The Weather Channel’s Hurricane Specialist, Bryan Norcross, does an excellent job of explaining what’s happening:
Isn’t it strange that a hurricane in the Bahamas would somehow turn into a monster mega-storm and slam into the Northeast at the end of October? Aren’t hurricanes supposed to weaken as they move north over cold water? What the hell is going on?
The answers are… yes, yes, and we’re not completely sure. This is a beyond-strange situation. It’s unprecedented and bizarre. Hurricanes almost always bend out to sea in October, although there have been some exceptions when storms went due north, but rarely. No October tropical systems in the record book have turned left into the northeast coast.
The strong evidence we have that a significant, maybe historic, storm is going to hit the east coast is that EVERY reliable computer forecast model now says it’s going to happen. The only way we can forecast the weather four or five days days from now is with the aid of these super-complex computer programs run on supercomputers. The two best, the European and the U.S. GFS (Global Forecast System) run by NOAA, are now in reasonable agreement that there IS going to be an extraordinarily unusual confluence of events that results in a massive storm.
The upper-air steering pattern that is part of the puzzle is not all that unheard of. It happens when the atmosphere gets blocked over the Atlantic and the flow over the U.S. doubles back on itself. Sometimes big winter storms are involved.
The freak part is that a hurricane happens to be in the right place in the world to get sucked into this doubled-back channel of air and pulled inland from the coast.
And the double-freak part is that the upper level wind, instead of weakening the storm and simply absorbing the moisture – which would be annoying enough – is merging with the tropical system to create a monstrous hybrid vortex. A combination of a hurricane and a nor’easter.
At least that’s what the models are saying. And since all of the independent models are saying something similar, we have to believe them and be ready. …
The hope we have is that the computer models are not handling this unusual situation well, and are predicting a stronger storm than we get. But, we can’t bet on it. Even a weaker version will likely mean a nightmare for millions.
Another excellent write-up about the ingredients creating this unique situation can be found at this site, courtesy of Bob Henson:
What makes Sandy so unusual? And how might it rewrite weather history?
As I write, Sandy is in the Bahamas…and projected to move northeast from the Bahamas, as hurricanes so often do. But then — thanks to a strong upper-level storm dipping into the eastern U.S., and an extremely strong center of high pressure toward Greenland — the storm is expected to take a grand counterclockwise loop and arc northwest, which would bring it to the U.S. coast by Tuesday and drive it well inland.
The official National Hurricane Center forecast issued at 5:00 p.m. EDT Thursday has the center of Sandy making landfall early Tuesday in New Jersey while moving toward the northwest. To say this path would be unusual is a major understatement. Few if any hurricanes in the last 100 years have struck the mid-Atlantic or New England with quite such a dramatically hooking path …
Unfortunately, this curving path would also maximize Sandy’s time close to the warm waters of the Gulf Stream. … [T]emperatures are running 1-2°C above average off the mid-Atlantic coast. …
Leading forecast models are producing spectacularly low pressures at the center of Sandy. … While a couple of hurricane landfalls in Florida have produced pressures in this range, most cities in the Northeast have never reached such values … [T]he landfall pressures with Sandy may be less extreme than predicted, because of known model difficulties in handling tropical systems moving into midlatitudes. However, all-time records could still be well within reach at some locations. …
Sandy’s hooking path and low pressure will help funnel mammoth amounts of seawater toward the Atlantic coast north of wherever the center makes landfall. Thanks to the massive high pressure to the north, and a separate low pressure center far to the east, models project a ribbon of easterly winds all the way from Europe to the mid-Atlantic as Sandy approaches. In addition, there’s a full moon on Monday afternoon, which will exacerbate any storm tides. All of these factors should help strengthen a rare and powerful wave- and surge-generating machine. …
New York City is at particular risk for serious impacts from storm surge. If Sandy moves inland on the New Jersey coast, huge amounts of water will flow toward New York’s harbor, so predictions of storm surge will be critical. … “I am personally very concerned about storm surges in New York City,” says [Philip Orton of the Urban Ocean Observatory of the Stevens Institute of Technology] … “City managers and scientists agree that we’re not ready for a 100-year flood event, in major part because we haven’t had one in well over 100 years,” Orton says. …
Inland flooding is also a serious threat with Sandy. NOAA guidance and recent model runs suggest that 5–10” of rain could fall along and near the storm’s path. Heavy snow is even possible along some of the highest terrain of the Appalachians. …
Perhaps the biggest potential threat from Sandy for millions of people inland is the risk of power outages and tree loss. Winds may not be sustained above hurricane force, especially inland, but strong gales of 50–60 mph with higher gusts could easily stretch across a vast swath of the most heavily populated part of the country. These winds may be enough to bring down many trees and power lines, especially when accompanied by soil-loosening rains. If enough utility customers are affected, it could take days if not weeks to restore power to some areas — potentially affecting the November 6 elections.
Mike Smith — again, a climate change skeptic, FWIW — puts it this way:
A very prominent and respected National Weather Service meteorologist wrote on Facebook last night, “I’ve never seen anything like this and I’m at a loss for expletives to describe what this storm could do.”
Yes, I’ve never seen anything like it either nor have our modern meteorological tools. As I wrote yesterday afternoon, we don’t know whether our tools are up to the task because no storm of this nature has occurred in the modern meteorological era.
That last bit is the hope we can still cling to — that the computer models are all getting it wrong, because they’ve never seen a storm like this before. But that’s not something we can assume. We can hope for it, but we have no right to presume it will be true. So folks in harm’s way have got to prepare.
How should they prepare? Well, listen to trusted local information sources, including local officials and media sources, for starters. In addition, here are some helpful resources:
Also, if you have travel plans that require you to be on an airplane in the affected region this weekend or next week, read this and take it to heart (more smart advice from Mike Smith). Maybe even print it out and bring it on your trip, if you aren’t just cancelling/postponing your trip altogether.
Above all, if you’re in a flood-prone coastal location, be prepared to “Get The Hell Out” if you’re ordered to evacuate ahead of the storm. And if you’re in a flood-prone inland location, be ready to leave on a moment’s notice if a rain-caused flash flood threatens.