Tuesday, September 6, 2011

September 6, 2011: Lee's Tropical Plume Soaks The Mid Atlantic

On Sunday, September 4, the broad circulation of Tropical Storm Lee came onshore along Louisiana.   The wind field around the storm's core rapidly weakened, but heavy rain has become problematic across the entire U.S. east of the Mississippi.  Ex-Lee's post-tropical circulation continues to import vast amounts of moisture northward out of the Gulf of Mexico.  This plume of tropical air, called an "atmospheric river", shows up in the satellite water vapor imagery, and is analyzed in terms of "precipitable water" (shown below).   Precipitable water is a simple measure of the total water vapor content of the air.   Professor Ray Hoff in Physics pointed out an atmospheric river impacting the West Coast, but we get them here in the East as well:
Deep, continuous river of tropical moisture extending from the Gulf of Mexico over the Mid Atlantic.
This plume of tropical moisture (dark green shades, up to 2.5" precipitable water) has its origins in the western Gulf of Mexico.  The plume is overriding a stationary front anchored along the spine of the Appalachians, stretching from Florida to Maine.  Lee's remnant circulation has become embedded in this front, and is moving slowly toward the northeast along the frontal boundary.   The sequence of two figures below shows how Lee interacted with this front (the first weather map shows conditions at 8 AM Monday, the second shows 8 AM today).   When a post-tropical weather system combines with a front in this manner, and a wave-type frontal cyclone structure develops, the evolutionary process is called extratropical transition (ET).  As ET unfolds, all types of severe and hazardous weather can ensue...sometimes hundreds of miles inland from the coastline. 
Surface weather map, 8 AM Monday, showing Tropical Storm Lee making landfall.  Note the extensive slow-moving cold front along the Appalchians.
Surface weather map, 8 AM Tuesday.  Lee's circulation is now embedded within the cold front, and is evolving into a classic mid-latitude wave cyclone.  The entire system is lifting slowly toward the northeast.
Another facet of inland, post-tropical evolution is tornado formation.  In the 8 AM Tuesday weather map above, note the red tornado watch boxes issued for the Carolinas.  The favored location for tornado formation in decaying tropical remnants is the right front quadrant of the storm.  This is where the low level wind shear, and potential for air to develop rotation, is greatest.

Here is a more detailed, regional (mesoscale) surface analysis of ex-Lee.  Early this afternoon, the storm center was located over the Smoky Mountains.  The pressure gradient is fairly weak, so widespread strong winds are not a concern.  Note several spiral rain bands sweeping over the Carolinas and southeastern Virginia.  Also note how the isobars (solid black lines) dip southward across Virginia.  This pattern is suggestive of Appalachian cold air damming.  In cold air damming, low-level winds get channeled from the northeast, from the cool North Atlantic, down the Virginia Piedmont.   This cool dome of surface air, trapped up against the Appalachians, is one reason why temperatures are struggling to climb out of the mid-60's today.
Detailed analysis of ex-Lee centered over Ashville, N.C.  Note the spiral rain bands on the storm's eastern side, some of which may be producing tornadoes.
In case you are wondering how much rain will fall, and where, the Hydrometeorological Prediction Center (HPC) offers this forecast for 5-day rain accumulation:
HPC prediction of 5-day total rain accumulation.
The heaviest pocket of rain, 10"+, is expected to fall west of the Allegheny Front.   This is significantly west of where Irene produced heavy rain, which amounted to 8"-10" along the coastal plain.   It seems that the combination of orographic rain enhancement (lifting of tropical air along the Appalachians) and the tendency for heavy rains to fall left of track in post-tropical, inland storms...are two explanations in support of HPC's prediction.

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