027
FXUS61 KBOX 121121
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
721 AM EDT Sat Sep 12 2026

.WHAT HAS CHANGED...
A Flood Watch has been issued for Rhode Island and southeast
Massachusetts for Sunday.

&&

.KEY MESSAGES...

- Showers, and possibly a few thunderstorms, with pockets of
  torrential rainfall possible early Sun am into the afternoon.
  A Flood Watch has been issued for RI and southeast MA.

- Any lingering scattered showers depart early Mon...otherwise
  dry/seasonable Mon-Tue but a brief bout of summer warmth and
  humidity possible Wed and/or Thu ahead of a cold front.

&&

.DISCUSSION...
KEY MESSAGE 1...Showers, and possibly a few thunderstorms, with
pockets of torrential rainfall possible early Sun am into the
afternoon. A Flood Watch has been issued for RI and southeast MA.

One more beautiful day is on tap with plenty of sunshine and high
temps this afternoon in the 70s.

Otherwise...the main forecast concern will be an approaching
shortwave and associated warm from very late tonight into Sunday. As
this shortwave approaches...it will result in a very strong
instability burst. Showalter indices drop near 20 units to near or
below zero inside 12 hours. That rapid change in stability is often
a good sign for a complex of widespread showers and the potential
for a few embedded thunderstorms. In addition...Pwats climb to
around 2.25 inches across RI/SE MA which is quite high for this part
of the country and supportive of pockets of torrential rainfall. The
GEFS situational awareness tables indicate Pwat values outside the
models climatology near the south coast Sunday morning.

So in a nutshell...the ingredients above are concerning for a narrow
band of torrential rainfall and the potential for localized flash
flooding. The HREF has probabilities for 6-24 HR precipitation
values to exceed 10 year ARI across parts of eastern CT/RI and
southeast MA. There are even very low probabilities to exceed 100
year ARI values near the south coast. That being said...whether or
not this actually materializes remains uncertain and will depend on
the mesoscale. And on top of that...even if it occurs trying to
determine the specific location is still beyond the state of the
science. But we do know this risk will hinge on the potential for
where a mesoscale low tracks...which would setup an area of enhanced
low level convergence for the southerly LLJ to impinge upon.

Given the above...we have opted to issue a Flood Watch very late
tonight into Sunday for RI and southeast MA. Whether or not this
narrow band of torrential rain falls across this region or remains
to our south is uncertain...but given the potential felt it is just
a Watch and worth highlighting the potential. Widespread rainfall of
0.50" to 1.50" expected across much of our region...But localized
narrow bands of 2"-4" of rain possible inside 3 hours. If this
occurs...the potential for localized flash flooding would increase
especially if it occurs over a vulnerable urban center.

KEY MESSAGE 2...Any lingering scattered showers depart early
Mon...otherwise dry/seasonable Mon-Tue but a brief bout of summer
warmth and humidity possible Wed and/or Thu ahead of a cold front.

The actual cold front crosses the region later Sunday night into
early Monday morning. This may be accompanied by a few brief
showers. Otherwise...high pressure builds back into the region Mon
and Tue bringing dry and seasonable weather. High temps will mainly
be in the upper 60s to the middle 70s with lows in the 40s and 50s
both days. This high pressure system moves east of the region by the
middle of next week. This may allow for a brief return of summerlike
warmth/humidity sometime Wed and/or Thu.

&&

.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

12z TAF Update...

Today...High confidence.

VFR. Winds becoming S at 6-12 knots except for sea breezes
developing along much of the immediate coast.

Tonight and Sunday...High confidence tonight with moderate
confidence Sunday.

Mainly VFR conditions tonight. A large cluster of showers and
potentially a few embedded thunderstorms will overspread the region
from west to east roughly between 9z-15z Sunday. This will result in
low end MVFR-IFR conditions developing with even brief LIFR
conditions in narrow bands of torrential rainfall. The bulk of the
heaviest showers should be exiting the southeast New England coast
by 20z/21z. Winds will be light from a S-SE direction. There also
may be a period of LLWS.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Mainly VFR, with local IFR possible. Breezy.
Slight chance SHRA, patchy BR.

Monday through Monday Night: VFR. Breezy.

Tuesday through Wednesday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Today through Sunday...High confidence.

High pressure over the waters today shifts to the east tonight. A
warm front than approaches and crosses the waters on Sunday. Surface
gradient should remain weak enough to keep winds/seas below small
craft advisory thresholds through Sunday. However...the main concern
for mariners will be heavy showers and potentially a few embedded
thunderstorms on Sunday with reduced visibility.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers, patchy fog. Local visibility 1 to 3 nm.

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers.

Monday Night through Tuesday: Winds less than 25 kt. Areas of
seas approaching 5 ft.

Tuesday Night through Wednesday: Winds less than 25 kt. Seas
locally approaching 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Flood Watch from late tonight through Sunday afternoon for
     MAZ017>024.
RI...Flood Watch from late tonight through Sunday afternoon for
     RIZ001>008.
MARINE...None.

&&

$$

DISCUSSION...Frank
AVIATION...Frank/Dooley
MARINE...Frank