510
FXUS61 KBOX 131124
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
724 AM EDT Sun Sep 13 2026

.WHAT HAS CHANGED...
No significant changes have been made to the forecast.

&&

.KEY MESSAGES...

- Showers with a few embedded thunderstorms overspread the region
  this morning. Pockets of torrential rainfall bring the potential
  for localized flash flooding with the greatest risk near the
  south coast.

- Overall the weather looks rather quiet and mainly dry for the
  upcoming work week with just the risk for a few showers sometime
  Thu and/or Fri. Seasonable temps Mon and Tue with a brief bout
  of summer warmth possible Wed and/or Thu ahead of a cold front.

&&

.DISCUSSION...
KEY MESSAGE 1...Showers with a few embedded thunderstorms overspread
the region this morning. Pockets of torrential rainfall bring the
potential for localized flash flooding with the greatest risk near
the south coast.

A fairly strong shortwave and associated mid-level warm front will
approach the region this morning. In response...the atmosphere will
undergo quite an instability burst with the Showalter indices
dropping nearly 20 units inside 12 hours. This is often a very
strong signal for a cluster of showers and a few embedded
thunderstorms along with the potential for localized torrential
rainfall. Pwats on the NAM/GFS climb to near 2.25 inches across
RI/SE MA today...which is extremely high for this part of the
country. In fact...the EPS situational awareness table has those
Pwat values outside the model/s climatology for this time of year.
In addition...the HREF/s worst case scenario indicates modest
probabilities for 6 HR rainfall to exceed the 10 year ARI/s for
RI/SE MA. Lastly...the HREF continues to indicate a 30 percent
probability for 3"+ of rain inside 3 hours across RI/SE MA. That is
often another signal to look for in the potential for flash
flooding. Keep in mind though...these are just favorable ingredients
and mesoscale factors will determine the outcome.

The above ingredients are favorable to produce a narrow swath of
torrential rainfall and potentially localized flash flooding...But
only if the mesoscale environment cooperates. That is often very
hard to forecast even in the near term. In this case...the track of
a meso-low and its surface boundary will likely be the main player.
If it tracks near the south coast and a surface boundary enhances
convergence with the extreme Pwat plume in place...the risk for
torrential rainfall and localized flash flooding will increase.
Current guidance is still favoring RI/SE MA and especially areas
near the south coast at the greatest risk for this occurrence.
However...the current state of the science still prevents us to pin
an exact area so we can not rule out this risk further north even
into the Boston and Worcester metro areas. And this is still only a
watch...so if the mesoscale environment does not setup the band of
torrential rainfall and localized flash flooding will not occur.

Lastly the overall severe weather threat is quite low today given
most of the instability will be elevated. However...there is a very
low risk for a brief spin-up/waterspout in the vicinity of the
mesoscale low and its associated triple point. Again...it is a
rather low probability but something we need to watch in these
setups.

So to sum up...The Flood Watch continues for the BOS-ORH corridor
points south and east with the highest risk near the south coast.
The potential exists for 2-4" of rain inside 3 hours within the
Flood Watch. Outside that region...rainfall amounts of 0.50" to
1.50" seem reasonable.

The system itself is fairly progressive...so expect the bulk of the
steady showers to come to an end from northwest to southeast roughly
between 2 and 4 pm. However...some of the steady activity may linger
until 5-6 pm towards the Cape and Islands. A cold front will likely
bring a round of brief scattered showers very late tonight into
early Mon morning...but not the widespread heavy rainfall we will be
dealing with today.

KEY MESSAGE 2... Overall the weather looks rather quiet and mainly
dry for the upcoming work week with just the risk for a few showers
sometime Thu and/or Fri. Seasonable temps Mon and Tue with a brief
bout of summer warmth possible Wed and/or Thu ahead of a cold front.

Any lingering scattered showers depart the southeast New England
coast by mid-morning Monday behind the cold front. 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 Wed. This may allow for
a brief return of summerlike warmth Wed and/or Thu. Much of the
upcoming work week looks dry...but the next cold front may bring a
few showers to the region sometime Thu and/or Fri but it does not
look significant at this time.

&&

.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...

Today...Moderate confidence.

Widespread showers and potentially a few embedded thunderstorms will
overspread the region from west to east roughly through 15z. This
will result in low-end MVFR to 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 21z/22z. Winds will be light but
changeable in direction with weak boundaries in the vicinity.
Some LLWS will be possible at times too.

Tonight...Moderate confidence.

Lingering MVFR-IFR with localized LIFR will gradually improve from
west to east tonight behind a cold front. However...some areas that
see low clouds scour out may temporarily fog in before the winds
shift to the NW and bring drier air into the region. There also will
likely be a brief band of scattered showers associated with the cold
front overnight. Light and variable winds will shift to the NW at 5-
10 knots toward daybreak behind the cold front.

Monday...High confidence.

Any lingering lower cigs across the Cape/Islands should improve to
VFR by mid morning. Otherwise...VFR conditions on Monday with
sustained NW winds of 10 to 15 knots and gusts of 20 to 25 knots.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Monday Night through Thursday/...

Monday Night through Tuesday: VFR. Breezy.

Tuesday Night through Thursday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Today...High confidence.

A warm front will cross the waters today. The main concern for
mariners will be heavy showers and potentially a few embedded
thunderstorms with reduced visibility. There is a very low
probability for a water spout or two mainly across the
southern waters. Winds/seas though should remain below small
craft advisory thresholds.

Tonight and Monday...High confidence.

Southwest winds will shift to the northwest overnight and continue
from that direction on Monday behind the cold front. Winds may gust
to between 20 and 25 knots at times on Monday...perhaps flirting
with marginal small craft advisory criteria. Highest risk for small
craft wind gusts may actually be Monday night in the cool air
advection pattern. We will let the next shift take a look at these
potential headlines.

Outlook /Monday Night through Thursday/...

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of seas approaching 5 ft.

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

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

Thursday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch until 6 PM EDT this evening for CTZ004.
MA...Flood Watch until 6 PM EDT this evening for MAZ012-013-015>024.
RI...Flood Watch until 6 PM EDT this evening for RIZ001>008.
MARINE...None.

&&

$$

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