General Synopsis
As of this writing, strong shortwave energy embedded within a seasonably strong jet stream was moving across Michigan with low pressure and associated warm front at the surface with the warm front having already moved across Michigan and headed towards the Northeast/New England. Forecast model guidance is in very good agreement in tracking this shortwave energy and associated surface low pressure and attendant warm front east across New England Thursday. The surface warm front is expected to move southwest to northeast across the region Thursday morning. This may be accompanied by isolated shower and thunderstorm activity.
As the warm front passes through, a very rich low-level theta-e airmass is expected to overspread much of New England with surface dewpoints climbing into the lower 70's with dewpoints as high as 73-76 likely across southern portions of the region. While there may be widespread cloud cover during the morning (more likely towards northern sections of the region), forecast model soundings and guidance suggest there will be enough dry air aloft to result in thinning clouds which should help temperature soar quickly, especially with 925mb temps around +25C and 850mb temps around +20C. Temperatures should soar well into the 80's region-wide with the hottest locations ranging between the upper 80's and lower 90's.
Instability
Combination of a very rich low-level theta-e airmass (theta-e between 360-370K) characterized by dewpoints into the 70's and surface temperatures well into the 80's to lower 90's should contribute to moderate levels of instability with mixed-layer CAPE values between 1,500 - 2,000+ J/KG. One inhibitor for stronger instability will be the presence of rather weak mid-level lapse rates (generally between 5.5-6 C/KM). While surfaced-based CAPE values will be high (potentially 2,500 - 3,500 J/KG) and mixed-layer CAPE values will be between 1,500-2,000 J/KG indicating moderate instability, the instability is largely a product of the rich low-level theta-e air - majority of instability is tied into the lower levels of the atmosphere and the CAPE profile is not exactly fat CAPE. This is evident on bufkit soundings within the region which have normalized CAPE values generally around 0.2. What all this means is with the rich low-level theta-e airmass, larger parcels accelerations will happen for surface parcels and will easily rise to the lifted condensation level (LCL) and then from the LCL to the level of free convection (LFC). Once parcels reach their LFC, they will continue to rise, however, the presence of weak mid-level lapse rates will result in slower parcel accelerations. This will limit more robust convection on a widespread scale. While weak mid-level lapse rates will hold back parcel acceleration above the LFC, it is possible dewpoints into the lower 70's (or even a bit higher) could help compensate for this a bit.
Wind Shear
A seasonably strong mid-level jet characterized by 500mb winds of westerly flow on order of 40-60 knots traverses the region as New England is situated towards the northern periphery of a strong upper-level ridge which has become established across the southern states. In the lower-levels of the atmosphere, an unseasonably strong low-level jet is expected as the 850mb jet strengthens in response to the approaching shortwave. 850mb winds are expected to increase to 35-45 knots across the region through Thursday morning from a west-southwest direction. At the surface, winds are expected to range anywhere from southwest to more south-southeast with proximity to the warm front a huge key in the direction. Closer to the warm front surface winds will be more south to south-southeast and farther away from the warm front wind will be more south-southwest to southwest. With winds increasing with height through the troposphere (speed shear) and changing direction with height through the troposphere (directional shear) there will be plenty of dynamical support (combined with the shortwave tracking across the region) for thunderstorms to become organized. Given the presence of directional wind shear, the more mature thunderstorms will take on supercell characteristics.
Inhibiting Factors for a Severe Weather Outbreak
While numerous strong-to-severe thunderstorms are possible, there are some inhibitors which may prevent a region-wide severe weather outbreak and higher-end severe weather (higher-end meaning wind gusts 74+ mph, hail 2''+ in diameter, or tornadoes EF2 or higher). As mentioned above, weak mid-level lapse rates may negate a more substantial threat for severe thunderstorms as this will yield slower parcel accelerations above the LFC. Steep mid-level lapse rates (> 7 C/KM) are a large discriminator between severe weather and higher-end severe weather, particularly in the Northeast. Larger parcel accelerations above the LFC can help thunderstorms become quite vigorous.
Forecast model soundings also indicate there may be an abundance of drier air in the mid-levels of the atmosphere. While dry air is good in the mid-levels, too much dry air can be bad, especially without the presence of an elevated mixed-layer as this drier air can mix into the lower levels of the atmosphere, subsequently reducing instability and it can inhibit mid-level storm development. Given rich theta-e air, however, with strong low-level instability, parcel acceleration may be vigorous enough to transport moisture deeper into the troposphere.
Forecast model soundings also indicate some pretty strong capping up around 500mb. This has some relation to the weak mid-level lapse rates which are a product of very warm 700mb temperatures (around +7C) and 500mb temperatures (around -8C) resulting in high freezing levels. The weak mid-level lapse rates and warmer mid-level temperatures significantly reduce parcel acceleration through these levels which negatively impacts storm strength potential.
What to Expect
Based on everything above, the expectation is scattered-to-numerous thunderstorms develop across New York mid-to-late morning and push east. With the majority of forcing tied into the approaching shortwave, combined with the mentioned instability and wind shear parameters, the thinking is storms will be discrete. This will enhance the potential for supercell thunderstorms capable of producing damaging winds and large hail (despite the weak mid-level lapse rates, warm mid-level temperatures, and high freezing levels) and risk for a few tornadoes. These discrete supercell thunderstorms progresses across Vermont, New Hampshire, and western/central Massachusetts. As this activity pushes east, enhanced forcing from a likely sea-breeze boundary and wind shear becoming more unidirectional as the warm front continues to surge north should yield activity organizing into a squall line. This will increase the potential for damaging straight-line winds across southern New Hampshire, southern Maine, and eastern Massachusetts.
One wild card is the state of Connecticut. Connecticut will be south of the strongest shortwave forcing. This combined with the capping mentioned above may mean much of Connecticut misses out on activity (especially western sections). However, the thinking is a sea-breeze front will develop through the afternoon and push north. This will help with thunderstorm initiation and thunderstorms may quickly develop along and east of I-84 during the late afternoon and quickly consolidate into a line with this moving across eastern Connecticut and Rhode Island producing some wind damage and hail (and of course a tornado can't be ruled out).
Overall, this is a pretty intriguing setup for some strong-to-severe thunderstorms across the region, but several limiting factors outlined here will likely preclude a region-wide severe weather outbreak and preclude higher-end severe weather from occurring. However, a concentrated swath of wind damage is possible is a squall line materializes and this may be more likely towards eastern sections.
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