textproduct: Boston / Norton

WHAT HAS CHANGED

Forecast remains on track.

KEY MESSAGES

- Humidity briefly trends down tonight, but then rises again Monday afternoon. Above normal temperatures continue through Tuesday.

- Warmer and less humid Wednesday, with a strong cold front moving in early on Thursday bringing a risk for showers and a low risk for storms.

- Nice stretch of later-summer weather Friday into the weekend.

DISCUSSION

KEY MESSAGE 1...Humidity briefly trends down tonight, but then rises again Monday afternoon. Above normal temperatures continue through Tuesday.

Expecting mainly rain-free conditions for this portion of the forecast. The exception should be Monday afternoon into the evening, when a round of diurnal showers and thunderstorms will be possible.

Humidity drops later tonight behind a weak cold front. This front should not go far, and return as a warm front Monday afternoon into the evening. Dew points increase again just to the south of this front. A passing cold pool across northern New England should help to generate CAPE values of 750-1,250 J/kg by late Monday afternoon. The greatest risk for any thunderstorms likely remains west of our region.

Once again, this warm front moves back south as a cold front Monday night into Tuesday. Not looking humid enough to keep a mention of showers in the forecast, especially that early in the day when this front should pass by. Humidity starts to fall once more Monday night, but really get into the comfortable range Tuesday afternoon and night.

KEY MESSAGE 2...Warmer and less humid Wednesday, with a strong cold front moving in early on Thursday bringing a risk for showers and a low risk for storms.

Summer warmth continues into Wednesday, but with a substantial decrease in humidity levels (e.g. aftn dewpoints in the upper 50s to mid 60s) with westerly non-convergent flow offering dry weather. Partly sunny with highs in the mid to upper 80s and lows in the lower to mid 60s.

A rather strong mid/upper trough and surface cold front moves through Southern New England on Thursday. This frontal system ushers in a true substantial change in airmass. Ordinarily, that would point to an active day, but the timing is the biggest mitigating factor, as the front looks to reach the eastern MA coast by early afternoon per loose global model consensus. Thus current expectation is for minimal to weak diurnal instability and an attendant low risk for thunderstorms, but if the front ends up slower than current thinking then the risk for storms would increase to go along with the risk for strong storms. Partly to mostly cloudy skies with highs in the lower 80s, with dewpoints falling through the afternoon and evening into low-mid 50s post- frontal, and lows in the 50s to around 60.

KEY MESSAGE 3...Nice stretch of later-summer weather Friday into the weekend.

Main story for Friday into the weekend is a much cooler and drier airmass that entrenches itself over the Northeast. Cool advection brings 850 mb temps down to around +8 to +10C, supportive of highs in the 70s to lower 80s with mostly sunny/partly cloudy skies and dewpoints falling into the 40s. Overall a nice stretch of late- summer weather if slightly cooler than normal for mid August. Next chance for showers or storms could arrive by late Sunday or Monday but the timing is still a question mark.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

Tonight...High confidence.

VFR conditions outside the risk for a brief period of lower VSBYS clipping ACK tonight. Otherwise, W winds may gust to around 20 knots or so this afternoon.

Monday...High confidence.

VFR. Scattered late afternoon/evening showers or thunderstorms, with the greatest risk across interior southern New England. SW winds 5-12 knots but sea breezes possible along portions of the very immediate coast.

Monday Night... High confidence.

Continued VFR with light W winds.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday through Friday/...

Tuesday through Wednesday: VFR.

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

Thursday: VFR. Breezy. Chance SHRA, slight chance TSRA.

Thursday Night: VFR.

Friday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Through Monday Night... High Confidence.

Winds briefly turn WNW this evening with a weak cold frontal passage before returning to SW by late Monday morning. Local sea breezes along the east coast of MA. Relatively light seas across the waters through Monday Night.

Outlook /Tuesday through Friday/...

Tuesday through Wednesday: Winds less than 25 kt.

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

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

Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers.

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

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