12/01/06 FCST: Northeast U.S.

The latest models and atmospheric setup favor squall line development during the day Friday with the likelihood of a Derecho/Widespread Damaging Wind event. Sufficient instability with MUCAPES ofr 400-900 J/KG, strong low level forcing and incredible vertical shear as high 75-90 knots in the 925-500mb layer should provide for an extremely volatile situation on Friday. As the strong cold front moves across PA and NY tomorrow morning/midday, it will encounter this instability and a large severe squall line (low topped) should rapdily develop into Central NY and PA. This is one of those situations where just about any convective shower or thunderstorm should transport strong, gusty winds to the surface. I do believe that the stronger parts of the line will be able to generate wind gusts of 60-80mph quite easily. In addition, whenever convection nears the Hudson Valley, it is normally enhanced by the South-Southeast surface wind up the valley and this many times causes supercell or isolated tornado threat enhancement.

A check of the latest SWOD2 shows 45% chance for severe and talk of an upgrade to Moderate Risk.
 
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PS: The 00z NAM shows that the warm front should lie between Albany, NY and Poughkeepsie, NY for much of the day. It finally pushes north of Albany by 00z (likely just before the squall line). At 00z, here are some highlights from the Forecast Soundings:

0-3km SRH: 644
1km SRH: 496
LI: 0
KI: -1
Storm Motion: 220 deg at 60 kts (= 69 MPH)
LCL: 954mb
0-6km Shear Value: 70 knots
 
Welcome to Meteorological Winter.

The new 13z SPC outlook has extended the MODERATE RISK of SVR through most of the NYC Tri-State area, including Eastern NY and ALbany, NY and many big cities in the Northeast. Things are still on track for a major severe weather outbreak today in the form of a Severe, Large Squall Line/Derecho. Instability numbers have actually gotten a bit more unstable and this should bode well for the development of the squall line that is now in the Ohio Valley. With forward speed of the line expected to be 50-70mph, it should blast Eastward in no time.

Hence, with the "astronomical" shear values, I would expect widespread 60-80mph gusts, with some possibly to 90-100mph this afternoon.
 
Have been monitoring this system and possibilites for severe for Virginia. This morning, we are seeing a bit conditional instability develop out ahead of the approaching convergence line, with some 68 to 70td readings across the state roughly from Franklin to Louisa and now a lifted index of -1 over the SE part of the state; I like the way the deep low seems to troughing down to the SSE and interspersion of some sunlight now and then through the very fast-moving lower level cloud flow. Over the past hour, some weak convection developing out ahead of the line. Obviously great-looking speed shear, and hopefully the flow will not become too unidirectional before we get some real convection firing.
 
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