NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Dec 10, 10:57 PM EST, 1 hour 11 minutes earlier.
Area forecast discussion, Dec 10, 10:57 PM EST, 1 hour 11 minutes before
Area Forecast Discussion
National Weather Service Boston/Norton MA
1057 PM EST Sun Dec 10 2023
.SYNOPSIS...
Multifaceted 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.
&&
.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Winds were starting to increase late this evening, but still
have not reached really unusual levels for this time of year.
00Z upper air soundings from OKX and ALB showed shallow
surface-based inversions. These inversions would need to be
overcome to really get the winds from the low level jet down to
the ground.
Rainfall so far has not been too bad in the grand scheme of
things. However, we are just starting to get into the period
where the heaviest rainfall is expected. Will need to monitor
the hydrologic conditions closely overnight.
Overall forecast continues with minimal changes this evening to
reflect observed trends.
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.
&&
.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
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.
&&
.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
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.
&&
.AVIATION /04Z MONDAY THROUGH FRIDAY/...
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
elevations.
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 Monday.
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.
&&
.MARINE...
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.
&&
.HYDROLOGY...
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.
&&
.TIDES/COASTAL FLOODING...
* 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.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
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 until 10 AM EST Monday for MAZ019>021.
Coastal Flood Advisory from 4 AM to 8 AM EST Monday for MAZ020.
High Wind Warning until 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 until 10 AM EST Monday for RIZ006-007.
Coastal Flood Advisory from 4 AM to 8 AM EST Monday for RIZ002-
004>007.
High Wind Warning until 1 PM EST Monday for RIZ008.
MARINE...Gale Warning until 7 PM EST Monday for ANZ230.
Storm Warning until 1 PM EST Monday for ANZ231-250-251-254.
Storm Warning until 1 PM EST Monday for ANZ232>235-237-255-256.
Storm Warning until 10 AM EST Monday for ANZ236.
&&
SYNOPSIS...KS/JWD
NEAR TERM...Belk/KS
SHORT TERM...Nocera/KS
LONG TERM...JWD
AVIATION...Nocera/KS/JWD
MARINE...KS/JWD
HYDROLOGY...KS/JWD
TIDES/COASTAL FLOODING...JWD