textproduct: Boston / Norton

WHAT HAS CHANGED

The overall threat for severe weather across southern New England has decreased. However, a risk for strong to severe storms remains for areas west of the Connecticut River through this evening.

KEY MESSAGES

- A few stronger storms are possible, mainly west of the CT River Valley today.

- Spot shower or thunderstorm possible Friday, otherwise a mainly dry and pleasant late-summer day.

- Seasonable temperatures and dry weather this weekend.

- Cloudy and unsettled early to mid next week with several opportunities for showers and storms, but the timing of them is uncertain.

DISCUSSION

KEY MESSAGE 1...A few stronger storms are possible, mainly west of the CT River Valley today.

A seasonably warm and humid day is ongoing, helping fuel thunderstorms from the Mid-Atlantic into the New York City metro. However, the threat for severe storms across southern New England is decreasing, albeit not zero. This is highlighted by the current outlook, which indicates the greatest risk is mainly west of the Connecticut River Valley, where a Marginal Risk (level 1 out of 5) exists. The greatest concern is across the Berkshires and points west, where a Slight Risk (level 2 out of 5) exists. Driving today's weather is a mid-level closed low over southern Hudson Bay and a neutral to slightly positively tilted trough. Dynamics are modest, with 0-6 km bulk shear around 35 kt, CAPE values of 1,000-1,500 J/kg west of the Connecticut River, and effective helicity of 50-100 m2/s2.

It is a tricky forecast, as there is sufficient instability across southern New England, with the greatest CAPE values remaining west of a Willimantic-Worcester-Fitchburg line. However, some high-res guidance suggests ongoing convection from the Mid-Atlantic into the NYC metro may work over the environment to our south, limiting the northward transport of higher instability into southern New England this afternoon. This could ultimately reduce the amount of instability available for storms later today. Nevertheless, any storms that do develop will have the potential for gusty winds, which remain the primary concern given DCAPE values around 800 J/kg. Small hail is also possible, and an isolated tornado cannot be ruled out. We continue to feel the greatest risk for any stronger storms is west of the Connecticut River Valley.

Today's 12z suite of guidance continues to support storm development across New York State by mid-afternoon, reaching western New England between 5-8 PM. The greatest risk for stronger storms will be during the late afternoon and early evening, primarily west of the Connecticut River Valley. Farther east, decreasing instability with the loss of daytime heating, marginal mid-level lapse rates around 5.5-6.5 C/km, and limited moisture should result in a less favorable environment. A few storms may survive eastward into central and eastern southern New England, but coverage and intensity should diminish.

Mild overnight with stratus advecting in from the south, which may lead to low clouds and areas of fog mainly across RI into southeastern MA.

KEY MESSAGE 2...Spot shower or thunderstorm possible Friday, otherwise a mainly dry and pleasant late-summer day.

Clouds early to start Friday, followed by seasonably warm and humid conditions. A cold front moves across the region during the morning into early afternoon, providing a focus for isolated to scattered showers and perhaps a garden-variety thunderstorm. Much of the day should remain dry, with any shower or thunderstorm activity brief and localized.

The cold front clears the coast Friday evening, with winds shifting to the northwest behind it. A more comfortable and less humid air mass arrives overnight into early Saturday.

KEY MESSAGE 3...Seasonable temperatures and dry weather this weekend.

Nice weekend shaping up for Southern New England as large high pressure cell centered over southeast Ontario early Sat shifts southeastward through Long Island and offshore by Sunday. Expect full sun and good mixing helping to temper dewpoints/humidity levels to tolerable levels. Seasonable highs in the upper 70s to low 80s with good radiational cooling both Sat and Sun nights supporting lows in the upper 50s to low 60s. Enjoy the weather this weekend as we turn more active and unsettled starting Monday - more on that to follow below.

KEY MESSAGE 4...Cloudy and unsettled early to mid next week with several opportunities for showers and storms, but the timing of them is uncertain.

Our weather pattern then turns more unsettled starting Monday into midweek, as the northern mid-Atlantic and Southern New England remains entrenched between cooler northerly flow over northeast New England and a strong surge of warm, humid air on southwest flow over the Ohio Valley and Upper Midwest. Quasi-stationary/backdoor sfc frontal boundary looks to be draped in vicinity of Southern New England Monday thru Wednesday, with embedded hard-to-time shortwaves in a fast NW flow in mid-levels, atop the stalled frontal boundary, favoring several opportunities for showers and thunderstorms along its extent.

Other than it being a pattern which favors cloud cover and periods of showers and storms, given the coarser-resolution ensemble guidance available at this point, confidence in the timing of showers/storms is lower than average. The position of the quasi- stationary front will also affect the timing and placement of where showers and storms are more favored day-by day; as we move though time, the initial lean is for the frontal boundary to adjust southwestward, but will wait to see incoming guidance before adjusting PoPs beyond NBM. Expect a narrower range in high/low temps Monday through Wednesday, with highs running a little cooler than normal and lows slightly milder than normal.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

18z TAF Update:

Through Tonight...Moderate confidence in thunderstorm coverage, higher confidence in overall trends.

Breezy SSW to SW winds through this afternoon with low-end VFR ceilings. Greatest risk for isolated TSRA is across interior southern New England, with a PROB30 at KBDL between 23z-02z. Otherwise, cannot rule out a stray SHRA farther east, but confidence is too low for inclusion in the current TAFs. Gusts subside between 22-00z. Ceilings lower overnight, mainly after 04-06z, to IFR/LIFR. Visibilities lower to 3-5SM, with localized areas below 2SM across southeast MA and RI.

Friday...High confidence in trends, lower confidence in storm coverage.

Conditions improve to VFR with ceilings 050-080 after 14-16z. Pop-up TSRA possible between 18-00z, which may result in brief MVFR/IFR conditions. Light W/SW winds become WNW by late afternoon.

Friday Night...High confidence.

VFR ceilings for most, though MVFR possible along the immediate south coasts of RI/MA, Cape Cod, and the Islands. Light NW winds.

KBOS Terminal...High confidence.

VFR and mainly dry this afternoon outside of a stray shower. Ceilings lower to IFR overnight with MVFR visibilities. Low probability of a sea breeze Friday, with hit-or-miss afternoon TSRA possible.

KBDL Terminal...Moderate confidence.

Low-end VFR this afternoon with a PROB30 for TSRA between 23-02z. Ceilings and visibilities lower to MVFR overnight.

Outlook /Saturday through Tuesday/...

Saturday through Saturday Night: VFR.

Sunday: VFR. Breezy.

Sunday Night: VFR. Slight chance SHRA.

Monday: VFR. Slight chance SHRA, slight chance TSRA.

Monday Night: VFR. Chance SHRA, slight chance TSRA.

Tuesday: Mainly VFR, with local IFR possible. Slight chance SHRA, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

Through Friday Night...High confidence.

Chances for showers and thunderstorms increase tonight, mainly south of the Islands. Southerly winds remain less than 20 kt, with seas 3 to 4 ft across the southern waters and 2 to 3 ft across the eastern waters. Another round of scattered showers and thunderstorms is possible nearshore Friday afternoon with the passage of a cold front. Winds shift to the west behind the front Friday afternoon, then become northerly Friday night into Saturday morning.

Outlook /Saturday through Tuesday/...

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft.

Saturday Night through Sunday: Winds less than 25 kt.

Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms.

Monday Night: Winds less than 25 kt. Chance of rain showers, slight chance of thunderstorms.

Tuesday: Winds less than 25 kt. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.