632
FXUS61 KBOX 211123
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
723 AM EDT Mon Sep 21 2026

.WHAT HAS CHANGED...
Inner southern waters added to the Gale Watch.

&&

.KEY MESSAGES...

- Light showers exit early this morning, pushing offshore of
  the Cape and Islands by late morning. Breezy and cool this
  afternoon.

- Dry and cool conditions persist through mid-week. Monitoring
  potential late-week coastal low that has potential to bring
  rain, strong winds, and coastal impacts depending on the
  track.

&&

.DISCUSSION...
KEY MESSAGE 1...Light showers exit early this morning, pushing offshore of
the Cape and Islands by late morning. Breezy and cool this
afternoon.

Light showers shift eastward across the eastern parts of the region
early this morning. The remaining showers likely push of the Cape
and Island by late-morning. Expect areas of patchy fog early this
morning. A strong surface shifts east across the upper Great Lakes
pushing high pressure in behind the exited system today. This will
bring in a dry and cooler than normal airmass into the region
starting today. Mid-level dry air pushes in quickly within the
northerly flow, with some drying in the lower levels through the
morning based on model soundings. This will result in gradual
improvements in the low clouds and eventual some breaks in the
clouds mid morning into the afternoon. Low clouds may linger longer
for the Cape and Islands. Another cooler day expected today with
850mb temperatures 10-13C yielding highs in the low to mid 60s. Some
spots may reach upper 60s in the valleys. Expect northeasterly
onshore flow today. As a strong high builds in to the northwest the
pressure gradient tightens supporting breezy N/NE winds for the
south coast extending to the Cape/Islands with gusts 20-25 mph.

Mostly cloudy conditions tonight. Despite the limited radiational
cooling, temperatures should fall into the 40s across most of the
interior. Urban areas and areas near the coast will trend warmer in
the 50s.

KEY MESSAGE 2...Mainly dry and cool conditions persist through
mid- week. Monitoring potential late-week coastal low that has
potential to bring rain, strong winds, and coastal impacts
depending on the track.

Ensemble guidance remains in good agreement that an anomalously
strong surface high will set up over Quebec and persist through
late week. This will advect in a below normal airmass, yielding
high temperatures in the upper 50s and 60s alongside a prolonged
period of northeasterly flow through the week. A weak low
pressure system moves through the Mid-Atlantic on Tuesday.
There remains uncertainty regarding its northern extent;
however, if it tracks north enough, a few light showers are
possible. The higher probabilities for any showers will be for
CT, south coast, and Cape/Islands. Otherwise, expect mainly dry
conditions through mid-week.

Attention turns to a developing coastal low late in the week.
Deterministic and ensemble guidance continues to highlight a
slow-moving low developing and tracking northward toward the
northeast coast by late-week. However, considerable spread
remains in the exact track and strength of the low. Ensemble
means have largely settled on a middle-ground solutions of the
low developing off the Mid-Atlantic coast Wednesday, followed by
a northward track into closer proximity of southern New England
Thursday and Friday. Looking at individual members, tracks range
from as far west as Long Island to near Cape Cod, to further
offshore southeast or east of southern New England. Recent ECMWF
ensemble runs have exhibited subtle shifts east/west in the
cluster of low tracks, while the GEFS has show a subtle but
consistent trend west. Regardless of the exact track, ensemble
means bring the low close enough to bring strong gusts, rain,
and coastal impacts(erosion/coastal flooding). It is important
to note that the exact track and low strength will strongly
influence the magnitude of gusts, QPF, and how far inland the
rain extends.

While the center of the low likely holds off until late week,
marine and coastal impacts may begin as early as Wednesday. As
the low deepens in the Mid-Atlantic, the robust high to the
north will aid in tightening the regional pressure gradient,
particularly across the southern coastal waters. This will
support development of strong winds by Wednesday AM, well ahead
of the primary low. The exact positioning and strength the
associated areas of high/low pressure will dictate how strong
the winds could be. There is growing confidence in Gale
force(40mph+) for the southern coastal waters which has been
heavily supported by the ECMWF and its ensemble, though the
GEFS/GFS remain slightly weaker. A Gale Watch has been issued in
these areas. As the system lifts northward expect strong gusts
to continue if not strengthen and expand across portions of the
waters. The ECMWF ensemble even shows low probabilities
(10-25%) for greater than 45 mph gusts Thursday into Friday.
Inland winds will also be gusty, though lighter than over the
waters with gusts 25-35 mph, strongest along the immediate
coast.

