Sandy found a favorable patch of atmosphere and sea water between Jamaica and eastern Cuba... so a burst of rapid strengthening dropped the central pressure dramatically and the satellite presentation looks circular and impressive. But, Cuba's mountains are ahead and the atmospheric pattern is changing in complicated ways.
The Sierra Cristal and Sierra Maestra - mountain ranges in eastern Cuba - should disrupt the core of Sandy so a somewhat weakened version of the storm moves on toward the Bahamas on Thursday. And then the atmospheric situation changes dramatically.
Just to the west of Sandy there is a robust dip in the subtropical (southern branch) jet stream that is pulling down dry air. In fact, some of that dry air is already affecting the west and south side of the circulation.
As the storm moves through the Bahamas on Friday, the jet-stream dip will start to interact with Sandy. A number of things may happen simultaneously. First, the upper-level winds of the jet stream will create an unfavorable environment for the kind of strengthening we saw Wednesday night. Second, if or while the core of the upper winds are just west of the center of Sandy, they can have the effect of enhancing thunderstorms on that side - the Florida side - of the storm meaning stronger winds and heavier rain at the coast. Third, the energy from the jet-stream can begin the transition of Sandy to a hybrid type of storm with a large center and strong winds spread out over a much larger area.
These large-diameter storms, whether they are tropical like Ike or Irene, or nor'easters like a big northeast blizzard, produce storm surge and other effects a long way from the center. In fact, the weather near the center is often not significant at all.
As a result, threatening storm surge is expected on many of the populated islands in the Bahamas. In Florida, extremely high seas will cause major erosion on the east-coast beaches, and some strong wind gusts may effect coastal sections, depending on how and where those west-side thunderstorms develop. More than local flooding from rainfall is not expected.
Then it really gets complicated. The jet-stream dip over Florida is expected to push Sandy north of the Bahamas on Friday and perhaps nudge it a bit to the west... as the high winds spread out from the center. The highest winds will likely be at or just below hurricane strength, but over a larger and larger area. It looks like Sandy will ride the Gulf Stream a long way north, so it should be able to maintain its strength as it passes the Carolinas with similar effects to Florida, but a longer duration of wind.
The most logical forecast is looking more and more like a direct hit on some part of the coast between the Delmarva and Maine. The American GFS computer model wants to turn the storm out to sea and then loop it back to Canada, but it seems to be heading directly into a strong, blocking high pressure system, which doesn't look likely. Most of the rest of the credible models, including the multi-run ensembles, bring the system directly to the coast.
The effects of the storm are likely to be widespread - many hundreds of miles - but there are still a lot of variables. How strong will it be? Maybe the upper-level winds over the Bahamas wound it so it can't recover. Does the southern jet-stream dip push it farther east, which changes the track father north? Where does the center make landfall, if it does? The biggest coastal threat, by far, will be north of that point. If landfall misses your location to the north, coastal impacts for you would be dramatically reduced, though you could still have a long duration of strong winds.
The key take-away is the same as it has been. The consensus of the best computer forecasts we have is an extremely strong storm on an unprecedented track into the Northeast or New England on Monday or Tuesday, depending on how far north it tracks before turning inland. We'll know more by Friday when we see how the interaction with the jet stream comes out and where it is at that time. But, for now, everybody along the East Coast needs to stay informed and be ready to get prepared for an extended period without power and all of the other problems cause by a hurricane-like storm.