Strong to Severe T'storms Possible Late Sunday Evening into Overnight Hours

A shortwave trough with a fairly strong piece of shortwave pushing east through the Great Lakes during the day tomorrow will allow for a pre-frontal trough and cold front to push east as well approaching southern New England late tomorrow evening.  Out ahead of the front a hot and extremely humid airmass will overspread the region with temperatures ranging from the mid-80's to the lower 90's and dewpoints into the lower 70's.  The combination of high heat/humidity will allow for an extremely unstable airmass to develop during the late morning and afternoon hours.

While typically we see the possibility of strong to severe t'storms decrease with the loss of daytime heating as this allows the atmosphere to stabilize, that won't be the case this time.  Computer forecast models have been rather consistent in an area of steep mid-level lapse rates associated with an elevated mixed-layer plume moving overhead on Sunday.  With the presence of steep mid-level lapse rates, combined with the hot/humid airmass, the atmosphere will remain quite unstable through the evening and into the overnight hours.

Below is the 18z run of the GFS from 7/18/15 showing surfaced-based cape values as high as 1500 J/KG valid for 11:00 PM Sunday evening:

Associated with the approaching shortwave trough will also be some pretty strong wind shear aloft.  Computer forecast models overspread as much as 40-50 knots of surface-500mb bulk shear values between 11:00 PM Sunday evening and 2:00 AM Monday morning:

With such an unstable airmass in place, strep lapse rates, increasing shear, and a source for lift, we will see showers and t'storms develop and move into the region during the evening hours tomorrow and lasting through the overnight.  A few of these storms will have the potential to become rather strong to severe and pose a threat for large hail and damaging winds.  In addition, storms will contain torrential downpours and vivid lightning.

The degree of the severe potential is still a bit in question as there are some uncertainties with regards to timing of all the features and whether or not all the best parameters align together.  The NAM computer forecast model also develops a quite a bit of activity during the afternoon on Sunday just off to our west and across New England and this would weaken lapse rates aloft reducing the severe threat after sunset.  Given how the atmosphere should be capped pretty well during the afternoon this scenario doesn't seem all that likely at this time.

The timeframe for t'storms and severe potential should be from 7:00 PM Sunday evening lasting through 3:00 to 4:00 AM Monday.  A second threat for strong to severe t'storms may be possible later Monday morning and early afternoon across southeastern New England as the cold front pushes through.