Flood and Coastal Flood in Massachusetts, December 18, 2023

Area Forecast Discussion
National Weather Service Boston/Norton MA
412 AM EST Mon Dec 18 2023

.SYNOPSIS...
A strong storm moving up the eastern seaboard will bring very
strong winds, heavy rain and coastal flooding along the south
coast today. Although conditions to improve late this afternoon
and early tonight, it remains unsettled into Tuesday with clouds
and perhaps a spotty shower. A prolonged dry period sets up
from Wednesday through the weekend. Temperatures will be
seasonable to slightly above normal by late next weekend. Those
hoping for a white Christmas will need to wait for another
chance in 2024, with little to no precipitation expected after
Tuesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
400 AM Update:

Key Points:

* Strong to Damaging Southeast Wind Gusts peaking during the mid
  morning to early afternoon. Highest gusts closest to the
  eastern coast.

* Steady/heavy rains this morning to mid afternoon. Rain could
  be heavy enough to produce street flooding and rapid creek and
  stream rises, especially in interior Southern New England.
  Larger rivers to experience rises long after rain has ended,
  especially on the CT River.

* Moderate Coastal Flooding in Narragansett Bay with Minor to
  Pockets of Moderate Coastal Flooding into Buzzards Bay,
  occurring around the noon high tide. Splashover to very minor
  coastal flooding late afternoon on the North Shore.

Strong, dual-barrelled low pressure around 980 to 985 mb was located
over the mid-Atlantic coast, with sfc observations indicating one
low near the Chesapeake Bay and another well south near the lower
portion of the Outer Banks of NC. An anomalously strong SE low level
jet was transporting a plume of rich moisture from the subtropics
northward. Step outside this morning and you'll feel that rich
moisture in the form of dewpoints in the middle to upper 50s! While
there has been periods of rain with brief downpours overnight, bands
of steady to heavy rains are falling from the lower Hudson Valley
and mid-Atlantic region, pinwheeling northward into western New
England. SE winds have also been increasing of recent as well, with
observed 30 to 35 kt gusts mainly near or SE of I-95.

Conditions will continue to go downhill as low pressure pulls
northward. We still expect this system to produce several weather-
related impacts to Southern New England. Those include strong to
damaging southeast wind gusts, heavy rains especially in the
interior which may lead to urban and poor drainage street flooding
and river/stream rises, coastal flooding along the southern coast
and into the North Shore, and dangerous marine conditions.

So what's changed? There hasn't been any significant change to
active headlines. With that said, biggest changes overnight has been
to increase the coastal flooding risk in Narragansett Bay; due to
observed surge values running a half to two thirds of a foot above
forecast, several tidal gages in Narragansett Bay are now expected
to reach Moderate flood stage around the noontime high tide and
could approach levels observed last December in 2022. We've also
increased rain amounts in interior portions of MA and CT due to the
expanding rain shield, and the heaviest rains are likely to coincide
with the morning commute in these areas.

Strong to Damaging Winds...

As mentioned - other than in the east slopes of the Berkshires where
cooler (40s) surface temps are permitting less mixing, gusts
elsewhere have been increasing with a peak of 40 mph observed at
PYM. Doppler wind profiler observations indicate the jet core is
still over the lower Hudson Valley and NJ vicinity with upwards of
80 kt shown at 925 mb! Expect gusts to continue to increase as we
get deeper into the stronger pressure falls and as this low level
jet skirts across New England. We still expect the peak of the SE
winds to occur around mid morning to early afternoon. Winds will
then shift from SE to SW/WSW with decreasing gusts. 00z HREF 10-m
minimum gusts generally support current forecast values with no
changes made to peak gusts from previous thinking. Impacts will
still be most concentrated in the high wind warning areas. Further
inland where wind advisories are in effect, the combo of soaked
ground combined with the increasing winds could lead to at least
isolated wind-related tree damage.

Rain/Flooding...

