Thursday, August 25, 2011

August 25, 2011: Irene's Threat Intensifies Across Baltimore Region

Overview:   Prediction models have trended toward pulling Irene's track back westward, bringing Irene's core close to the Maryland coastline.   While the coast is 125 miles to our east, Irene has a very large wind circulation, larger than average.   Presently a major category hurricane, Irene is expected to weaken to Cat 2 or 1 by the time the storm passes through eastern Maryland.  However, Irene may be slow to wind down because of its high initial intensity, and the fact that much of the circulation will still be over warm Gulf Stream water.  And, as hurricanes weaken, their wind circulation tends to while winds may weaken in the core, winds on the fringe (i.e. central Maryland) may intensify. 

An additional factor is that this storm will likely undergo extratropical transition as it moves through Maryland.   The prediction models are swinging a jet stream trough through the Great Lakes on Saturday.  This is a ripple or disturbance that creates rising motion in the atmosphere.   It is expected to merge with Irene's circulation over eastern Maryland and New Jersey.   The additional energy source that this trough provides may help to re-energize Irene, countering some of the weakening due to cooler ocean water.   The additional uplift of air will squeeze additional tropical moisture out of the storm, creating heavier rains.   And Irene is also expected to interact with a weather front over southern Maryland, from which it can draw additional energy.   There is precedent for this type of extratropical transition, namely, Hurricane Floyd of 1999.

Let's talk about likely impacts in the Baltimore region.  The main time frame for these impacts is Saturday afternoon through Sunday afternoon, with the most dangerous conditions early Sunday morning.

Storm Surge on the Bay:  Initially, strong winds from the north will blow water down and out of the Bay.  However, on Sunday afternoon, the winds will shift direction, blowing strongly from the west and southwest.  This may cause flooding along the Bay's eastern shore - particularly since this weekend we are experiencing spring tides (larger high tides than normal).

Heavy Rainfall:  These rains will likely start falling well before the actual storm center passes by.  These "rains in advance" are part of the extratropical transition process.   Light rain may start to fall Saturday morning.  It will intensify through the day, and will be showery in nature.  The heaviest rains will fall overnight Saturday into Sunday, and may taper early Sunday afternoon.  Below is the latest NWS guidance on rain accumulation.  Amounts in the 3"-6" range are possible in and around Baltimore.  Notice the heavy axis of 13"+ rains along the Delmarva:

Winds:  With the storm potentially tracking closer to the coastline, and because of the storm's large size and intensity, the wind threat across Baltimore has increased.   Being on the right side of the track diminishes the winds somewhat, but winds of tropical storm force (40 mph and greater) are possible across Baltimore.  Below is a sequence of wind intensity forecasts from the NWS, showing the maximum expected wind gusts:

Wind Gust Forecast, Knots, 2 PM Saturday

Wind Gust Forecast, Knots, 2 AM Sunday

Wind Gust Forecast, Knots, 8 AM Sunday

Wind Gust Forecast, Knots, 2 Pm Sunday
Notice the enhancement of winds along the Bay.  Peak winds may gust to 70 mph in and around Baltimore early Sunday morning.   Hurricane force wind gusts (75-80 mph) may occur along the Bay.  Given saturated soils from recent rains, and trees in full foliage, all area utility companies (BGE, PEPCO) are advising customers to prepare for possible widespread, multi-day outages.   The last time our region experienced power outages of this magnitude was during Hurricane Isabel of 2003.

1 comment:

  1. Should the second image of wind gust predictions be captioned "2AM Sunday" rather than "2AM Saturday"?