Take a hurricane moving up from the south. Mash in a colder storm moving in from the west. Add a ridge of high pressure extending through the atmosphere above the northeastern Atlantic Ocean and Greenland, blocking the typical flow of the jet stream. That's the recipe for what will become "Post-Tropical Storm Sandy" or, as it has more colloquially been dubbed: "Frankenstorm."
The result of all that atmospheric blocking is that instead of turning away from land and heading out into the northeastern Atlantic Ocean, this particular storm is going to be pushed ashore somewhere between Delaware and Long Island, New York. At the same time, it will be merging with the cold air coming in from the west—and that means Sandy will be the unusual hurricane that ends up producing snow in its western reaches.
And what a reach. Sandy's swirling circulation and high winds will reach from Ohio and the Great Lakes region all the way to the New England Coast and down into the Carolinas. "A large number of folks over a very large area, many states, are going to experience a significant wind event, strong tropical force winds to perhaps hurricane winds," warns James Franklin, the branch chief of NOAA's National Hurricane Center on a conference call with reporters. The high winds increase the chances of downed trees and power lines; several days of ground-softening rain are likely to increase the risk. As much as 10 inches of rain may fall to the east of wherever the center of the storm comes ashore, and NOAA is currently predicting as much as two feet of snow for the mountains of West Virginia and Pennsylvania.
The unusual combination of rain and snow are not likely to hit until early next week. NOAA is currently predicting landfall by Monday night, which is bad timing for storm surges; the full moon is on October 29 so high tides will be even higher than other times of the month. The high pressure off to the east means the storm will land directly perpendicular to the coast, rather than delivering a more typical glancing blow the way Hurricane Irene last year. It's possible a city like New York could see storm surges of as much as 10 feet.
Plus, Sandy is a huge, lumbering beast of a storm, likely to linger for two or even three days. That means much like the "Perfect Storm" of 1991 the danger is not just coastal flooding, but also inland flooding as the system dumps rain on creeks, streams and rivers.
And this storm will be picking up extra fuel courtesy of the buildup of greenhouse gases in the atmosphere. Waters off the Eastern Seaboard are a degree or two warmer than usual, helping fuel the storm. "It will get a little bit of extra kick from the slightly warmer than the normal waters it will be tracking over," Franklin says. Plus, the meltdown of Arctic sea ice this summer may help to trap the storm in place for a while.
It remains to be seen what, if anything, Sandy will do, and where. It has the chance to be the storm that comes ashore with the lowest pressure ever recorded, a sign of strength that will spread strong winds across the whole post-tropical storm. It is a testimony to the computer modeling capacity of government weather forecasters as well as the eyes in the sky that can track such storm that warnings of this storm are appearing almost a full week before it hits. (Mind you, these satellites are nearing the end of their useful lifespan and budget woes are delaying their replacement.)
With the storm still three or more days away from landfall, the margin of error for NOAA's track forecast is as much as 200 miles, or just a little bit less than the distance between Washington, D.C. and New York City. But it is already clear according to Craig Fugate, the administrator of FEMA, that "it is going to be a challenging storm because it is going to continue to grow in size and the impacts are going to be very widespread and go well inland." In other words, batten down the hatches, get your supplies in order and be ready for technology failures, whether electricity outages or a loss of cellphone service.
Oh and, as Franklin adds: "After Sandy has come and gone, we'll still have another whole month of hurricane season to go. This may well not be the last system we deal with this year."