Caution: this discussion could be a bit technical for the casual reader. If you're not interested in the inner details of the severe weather threat, just scroll to the end of the post for the bottom line! The forecast models have honed in on the potential for a severe weather event on Friday throughout the Northeast States. A surface low is forecast to dive southeast in response to a shortwave and vorticity max currently located over the Western Great Lakes. In response to this feature, kinematic support will increase throughout the day on Friday. In fact, the NAM has a widespread area of of 0-500mb bulk shear over 40 kts with isolated values over 50 to near 60 kts by 21z on Friday. Featured image: 12z NAM model sfc-500mb bulk shear values at 21z Friday. 

In addition, there is quite a bit of low level shear evident on forecast soundings throughout the Northeast. This is especially true over New England, closer to the surface low, but is even evident as far south as the Northern Mid-Atlantic towards Central New Jersey. This is confirmed on the graphical forecast 0-3km helicity parameters, with values in excess of 200-250 m2/s2 forecast over New Jersey and New York, increasing to 300-400 m2/s2 over New York State and New England.
Low level shear values are important in helping to develop rotation within thunderstorms, and the eventual capability for the thunderstorm to produce high winds/damaging winds and possibly tornadoes. Featured image: 12z NAM models forecast 0-3km helicity (low level shear) for Friday evening. 

The forecast models also have a moderately to severely unstable environment (probably more severely unstable given the degree of shear and forcing present in the environment) in place by the afternoon across the area...with surface cape values over 2000 joules. This instability is a bit less to our north--where the better shear is--and a bit higher to our south, although they lack the better kinematics. With the degree of forcing for ascent being dramatically increased with the shortwave and surface low in the vicinity, the potential certainly exists for a widespread convective event Friday afternoon and evening throughout the Northeast states. The juxtaposition of kinematic and thermodynamic support seems to be best over a small area from New York State moving southeast into Western SNE by evening--and it is here that I would not be surprised to see a few tornadoes. Otherwise--damaging wind reports could be widespread if the forecast models interpretation of the synoptic setup is correct.  The Storm Prediction Center has outlined our area in a Slight Risk for severe thunderstorms on the Day 2 Outlook. Featured image: Storm Prediction Center Day 2 Outlook, showing portions of our area in a Slight Risk for severe thunderstorms. 


Be sure to stay tuned here, and of course to our Twitter and Facebook accounts for rapid updates. We'll have a new blog post in the morning (or potentially slightly after midnight) to detail an update on the threat. Have a terrific evening! Article written by John Homenuk, published 6:20pm July 28th, 2011.