Sunday, June 23, 2024 New England Thunderstorm Discussion

Synopsis

Scattered showers and thunderstorms are expected to develop within New York and Pennsylvania by early Sunday afternoon with this activity spreading east through the day with additional thunderstorm development within New England. The potential will exist for some of these thunderstorms to become strong-to-severe and capable of producing localized damaging wind gusts and/or hail. There is also potential for a few tornadoes within the region. In addition, torrential downpours with any thunderstorms may contribute to flash flooding within typical, low-lying, flood-prone areas.

As of Saturday morning, an elongated stationary front stretched from off the New England coast westwards through the Great Lakes region and into the upper-Midwest. Across New England, the north of the boundary is characterized by a much cooler and less humid airmass than what is in place south of the boundary.

Also as of Saturday morning, multiple pieces of shortwave energy were embedded within the jet stream across the north-central Plains and upper-Midwest with multiple weak areas of low pressure at the surface.

Forecast model guidance is in general agreement this shortwave energy will amplify (strengthen) some as it traverses the Great Lakes region and advances towards New England. This will result in a deepening low pressure at the surface and help to lift the stationary front across New England northwards as a warm front, advecting much warmer and more humid air into northern New England on Sunday morning. This will help set the stage for scattered afternoon and evening thunderstorms.

Dynamics

With a strengthening weather system, we’ll see an increase in winds throughout the troposphere. Forecast model guidance overspreads 40-50 knots of bulk shear by Saturday night through Sunday. These values are more than sufficient for thunderstorms to become organized, and these values are more than sufficient for the more mature storms to take on supercell characteristics.

In addition to winds increasing with height through the troposphere (speed shear) winds will also change direction through the atmosphere (directional shear) with the greatest turning of the winds occurring within the lowest 3 km thanks to the presence of a surface warm front. Forecast model guidance suggests 0-1km helicity (a measure of how winds are changing with speed and height) values on the order of 150-200+ m2s2 with 0-3km helicity values on order of 200-250+ m2s2. These values are more than sufficient for any mature, discrete thunderstorms to become supercells.

Thermodynamics

The combination of temperatures climbing into the 80’s with dewpoints into at least the lower 70’s will contribute to a moderately unstable airmass characterized by mixed-layer CAPE values ranging between 1500-2500 J/KG. What will hold back from an even more volatile atmosphere will be the lack of stepper mid-level lapse rates (change of temperature with height) in the 700-500mb level (typically associated with an elevated mixed-layer). One thing to watch for, however, is the potential for dewpoint pooling, especially within the Connecticut River Valley. Should this occur, a narrow corridor of dewpoints rising into the 74-76°F range would be likely, this could further boost mixed-layer CAPE values (in the 2500-3000 J/KG range) and compensate some for the weaker mid-level lapse rates.

Expectation

Combination of approaching shortwave energy, moderate instability, and sufficient wind shear will result in scattered-to-numerous showers and thunderstorms develop by early afternoon across New York and Pennsylvania with this activity spreading east through the day with showers and thunderstorms firing across New England by early-to-mid afternoon. While there is some potential for widespread strong-to-severe thunderstorms, the overall potential will be held back due to;

1)        The lack of an elevated mixed-layer and associated very steep mid-level lapse rates (> 7-7.5 C.KM).

2)        Assessing some forecast soundings within the region, it is noted there are some subtle warm layers aloft. These subtle warm layers (contributing to weaker lapse rates) will have a negative impact on parcel upward acceleration which can make it difficult for thunderstorms to become explosive and very intense, at least on a widespread scale.

The greatest potential for any thunderstorms to become strong-to-severe will be eastern eastern New York, Vermont, New Hampshire, western and central Massachusetts, and northwestern Connecticut. This is where the strongest thunderstorms will be capable of producing localized damaging wind gusts and hail. There will also be potential for a few tornadoes and some localized flash flooding.