Thursday, October 25, 2012

October 25, 2012:  Countdown to Hurricane Sandy

After a long hiatus (I have been on sabbatical) I have decided to re-activate the UMBC Storm Page, for obvious reasons.  I will be updating this daily as long as Sandy remains a threat.

Track Uncertainties 


Let's start with the Hurricane Center's 11 AM advisory:

NOAA Hurricane Prediction Center

Sandy is a very late season hurricane, and an intense one at that - presently a high-end Cat 2, with tropical storm force winds extending across a 200 nmi diameter vortex.  The forecast is for Sandy to accelerate northward this weekend and gradually weaken (from a combination of cooler ocean water and increased wind shear).  It is expected to dip just below hurricane intensity once it passes north of the Carolina Outer Banks.

But DO NOT focus on the exact center-line of the forecast track, but rather on the large envelope of track uncertainty, which expands with time.   The numerical prediction models have been struggling for days to reach consensus on exact track, and point of landfall.  Some of this is to be expected, since there are a great many different types of track models being used, and large errors are typical in the 5-7 day range of prediction.   To provide an example of the disagreement, the plot below (called a "spaghetti diagram") illustrates the possible track solutions from a large number of these models, compiled at 2 pm today:

South Florida Water Management District
Most models curve Sandy, as a post-tropical storm, back TOWARD the U.S. mainland.   But there is tremendous divergence, or spread, in where the storm will make landfall.

Another type of track prediction comes from what's called an "ensemble model".  This is where a single forecast model is run with many different meteorological starting points, in order to better characterize the error inherent in the simulation.   An ensemble of individual runs is generated.  An example of this is shown below, which is the ensemble model forecast for a widely used model called the GFS:

Weather Underground

Once again, lots of possibilities exist for various points of landfall, ranging from the Mid-Atlantic, New England and Canada.   The white track indicates the "center of mass" of all these various ensemble solutions.  Notice the odd kink over Long Island.  This is created when the storm slows to a stop (stalls) and essentially loops on itself, before making final landfall.

What Will The Impacts Be? 


With all the uncertainty in the track, it's still premature to nail down specific impacts for the D.C.-Baltimore region.   Right now we can only speak in general terms:

1.  Timeframe will be Sunday evening through Tuesday, with greatest impacts on Monday;
2.  Expect, at a minimum, periods of heavy rain (squalls) with sustained windy conditions

It's still too early to foretell (1) exact amount of rain, and (2) speed of maximum winds - there are too many variables involved without knowing (a) exact track;  (b) vortex intensity as it enters the post-tropical phase;  (c) storm speed (forward motion);  (d) vortex size.

Suffice it to say, given the POSSIBILITY of a landfall as close as Delaware (bringing the storm center very close to Baltimore), it's prudent to begin preparing for rapidly deteriorating conditions Sunday night, the possibility of urban and small stream flooding, and extended power outages.
The area utilities (Dominion, BGE, Pepco) have already started their initial preparations.

The Nature of The Beast 


It's important to understand that the storm that impacts the Mid Atlantic and New England is not expected to be purely tropical, but rather a hybrid type of system that combines elements of a hurricane and a nor'easter (nor'easters are intense wintertime coastal cyclones).    As Sandy approaches our area, it is expected to interact with a vigorous trough in the jet stream (currently over the western U.S.).   So while we have to not only forecast the track and intensity of the tropical storm, we also must accurately predict how the jet stream trough evolves over the next few days, and the manner in which it will phase with post-tropical Sandy.   Some hurricane-trough interactions can intensify a vortex, while others can lead to its demise.   Sandy's interaction with this trough is expected to (1) maintain a strong post-tropical storm against factors that would otherwise cause it to spin down (decay);  and (2) draw the storm westward toward the East Coast.

The process by which a tropical cyclone morphs into an extratropical system is termed extratropical transition, and a lot about this process remains poorly understood.   Transitioning storms change character in many ways - for instance, they (1) acquire weather fronts;  (2) develop strong asymmetries in the wind and precipitation (heaviest rains shift to the north and west side, highest winds develop on the east side);  and (3) expand considerably in diameter.   Some storms accelerate rapidly once they become embedded in the jet stream.  If a constructive type of phasing between a trough and vortex occurs, the storm can even re-intensify, as it begins feeding from dual energy sources (both the warm ocean and the jet stream);  this process is called rejuvenation.  These transitions are tricky to forecast because meteorological data near the surface and aloft is non-existent, or sparse, over the open ocean.

Some of you may remember Hurricane Hazel from 1954, which created quite a mess across the Mid Atlantic.  While the meteorological situation with Sandy is different in many respects, the Hazel that struck Washington-Baltimore was a storm that underwent extratropical transition in mid-October, interacting with a very strong trough in the jet stream.   The post-tropical vortex over land drew a tremendous amount of energy from the jet stream, and allowed Hazel to generate hurricane-force winds all the way to Toronto, Canada (the storm made landfall in the Carolinas).  Again, I stress that what we will experience with regards to Sandy is not a Hazel repeat, but Hazel serves as a reminder of how potent a tropical-extratropical interaction can be.

For additional meteorological perspectives on this storm, check out the blog of my colleague and hurricane expert Dr. Michael Folmer at NOAA:

No comments:

Post a Comment