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Weather Nerd

Sandy Now a Hurricane, and a Serious U.S. Threat

October 24th, 2012 - 12:22 pm

Tropical Storm Sandy was upgraded to a hurricane at 11:00 AM Eastern Time, and its central pressure has dropped another 5 millibars in the 90 minutes since the last reading, indicating a rapidly strengthening storm. Thankfully, Sandy will soon run out of time to strengthen further. Unfortunately, the reason for this is that Hurricane Sandy is nearing landfall in Jamaica, where the impact will be worsened by the storm’s ongoing intensification — all else being equal, strengthening hurricanes are more damaging than weakening or steady-state hurricanes of the same stated intensity at landfall.

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After Jamaica, Sandy will briefly re-emerge over water, but will then hit eastern Cuba. By the time it emerges from its passage over Cuba, the upper-level wind shear will have increased, likely preventing much further strengthening — at least as a pure warm-core tropical system. (More in a moment on what I mean by that.) All things considered, it is unlikely Sandy will, in its purely tropical phase, ever be worse than a Category 1 hurricane.

However, after Sandy crosses Jamaica and Cuba, things get really interesting — and dangerous — because the atmospheric setup is uniquely conducive for Sandy to become a bizarre and, possibly, extremely destructive hybrid storm, injecting its tropical moisture, warm core, and low barometric pressure into a dynamic atmospheric situation involving a diving upper-level trough, driven by the jet stream, and the resulting clash between warm and very cold air. We could end up with a “subtropical hurricane” — a category that isn’t even supposed to be able to exist — bashing the U.S. East Coast with fierce wind, rain and surge, while its back side produces extremely heavy snow over the northern Appalachians. It would be like a nor’easter on steroids.

This computer model forecast map tweeted by Dr. Ryan Maue, shows the temperature contrast well. This is not something you would see with a typical hurricane.

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Dr. Jeff Masters believes Sandy has the potential to be a billion-dollar disaster for the Northeast and Mid-Atlantic:

On Friday, a very complicated meteorological situation unfolds, as Sandy interacts with a trough of low pressure approaching the U.S. East Coast and trough of low pressure over the Central Atlantic. The Central Atlantic trough may be strong enough to pull Sandy northeastwards, out to sea, as predicted by the official NHC forecast, and the 06Z GFS, 00Z UKMET, 00Z Canadian, and 06Z HWRF models (00Z is 8 pm EDT, and 06Z is 2 am EDT.) However, an alternative solution, shown by the 00Z ECMWF, 06Z GFDL, and 06Z NOGAPS models, is for Sandy to get caught up by the trough approaching the Eastern U.S., which will inject a large amount of energy into Sandy, converting it to a powerful subtropical storm that hits the mid-Atlantic or New England early next week with a central pressure below 960 mb and sustained winds of 60 – 70 mph. Such a storm would likely cause massive power outages and over a billion dollars in damage, as trees still in leaf take out power grids, and heavy rains and coastal storm surges create damaging flooding. The full moon is on Monday, which means astronomical tides will be at their peak for the month, increasing potential storm surge flooding. A similar meteorological situation occurred in October 1991, when Hurricane Grace became absorbed by a Nor’easter, becoming the so-called “Perfect Storm” that killed 13 people and did over $200 million in damage in the Northeast U.S.

Depending on Sandy’s eventual track, strength and structure — all of which are very much in flux right now — there is some potential for this storm to bring a real nightmare scenario to the highly vulnerable New York City region, with storm surge and extremely high surf getting funneled into the harbor (at astronomical high tide, no less!). This is just a hypothetical at this point, and it probably won’t happen precisely that way, but it’s concerning to me that the scenario is on the table, and is one of many reasons that Sandy bears watching.

Not to get all Nate Silver on you, but it’s important to recognize and articulate the vast uncertainties here. There are a variety of scenarios in play, ranging from good (out to sea!) to bad to very bad to potentially catastrophic, but there are also an enormous number of variables, particularly in a dynamic and highly unusual atmospheric environment like this. Geoff Fox, an excellent TV meteorologist at Connecticut’s one major news station based on the coastline, WTNH Fox affiliate, WTIC, gets at this in his latest blog post:

Here’s my problem with Sandy. I can look at all the computer runs and sense something’s wrong. Too many things are happening I’ve never seen before. You’d think in nearly 30 years here I’d seen it all.

I spoke to Bob Hart on my way home. He’s a professor at FSU. No one knows more about tropical weather. … Bob said there were some storms in the 1800s that produced what is Sandy’s worst case scenario.

There’s a reason stuff like this happens infrequently. A huge number of conditions must come into alignment. If one or two aren’t as forecast it all goes to hell! With Sandy this far away in time and distance there’s no parameter that’s not suspect. …

We talked about the weaknesses of models. … Tropical systems are really small compared to the weather we most often see. The computer models are often too coarse to understand the complexity of these tightly wound storms that mathematically can fall between the cracks.

Dr. Masters thinks, or at least hopes, the computer models will start to get a better handle on Sandy soon:

The models vary significantly in their predictions of when Sandy might arrive along the U.S. coast. The 06Z NOGAPS model predicts Sandy’s heavy rains will arrive on North Carolina’s Outer Banks on Saturday, then spread into the mid-Atlantic and New England on Sunday. The 00Z ECMWF model predicts that Sandy’s rains won’t affect North Carolina until Sunday, with the storm making landfall in New Jersey on Monday night. The GFDL model is in-between these extremes, taking Sandy ashore in Delaware on Monday morning. The trough of low pressure that Sandy will be interacting with just moved ashore over the Western U.S. this morning, and got sampled by the 12Z (8 am EDT) set of land-based balloon-borne radiosondes for the first time. One of the reasons the models have been in such poor agreement on the long-term fate of Sandy is that the strength of this trough has not been very well known, since it has been over the ocean where we have limited data. Now that the trough is over land, it will be better sampled, and the next set of 12Z model runs, due out this afternoon between 2 pm – 4pm EDT, will hopefully begin to converge on a common solution.

Stay tuned. I’ll be tweeting about Sandy as well at @brendanloy. (Fair warning: most PJM readers may not care for my political tweets. If so, just ignore those, and look for the hurricane-related tweets. :))

AFTER THE JUMP: Sandy’s potential impact on the presidential election.

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