Isolated Strong to Severe T'storms Possible Tomorrow

Looks like March will be coming in like a lion as a rather anomalous (for the time of year) meteorological setup will be in place.  A secondary warm front will be pushing into and through parts of southern New England overnight tonight into tomorrow morning.  Once the warm front passes, areas to the south of the warm will see temperatures climb well into the upper 50's and perhaps even lower 60's with dewpoints perhaps near 60F as well.  There are some uncertainties, however, with regards to how high temperatures away from the coast get tomorrow.  We will have numerous clouds to contend with which will help to keep temperatures back, however, the exact direction of the surface wind will play a crucial role as well.  If surface winds end up more out of the southwest, this could allow for surface temperatures to perhaps spike into the mid-60's while a more southerly component to the winds will keep temperatures closer to 60F.  This will be one of the key's when dealing with the thunderstorm potential late tomorrow afternoon and evening.

It is very, very unusual to ever talk about the possibility for strong to severe thunderstorms in February or even March (we dealt with some the other day) but that potential is looking like a possibility tomorrow afternoon.  An area of low pressure passing to our north and west will work to swing the above mentioned warm front through the region.  The system will also swing a cold front through the region overnight tomorrow and early Thursday morning.  Out ahead of this cold front the atmosphere is expected to become modestly unstable thanks to temperatures and dewpoints near 60F.  There will be one other player which will enhance the instability of the atmosphere and that will be the presence of rather steep mid-level lapse rates associated with an elevated-mixed layer plume.  For more info on an elevated-mixed layer check out the following link:

http://weatherwiz.blogspot.com/2015/06/what-is-elevated-mixed-layer.html

Computer forecast models are indicating that we will see a plume of 7 C/KM to 7.5 C/KM 700-500mb lapse rates push into this area (this indicates a temperature decrease of 7-7.5C per km between the 700-500mb level which is a pretty impressive decrease) and this will really help to destabilize the atmosphere:

The combination of near 60F temperature, dewpoints, and the steep mid-level lapse rates will yield rather modest instability for this time of year.  Computer forecast models are indicating perhaps as much as 500-1000 J/KG of mixed-layer cape (a measure of a potential storm's updraft).  Typically this values isn't high, however, we will have very strong wind shear in place (along with a few other factors) which can help to compensate for the lack of higher cape and enhance updraft strength:

Winds aloft are expected to be rather strong as well with 500mb winds in excess of 80-90 knots and 700mb winds in excess of 70-80 knots.  These values will help to contribute to bulk shear values in excess of 70 knots!  This is extremely impressive:

The combination of instability and wind shear aloft will help set the stage for the development of some showers and t'storms during the afternoon and evening hours as some shortwave energy moves overhead.  Given the mentioned parameters, some of these thunderstorms will have the potential to become strong to locally severe and have the potential to produce strong to damaging wind gusts, some hail, and an isolated tornado can't even be ruled out.

Looking at the 12z NAM computer forecast model bufkit profile for Waterbury, CT shows a profile which is certainly favorable for strong to severe t'storms so long we get a forcing/lifting mechanism for storms to develop:

The NAM is projecting between 200 and 300 J/KG of 0-6km cape.  Again while this value isn't overly impressive, when combined with the strong shear this could be compensated for.  One flag to watch for, however, is given how strong the shear is, we would have to develop enough low-level buoyancy for any updrafts which develop to be able to sustain the strong winds aloft.  Higher cape values help to lead to stronger updrafts so we will need some cape.  If there is not enough cape, any updrafts would likely be toppled over and not grow tall enough to utilize the strong winds aloft and the threat for strong to severe t'storms would be vastly reduced.  As for as isolated tornado potential...looking at the hodograph (circled) we see a curved look to it...this indicates very strong speed shear aloft (increase of winds with height).  However, notice how this hodograph just begins to curve right away...this means there is little directional shear (change of wind direction with height) and that is critical for the possibility of tornado formation.  If the hodograph sort of went out in a straight line and then curved this would indicate more of a change of wind direction with height.
On the bottom left we see helicity values well over 400 m2s2 which is VERY impressive.  Typically when talking about the possibility for a tornado you want to see values approach 150 m2s2 so we're way higher than that.  Due to the lack of directional shear this could limit the potential for an isolated tornado, however, this could enhance the threat for damaging straight line winds and even large hail.  Given the degree of the shear/helicity, however, an isolated tornado certainly can't be ruled out.

This is a very delicate forecast (as typically is when dealing with severe weather here in New England) and very unusual for this time of year which raises even more questions and uncertainties.  The situation will have to be evaluated through the day tomorrow as many of our questions will be answered on the extent of the threat depending on whether we see any morning showers/t'storms, how much cloud cover, and surface winds.