Significant Severe Weather Event Possible: Thursday, August 27, 2020

A widespread and perhaps significant severe weather event is possible across parts of southern New England Thursday. There are uncertainties which exist, however, the ingredients will be there for what could be a widespread damaging wind event. A few tornadoes can't be ruled out either. 

The setup:

At the surface it will be very warm to hot with temperatures pushing into the 80's to perhaps near 90 in spots. Humidity will be moderate-to-high with dewpoints into the upper 60's to 70's. This combined with a plume of steep mid-level lapse rates (perhaps in excess of 6.5 C/KM to 7 C/KM) would result in a moderately unstable airmass characterized by mixed-layer CAPE values approaching 2000-2500 J/KG. 

Aloft, New England will be on the downstream crest of a upper-level substantial ridge which is centered over the heart of the country. This will place southern New England in a northwesterly flow in the mid and upper-levels of the atmosphere and also place southern New England in a zone of unseasonably strong wind shear with the mid-level jet stream on order of 40-60+ knots:

A cold front approaches by late Thursday afternoon. The combination of a moderately unstable airmass ahead of the cold front and strong wind shear aloft will yield the potential for numerous thunderstorms; likely in the form of several line segments which will be capable of producing wind damage...which could be widespread. Given a wind direction of northwest in the mid-levels of the atmosphere, more west to west-southwest in the lower levels of the atmosphere, and more southwest at the surface this will create directional wind shear which is evident on this forecast hodograph at Windsor Locks, CT from one of the computer forecast models for late Thursday afternoon:

This changing of wind direction with height indicates the potential could exist for a few tornadoes, especially if any discrete thunderstorms develop. 

Limiting factors: 

This event is not set in stone and there are a few wild cards which could hinder this event and overall potential;

1) Morning shower and thunderstorm activity. A warm front approaches from the southwest overnight Wednesday and lifts through southern New England Thursday morning. A complex of thunderstorms is expected to develop across southeastern Canada on Wednesday and then dive southeast through NY and parts of southern New England Thursday morning. This could result in extensive cloud debris (yielding a lower temperature forecast and much lower instability) along with subsidence (rising air) behind the complex. Subsidence could help erode cloud debris enhancing potential for maximum instability to be realized but if that subsidence is not able to be overcome (strong forcing mechanism such as a warm front or falling heights) this could suppress thunderstorm activity. This is something we will not have a clear picture of until Thursday morning. 

2) Lack of stronger height falls . Without going into a warm and piece long article to explain this...heights are basically the height above the ground you would need to go to find a particular pressure level. Falling heights over time is indicative of rapid upward vertical motion (excellent for thunderstorm growth) while more neutral or rising heights aren't very supportive for large-scale lift. 

3) Forecast soundings also indicate potential for some subtle capping layers (areas of the atmosphere where temperatures warm with height instead of cool). This can inhibit parcels from accelerating upward which is not good for establishing deep convection.

All in all, the ingredients are there Thursday for what has the potential to be a widespread severe weather event with damaging winds, large hail, and perhaps the risk for a few tornadoes.