textproduct: Boston / Norton

WHAT HAS CHANGED

Issued a Coastal Flood Advisory for the Cape and Islands for the threat of minor coastal flooding with this afternoon's high tide cycle. Otherwise, the forecast remains on track.

KEY MESSAGES

- Early morning showers and cloud cover give way to a mostly sunny albeit cooler day. Coastal hazards including the potential for minor coastal flooding and rip currents continues for east-facing beaches.

- Dry conditions Labor Day through much of Wednesday. Pattern becomes more unsettled Wednesday night into late-week with periods of showers possible and gusty winds possible Thursday.

KEY MESSAGE 1

Early morning showers and cloud cover give way to a mostly sunny albeit cooler day. Coastal hazards including the potential for minor coastal flooding and rip currents continues for east-facing beaches.

Any lingering showers early this morning dissipate by mid to late morning as an area of subsidence or sinking air moves overhead. The subsidence will also result in a clearing trend through the day. Weak cold advection and 925 mb temps of 12-14C will keep highs in the upper 60s near the eastern coastline to the lower 70s elsewhere. Another day of high astronomical tides and an onshore flow brings the chance for minor coastal flooding back for the afternoon high tide cycle. As a result, a coastal flood advisory has been issued for this afternoon's high tide cycle. The advisory includes the Cape and Islands from 19-23z. As was the case with yesterday's high tide, anticipating minor impacts in typically more vulnerable spots (Easy Street on Nantucket and Sesuit Harbor in Dennis). There is also an high risk for rip currents along east- facing shorelines in eastern MA, as offshore seas rise to around 4-6 ft overnight into Monday.

KEY MESSAGE 2...Dry conditions Labor Day through much of Wednesday. Pattern becomes more unsettled Wednesday night into late-week with periods of showers possible and gusty winds possible Thursday.

The deeper mid-level trough shifts further eastward through the day Monday while higher heights advect in as a mid-level ridge builds to the west. This will bring in warmer temperatures aloft with 925mb temperatures +15-17C. NW flow and a sufficiently mixed boundary layer accompanied mainly clear skies should help yield high temperatures in the the mid to upper 70s for most. High pressure will support mainly dry conditions Monday with similar conditions and temperatures likely for Tuesday under the same pattern.

Ensemble guidance shows a signal for a transition to a more unsettled pattern mid to late week. Flow aloft becomes more zonal with a few embedded shortwave troughs moving through late Wednesday through late-week. This should be accompanied by a plume of above normal moisture (120-150% of normal) and a jet positioned to supply lift across the region. This should be enough to support periods of showers later Wednesday/Wednesday night into Friday. There is spread across guidance in the finer details of the timing of the waves which will influence timing of the showers. Overall, there isn't a strong signal for flash flooding at this time. Marginal instability in place may owe to a storm or downpour depending on the timing.

Another thing we will need to monitor is the potential for a period of gusty winds Thursday. An 850mb 35-45kt LLJ positions across southern New England into Thursday. If winds mix down efficiently, this would support gusty winds. There is a signal for anomalously gusty winds on the ECMWF EFI (still fairly far out). Flow looks to become more westerly behind the passage of the first wave with a signal for CAA. Both things that could facilitate the mixing down of the higher winds aloft (to some extent at least). As mentioned, timing of these waves are lower confidence and this could have influence on when we get the better mixing and thus the gusty winds. So plenty of details to be worked out. If things align, ensembles show potential for gusts in the 30-40 mph range on Thursday across the land and coastal waters. Still plenty of details to work out in the pattern before we come into better agreement about the gusts, but something to watch for now.

Temperatures Wednesday/Thursday lean near to slightly above normal with temperatures in the mid to upper 70s. A cooler airmass pushes in sometime later Thursday into Friday with cooler temperatures arriving by Friday (upper 60s/low 70s).

AVIATION /14Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

15z TAF Update:

Today: High confidence.

SCT-BKN VFR/MVFR ceilings (020-040 bases) continue as of mid- morning, with widely SCT light -SHRA around pushing southward. While showers come to an end shortly, anticipate some lifting of cloud bases closer to VFR levels by afternoon. Another surge of moisture comes down now over central Maine could bring an increase in clouds toward BKN later this afternoon for eastern terminals. North to northeast winds around 5-10 kt.

Tonight: High confidence.

Mostly VFR as cloud cover decreases. Some risk for patchy fog in the river valleys prior to sunrise (08-12z) but its development is uncertain. Winds decrease to light northerly.

Monday and Monday Night: High confidence.

VFR. NW winds increase to around 8-12 kt during the daytime, easing and becoming WNW with an increase in VFR clouds after 06z Tue.

KBOS Terminal...High confidence in TAF. Mix of SCT-BKN VFR/MVFR ceilings, but should trend VFR by early afternoon. NE winds around 5-10 kt. Seabreeze possible but could be late in the day.

KBDL Terminal...High confidence in TAF.

Outlook /Monday Night through Thursday/...

Monday Night through Tuesday Night: VFR.

Wednesday: VFR. Breezy.

Wednesday Night: VFR. Windy with gusts up to 30 kt. Chance SHRA.

Thursday: VFR. Windy with local gusts up to 30 kt. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Sunday Night: High confidence.

SCAs remain in effect for the eastern outer waters through 8 pm this evening as seas build to around 4-6 ft. NE gusts around 15-20 kt through this afternoon, before becoming northerly the rest of Sunday around 10-15 kt. Off-and-on light rain showers this morning before clearing this afternoon. but only minor reductions to visibility, at worst to 4 miles, are expected. Calmer ocean conditions return overnight as high pressure builds in. Seas fall to 2-3 feet.

Outlook /Monday Night through Thursday/...

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

Tuesday through Tuesday Night: Winds less than 25 kt.

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft.

Wednesday Night: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 11 ft. Chance of rain showers.

Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 11 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Risk for Rip Currents until 8 PM EDT this evening for MAZ007- 014>016-019-022>024. Coastal Flood Advisory from 3 PM this afternoon to 8 PM EDT this evening for MAZ022>024. RI...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ250- 254-255.