Friday, October 26, 2012

October 26, 2012: Sandy's Baltimore Impact Becoming More Certain And More Severe

What The Prediction Models Say:

The prediction models continue a westward trend in the point of landfall;  the official hurricane center forecast issued at 11 AM today calls for landfall of a Cat 1 storm over Delaware early Tuesday morning:
National Hurricane Center
As we discussed yesterday, note the very broad cone of uncertainty.  The various models still remain widely divergent:
South Florida Water Management District do the various ensemble runs of individual models:
Weather Underground
Nonetheless, because of the consistent wholesale westward shift in track guidance - closer to Baltimore - confidence is growing for increasingly severe impacts in our region.   Bear in mind a couple things:  (1)  don't focus on the center point of the hurricane center prediction, as this is a very large storm, and the most severe effects will not be confined close to center;  (2) we still have another 5-6 cycles of model runs to go before landfall.  These runs are made every 12 hours, and I would not be surprised to see further shifts in the landfall point, perhaps even back toward the eastern side of the guidance spread.  Some back-and-forth movement of tracks is typical for this timeframe in the numerical track guidance.

Likely Impacts In the Baltimore Metro:

Again, because we are 2-3 days out from passage of the vortex past our region, we can't get too specific here.   But some generalities are starting to emerge.

(1)  Tropical Storm force winds.  If we remain on the LEFT SIDE of the track, the overall energy will be less than on the right side.   This means lower wind speeds and gusts.   This applies as long as the storm is moving briskly.  Here is a very preliminary estimate of the sustained winds on Tuesday morning and early afternoon, the likely period of peak activity:
National Weather Service
These winds are tropical-storm intensity (40+ mph sustained, with higher gusts, perhaps in the 50-60 mph range).  The highest winds will be over the Bay and along the eastern shore of the northern Bay.  These winds will initially push water OUT of the tidal mainstem rivers draining along the western shore of the Bay, but pile up a surge of several feet along the Bay's eastern shores and mouths of major tributaries.  Winds will also be stronger in the high elevations of the Maryland Piedmont and Blue Ridge;

(2)  Heavy rain is expected, to the tune of 6"-10".  Here is the latest graphic illustrating likely amounts:
National Weather Service
These rains will be fairly steady with intermittent, heavy bursts, for a 24+ hr period.  So there is certainly the possibility of urban and small stream flooding, as well delayed flood crests in the main stem rivers

(3)  Timing of impacts.  Because this is a large storm, impacts will begin long before the center moves onto Delaware.  Conditions will begin deteriorating Sunday night...meaning arrival of rain, and a freshening of the breeze.   Winds and rain intensity will continue to ramp up through Monday, peaking early Tuesday AM.   As long as the storm remains on the move, winds should begin to abate early Wednesday AM, and rain will slacken.    The winds may die down particularly quickly, from a combination of the storm moving away from Baltimore, and fairly rapid weakening of the storm once inland.

All of this information is predicated on the latest (11 AM) hurricane center track and landfall information, which is the best guess offered by the hurricane prediction pros - a mental consensus of the dozens of track and intensity forecast models.  There are models that suggest alternate scenarios, including (1) a stronger storm at landfall than the hurricane center is forecasting;  (2) storm tracks further north and east of our region;  (3) the storm becoming stationary for a time along the coast, and even executing a tight loop - which would greatly prolong the window of severe weather.   So much of the outcome depends on how the post tropical phase of Sandy interacts with the large, intense jet stream trough, and the vagaries of its poorly-understood extratropical transition process.  This interaction has not yet started.  There are few historical precedents for this type of unusual tropical-extratropical merger to gain insight from.  There is one model (the European Center model) that continues to portray an absolutely frightening storm, the intensity of which has never been experienced in our region - but there is some reason to discount this scenario as unlikely.

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