A look at weather extremes and the big-picture climate connections.

Hurricane Sandy Poses Growing Threat To East Coast

Tropical Storm Sandy turned into Hurricane Sandy on Wednesday morning and the possibility that it will interact with several other weather systems to create a rare and damaging storm for the Mid-Atlantic or New England early next week has become more credible since Tuesday, as new computer model runs now show a stormier scenario. However, this is still an extended-range forecast, and it could change dramatically during the next few days.

A range of forecast tracks generated by different runs of the GFS computer model. 
Click to enlarge the image.
Credit: NCAR.

What forecasters know with relative certainty is that Sandy is going to make landfall in Jamaica on Wednesday, lashing that island with hurricane force winds and heavy rains, before charging northeastward into eastern Cuba. It will also dump heavy rainfall on Haiti and the Dominican Republic, which could cause life-threatening flooding. On Friday, Sandy is expected to be churning through the eastern Bahamas, sparing southeast Florida the worst of its impacts, but still bringing strong winds to the east coast of the state. Tropical storm watches are up for coastal areas of southeastern Florida, including Miami and West Palm Beach.

After Sandy moves away from the Bahamas, the are two main scenarios that meteorologists are wrestling with, and forecasters' dilemmas illustrate the difficulties involved in interpreting computer projections. This is especially the case right now, since one of the most reliable medium-range forecasting models has repeatedly predicted a worst-case scenario for New England, with a storm of hurricane intensity — although with a structure more like a conventional nor'easter — bearing down on Long Island. With a full moon coming up to raise tides, it won’t take much onshore wind to cause significant coastal flooding, and the storm could also bring a large blast of damaging wind inland, along with very heavy rainfall.

Such a scenario looks plausible partly due to an unusual, independent weather pattern projected for early next week: a large dome of high pressure between the Canadian Maritimes and Greenland, which may act as a block (it's tecnically known as as a “blocking high”), preventing Sandy from moving out into the open ocean, and instead helping to direct it northwestward, back toward the U.S.

Recent studies have shown that blocking highs have appeared with greater frequency and intensity in recent years, which some scientists think may be related to the loss of Arctic sea ice as a result global warming.

Computer model simulation from the European model, showing a very intense storm located near Southern New England on October 30.
Click to enlarge the image.
Credit: Weatherbell Analytics.

In addition, a deep dip in the jet stream, which will trigger the coldest air of the fall season, is expected to set up near the eastern seaboard. This jet stream, and weather disturbances riding along with it, could help pick up Sandy, give it an additional shot of energy, and turn it into a non-tropical superstorm similar to major nor’easters of the past, such as the “Perfect Storm” of 1991, which also occurred in late October.

That's the bad news: the far more benign forecast scenario would take Sandy out to sea, while a much weaker and more ordinary nor’easter would hit the Eastern Seabord early next week.

As was the case on Tuesday, both forecast scenarios — along with others — are very much in play, but the odds of a major strike on New England have increased somewhat since then.

On Wednesday morning, the Hydrometeorological Prediction Center, which is part of the National Oceanic and Atmospheric Administration (NOAA), said that confidence in the stormier scenario was growing after new computer model runs, but that much will depend on the interaction between the coastal storm and a ripple of upper level energy riding the jet stream, which is known as a short wave. “THE DIFFERENCE IN THE SPACING BETWEEN THE COASTAL LOW AND THIS SHORT WAVE IS SMALL IN THE GRAND SCHEME . . . BUT WILL MAKE ALL THE DIFFERENCE IN WHETHER THE FORECAST AREA IS IMPACTED OR NOT. THIS REMAINS A LOW CONFIDENCE . . . BUT POTENTIALLY HIGH IMPACT FORECAST,” HPC forecasters said.

Later on Wednesday, the HPC issued a more ominous extended forecast discussion that indicated forecasters' growing concerns (note that the ALL CAPS style is standard in NWS discussions). "THERE IS INCREASING CONSENSUS AMONG THE GUIDANCE FROM THE VARIOUS MODELING CENTERS...  THE ENTIRE ATLANTIC BASIN SEEMS DESTINED TO BECOME DOMINATED BY BLOCKY HIGHS AND LOWS, WITH DECREASING WIGGLE ROOM FOR EVEN LARGE FEATURES LIKE SANDY. THE BLOCKING IS KEY TO THE OPPORTUNITY FOR THE INTERACTION BETWEEN THE ATLANTIC STORM OF TROPICAL ORIGIN AND THE NORTH AMERICAN VORTEX OF POLAR ORIGIN."

Weather Channel hurricane specialist Bryan Norcross wrote that he has never seen a storm behave like what many computer simulations are showing. “What seemed like a fluke of an idea — a hurricane-like system hitting the northeastern U.S. — is gaining credibility,” Norcross wrote. “The spectacularly unusual confluence of events is the shape and orientation of the dip in the jet stream that is forecast to develop over eastern North America over the weekend — oriented in such a way to pull Sandy inland instead of pushing it out to sea, and the presence of a strong tropical or subtropical system where it can get pulled in. That's so bizarrely unusual that I can't think of another event like it.”

The bottom line is that if you live anywhere along the East Coast of the U.S., particularly in the Mid-Atlantic or Northeast, you should be keeping close tabs on weather forecasts during the next few days.

Stay tuned to Climate Central as this storm threat evolves. We'll have a roundup of forecasts and computer model projections on Wednesday afternoon, with updates as needed.

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