Monday, November 5, 2012

East Coast Nor'easter

The eastern seaboard can't buy a break.  Another cyclone, this one a more traditional nor'easter, is expected to impact the area affected by Sandy on Wednesday and Thursday.

We discussed last week how deep extratropical cyclones often feature a warm-core seclusion, an isolated pocket of warm air near the center of the system.  Indeed, the graphics below show the  development of a warm-core seclusion as the nor'easter intensifies off the eastern seaboard through midweek.

GFS sea level pressure (every 4 mb) and 925-mb temperature (every 2ºC)
forecasts valid 0300 UTC 7 Nov, 1500 UTC 7 Nov, and 0600 UTC 8 Nov 2012

The close-up below better highlights the warm core seclusion, which at its center is about 6ºC warmer than the surrounding airmass.

GFS Sea level pressure (every 4 mb) and 925-mb temperature (every 2ºC)
forecast valid 0600 UTC 8 Nov 2012 
The evolution of the storm above is very reminiscent of the Shapiro-Keyser cyclone model, which is named for atmospheric scientists Mel Shapiro and Dan Keyser.  In their model, the cold front does not "overtake" the warm front and form an occlusion, as it does in many text book descriptions that derive from the so-called Norwegian Cyclone Model.  Instead, it remains perpendicular to the warm front, forming what they call a frontal "T-bone."  The warm front migrates into the airstream behind the low center, which it eventually encircles, forming the warm-core seclusion.  The portion of the warm front that extends behind the low center is called a bent-back front.

The Shapiro-Keyser Model
The nor'easter in the color images above closely mimics this evolution.  The cold front never overtakes the warm front, but instead remains perpendicular to it.  The warm front migrates into the airmass behind the low-center, which it encircles to form the warm-core seclusion.   

The nor'easter will bring another bout of hazardous weather for the vulnerable eastern seaboard.  Let's hope the the latest model forecasts are wrong and the storm shifts eastward and farther out to sea.

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