By Joe D'Aleo
Monday, November 24, 2008
East coast cyclones in winter can produce prodigious snowfalls burying the major cities of the east. They come in different flavors – as waves along fronts offshore or as storms that develop secondary to inland lows.
My Masters Thesis at the University of Wisconsin was on Explosive Development in East Coast Cyclones. In that study I found the most common precursor was a cold outbreak into the Atlantic ahead of the trough and storm development. This generates a considerable reservoir of available potential energy that gets released when the upper conditions become more favorable. You can see visible signs of this with the clouds that form in the cold air stream off the coast over the warmer water.
This vertical development is limited by subsidence in the negative vorticity advection (NVA) area ahead of the short-wave ridge between systems.
This produces an inversion that acts as a lid on vertical motion. After the passage of the short wave ridge and the start of positive vorticity advection, the lid is removed and penetrative convection begins. Often this takes the form of thunderstorms which transfer sensible and latent heat into the storm and help pressures fall rapidly. Pressure falls of up to 40mb in 24 hours have been recorded.
The dynamics of the upper systems of course play a role in how much development occurs. Kocin and Uccellini in their wonderful two volume monograph available from the AMS discussed the schematics of the jet stream circulation in many or most east coast cyclones.
They show a departing trough (a low near 50N, 50W empirically) and a neck of cold high pressure pressing in to the northeast and offshore. The right rear quadrant of the jet streak with the departing low and the left front quadrant of the jet with the approaching jet and trough both favor rising motion near and off the coast. This facilitates that convection and latent and sensible heat release.
In my thesis, one of my case studies was the storm of March, 1960, shown below. Note the preceding cold air mass flooding cold air offshore. Note the approaching surface low reflecting the approaching trough and southern jet. Note how the coastal low quickly takes over the circulation as it explosively deepens on March 3rd.
The storm left behind a foot and half of snow over New York City and nearly 2 feet in the Boston metro area.
It was a classic case of an explosive east coast cyclone and the role of both dynamics and the contribution of sensible and latent heat to the development.