textproduct: Boston / Norton

KEY MESSAGES

- Cloudy and drier today with an increasing chance of showers and thunderstorms late tonight into Thursday morning.

- Rain continues Thursday with a chance for a strong thunderstorm or two during the afternoon.

- Mix of clouds and sun with diurnally driven spot showers Friday into the early part of Labor Day Weekend, but drier weather most of the time. Dry and seasonable Sunday and on the Monday holiday, with onshore breezes.

DISCUSSION

KEY MESSAGE 1...Cloudy and drier today with an increasing chance of showers and thunderstorms late tonight into Thursday morning.

Expecting a mainly dry but cloudy day today as a 1026 mb surface high builds over the Canadian Maritimes and extends SE toward s southern New England. Cloud cover will keep high temperatures in the upper 60s to low 70s for most locations. Onshore flow means that coastal areas likely see the coolest temps while the warmest conditions will be found across the interior. The day ends on an unsettled note as a frontal wave approaches from the Great Lakes. At the surface, a warm front approaches southern New England overnight bringing the return of a moisture-rich airmass to the region. Precipitable water values rise to 1-1.8" overnight. Lift provided by the jet positioned to the west and warm front will support the return of showers and perhaps some embedded, mostly elevated storms.

Showers/storms more likely arrive in west and central portions of southern New England 11 PM to 2 AM arriving in the I-95 corridor 4 AM- 7 AM. There are indications of marginal elevated instability that accompanies the plume of richer moisture. While instability looks to be marginal (MLCAPE 400-700 J/kg), a strengthening low level jet on the order of 35-45 kts helps to increase the low-level wind field. Thus, any storms that form to the west of our CWA likely continue as they head east despite lower instability levels.

Steering flow appears strong enough to support quicker storm motion which would limit the risk for flash flooding. If there are embedded storms and downpours, can't rule out the potential for poor drainage/urban flooding in the more vulnerable spots. As a result, there is a marginal (level 1 of 4) risk for excessive rainfall and localized flash flooding.

KEY MESSAGE 2...Rain continues Thursday with a chance for a strong thunderstorm or two during the afternoon.

Yet another shortwave impulse dives S from eastern Canada Thursday. An associated surface cold front provides lift for showers and thunderstorms by the afternoon The main question mark is on if storms could become strong in this timeframe. Although the airmass is plenty humid and mid and upper level wind fields are seasonably strong, considerable cloud cover could temper how much destabilization occurs. GFS-based CAPE profiles show about 800-1600 J/kg of most-unstable CAPE with moist-adiabatic mid-level lapse rates also tempering instability. It's still debatable if storms could become strong but the chances seem low with more tempered instability favored. Still, Thursday looks to be a washout with good chances for rains during the daytime hours, with more substantial clearing taking place Thursday evening.

KEY MESSAGE 3...Mix of clouds and sun with diurnally driven spot showers Friday into the early part of Labor Day Weekend, but drier weather most of the time. Dry and seasonable Sunday and on the Monday holiday, with onshore breezes.

Still under cyclonic flow aloft for Friday into the early part of Labor Day Weekend. This will keep weather pattern somewhat unsettled with a mix of clouds and sun. Spot showers are possible Friday and Saturday afternoon but drier weather prevails most of the time. much of the time. Broad high pressure then works in from the north Sunday into Labor Day, leading to onshore breezes, but with partly sunny skies and dry conditions.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

This Morning: Moderate to high confidence.

MVFR-IFR conditions continue through at least 12z across interior terminals like BDL. Elsewhere, MVFR-VFR categories for BED, BOS and the Cape and Islands should trend BKN/OVC VFR by daybreak. Light E/SE winds.

Today...High confidence.

VFR after 15z, however, mid-level clouds remain throughout the day. Easterly to east-southeasterly wind prevail.

Wednesday Night...High confidence in trends, moderate confidence in timing.

Showers returning overnight with embedded thunder, ceilings lowering to IFR/LIFR. Southeast winds.

KBOS Terminal...High confidence in TAF. BKN/OVC VFR ceilings/visbys with light ESE winds tonight. MVFR ceilings return around or after 00z Thursday.

KBDL Terminal...Moderate confidence in TAF. MVFR-IFR in stratus and mist, not likely to improve until mid morning Wednesday, then VFR. MVFR ceilings return around or after 00z Thursday.

Outlook /Thursday through Sunday/...

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

Thursday Night through Friday Night: VFR.

Saturday: VFR. Breezy. Chance SHRA.

Saturday Night through Sunday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Through Tonight...High confidence.

SCA gusts (~25 kts) continue through 12z this morning for the southern waters tonight with seas 3-5 ft. A SCA is in effect for the southern waters through sunrise this morning. Marginal to near SCA conditions continue through much of the day today with E winds between 15 and 20 kts and seas between 2 and 4 feet. Conditions eventually relax overnight ahead of low pressure approaching from the west.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Chance of rain showers, slight chance of thunderstorms, patchy fog.

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

Friday through Friday Night: Winds less than 25 kt.

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

Saturday Night through Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 AM EDT this morning for ANZ235- 237-255-256.