Saturday, August 27, 2011

Aug 27, 2011 6 PM: Irene Weakens to 80 mph But Moving Slowly

The 5 PM update from the Hurricane Center has the storm moving slowly N-NE at about 14 mph with sustained winds of 80 mph.   Irene is predicted to be directly over Ocean City at 2 AM, also the time when maximum winds (sustained near 40 mph) will be impacting the Baltimore region.
Hurricane Center Irene position at 2 AM Saturday

Here is the surface weather map at 2 PM today, showing the large, asymmetric and mainly inland rain shield.  Note that while Irene is likely pulling in dry air off the U.S. along its southern side (which is helping to weaken the storm), it is not interacting directly with the stationary front (along the Appalachians), as I thought might happen.   It is also not fully embedded in the jet stream trough coming through the Great Lakes.  Thus, extratropical transition - a process by which tropical cyclones acquire weather fronts and morph into an extratropical cyclone - may not occur until the storm is over New England.

Surface weather map at 2 pm Saturday

The final image from late this afternoon shows the rainfall overlaid on top of the detailed surface pressure distribution.   Note the intense pressure gradient, and the enormous size of the pressure field.   A note about the winds:  The surface winds to the left of track, over land, are significantly weaker than right of track, over water.   This large asymmetry is a consequence of land friction, which slows the winds, and the motion of the storm.   However, just a few thousand feet above the surface, the winds are not slowed by friction.  These strong winds are brought down in occasional pulses to the surface within spiral rain bands.  Corridors of intense wind gusts tend to occur within the strongest rain bands.   This process contributes to the localized nature of most intense wind damage in a hurricane over land, which tends to be streaky in nature.

Rainfall and pressure field at 5 pm Saturday

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