Wednesday, September 7, 2011

September 7, 2011: Serious Hydrological Event Underway

The remnants of ex-Lee continue to impact the Baltimore region.  The large-scale storm circulation has become nearly stationary, and is located over the Great Smoky Mountains...where it was yesterday.   On its eastern side, a warm front remains draped across the Washington-Baltimore region.  A tropical plume of high humidity is being lifted along this front.
Surface map, 8 AM, showing ex-Lee's circulation situated over Appalachia.  A warm front extends northward across Washington-Baltimore.
Plume of high tropical moisture streaming from south to north over the Washington-Baltimore region.
The movie below shows a phenomenon called echo training...whereby storm cells form within the tropical moisture plume, and move repeatedly over the same locations.   The cells are developing south of Baltimore, and moving northward across the city, one after another.  Each cell dumps a load of rain.  Then the next one arrives.  The "train" sets up whenever the winds aloft blow parallel to a stalled front.  In this case, very humid flow from the south is running parallel to the warm front, which is also oriented south to north.   The result is a very wet conveyor belt:
video
Shown below are the flash flood warnings that have been posted for our region...note how they all run along the axis of the echo train:

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