A severe thunderstorm watch has been posted for a good chunk of New England this afternoon, which is in effect until 9:00 PM EDT.
A cold front is approaching the region from the west, and is currently located roughly around the US/Canadian border. As it continues to move east through the day, it will serve as a focus to fire off some nasty thunderstorms across parts of the northeast. Some of the storms will be severe with damaging wind gusts and large hail -- some hail exceeding the size of golf balls.
The current SPC mesoanalysis for the northeast shows a pool of moisture extending up through southern New England up into parts of Vermont and New Hampshire. As southerly flow ahead of the front continues, this pool of moisture will continue to feed northward deep into VT and NH as temperature increase due to daytime heating.
Dark green shading shows dew points between 60-64F, red contours are isotherms (temperature contours), and black lines are isobars (pressure contours).
Daytime heating will be no issue in the area. The 1:15PM EDT visible satellite image shows a nice area of clearing between stratus to the east and precipitation associate with the cold front off to the west. The areas that get the most sunlight (and, subsequently, the most heating) will be at highest risk for severe weather this afternoon.
(The stuff below is for the weather geeks, I'll summarize it a scroll or two down)
The RAP Model shows CAPE pproaching 2000-3000+ J/Kg across the areas currently getting the most heating:
Combine that amount of CAPE with bulk wind shear values of 40-50 knots, and you've got yourself a setup for supercells to form across the areas currently under the severe thunderstorm watch.
In addition to the damaging wind threat, the big story today will probably be the hail. Although helicity isn't really high across the area (3PM helicity of 50-100 m2s2 as forecast by the RAP model), the 12z NAM KALB Bufkit sounding shows WBZ heights hovering between 8000-10000 feet this afternoon, which is ripe for severe hail development. The SPC points this out as the largest threat today, going so far as to say hail up to 2.00" in diameter is possible. Given WBZ heights in the perfect spot with supercells in the area, that's a very real possibility.
Even with helicity below the minimum usually needed for even supercells to develop, I wouldn't rule out a tornado or two just based on the fact that supercells will be present. The SPC says it's a low 20% probability, but it's always worth a mention.
(Non-weather geek summary)
All the severe weather parameters used to forecast storm type and strength point to supercell thunderstorms forming over the areas under the severe thunderstorm watch right now, with the highest risk occurring over eastern New York and western Vermont. The biggest risks today will be damaging winds, a low chance of an isolated tornado, and pretty large hail for the area -- 2.00" in diameter, which is slightly larger than a golf ball.
If you live in the northeast or know someone who does, tell them to keep an eye on the weather this afternoon. The links below are great ways to keep track of the weather:
Storm Prediction Center
NWS Binghamton NY
NWS Albany NY
NWS Burlington VT
NWS New York NY
NWS Boston MA
NWS Caribou ME
Wunderground Radar Page
NWS Buffalo NY
iMapWeather Radio App for iPhone/iPod Touch (costs $9.99)
Free Severe Weather Text Alerts from WNYT Albany
Free Email Severe Weatre Alerts from WPTZ Burlington VT