Wednesday, August 24, 2011

August 24, 2011: What Will Irene Bring To Baltimore? Epic Rainstorm Possible Across New England

We're getting to the point where we can begin contemplating some local effects of Irene.

Winds:  Here is an image from NWS Sterling that is very interesting.  It is this morning's prediction of peak sustained winds across our region on Sunday as Irene slides by to the east.   This shows the very tight gradient in wind.  According to this forecast, those in Baltimore won't have to go very far east to experience tropical storm force winds (in this image, 43 kts = 50 mph).  This is a great example of how the Bay enhances the winds in a hurricane.  On the west side of the storm, Irene's winds will be northerly, and notice how the winds really scream down the Bay.  This is in contrast to Isabel, which slid by to our west, sending strong winds and a surge moving up the Bay from the south.

Could Baltimore still experience tropical storm force winds (> 39 mph)?  Possibly.  This depends critically on track.  It also depends on Irene's intensity when she moves by.  It also depends on the size of the storm, which is actually linked to intensity:  When a hurricane weakens, the winds near the center diminish, but the radius of tropical storm force winds actually expand outwards. depends on the storm's speed of forward motion.   To the east of the storm's track, you add the wind speed to the speed of movement (both effects reinforce).  To the west of the storm track, you subtract the speed of movement from the wind speed (the effects tend to cancel).  So to minimize the wind speeds over Baltimore, we actually want Irene to move faster!

Regional prediction of winds on Sunday morning due to Irene's passage to our east.

Rainfall:  I still expect this could be a concern, starting as early as Saturday morning.  Big storms like Irene, even when along the coast or offshore, can circulate Atlantic moisture inland well in advance of the actual storm center. Moderate to heavy rain may begin spreading up the East Coast 24-36 hours ahead of the storm. The models are certainly presenting this scenario:

GFS model forecast for Sunday morning showing moderate-heavy rain spreading along the East Coast hundreds of miles north of Irene's center.
But the other aspect to note is the very tight gradient in rainfall.  Just a few miles can make all the difference between drizzle and a real soaker.   This is typical of hurricanes, which tend to generate very compact regions of hazardous weather.

Now slightly further north and east of Baltimore, the ingredients may come together for a monstrous rain storm - from NJ northward.   By this time, Irene will likely be undergoing a process called extratropical transition, whereby the storm's energy and tropical moisture begin interacting with mid-latitude weather systems.  The GFS model (below) suggests that Irene will be absorbed into a trough in the jet stream swinging across the Great Lakes.  This adds a lot of energy to the system.  Additionally, a jet streak over New England will enhance the ascent of moist air.  And...Irene will likely join forces with a stalled cold front over the Mid Atlantic.  Vigorous uplift along the front, which may become reconfigured as a coastal front, will further enhance the rain. 

GFS model forecast for Sunday morning, showing Irene linking up with a jet stream trough and jet streak.
Below is a figure from the Hydrometeorological Prediction Center that shows the predicted rain accumulation for the next five days...the swath of rain in excess of one foot will pose serious problems for New England:

HPC heavy rain prediction during Irene's expected extratropical transition.
You might recall what Floyd did in 1999 - massive flooding (12"-15") along the Eastern Seaboard from North Carolina northward.  As you can see below, Floyd's (1999) track is virtually identical to that predicted for Irene:

Storm Surge:  Not a concern for the Bay;  the strong northerly winds will actually push the water out of the Bay, creating an abnormally low tide.   By the way, it's very important to consider the height of the tides when predicting storm surge.  This Sunday, with the new moon, Earth will experience a spring tide, which pushes high tides even higher.  This could exacerbate ocean inundation along the coast.

Tornadoes:  Not a concern for Baltimore.  Most hurricane-spawned tornadoes develop only when the right front quadrant lies over land. 

Bottom Line:  With the storm predicted to be tracking so close to the coastline, heading toward the beach this weekend is a very bad idea!

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