Monday, July 18th, 2016 Thunderstorm Potential.

Another hot and steamy day is in store for Monday as temperatures ahead of an approaching cold front are expected to soar into the upper 80's to lower 90's with dewpoint temperatures in the 60's and perhaps rising into the lower 70's as the front approaches (thanks to increasing theta-e and theta-e pooling).  The combination of heat and humidity will lead to a quite unstable airmass tomorrow across southern New England.  While mid-level lapse rates may be modest at best (~6 C/KM), dewpoints climbing up to around 70F (or perhaps a little higher) could help to offset the weaker lapse rates and yield as much as 1500-2000+ J/KG of mixed-layer cape tomorrow afternoon:

With such an unstable airmass in place and an approaching weather system, the question becomes do we see showers and thunderstorms tomorrow and if so, what are the chances for strong to severe thunderstorms?  With the main cold front pushing through western and central New York during the late morning hours, a line of showers and thunderstorms is expected to develop back across central New York state during the late morning and early afternoon hours and push southeast  with the storm motion from southeast to northeast.  This line would approach southern New England during the early evening hours.  This would result in some unfavorable timing for our region, however, what we will have to watch for is a pre-frontral trough setting up across eastern New York and sliding through southern New England during the early to mid afternoon hours.  It is this feature along with modest height falls which could provide the focus for the develop of showers and thunderstorms across southern New England during the afternoon hours.  

Severe Weather Potential:  

While a widespread severe weather outbreak is not anticipated, the possibility does exist for several thunderstorms to become strong with a few reaching severe limits.  The degree of instability is certainly not a question tomorrow nor is the degree of wind shear aloft, however, there are some uncertainties with regards to where the best mid-to-upper level forcing is located.  By late afternoon, we see as much as 35-45 knots of wind at 500mb (~18,000ft AGL) overspreading southern New England which is more than sufficient for thunderstorms to become organized.  However, we see the strongest winds back further to the west and subsequently the strongest mid-level forcing as well:

One thing to watch tomorrow is perhaps the degree of low-level directional shear that a few of the computer forecast models are showing.  Computer forecast models are showing southerly winds at the surface, then turning to southwesterly in the lower-levels of the atmosphere and then more westerly in the mid-levels of the atmosphere.  This is indicative of good directional shear and with good speed shear (increase of winds with height through the troposphere), this can allow for strong thunderstorms with have strong and mature updrafts to actually begin rotating which can increase the potential for both larger hail and even a tornado...just something that needs to be watched.  The best potential for this possibility would be the CT River Valley where the surface winds could remain more southerly to even southeasterly:

While the more widespread thunderstorm activity may be confined back across much of New York into PA and into VT along/just ahead of the cold front and closer to the best mid and upper level forcing, the pre-frontal trough sliding through southern New England during the afternoon should provide a focus for showers and thunderstorm development as well.  While the setup really doesn't favor a big severe weather event right now, some of these thunderstorms across southern New England may become strong with a few reaching severe limits.  These thunderstorms will be capable of producing strong to damaging wind gusts, some hail, and while a low potential, an isolated tornado can't be ruled out given the degree of directional shear being advertised.  All storms will produce cloud-to-ground lightning and with precipitable water values approaching 2'' torrential downpours are likely.  The best threat for strong to severe thunderstorms may be confined to NY, PA, VT, western MA, and northwestern CT.  However, if the better forcing pushes east faster than modeled and overlaps the strongest instability and wind shear, then a bigger severe weather threat may materialize.