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Saturday, October 27, 2012

Hurricane Sandy forecast from meteorologist Paul Douglas

Here is the Alerts Broadcaster Update I just sent out to our corporate clients. In the name of public safety and erring on the side of caution I wanted to share this with the general public:

The stage is set for an historic storm over the Mid-Atlantic and southern New England early next week, an unfortunate convergence of meteorological (and astronomical) events, leading to the formation of a huge storm capable of widespread damage, from winds, waves and inland flooding. I am not an economist, but based on where the highest impacts are predicted, I see no reason why this won’t be a billion dollar storm, capable of extended power outages, beach erosion and coastal flooding that may take weeks to repair and restore.

* Maps below include “SLOSH” predictions for a Category 1-2 hurricane for New York City, courtesy of the New York City Natural Hazard Mitigation Plan (2009).

What makes Hurricane Sandy different?

1). Size of the storm. Sandy will be nearly 500 miles wide within 48 hours, with a 300 mile radius of tropical storm force winds, and a 100-200 mile radius of hurricane-force wind gusts Sunday night into Tuesday.

2). Duration of the storm. Unlike Irene, which moved quickly ashore late August, 2011, Sandy, a hybrid hurricane/Nor’easter, will move slowly, “hooking” westward, prolonging the amount of time sustained, high winds can carve out a significant storm surge, and extending the period of time when flooding rains will fall well inland.

3). Astronomical Bad Luck. A full moon is coming Monday afternoon, about the time Sandy will be pushing toward coastal Delaware or New Jersey, increasing tides, making the resulting storm surge even more severe.

Hurricane Sandy has weakened slightly in the last 24 hours, now a minimal Category 1 storm with 75 mph sustained winds. But don’t be fooled – a jolt of energy from an approaching surge of much colder, Canadian air will reenergize Sandy, causing it to strengthen significantly into the weekend. The analog is Hurricane Grace in late October, 1991 – some of the energy and moisture from Grace went on to fuel “The Perfect Storm”, which battered New England with hurricane-force winds on Halloween, 1991. There is growing speculation that “Sandy” could be considerably worse than “Grace” in 1991, and Irene in late August of 2011, due to the size and duration of the storm, as well as a full moon (Monday) - and the population centers impacted. Image: Naval Research Lab.

Tropical Storm Watch. NOAA NHC has extended a Tropical Storm watch from Savannah northward to the Outer Banks of North Carolina. I expect this to be upgraded to a Hurricane Watch for the Outer Banks by tomorrow morning, and Hurricane Watches/Warnings may have to be issued as far north as Long Island, Providence and Cape Cod by Sunday night and Monday, depending on the final track and intensity. It’s important to follow instructions from local authorities, and now wait until the last moment to evacuate. I’m especially concerned about barrier islands, from North Carolina to Long Island, and the amount of time necessary to safely evacuate residents from the most vulnerable regions, within 5-8 feet of sea level. My advice: get out ahead of the curve and move people faster, especially those near sea level. Image:Ham Weather.

Model Ensemble. There has been a slight northward shift in the models in the last 12 hours. Although the most likely track of Sandy brings the core/eye of the storm into coastal Maryland or Delaware late Monday or very early Tuesday, a slight northward wobble in the track would bring the strongest winds/surf into New Jersey and metropolitan New York/Long Island. It’s this scenario that I’m most concerned about. Map:Ham Weather.

* The odds of serious flooding/storm surge impacts in New York Harbor and the southern coastline of Long Island have increased from 30 to40% in the last 12 hours. Your contingency plans need to include metro New York and Long Island.

Most Likely Landfall. Based on model trends the most likely landfall is near Bethany Beach or Rehoboth, Delaware during the very early morning hours on Tuesday, September 30. But keep in mind that the strongest winds – highest storm surge – will come within 24 hours of landfall, so conditions will deteriorate Sunday, with the worst wind/wave/flooding Monday into Tuesday. Model guidance: NHC and Ham Weather.

