976
FXUS61 KBOX 212026
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
426 PM EDT Mon Sep 21 2026

.WHAT HAS CHANGED...
No changes.

&&

.KEY MESSAGES...

- Dry and cool weather through at least Tuesday night. Breezy,
  especially across Cape Cod and the Islands.

- Monitoring the potential for a strong coastal low to form mid
  to late week, bringing chances for strong winds, rainfall, and
  coastal flood/erosion concerns heading into the weekend.

&&

.DISCUSSION...
KEY MESSAGE 1...Dry and cool weather through Tuesday night.
Breezy, especially across Cape Cod and the Islands.

Anomalous 1030+ mb high pressure over southeast Ontario Monday,
builds eastward into Quebec Tuesday, providing dry and cool
weather for Southern New England through at least Tue night.
Northeast gradient will maintain breezy conditions, especially
across southeast MA, including Cape Cod and Islands. Model
soundings here support NE winds 15-20 mph with gusts up to 25
mph at times, less wind away from the coastline.

Abundant sunshine Monday afternoon, except post frontal strato-
cu over southeast MA from departing frontal wave, combined with
cool northeast flow over the relatively mild ocean. Mid/high
clouds will preclude radiational cooling tonight, but a very dry
airmass (dew pts in the 40s and 50s) will be sufficient for min
temps to fall into 40s, except low to mid 50s along the coast,
including metro Boston, Providence and Hartford.

Tuesday, short wave over the OH valley today spreads mid/high
clouds into SNE tonight and Tue. However mid level confluent
flow over the region will provide sufficient subsidence for dry
weather to prevail. These mid/high clouds will yield filtered
sunshine Tue. Cooler temps aloft (+7C to +9C) combined with
subsidence inversion from 1030+ mb high, will yield another day
of cooler than normal highs Tue, with highs only in the low to
mid 60s, about 10 degs below normal.

KEY MESSAGE 3...Monitoring the potential for a strong coastal
low to form mid to late week, bringing chances for strong winds,
rainfall, and coastal flood/erosion concerns heading into the
weekend.

Deterministic and ensemble guidance continue to indicate
potential for a strong coastal low to form Wednesday into the
later half of the week. While there is still a large amount of
uncertainty in the guidance, the overarching trend is indicating
the low slowly meanders north into the waters south of
Nantucket; however, each subsequent model run has trended the
track further east. The ECMWF ensemble is showing about a 50/50
member spread on if the low tracks NW of the 40N/70W benchmark
while the GFS ensemble is much more spread, stretching from the
DelMarVa coast to Georges Bank, and the Canadian ensemble
members are majority far east of the benchmark. Regardless, this
system is likely to bring strong winds, rough seas, rain, and
the potential for erosion/coastal flooding. It is important to
note that the exact track and strength of the low will strongly
influence the magnitude of gusts, QPF, and how far inland the
rain extends.

Highest confidence at this time pertains to winds, especially
over the waters. As the low deepens and approaches to the south,
the Canadian high over Quebec is also likely to strengthen and
expand, leading to a tighter pressure gradient over New England.
This will support the development of gusty winds along the
south coast, Cape/Islands, and the waters by Wednesday
morning... before the arrival of the low. Confidence continues
to increase in at least Gale Force winds (34kts/40mph) across
the southern waters, and a Gale Watch continues for Tuesday
evening through at least Thursday morning. As the low continues
to deepen on approach, winds are likely to increase, especially
across the outer waters. By Friday night/Saturday morning, the
latest ECMWF ensemble shows widespread 20 to 30 percent
probabilities of 50 knots (57 mph) across the waters, with a 30
to 40 percent bullseye over Cape Cod Bay. While the winds over
land along the south coast, Cape, and Islands throughout the
week are more likely to be a touch lighter than the marine
winds, they`re still expected to be gusty... more on the order
of 25-35 mph along the immediate coast.

