150
FXUS61 KBOX 201138
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
738 AM EDT Sun Sep 20 2026

.WHAT HAS CHANGED...
No significant changes.

&&

.KEY MESSAGES...

- Rain arrives from west to east through the morning continuing
  overnight.Rain exits most areas Monday morning with the
  exception of a few lingering showers near the Cape and
  Islands.

- High pressure will yield mainly dry conditions and cool
  temperatures the first half of the week. Potential for an
  offshore coastal low late week. There is still uncertainty in
  the track and strength at this time.

&&

.DISCUSSION...
KEY MESSAGE 1...Rain arrives from west to east through the morning
continuing overnight.Rain exits most areas Monday morning with
the exception of a few lingering showers near the Cape and
Islands.

Expect the second-half of the weekend to be a soggy one. A shortwave
trough coupled with an 850mb jet will advect in ample moisture and
increase lift across the region. Precipitable water values increase
to 1.3-1.8" which equates to 160-190 percent of normal. Despite the
anomalous moisture, weak forcing and a lack of instability will
limit heavy rain rates, keeping the flash flood threat low. This
event will primarily feature a prolonged period of more stratiform-
like rain driving by the WAA rather than convective downpours that
are fueled by instability.

High-resolution guidance shows rain likely arriving mid to late
morning for most areas as rain spreads from west to east. Rain may
not arrive in the more eastern parts of southern New England until
around noon. Rain continues overnight, coming to and end between 2AM-
8AM for most areas. High-res guidance hints at the potential for
showers to linger for the Cape and Islands through the afternoon
(latest).

As mentioned earlier, there isn`t a strong signal for widespread
heavy rain/downpours with this system. However, there are
indications of very marginal instability along the south coast which
may be enough to support an embedded rumble of thunder and a brief
heavier rainfall. Areas of increased convergence will also be
favored for enhanced rain rates. There are a couple areas of
enhanced convergence as well that have been highlighted across model
guidance with areas along the Mass Pike and higher terrain in
western and central southern New England. These areas would at least
be more favorable for more enhanced rates and thus higher end
totals.

For total amounts, HREF PMM shows a range of 0.50 to 1.2" with the
higher end of the range being more localized and in those favored
areas discussed above (terrain, south coast). HREF ensemble max
shows localized amounts of 1.5-1.75" which would be a low
probability (< 10%) given it is a max of that guidance.
Global/ensemble guidance leans slightly lighter overall with amounts
0.30-0.70". Outside of the localized moderate to briefly heavier
rates, the lower rain rates should keep the flash flood threat low.
Can`t rule out some minor ponding in the usual poor drainage spots,
but otherwise this will be prolonged soaking rain.


KEY MESSAGE 2...High pressure will yield mainly dry conditions
and cool temperatures the first half of the week. Potential for
an offshore coastal low late week. There is still uncertainty in
the track and strength at this time.

There is good agreement across model guidance with a fairly
strong surface high shifting over Quebec Tuesday into late-week.
This will put the region under a cooler than normal airmass
with consistent NE flow. This will yield cooler than normal
highs in the 60s through the week. There is a low probability
that a system just brushes southern New England Tuesday bringing
a chance for showers mainly confined to the immediate south
coast and Cape/Islands.

Deterministic and ensemble guidance continues highlight potential
for a slow moving offshore coastal storm developing Wednesday into
late-week. There is a general consensus with an upper low and trough
first tracking eastward across the upper mid-west, although cluster
analysis does highlight some variance in the amplitude of the trough
and track of the  low. As this pushes to the east coast, a coastal
low develops in the Atlantic then tracking northward toward the
northeastern coastline. As with most coastal storms this far out,
there is still a notable amount of variance across guidance in how
this low tracks and its strength. This will directly feed into the
uncertainty in elements such as winds, rain chances/amounts, and
coastal erosion/flooding concerns.

With a strong high to the north and an incoming upper low, the
pressure gradient will tighten Tuesday through the week. This will
result in strengthening winds/gusts across the coastal waters,
especially the southern waters in closer proximity to the stronger
gradient. There is some question on how strong the gradient gets and
where the strong part of the gradient positions and this will
likely depend on the track and strength of the low. All that
said, ensemble guidance is signaling potential for gale force
gusts for the southern waters. Strong gusts may develop as early
as Tuesday night, but more likely Wednesday morning. Strong
winds are likely to stay as the coastal low tracks northward;
however, the magnitude will depend more on the track and
strength of the low itself which remains less certain at this
time. For land, winds will be gusty as well although not as
high in magnitude as the coastal waters(25-35 mph) Wed- late
week. Cape and Islands will likely be on the higher range of the
gusts. If the low ends up tracking more west, periods of rain
will be possible sometime in the Thursday to Sat timeframe.

&&

.AVIATION /12Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

12Z TAF Update:

No major changes from the previous forecast. Trends remain on
track.

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

VFR to start, trending downward to MVFR from west to east as
rain overspreads the region 13-17Z. Potential for IFR ceilings
to develop this afternoon-evening although coverage is lower
confidence. The threat for LLWS has lessened, although can`t be
ruled out as a weaker low level jet shifts across southern New
England between 18Z-06Z. Otherwise winds generally SE to ESE at
5-10 kt. Chances for embedded thunderstorms are very low to
near zero.

Sunday Night...High confidence in trends, moderate confidence
in timing and ceilings.

IFR ceilings with LIFR possible in areas. Visibilities 2-5SM
with -RA continuing overnight. -RA ends from west to east
mainly between 06Z-12Z. Winds shift to the N at 8-12 kt.

Monday...Moderate confidence.

IFR and MVFR improving to VFR through the day. Exact timing
somewhat uncertain. NE winds 10-20 kt with gusts to 25 kt
possible towards the Cape and Islands along with the higher
sustained winds. These diminish slightly heading into Monday
night.

KBOS Terminal...Moderate confidence in TAF. Timing for onset of
IFR is somewhat uncertain, but generally expected to be after
18z. Hinted at the possibility with SCT groups at IFR levels for
the time being, at least until 05z.

KBDL Terminal...High confidence in TAF.

Outlook /Monday Night through Thursday/...

Monday Night: VFR. Breezy.

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

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

Wednesday: Mainly VFR, with local MVFR possible. Windy with
gusts up to 35 kt.

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

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

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Monday...High Confidence.

A low pressure system will bring rain today arriving into the
coastal waters in the afternoon, continuing into Monday There
is a low chance for a few rumbles of thunder, primarily across
the southern waters. The bulk of the rain ends early Monday
morning, though a few showers may linger through the afternoon
across the waters. Seas remain between 2-4 ft. Southeast winds
today less than 25 kts. Front moves through early Monday morning
shifting winds to the N/NE. Gusts increase into the 20-30 kt
range with the highest gusts across the southern waters Monday
morning into the evening. Seas increase to 5 ft across the
southern and eastern outer waters. Small Craft Advisory issued
12Z-00Z for those areas.

Outlook /Monday Night through Thursday/...

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

Tuesday: Strong winds with gusts up to 35 kt. Seas up to 5 ft.
Chance of rain showers.

Tuesday Night: Strong winds with gusts up to 40 kt. Rough seas
up to 11 ft. Slight chance of rain showers.

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

Wednesday Night: Strong winds with gusts up to 45 kt. Rough
seas up to 16 ft. Slight chance of rain showers.

Thursday: gale force winds with gusts up to 45 kt. Rough seas
up to 20 ft. Chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 AM to 8 PM EDT Monday for
     ANZ232>235-237-254>256.

&&

$$

DISCUSSION...Mensch
AVIATION...Hrencecin/Mensch
MARINE...Mensch