This prolonged period of gusty northeasterly winds will build
rough seas and elevate the threat for coastal erosion and
flooding depending on the track. Given the aforementioned spread
in track and intensity across models, rain amounts have also
remained uncertain. Most guidance suggests the rain shield will
clip at least the far eastern portions of MA with the 50th and
upper percentiles shifting the shield further west. Some
deterministic run and individual ensemble members indicate
amounts > 1.0" which will require monitoring given this is a
slow moving system.

In summary, confidence is steadily increasing that a coastal
will at least impact the coastal waters and coastal areas late
this week. However, this is still far enough out that the
details (track, strength, upper level pattern), so the exact
impacts and magnitude are still uncertain at this time. Stay
tuned.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

12Z TAF Update:

Today...High confidence in trends, moderate confidence in
timing and ceilings.

IFR/MVFR improving to VFR through the day, though exact timing
is lower confidence. General trend is ceilings improving to VFR
09-14Z for interior terminals and 13-16Z for more east terminals
(BOS, PVD). Improvement trend will be likely slow for
Cape/Islands potentially not VFR until 18Z-21Z. NNE to NE winds
increase to 10-15 kt, with gusts up to 25 kt possible across
the Cape/Islands. Gusts diminish by 00Z, although sustained
winds 10-12 kts possible for Cape/Islands early evening.

Tonight...Moderate confidence.

VFR. Lower-end VFR ceilings are possible near the southern
coast and Islands closer to 12Z. NNE winds 8-12 kt, increasing
to 10-15 kt across the Cape and Islands.

KBOS Terminal...Moderate confidence in TAF.

Ceilings improving to VFR 14-16Z. NNE winds 8-12 kts with gusts
up to 20 kts developing after 15Z through around 21Z.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday through Friday/...

Tuesday: VFR. Breezy.

Tuesday Night: VFR. Windy with local gusts up to 30 kt.

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

Thursday: VFR. Windy with gusts up to 35 kt.

Thursday Night: VFR. Strong winds with local gusts up to 40 kt.
Slight chance SHRA.

Friday: VFR. Strong winds with gusts up to 45 kt. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Monday Night...High Confidence.

Any remaining showers end mid to late morning across the coastal
waters. Winds N/NE today with gusts prompting Small Craft
headlines for all southern waters and Cape Cod Bay through the
afternoon. Seas build to 3-5 ft, with the highest seas across
the outer waters. Winds subside Monday evening, but only for a
brief time as winds increase Tuesday with rougher seas (4-7 ft).
Winds continue to strength into mid-week and furthermore into
late week ahead of an approaching coastal low. This has promoted
a Gale Watch for the southern coastal waters. The inner southern
waters have been added into the Watch in the most recent update.
There is still uncertainty in the timing of the Gale force gusts
and how widespread they will be. Strong winds and rough seas
continue through late week.


Outlook /Tuesday through Friday/...

Tuesday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Seas up to 5 ft. Slight chance of rain
showers.

Tuesday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 10 ft.

Wednesday: Strong winds with gusts up to 35 kt. Rough seas up
to 13 ft.

Wednesday Night: Moderate risk for gale force winds with gusts
up to 40 kt. Rough seas up to 14 ft.

Thursday: Strong winds with gusts up to 45 kt. Rough seas up to
15 ft. Slight chance of rain showers.

Thursday Night: Moderate risk for gale force winds with areas
of gusts up to 45 kt. Rough seas up to 16 ft. Chance of rain
showers, slight chance of thunderstorms.

Friday: gale force winds with gusts up to 45 kt. Rough seas up
to 18 ft. Rain showers likely, slight chance of thunderstorms.
Local visibility 1 to 3 nm.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
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...Mensch
AVIATION...Dooley/Mensch
MARINE...Mensch