Surface low pressure in southern Ontario will develop into western New York this afternoon. An associated cold front currently extends across the eastern Lakes region into western PA. This front is the focus for severe thunderstorm activity ongoing over western NY. The SPC has issued a Severe Thunderstorm Watch for all of central and northern NY state into western VT. The right entrance region of a 125kt upper level jet max will advance into the region this afternoon with a 30kt southwesterly low level jet. This will boost helicity values up to 300 to 500J/kg, supporting supercells and bowing segments that could produce thunderstorm wind gusts to 70mph.
The main inhibitor will be instability, which is limited over much of New England by persistent cloudiness. Visible satellite imagery shows breaks over NY state into northern PA. These areas currently have temperatures up in the 70's, with dew points of 57 to 63. Across New England, surface temperatures are being held in the 60's, but still with dp's in the high 50's. Model guidance is indicating CAPE this afternoon reaching 500 to 1000J/kg with mid level lapse rates between 6 and 6.5C/km.
Secondary concerns include marginally severe hail with freezing levels around 650mb, as well as a slight risk of a tornado or two mainly to our west.
Based on WRF guidance, timing for storms in the Keene area is between 7pm and 9pm, with secondary rounds of weaker shower/storms later at night.
Stay tuned.
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