Saturday, October 27, 2012

October 27, 2012: The Beast of the East

We're starting to get down to details:

1.  Model Track/Landfall Location

With a bit of waffling north and south (as expected) the models appear to be converging on a solution that brings the storm into the strip of real estate from Delaware-New Jersey.   I expect that the track forecast will tighten up in the next 24 hours.   As long as this is the case, Baltimore-D.C. stay in the less energetic left-of-track semicircle:

National Hurricane Center 11 AM Track Forecast
Obviously, we would prefer to be farther away from the center...but this vortex is so large, you'll need to be 100-150 miles away before significant effects are no longer felt!    Here is the afternoon spread in track forecast (still highly divergent):

South Florida Water Management District
2.  Intensity Forecast

I believe this aspect will remain problematic.   The tropical core itself has become disorganized and has opened up;  Sandy is barely clinging to hurricane status.  The energy-yielding thunderstorms are no longer sitting over its core.   The storm is becoming tilted over by strong vertical wind shear, and dry air is invading from the south and west.   The storm is likely in its early phases of extratropical transition.   The models historically do not demonstrate as much skill with intensity as they do with track.  I'm going along with the Hurricane Center's general idea, which suggests a bit of intensification is likely prior to landfall. This is because of the anticipated jet stream interaction, which adds energy, and also the energy contained in the strong contrast between air masses (cold continental air abutting with warm tropical air).  Will the storm remain organized enough to efficiently tap this new source of energy?  I would still plan on ex-Sandy maintaining tropical storm status at landfall.

Here is a portrayal of the three elements beginning to interact, creating a hybrid vortex out of post-tropical Sandy:  (1)  strong cold front moving across the Mid Atlantic;  (2)  large trough (southward dip) in the jet stream (over the Upper Midwest);  (3) the tropical vortex itself:
Surface Weather Map, 11 AM Saturday (Intellicast)
Jet Stream Chart, 8 AM Saturday (NCAR)
3.   Wind Forecast

The left semicircle usually experiences weaker winds, because of several factors:  (1)  the storm's forward movement, or translation, which is in a direction opposite the counterclockwise swirl of wind (the storm is moving W-NW, but the winds are blowing from the W-NW - the opposing effects cancel somewhat);  (2) in the right semicircle, E-SE winds add to the direction of movement, which is from the E-SE;  (3) on the left side, winds are blowing over land (where surface friction is large) but on the right side, winds continue to scream in off open ocean, where they have built up momentum;  (4)  gustiness is reduced on the left side, because of the cooler, stable air mass in place over land.   When the air layer near the ground is stable, it resists vertical stirring, meaning less of the higher momentum air aloft gets mixed down to the surface.

Here is the NWS maximum gust forecast during the window of maximum storm impact (Tuesday morning-evening).  Notice how the Bay really amplifies the wind speed, and the highest winds shift to north-central MD as the storm tracks by to the N&W:

National Weather Service
National Weather Service
How fast will the winds diminish?  (1) If the storm stalls inland over PA, strong winds remain longer;  (2) if the storm makes progress toward the N&W, the winds will begin tapering Wed AM.   Winds will start to taper once inland, as the hybrid vortex "occludes" (the low pressure fills in and loses its jet stream support) and the supply of oceanic heat and moisture is lost.   However, this large vortex will take a while to wind down, because of its tremendous angular inertia (think of it as an enormous atmospheric flywheel).

4.  Big Rains

We're still on track for significant rainfall, in the 6" range.  This could begin falling as early as Sunday afternoon.  When large, landfalling storms approach the East Coast, the strong onshore flow of warm ocean air induces a frontal boundary along the coast.  In Sandy's case, the cold front presently moving across the Mid Atlantic will set up shop along the coast, and will become reinforced...becoming the coastal front.  There are myriad factors contributing to heavy rain:  (1) Heavy rains tend to concentrate not just around the core of this storm, but also focus along the coastal front, where there is strong, sustained uplift of air.  Heavy rains will likely spread all the way up to New England well in advance of landfall, along this front;  (2) if storm movement slows, the higher the local rain accumulation; (3) uplift of tropical air along the east slopes of the Appalachians will create "orographic enhancement" of heavy rain;  (4) the strong uplift of air in the jet stream trough will generate heavy rainfall over a wide area;  (5) a mitigating effect for Baltimore may arrive Wednesday, in the form of a surge of dry air from the southwest...shutting down precipitation over parts of the interior Mid Atlantic.


5.  Tidal Surge

Surge effects, coinciding with astronomical high tide, should be in the 3'-4' range, mainly in the northern reaches of the Bay and large main stems feeding toward Baltimore and Annapolis.   As the storm approaches from the east, W-NW winds will initially push water out of the river mouths and toward the eastern shore of the Bay.  As the storm passes to the N&W, the water will slosh back, and E-SE winds will push water into the mouths of main stems, and along the Bay's western shore.
This is NOT another Isabel, provided the storm track remains to the north of Baltimore.

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