textproduct: Boston / Norton

WHAT HAS CHANGED

High Surf Advisory issued along the east coast of MA Friday through the weekend.

KEY MESSAGES

- Dry, cool, and breezy today with rough seas.

- A strong coastal low pressure system will bring a long duration of rain, rough seas, strong winds, and coastal flooding/erosion Friday through the weekend. Exact magnitude and placement of the winds and rain remain uncertain.

DISCUSSION

KEY MESSAGE 1...Dry, cool, and breezy today with rough seas.

The dry and cooler than average weather we've been enjoying for the last several days continues for at least one more day thanks to a sprawling high pressure centered over Quebec. Placement of southern New England between this surface high and building low pressure to the north will continue the persistent and increasingly steady northeast flow keeping temperatures in the low 60s (east) to mid/upper 60s (CT valley). Thanks to those persistent winds over a long fetch we'll continue to see dangerous seas with waves a high as 12-15 ft over the southern outer waters. Rough seas will only increase in magnitude and area going into the weekend. Over land, we'll continue to see breezy winds gusting to 25-35 mph.

KEY MESSAGE 2...A strong coastal low pressure system will bring a long duration of rain, rough seas, strong winds, and coastal flooding/erosion Friday through the weekend. Exact magnitude and placement of the winds and rain remain uncertain.

Bottom line up front: prolonged rain and wind is expected this weekend, with the worst of it along the coast. Coastal impacts will also be felt from coastal flooding and erosion, especially east facing coastlines. Guidance is ever so slowly coming into agreement on the storm's track, leading to still only moderate confidence in exact numbers/times with regard to strongest winds, peak coastal flooding etc. For this reason, watches continue. However, confidence is high that this will be an impactful early season nor'easter for southern New England as a coastal low meandering off the eastern seaboard is captured by another feature and pulled back to the west after passing near the 70/40 benchmark. Latest GFS guidance seems to be caving toward a more western track in line with the ECMWF/UKMET. It's yet to be seen if this is the start of a trend or an outlier, but the 00Z GEFS continues to have quite a wide E-W spread in low centers by Saturday morning.

A western track would have an impact on coastal flooding, potentially expanding minor flooding to more southern facing coastlines toward the later half of the weekend; this is hinted at in the Stevens Institute coastal flooding guidance and will need to be monitored as we settle on an eventual low track. Regardless, we will no doubt be dealing with a good deal of coastal erosion and at least some minor coastal flooding on the east coast, especially south of Boston. This will be a prolonged coastal flooding event from late Fri-Sun which could exacerbate flooding concerns as water may not be able to fully evacuate between high tides.

The latest ensemble probabilities continue to indicate the heaviest rainfall over eastern MA and RI where probabilities of 2+" have increased (80%+) as have probabilities of 4" (40-60%) centered over eastern MA. The slow moving nature of this pattern means that this is spread over 48+ hours which should limit flash flooding concerns.

The strongest winds will be over the waters where Storm Watches remain in effect starting on Friday. On land the strongest winds will be along the coast where the presence of leaves on trees will exacerbate potential tree damage compared to a late season storm. Ensemble guidance indicates >70% chance of gusts over 40 mph, while the chance of gusts nearing 60 mph over Cape Cod and the islands has decreased in the last 24 hours for both the EPS and GEFS likely owing to the further west track. The low center lingers, weakening, Sunday and even into Monday before finally exiting east by Tuesday.

AVIATION /06Z THURSDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

06Z update...

Through 12z...High confidence.

VFR. NE winds of 10 to 20 kt with gusts of 20 to 30 kt, strongest towards the southeast New England coast. Gusts up to 35 kt for Nantucket.

Thursday and Thursday Night...High confidence.

VFR, outside of MVFR cigs over ACK and some of the outer Cape late Thursday into the night. NE winds of 10 to 20 kt with gusts of 20 to 30 kt, strongest towards the southeast New England coast. Gusts near 35 kt for Nantucket.

Friday...High confidence.

VFR most of the day, with MVFR cigs moving in from the southeast, impacting ACK as early as 16z and FMH/HYA by 00z. Lowered cigs come following RA moving in throughout the day from SE to NW. Strong NE winds, 15-25 kts with gusts 30-45 kts, strongest over southeast MA terminals.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Friday Night through Monday/...

Friday Night: Mainly VFR, with local IFR possible. Strong winds with gusts up to 50 kt. SHRA likely.

Saturday: Mainly MVFR, with local IFR possible. Strong winds with gusts up to 45 kt. SHRA likely.

Saturday Night: Mainly MVFR, with local IFR possible. Strong winds with gusts up to 40 kt. SHRA, slight chance TSRA.

Sunday: Mainly MVFR, with areas IFR possible. Strong winds with local gusts up to 40 kt. SHRA, slight chance TSRA.

Sunday Night: MVFR/IFR conditions possible. Windy with gusts up to 35 kt. SHRA likely, slight chance TSRA.

Monday: Mainly VFR, with areas IFR possible. Windy with local gusts up to 30 kt. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Friday...High Confidence.

Gusty NE winds resume Thursday and continue well into the weekend. Winds 25-35 kt are expected on Friday, gusting to 40-50 kts. This northeast fetch will yield rough seas on the ocean waters, 6-12 ft Thursday, 10-18 ft Friday. Rain arrives from the SE as early as late Thursday night/early Friday morning for the waters south and east of Nantucket.

Outlook /Friday Night through Monday/...

Friday Night: gale force winds with gusts up to 45 kt. Rough seas up to 21 ft. Rain showers, slight chance of thunderstorms.

Saturday: gale force winds with gusts up to 45 kt. Rough seas up to 19 ft. Rain showers, slight chance of thunderstorms.

Saturday Night: gale force winds with local gusts up to 45 kt. Rough seas up to 17 ft. Rain showers, slight chance of thunderstorms.

Sunday: Strong winds with gusts up to 40 kt. Rough seas up to 13 ft. Rain showers likely, slight chance of thunderstorms. Local visibility 1 to 3 nm.

Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 11 ft. Rain showers likely, slight chance of thunderstorms.

Monday: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Seas up to 5 ft. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Surf Advisory from 5 AM Friday to 8 PM EDT Sunday for MAZ007-015-016-019-022>024. High Wind Watch from Friday morning through Saturday evening for MAZ007-015-016-019>024. Coastal Flood Watch from Friday afternoon through Saturday afternoon for MAZ007-015-016-019-022>024. RI...High Wind Watch from Friday morning through Saturday evening for RIZ005>008. MARINE...Small Craft Advisory until 8 AM EDT Friday for ANZ230-236. Gale Warning from 8 AM Friday to 8 PM EDT Saturday for ANZ230- 236. Gale Warning until 8 AM EDT Friday for ANZ231-254-281. Storm Watch from Friday morning through Saturday evening for ANZ231-254-281. Gale Warning until 2 AM EDT Friday for ANZ232>235-237-255-256- 282-283. Storm Watch from late tonight through Saturday evening for ANZ232>235-237-255-256-282-283. Small Craft Advisory until 2 AM EDT Friday for ANZ250-251-280. Gale Warning from 2 AM to 11 AM EDT Friday for ANZ250-251-280. Storm Watch from Friday morning through Saturday evening for ANZ250-251-280.