textproduct: Boston / Norton

WHAT HAS CHANGED

Increasing concern for dangerous rip currents for the Holiday Weekend...particularly by Sunday into Monday.

KEY MESSAGES

- Widespread showers and thunderstorms early Thursday morning and possibly Thursday afternoon

- Mainly dry & very pleasant Fri into early next week except for some diurnally scattered showers mainly on Sat. Pleasant temps too with highs in the 70s...but lower-middle 80s Fri.

- Elevated risk for dangerous rip currents for the Holiday Weekend especially by Sun into Mon.

DISCUSSION

KEY MESSAGE 1... Widespread showers and thunderstorms early Thursday morning and possibly Thursday afternoon

A frontal wave traverses over southern New England late tonight into tomorrow morning. A surface warm front lifts northeast with a rich of plume of moisture containing PWATs between 1.5 and 2". This will support a period of widespread showers with embedded thunder early tomorrow morning across southern New England. Shower activity begins around 2AM across the western areas and reaches the east coast around 4-6AM. Heavy downpours are likely with rich moisture in place. There is a marginal risk for flash flooding across western MA/CT in the Springfield/Hartford areas, but there is no strong signal for anything overly impactful at this time. Elevated instability will support some rumbles of thunder as showers move across the region, but deep layer shear and low/mid-level lapse rates do not appear to be favorable for deep moist convection capable of producing strong to severe storms.

As the frontal wave moves offshore, some mid-level dry air intrudes from the west which should support some clearing in the late- morning to early afternoon hours. With the clearing will come diurnal heating and instability which will introduce another potential round of isolated to scattered showers and thunderstorms Thursday afternoon. Hi-Res model guidance has been inconsistent with afternoon convection Thursday, so we leaned on a uniform 40% PoP for southern New England after 2PM. Expect clearing Thursday night into Friday with warm/sunny conditions expected Friday afternoon.

KEY MESSAGE 2...Mainly dry & very pleasant Fri into early next week except for some diurnally scattered showers mainly on Sat. Pleasant temps too with highs in the 70s...but lower-middle 80s Fri.

Upper level shortwave energy will close off over the Canadian Maritimes over the Holiday Weekend. This will carve out an upper trough over the northeast resulting in pleasant weather. Mainly dry weather expected over the majority of this time. However...some northern stream shortwave energy will probably result in diurnally driven scattered showers mainly on Sat...but can not rule out a few on Fri. High pressure building southward from eastern Canada Sun into Mon should bring an end to this risk for the latter half of the Holiday Weekend. Highs on Fri will mainly be in the lower to middle 80s...but generally 70s to near 80 this weekend into early next week. In fact...coolest days will be Sat and Sun as the eastern MA coast/Cape and Islands probably only see highs in the upper 60s to the lower 70s with a stiff northeast afternoon breeze. Overnight lows mainly in the 50s to the lower 60s...but there might be some 40s in the normally coolest outlying locations by Sun night/Mon AM as high pressure builds across the region.

KEY MESSAGE 3...Elevated risk for dangerous rip currents for the Holiday Weekend especially by Sun into Mon.

Given persistent NE low level flow and swell...we do expect an elevated risk for dangerous rip currents for the Holiday Weekend. This is especially true by Sun into Mon on east facing ocean exposed beaches.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

Through 00Z...High confidence.

VFR. Steady east winds becoming more southeast by 00Z.

Tonight...High confidence in trends, moderate confidence in timing -SHRA/CIGS

Showers arrive overnight with embedded thunder possible from west to east between 06Z and 12Z. Lower ceilings accompany the showers/few storms falling to MVFR/IFR. SSE winds 5-10 kts.

Thursday...High confidence in trends.

MVFR/IFR to start with -SHRA and a few embedded storms exiting 12-15Z. Winds turn to SSW in that timeframe at 6-10 kts. Expect improvements toward VFR 15Z into mid-afternoon. MVFR ceilings may linger for Cape/Islands through around 00Z. Additional isolated to scattered showers with embedded thunderstorm possible Thursday afternoon.

Thursday Night...High Confidence.

VFR. Light northwest winds.

KBOS Terminal...Moderate confidence. Showers arrive overnight, likely arriving after 06Z accompanied by MVFR ceilings/vsbys, potentially IFR. More widespread showers early tomorrow morning with embedded thunder possible.

KBDL Terminal...High confidence in TAF. Showers arrive by around 04Z with a few embedded heavier SHRA/embedded storms possible early Thursday (08-12Z).

Outlook /Friday through Monday/...

Friday: VFR.

Friday Night: VFR. Breezy.

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

Saturday Night through Sunday: VFR. Breezy.

Sunday Night through Labor Day: VFR.

MARINE

Forecaster Confidence Levels...

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

Tonight through Friday morning

A frontal wave moves over the coastal waters early Thursday morning. This will be accompanied by heavy downpours and possibly some thunder/lightning. Some locally stronger wind gusts to 20 knots are possible, but otherwise expect modest but steady southeast flow around 10 knots with gusts from 15 to 20 knots possible. Seas stay at sub SCY levels. Winds shift to the west/northwest tomorrow afternoon and carry into Friday with generally tranquil conditions.

Outlook /Friday through Monday/...

Friday through Friday Night: Winds less than 25 kt.

Saturday: Low risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Areas of seas approaching 5 ft.

Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas.

Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas.

Sunday Night: Winds less than 25 kt. Areas of rough seas.

Labor Day: Winds less than 25 kt. Seas up to 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

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