Marine Weather and Tide Forecast for Sandwich, MA

KEY MESSAGES

- A cold front brings the return of heavy rain and gusty winds Sunday night and Monday. Localized flash flooding possible.

- After a quiet Tuesday unsettled weather returns mid week through the weekend with periods of showers and storms.

DISCUSSION
KEY MESSAGE 1...A cold front brings the return of heavy rain and gusty winds Monday.

An anomalously deep upper trough digging into the Ohio Valley will become cutoff as the broader longwave trough axis works its way slowly east. Its trough axis will cross over southern New England Monday night into Tuesday ushering in a much quiet and drier period.
Before that, however, we'll be dealing with a very moist and dynamic system for the next 18-24 hours. This, as the shortwave and surface reflection pull copious amounts of moisture north into New England on strengthening SW flow (PWATs 2+ inches) with a 40-55 kt LLJ strengthening as it's core moves over the Cape and islands.
Fortunately, even though BUFKIT soundings show 55kts down to about 925mb, the low level inversion should keep the worst aloft.
Regardless, it will be a gusty day, especially over southeast MA.
While some brief stronger gusts can't be ruled out, they are more than likely to top out around 40 mph on Cape Cod.

Regardless of its impact at the surface, said LLJ will certainly act as a strong forcing mechanism for our heavy rain. This together with divergence aloft/placement beneath the RRQ of an upper jet will lead to broad, deep layer ascent and widespread rain showers and thunderstorms. Continuing to expect an initial round of moderate to heavy rain showers this morning into the afternoon which has already begun to materialize. The atmosphere is primed for some heavy downpours and while we can't nail down exactly who will see these downpours, any convection would lead to a better chance of those higher rain rates hinted at by the HRRR (low probability of as much as 1+/hr).

There's unlikely to be a real break in the showers and embedded thunderstorms but any pockets of clearing skies between showers may lead to increased instability parameters and a better chance for one of these thunderstorms to become severe as a final source of forcing arrives in the evening in the form of a cold front. Any storms that do get going will be in an environment with 30-45 kts of 0-6km bulk shear meaning potential for organization and some damaging wind potential.

When all is said and done (ending in western MA by 8pm, eastern MA by midnight) a widespread 0.5 to 1.5 inches of rain is expected.
That being said, convective enhancement and potential for training thanks to boundary-parallel mean flow mean that localized areas could see significantly more, on the order of 3-4 inches. This amount, especially if it falls in a short period, may lead to street flooding and even isolated flash flooding situations. However, we don't expect a widespread flash flooding risk with this system.

KEY MESSAGE 2...After a quiet Tuesday unsettled weather returns mid week through the weekend with periods of showers and storms.

After a beautiful, mostly sunny, less humid day on Tuesday under high pressure ridging, unsettled weather returns Wednesday into the weekend. A broad mid level trough sets up over the eastern U.S. with several shortwaves rotating through the base of the broader trough and this will bring periodic rain and thunderstorm chances each day, especially in the afternoon/evening.

AVIATION /18Z MONDAY THROUGH SATURDAY/
Forecaster Confidence Levels...

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

18Z TAF Update:

Today: Moderate confidence.

Periods of RA this afternoon decrease fromt west to east 21-02Z.
Pockets of moderate to heavy rates possible. IFR/LIFR in RA/+RA. A last round scattered showers and thunderstorms likely pushes through from west to east 18Z-00Z ahead of the approaching cold front. SW winds 10-20 kts with gusts 20-30 kts, highest gusts for the Cape & Islands.

Tonight: High confidence.

Last of the scattered showers and storms push off the east MA and the Cape 23-02Z. Otherwise expect a trend of improving conditions with ceilings improving to VFR between 22Z(west) and 02Z (Cape/Islands). Patchy fog possible in valleys and for Cape and Islands once they clear. Winds shift to the W/WNW behind the front (22-02Z) at 5-10 kts.

Tuesday: High confidence.

VFR. Winds 5-10 kts, first W then becoming SW after 18Z.
Potential for a late-day sea breeze for BOS.

KBOS Terminal...Moderate confidence in TAF.

-SHRA/SHRA this afternoon. A trailing line of scattered showers and thunderstorms is possible 21-00Z. Lower confidence on the potential for TSRA, but enough to include as a PROB30 given the unstable environment today. Conditions improve to VFR 23-01Z with the front as winds shift toward the W by 03Z. Winds become more SW tomorrow morning with potential for a late-day sea breeze.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Tuesday through Saturday/...

Tuesday through Tuesday Night: VFR.

Wednesday through Wednesday Night: VFR. Slight chance SHRA.

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

Thursday Night: VFR. Slight chance SHRA, slight chance TSRA.

Friday: VFR. Breezy. Chance SHRA, chance TSRA.

Friday Night: VFR. Breezy. Slight chance SHRA, slight chance TSRA.

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

MARINE
Forecaster Confidence Levels...

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

Winds and seas increase into early this morning across the southern and eastern waters eventually increasing across the northern waters. Seas 5-9 ft for the outer coastal waters, especially south. 5-7 ft seas stretch into portions of the inner souther waters. SW winds 15-20 kts with gusts around 30 kts.
Can't rule out a few isolated gusts near Gale-force late morning-afternoon if winds mix down from the low level jet overhead. SCAs are in effect for all the coastal marine zones.
Elevated seas will keep SCAs around through Tuesday for the southern waters.

A high surf advisory is in effect for the south coast and islands Tuesday. A strong 35-45 kt southerly low level jet will help build waves 5-9 ft across the waters of RI and south of MVY-ACK Monday. Rough surf and strong rips are likely to linger into Tuesday. Will have to watch this as Tuesday is shaping up to be an ideal beach day.

Outlook /Tuesday through Saturday/...

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

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

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

Wednesday Night through Thursday Night: Winds less than 25 kt.
Areas of seas approaching 5 ft.

Friday through Friday Night: Winds less than 25 kt. Seas up to 5 ft.

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

BOX WATCHES/WARNINGS/ADVISORIES
CT...None.
MA...High Surf Advisory until 9 PM EDT this evening for MAZ023-024.
RI...High Surf Advisory until 9 PM EDT this evening for RIZ006>008.
MARINE...Small Craft Advisory until 5 PM EDT this afternoon for ANZ230- 231-236-251.
Small Craft Advisory until 2 AM EDT Tuesday for ANZ232>234.
Small Craft Advisory until 2 PM EDT Tuesday for ANZ235-237-250- 254.
Small Craft Advisory until 8 PM EDT Tuesday for ANZ255-256.