European Solution. The ECMWF has done the best job with Sandy, to date, so I’m putting more weight on this solution. The latest guidance shows Sandy tracking almost due west Monday into early Tuesday, pushing a surge of water into southern New England, Long Island and New York Harbor, before the eye/center of this hybrid storm comes ashore over southern New Jersey or Delaware Tuesday morning. This constitutes a slight northward shift to the track from previous model runs. It’s worth pointing out that NHC, the National Hurricane Center, is putting much more weight on the ECMWF and the GFS models for their (official) forecasts, in essence splitting the difference between the two. Images above: WSI Corporation.

NOGAPS Model. The Navy’s NOGAPS model has a more northward solution, one that prove much worse for southern New England, from Cape Cod to Martha’s Vineyard, Long Island and metro New York City. As such it falls fairly closely into line with the GFS model (below).Map above: Weather Bell.

Why Boston Residents Can’t Let Their Guard Down Just Yet. There’s still a (disturbingly) large spread in model solutions. By this time, within 72-82 hours of landfall, you would expect more agreement between the models, but that’s not to be, at least not yet. The GFS is something of an outlier, with a (much) farther northward track, one that would prove damaging for Boston and Cape Cod. With a sustained (offshore) wind flow over New York this solution would actually spare New York Harbor and Long Island from the worst storm surge, but Boston might wind up enduring a 1 in 100 year storm/flood. Right now our official track forecast splits the difference between the ECMWF and the GFS, with probable landfall from Delaware into southern New Jersey.

Near-Blizzard Conditions. Well inland, enough cold air may be in place for a foot of snow or more over the hills of West Virginia, enough snow to plow and shovel over parts of western Pennsylvania. Map:

Why Sandy Is No Irene. Irene was a big fizzle, at least for New York City, where the storm surge wasn’t nearly as severe as many had feared. Irene moved quickly, 30 mph to the north/northeast. There was less time for extended winds to create a powerful storm surge for New York Harbor and Long Island. By comparison Sandy, or what Sandy mutates into, will move slower, not racing northward, but curving westward, leaving more time to generate a significant storm surge. The surge is created by the combination of sustained winds, low pressure and underwater geological considerations, the slope of the continental shelf. The result: a bulge of water some 5-15 feet above normal high tide that pushes ashore near the eye/center of the storm. Image: Ham Weather.

The New York Bight. New York City is at a geographical disadvantage; the combination of sustained east/southeast winds can funnel water toward the Hudson Valley Shelf. This “piling effect” can compound a storm surge, especially when a major storm tracks 25-100 miles south of Manhattan. Expect to hear more about the New York Bight and how this might magnify a predicted storm surge Monday into early Tuesday.

“SLOSH” Analysis. The Office of Emergency Management utilizes a “SLOSH” Analysis to predict which regions of New York City are most vulnerable to a storm surge. Here’s a description from the New York City Natural Hazard Mitigation Plan (2009):

Vulnerable Areas In A Category 1 Hurricane. Lower Manhattan, portions of Brooklyn, Queens and Staten Island would be inundated during a Category 1 storm, especially one that passes just south of New York, with southeast winds at the surface, funneling water into the New York Bight. Maps courtesy of the New York City Natural Hazard Mitigation Plan (2009).

Vulnerable Areas In A Category 2 Hurricane. The area at risk expands from a Category 2 storm, with sustained winds over 95-100 mph. This seems unlikely right now – if Sandy does impact metro New York and Long Island it will probably be as a Category 1 hurricane, or equivalent Nor’easter.

Summary: Meteorological and astronomical conditions are aligning for a 1 in 100 year storm for much of the Northeast and Mid-Atlantic region. Although a precise landfall estimate from Sandy remains unclear, residents from coastal Maryland and Delaware northward to New Jersey, New York City, Long Island, Providence, Cape Cod and Boston need to be on high alert, with contingency/evacuation plans ready to go. The greatest probability of landfall from Sandy is coastal Delaware or south Jersey Monday night and early Tuesday, but the trend is for models to nudge this landfall zone north over time.

We know Sandy is going to be an historic storm – and in spite of lingering uncertainty focused on the exact location of landfall, it would be prudent to take all necessary precautions to safeguard life and property, especially coastal regions, within 5-8 feet of sea level, from Ocean City, Maryland and Rehoboth, Delaware northward to coastal New Jersey, metro New York, Long Island and Cape Cod. The impacts from Sandy may be felt well into November.

Paul Douglas, Senior Meteorologist, Alerts Broadcaster LLC, Minneapolis, MN

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