This prolonged period of gusty northeasterly winds will build
rough seas and raise the threat for coastal flooding and
erosion, especially for east facing beaches. Latest coastal
flooding guidance from the Steven`s Institute shows chances for
Nantucket and Boston nearing/entering Action Stage. Given the
previously discussed uncertainty in track and strength,
confidence remains on the lower end for rainfall totals.
Ensemble guidance has ticked up slightly in totals compared to
previous runs, with the ECMWF ensemble showing 50 to 70 percent
probability for 2"+ in 48 hours (20 to 40 percent for 4"+ in 48
hours) over the Cape and Islands, but the GFS ensemble only
shows 30 to 50 percent probability (10 percent for 4"+ in 48
hours).

&&

.AVIATION /20Z 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...High confidence forecast.

VFR conditions throughout the region at 17z, except VFR cigs
over southeast MA, including Cape and Islands. Trend is for VFR
to overspread all regions later today and tonight, but MVFR cigs
may return Tue across southeast MA, as a reinforcing surge of
cool air aloft overspreads this area on NE flow. NE winds 10-15
kt with gusts up to 20-25 kt at times across southeast MA
terminals.

KBOS Terminal...high confidence in TAF.

VFR and dry weather, with NE winds 10-15G20 kt at times.

KBDL Terminal...High confidence in TAF. VFR and dry weather with a
modest NE wind 10-15 kt, gusting up to 20 kt at times.

Outlook /Wednesday through Saturday/...

Wednesday: VFR. Windy with gusts up to 30 kt.

Wednesday Night: VFR. Windy with gusts up to 35 kt.

Thursday: VFR. Strong winds with local gusts up to 40 kt.

Thursday Night: VFR. Strong winds with areas of gusts up to
40 kt. Chance SHRA.

Friday: Mainly VFR, with local MVFR possible. Strong winds with
gusts up to 45 kt. Chance SHRA.

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

Saturday: Mainly VFR, with local MVFR possible. Strong winds
with gusts up to 45 kt. SHRA likely, slight chance TSRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Tuesday Night...High Confidence.

NE winds 15-20 kt with gusts up to 25 kt at times will
continue, as strong 1030+ mb high pressure over southeast
Ontario on Monday, advects eastward into Quebec. This northeast
fetch will yield rough seas on the ocean waters. Dry weather and
good vsby prevail. Potential for gale force gusts appear to
hold off until Wed and beyond.

Outlook /Wednesday through Saturday/...

Wednesday: Strong winds with gusts up to 40 kt. Rough seas up
to 13 ft.

Wednesday Night: Strong winds with gusts up to 40 kt. Rough
seas up to 14 ft.

Thursday: Moderate risk for gale force winds with areas of
gusts up to 45 kt. Rough seas up to 15 ft. Slight chance of
rain showers.

Thursday Night: gale force winds with gusts up to 45 kt. Rough
seas up to 17 ft. Chance of rain showers.

Friday: gale force winds with gusts up to 45 kt. Rough seas up
to 19 ft. Rain showers likely, chance of thunderstorms. Local
visibility 1 to 3 nm.

Friday Night through Saturday: gale force winds with gusts up
to 45 kt. Rough seas up to 20 ft. Rain showers likely, slight
chance of thunderstorms.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Risk for Rip Currents from Tuesday morning through Tuesday
     evening for MAZ007-015-016-019>024.
RI...High Risk for Rip Currents from Tuesday morning through Tuesday
     evening for RIZ002-004>008.
MARINE...Small Craft Advisory until 5 PM EDT this afternoon for
     ANZ231>235-237.
     Gale Watch from Tuesday afternoon through Thursday afternoon
     for ANZ235-237-255-256-282-283.
     Small Craft Advisory until 3 AM EDT Tuesday for ANZ254>256.

&&

$$

DISCUSSION...McMinn/Nocera
AVIATION...McMinn/Nocera
MARINE...McMinn/Nocera