textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

- Plenty of sunshine with comfortable humidity today & Sat and highs in the 70s. Lows tonight in the 40s & 50s.

- Showers and possibly a few thunderstorms with brief torrential rainfall possible late Sat night into the first half of Sun. Low probability for localized flash flooding.

- Dry and seasonable Mon-Tue but a brief bout of summer warmth and humidity possible Wed and/or Thu ahead of a cold front.

DISCUSSION

KEY MESSAGE 1...Plenty of sunshine with comfortable humidity today & Sat and highs in the 70s. Lows tonight in the 40s & 50s.

Large high pressure builds over the region today into Sat. This will result in plenty of sunshine and very pleasant September weather with comfortable humidity. Highs today and Sat will be well into the 70s with much less wind than yesterday. Mainly clear skies, light winds, and a dry airmass in place will result in ideal radiational cooling conditions tonight. Low temps tonight will bottom out in the 40s to the lower 50s...But middle to upper 50s in the urban centers of downtown Boston/Providence.

KEY MESSAGE 2...Showers and possibly a few thunderstorms with brief torrential rainfall possible late Sat night into the first half of Sun. Low probability for localized flash flooding.

High pressure moves off the coast for the second half of the weekend. At the same time...a shortwave and an associated warm front will approach from the southwest. This will result in quite the mid level instability burst. Showalter indices drop nearly 20 units to near or below zero inside about 12 hours. That rapid change in stability supports the development of a complex of showers and perhaps a few thunderstorms. This coupled with Pwats rising to around 2 inches support the potential for pockets of torrential rainfall. This will be dependent on where a meso-surface low tracks and sets up a boundary for enhanced convergence. This will of course be a nowcast/mesoscale type of forecast...but given the Pwat plume/low level boundary there is a low probability for localized flash flooding. Usually for this to occur...several things need to happen on the mesoscale but it is something we need to watch with the main concern near and south of I-90.

The timing still needs to be refined...but favoring late Saturday night into Sunday morning as to when the bulk of the rainfall occurs. Rainfall amounts are highly uncertain and potentially extremely variable. Perhaps just a tenth to half inch in many locations...but the potential exists for a very localized narrow band of 2-3"+ of rainfall in a short amount of time. The models that indicate this risk have it in a different locations too. And even if this occurs it may not end up in our forecast area. Nonetheless...it is something we certainly need to watch. It will be also become quite muggy by Sunday afternoon with highs in the 70s.

KEY MESSAGE 3...Dry and seasonable Mon-Tue but a brief bout of summer warmth and humidity possible Wed and/or Thu ahead of a cold front.

High pressure builds back into the region Mon and Tue bringing dry and seasonable weather. High temps will mainly be in the upper 60s to the middle 70s with lows in the 40s and 50s. This high pressure system moves east of the region by the middle of next week. This may allow for a brief return of summerlike warmth/humidity sometime Wed and/or Thu.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

12z TAF Update...

Today...High confidence.

VFR. NW winds 7-13 knots except becoming SW after 19z/20z towards the Cape and Islands.

Tonight...High confidence.

VFR. Calm/light NNW winds.

Saturday...High confidence.

VFR. S winds 6-12 knots except for sea breezes along much of the coast.

KBOS Terminal...High confidence in TAF. Gradient should remain strong enough to keep winds NW today. However...sea breeze expected on Sat.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, slight chance TSRA, patchy BR.

Sunday: Mainly VFR, with areas IFR possible. Breezy. SHRA likely, slight chance TSRA, patchy BR.

Sunday Night through Monday: VFR. Breezy.

Monday Night through Tuesday: VFR.

MARINE

Forecaster Confidence Levels...

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

Today through Saturday...High confidence.

Large high pressure will be building over the waters today through Saturday. This will keep winds/seas below small craft advisory thresholds along with excellent visibility for mariners.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Winds less than 25 kt. Chance of rain showers.

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Rain showers likely, slight chance of thunderstorms, patchy fog. Local visibility 1 to 3 nm.

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

Monday through Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft.

Tuesday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

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