Sunday, June 30, 2024 southern New England Thunderstorm Discussion

 Currently, a northwest to southeast oriented warm front resides from the northern Great Lakes region into Canada southeast across Pennsylvania and into portions of the mid-Atlantic. This warm front accelerates northeast through the remainder of Saturday, advecting in a moisture rich, high theta-e airmass into New England with dewpoints likely into the lower 70's by early Sunday morning. 

Warm front position - early Saturday afternoon

Through the day today, a shortwave trough dives into the upper-Midwest and amplifies as it progresses across the northern Great Lakes region towards the northeast. The result will be unseasonably strong wind shear overspreading the region characterized by 40-50+ knots of bulk shear. In addition, winds in the lowest levels of the atmosphere in the wake of the warm front may remain more southerly with winds more west-southerly aloft. Where low-level winds remain more backed, large helicity values in the 0-1km and 0-3km are likely with as much as 150 m2s2 in the 0-1km layer and 150-200 m2s2 in the 0-3km layer. 

Forecast bulk shear values of 40-50+ knots. More than sufficient for thunderstorm organization.

At the surface, temperatures are expected to climb into the upper 70's to lower 80's with dewpoints into the lower 70's (even as high as 73-75°F should dewpoint pooling result). Depending on the extent of morning cloud cover and degree of heating, temperatures, especially in the valley, could soar as high as 83-85. Despite marginal mid-level lapse rates (~6 C/KM), the combination of surface temperatures into the lower 80's and dewpoints into the lower 70's may yield moderate instability characterized by mixed-layer CAPE values in the 1500-2000 J/KG range and surfaced-based CAPE values around 2000-2500 J/KG. It should be interesting to note some forecast model guidance is suggesting mixed-layer CAPE values could approach 2500+ J/KG with surface-based CAPE values exceeding 3000 J/KG. These values are difficult to obtain without mid-level lapse rates ~7 C/KM and greater, however, if surface temperatures can near 85 and dewpoints climb into the 74-75°F range, these values could become obtained. 

NAM forecast model developing 1500-2000+ J/KG of mixed-layer CAPE Sunday afternoon.

The combination of moderate instability and strong wind shear ahead of an approaching cold front and likely pre-frontal trough development will be more than favorable for scattered-to-numerous thunderstorms developing by early afternoon and organizing into one or multiple convective lines. This combination of ingredients is also supportive for the strongest thunderstorms to produce locally damaging wind gusts and perhaps even a tornado. 

There are some caveats, however, which may reduce a more organized severe weather episode from occurring. In addition to the marginal mid-level lapse rates, the strongest shortwave forcing remains displaced well to the west of the best overlap of shear/instability with forecast model guidance showing the best shortwave forcing still in southeast Canada between the mid-to-late afternoon. 

All in all, the expectation is scattered-to-numerous thunderstorms develop across interior southeast New York by early afternoon with activity quickly organizing into one or more convective lines. Should the more extreme instability scenarios verify (mixed-layer CAPE > 2000-2500 J/KG) potential for a more widespread damaging wind producing event will increase. Given the larger helicity values, there would even be potential for an embedded tornado or two. Any tornado potential would be highest with any discrete cells which form initially or out ahead of the line. Localized flash flooding will be a concern as well given PWAT values approaching and exceeding 2''. 

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.