Friday, August 26, 2011

August 26, 2011 Baltimore's Impacts From Irene Remain On Course

Here is the current surface map (from Intellicast).  Irene is most conspicuous, and notice the weather front the storm will interact with:

But there will be other interactions with Irene, including the jet stream level;  the storm will become embedded in a trough (region of low pressure) moving towards us from the Great Lakes.  Both the front and the trough will energize the storm, even as it is moving over cooler water, and this effect could sustain the storm as it moves into New England, and enhance the rainfall.   Here is the forecast map for tomorrow (Saturday) evening:

Notice how the weather front begins to reorient along the coastline.  As tropical humidity is lifted along the front, very heavy rains will develop.

All of this is part of the complex process called extratropical transition - a tropical storm begins to lose some of its tropical identity, and take on characteristics of a mid-latitude weather systems.

Now let's look at the likely impacts - using NWS graphics.

Official Watches and Warnings - As of 10 AM today:

Winds:  As of 10 AM today:

Baltimore is on the edge of low and moderate wind impacts...therefore a slight shift in storm track or change in storm intensity will change the impact level.   We are expecting sustained winds 30-40 mph over Baltimore's metro west, 35-45 mph sustained over metro east, with higher gusts.

Rainfall:  We've recently had considerable rain, so soils are moist.  Given the intense rain rates in hurricane rain bands, and frequent rain band passage, it won't take much to cause local flash flooding.  I expect 2"-4" broadly over Baltimore, more to the east (3"-6"), much more over the Delmarva (12" or more) - click on the map below to zoom in:

Storm Surge on the Bay:   This is complicated somewhat by the fact that wind direction during the strong wind period will be shifting throughout the event.  Initially, northeasterly winds may push water along the Bay's western shore, through the day on Saturday.  Saturday night and early Sunday morning, the strongest winds blow from the north, pushing water down and out of the Bay.  During the day on Sunday, the winds switch to westerly and begin to slacken, but this may lead to flooding of the Bay's eastern shore for a time.

Timing of Impacts:   Irene will likely be a Cat 1 storm as the core passes up Maryland's coastline tomorrow night and early Sunday.  It has a large circulation, so the weather impacts will take time to unfold, and time to wind down.  Rains will begin tomorrow afternoon.   The most intense period of rain and wind will be after midnight Saturday.   But the storm is progressive i.e. it will not stall, as it will get pushed along by south-southwesterly winds in the jet stream;  in fact, it will begin to accelerate toward New England.

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