grand scheme of things, but it is a real threat. With such intense upward motion and high precipitation rates, and with the benefit of elevation, the target area of heavy snow would be central and western Pennsylvania, and perhaps portions of eastern Ohio, northern West Virginia, and the Maryland panhandle.

5) Downed trees and power lines. Add all of the above up - high winds, heavy rain, and, in some place, heavy snow with leaves still on many trees, and there would be widespread trees downed and power lines down. It could eclipse some of the problems with the storm just one year ago in the Northeast.

I am sure there would be more adverse impacts should this storm turn left and slam the coast, but I really, really, really don't want to think about all that. As I stated yesterday, this could become an economic disaster if it unfolds. Forget my petty concerns about whether or not I get to ride for a couple of days. Pure chicken feed. From a meteorological standpoint, this could be a fascinating event. But from a human standpoint, as well as a business and community standpoint, it could be catastrophic.

More updates tomorrow and Friday!

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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About This Blog

Joe Lundberg
Joe Lundberg, a veteran AccuWeather.com forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.