Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 110026

Area Forecast Discussion
National Weather Service Boston/Norton MA
726 PM EST Sun Dec 10 2023

Multi faceted storm with heavy rain leading to flash flooding and
potentially damaging wind gusts will impact southern New England
through Monday morning. Behind the departing system, the week
ahead looks dry, cooling down mid week before warming up once
again towards next weekend.



7:25 PM update...

No major changes with this forecast update, as previous forecast
remains on track. True warm sector air across SNE with 60-65
temps at 7 pm across RI and southeast MA! Peak wind gust last
hour, 34 kt/39 mph at ACK. Strongest winds overnight occur
roughly in the 08z-14z window from west to east, as
strengthening low level jet traverses the region and is aided by
strong pressure falls/isallobaric acceleration. Brief break in
rain intensity next few hours before next band of heavy rain
currently across eastern PA and NJ arrives here in SNE later
this evening. As of 7 pm, all rivers and streams remain well
below action stage along with FLASH not concerning. Therefore
flood concerns will hold off until midnight and thereafter,
especially into Monday morning. Previous discussion below.


Rain associated with this evening`s much anticipated event has
begun to overspread southern New England with showers blossoming
across the entirety of the region. While generally light, we
have seen a few tenths of QPF over the last hour at sites across
central Connecticut, such as Willimantic.

Low pressure continues to push into southern New England this
evening as mid level trough tilts negative, amplifying pressure
falls as well and the low level jet, which at 925mb will reach
peak strength of 80-90kt early Monday morning.

Overall, anticipated hazards remain consistent with previous updates
with the low level jet mixing potent gusts to the surface across
southeast MA, Rhode Island, and Cape Cod. Gusts will build towards
40kt by the midnight hour, growing in strength as the low closes in
on southern New England by 12Z, at which time peak gusts near 55kt
will be experienced across the Cape and Islands. After 12Z, the
cold front that sweeps the low into the Gulf of Maine will
cause an abrupt wind shift from the south to the northwest. With
the jet offshore, will see winds diminish by half behind the
front, to 25-35kt between 09-15Z.

While not diminishing the damaging wind threat for coastal southern
New England, we wish to highlight that excessive rainfall leading to
rapid small river and stream rises as well as urban, poor drainage,
and flash flooding is likely overnight as rain becomes more intense
after sunset this evening. This system will feed off of a stream of
moisture advecting from the Gulf of Mexico, causing PWATs to rise to
1.5" plus (very high by December standards), combined with
southerly, parallel to the approaching front, flow some four to
five standard deviations of normal, which will amplify warm
rain processes. Additionally, the freezing level is some ten to
twelve thousand feet, indicating that this tropically infuses
system will pack a punch across southern New England, even
though it will lack significant convective elements with little
to no instability present. Several pieces of guidance show the
possibility of the low stalling, or even backbuilding a bit,
overnight, which would prolong the duration of heavy precip.

The overall QPF forecast remains consistent with the previous shift,
with a maximum of 4-6", perhaps locally higher, forecast over east
central Connecticut and central Massachusetts. HREF ensemble
consistency has been rather remarkable over the last few runs, with
little to no wobble in the anticipated axis of heaviest
precipitation and consistency in the 24 hour LPMM (best reasonable
guess for maximum precip potential) showing a widespread swath of 5-
7" across the aforementioned region. While it may seem
insignificant, both the CMC and GEFS ensembles show bullseyes of
10-30% probabilities of 4" of QPF or greater over
Tolland/Windham County Connecticut; which is marked as a
considerable signal for significant precip in terms of global
guidance. Thus, with all things above considered, collaborated
with WPC and our neighboring WFOs to upgrade the Excessive
Rainfall Outlook to MODERATE, to highlight the increased
potential for flash flooding. Given much of this precipitation
will fall overnight and to start the morning commute, motorists
are urged to heed extreme caution when encountering flooded
roadways as hazards may be difficult or impossible to see. Turn
around, don`t drown!

Aforementioned cold frontal passage will begin to infiltrate
far western southern New England around 09Z which will allow for
a transition to snow across the high terrain as temperatures
drop some 20F behind the front. Accumulation of snow will be
difficult outside of the high terrain given very warm antecedent
conditions, a very saturated ground, and borderline temps in
the mid 30s, thus, again relied heavily on positive snow depth
change forecasts rather than 10:1 or Kuchera snow accumulations
to derive our forecast. Overall, 1-4" is forecast for areas of
Franklin, Hampshire, and Hampden county above 800ft, with minor
accumulations possible between 600-800ft. Rain will likely
transition to snow across the higher terrain of Worcester County
for a very brief period between 12-15Z, but will have trouble
accumulating; again, for the above reasons. By 15Z, precip will
begin to wrap up everywhere with clearing beginning from
southwest to northeast.



It will be a tale of two forecasts on Monday as cold front pushes
offshore between 14-16Z, allowing for a much colder, drier airmass
to infiltrate southern New England. Fortunately, temperatures
in the mid 30s to mid 40s tomorrow afternoon accompanied by a
drying, blustery NW wind will preclude a hard freeze. Sunshine
looks to become widespread by the lunch hour/early afternoon.

Will note that there is still considerable uncertainty regarding
wind gusts tomorrow afternoon as guidance is split into two
campaigns about the strength of the low level jet. The GFS and NBM
are amped up on the potential for a 40-45kt jet maintaining position
over western MA tomorrow afternoon, with the NAM, ECWMF, and CMC
highlighting a weaker jet around 30kt. Thus, while it is possible
that gusts of 40kt+ mix to the surface across the Berkshires on
downsloping NW CAA, which would reach wind advisory criteria,
confidence is too low to issue a headline at this time.



Key Points:
 - Mainly dry weather expected
 - Below normal temps give way to another warmup by next weekend

Upper level pattern should feature a broad trough over much of
eastern US through midweek, but with lack of downstream blocking,
pattern remains progressive and trough will exit to Maritimes. This
will allow for a pattern change later in week as upper ridge takes
hold and brings yet another warmup into next weekend. This pattern
also favors dry weather overall.


Forecast Confidence Levels:

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

00z update: high confidence in trends, slightly lower on exact
timing of details. Strong frontal passage 07z-13z roughly from
west to east, with wind shift from SSW to west. Periods of heavy
rain in IFR/LIFR along with strong low level wind shear. No
change to TAFs as previous forecast remains on track. Earlier
discussion below.


Light rain gradually overspreads region this afternoon as
conditions lower to MVFR/IFR. Steadier and heaver rain arrives
this evening and continues overnight with widespread IFR/LIFR.
Should see rapid improvement to VFR as rain ends 15-18Z Mon but
with areas of MVFR ceilings lingering near the higher

Strong low level jet of 80kt passing through SE New England will
result in significant LLWS, mainly near coast, starting late
this afternoon and continuing through late tonight.

S winds increase through evening with peak gusts 40-60kt
overnight, strongest near Cape Cod and Islands, then winds
shift to W/NW overnight but still remain gusty through day on

KBOS Terminal...Moderate confidence. Timing of lowering ceilings
may be too fast through this evening. Higher confidence on winds
and LLWS.

KBDL Terminal...High confidence.

Outlook /Monday Night through Friday/...

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

Tuesday through Tuesday Night: VFR. Breezy.

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

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

Thursday: VFR. Windy with local gusts up to 30 kt.

Thursday Night through Friday: VFR. Breezy.


Forecaster Confidence Levels:

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

Dangerous storm for mariners expected to impact the waters
mainly tonight into Monday. Storm Watches remain in effect for
all waters except Boston Harbor where a Gale Warning is in
place. Rain will continue across the waters overnight and will
be heavy at times, reducing visibility. There is even a low
risk (10-20%) for thunderstorms, especially across the southern
coastal waters.

An abrupt and significant shift in winds is expected mid to
late day Monday as a cold front crosses the waters. Seas
gradually diminish through the day Monday, but will still be
rough on the waters.

Outlook /Monday Night through Friday/...

Monday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 13 ft.

Tuesday: Winds less than 25 kt. Seas up to 5 ft.

Tuesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft.

Wednesday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Seas up to 5 ft.

Wednesday Night: Low risk for gale force winds with gusts up to
35 kt. Seas up to 5 ft.

Thursday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas.

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas locally approaching 5 ft.

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Areas of seas approaching 5 ft.



The risk for rapid brook and stream rises, as well as poor
drainage and urban flooding, is elevated from tonight into
Monday. Most locations within interior southern New England
should see 2-4 inches of rainfall. Farther SE, rainfall totals
should be 2 inches or less, mainly due to the duration of
rainfall. The risk for 5"+ of rain has shifted a bit southeast
into central/eastern Connecticut and Central Massachusetts, and
has been highlighted by a Moderate Excessive Rainfall outlook.

Rainfall of this magnitude during this time of year, where the
ground may be partially frozen or debris may be clogging storm
drains, only magnifies the runoff and lesses the time before
rises on smaller stream and creeks.



* Coastal Flood Advisory for Narragansett Bay and south coast

Latest guidance suggests the peak surge for Providence will be
near the time of high tide. There is still some uncertainty with
exact timing and it could end up a few hours earlier or later
depending on the timing of the cold front and wind shift.
However, our confidence is high that peak surge will reach up to
3 ft at Providence and 1.5 to 2.5 ft elsewhere along the RI and
MA south coast. We think the ETSS and ESTOFS surge guidance is
underdone and we leaned toward PETSS 90th percentile given the
strong southerly LLJ and 24 hr pressure falls of 24-26 mb,
which will enhance the surge. The highest confidence for minor
flooding will be along Narragansett Bay due to
funneling/channeling up the Bay with southerly flow. The risk
for moderate flooding is rather low so we converted the coastal
flood watch to an advisory. We would need a 4+ ft surge to reach
moderate flooding and this appears unlikely at this time.

The flood threat is lower outside of Narragansett Bay where
surge will be less. Worst case scenario would be very minor
flooding or splashover along the RI south coast west of
Narragansett Bay as well as Buzzards Bay and the south coast of
Cape Cod.


CT...Flood Watch through Monday evening for CTZ002>004.
MA...Flood Watch through Monday evening for MAZ002>021-026.
     Wind Advisory until 1 PM EST Monday for MAZ007-013-015>018.
     High Wind Warning from 10 PM this evening to 10 AM EST Monday
     for MAZ019>021.
     Coastal Flood Advisory from 4 AM to 8 AM EST Monday for MAZ020.
     High Wind Warning from 10 PM this evening to 1 PM EST Monday
     for MAZ022>024.
RI...Flood Watch through Monday evening for RIZ001>007.
     Wind Advisory until 1 PM EST Monday for RIZ001>005.
     High Wind Warning from 10 PM this evening to 10 AM EST Monday
     for RIZ006-007.
     Coastal Flood Advisory from 4 AM to 8 AM EST Monday for RIZ002-
     High Wind Warning from 10 PM this evening to 1 PM EST Monday
     for RIZ008.
MARINE...Gale Warning until 7 PM EST Monday for ANZ230.
     Storm Warning from 1 AM to 1 PM EST Monday for ANZ231-250-251-
     Storm Warning from 10 PM this evening to 1 PM EST Monday for
     Storm Warning from 10 PM this evening to 10 AM EST Monday for


TIDES/COASTAL FLOODING...JWD is the U.S. government's official web portal to all federal, state and local government web resources and services.