Growing signal noted late evening of heavy rains during the morning
commute especially for our western interior CT/MA areas. Hourly rain
rates underneath this band of heavier rains in the mid atlantic and
lower Hudson Valley have been as much as 3/4ths an inch. These rains
will continue to spread northward today. There is high confidence in
the highest rain totals being over western interior MA/CT. Further
east into RI and eastern MA, we did have quite a bit of rain
overnight from passing downpours; although it looks as though this
area may see less overall rain compared to central/western MA and
northern CT. Rain amts were increased by about a half inch in
western sections but were reduced slightly in eastern MA/RI. Rain
generally tapers off to intermittent showers by mid afternoon and
comes to an end from SW to NE later today.

Flood Watch remains valid and the risk for urban and poor drainage
street flooding and potentially rapid creek and stream rises is
growing, especially into western MA/CT. Larger mainstem rivers will
also experience rises, supported by NERFC forecasts and the latest
MMEFS. Note that e.g. the Connecticut River is likely to experience
rises even well after rain ends in light of rain/snowmelt occurring
upriver.

Coastal Flooding...

See the updated coastal flooding section.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
355 AM Update:

Rapid clearing takes place early tonight with cold advection
occurring on WSW winds. This CAA will maintain some WSW breezes
tonight through mixing/steeper low level lapse rates. There could be
a considerable amt of cloud cover around, especially in the interior
high terrain as a deep upper level trough moves in from western NY.
Another area that may see increased cloud cover is along the
southeast coast associated with a passing wave of low pressure
brushing our far southern outer waters. Lows tonight in the mid 30s
to lower 40s, kept up by the WSW breezes.

Deep upper trough and its associated cold pocket of air aloft
establishes itself over Southern New England on Tuesday. Cold
temperatures aloft will probably fill cloudiness back in towards a
partly to mostly cloudy look. Wouldn't rule out an isolated shower
or higher-terrain snow shower but think this would be the exception
vs the rule. Temps upper 30s to mid/upper 40s, warmest east.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Highlights

* Dry and seasonable mid-week through the end of the weekend as high
  pressure builds over the eastern US

* Deep-low pressure system well offshore will support enhanced
  northwest winds along the east coast Wednesday through Friday

Wednesday through Friday

Large surface high pressure system slowly builds east between
Wednesday and Friday. This will be accompanied by a cooler/drier air
mass that could be described as seasonable for late December in
southern New England. Expect highs/lows during this time frame to
range from the upper 30s to mid 40s and low to upper 20s
respectively. Probably the most notable aspect of the long-term
forecast is the expected presence of a deep low pressure system off
the east coast of the US mid to late week. With high pressure
building in from the west, this will support a strengthening
pressure gradient over southern New England. With a northwest flow
and CAA regime in place, this may support a period of gusty winds
along the east coast between Wednesday night and Friday morning.
Gale force gusts look likely over the east coastal water with wind
gusts along the east coast, Cape, and Islands possibly exceeding 40
mph. Latest suite of ensemble products highlights a 20 to 30 percent
chance of wind gusts exceeding 40 mph Wednesday night into Thursday
morning over The Cape and Islands. While there looks to be no
substantial precipitation in the long-term forecast, northwest flow
over Cape Cod Bay and a cold pool aloft may support some ocean
effect rain/drizzle over The Cape as well mid to late week.
Elsewhere across the interior expect dry/seasonable conditions and
modest northwest winds.

Next Weekend

High pressure shifts far enough east by the weekend to allow the
pressure gradient to relax and bring an end to the expected period
of gusty winds along the east coast. Column remains dry enough to
support a quiet/seasonable Christmas weekend. Next signal for active
weather appears to be early to mid next week when a low pressure
system may be in the vicinity of the eastern US. Very vague details
at 10 days out, but we'll have more information on what can be
expected by the end of this week.
&&

.AVIATION /09Z MONDAY THROUGH FRIDAY/...
Forecast Confidence Levels:

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

06Z TAF Update:

Overnight Through Today: High confidence.

Active aviation weather in this period with IFR-LIFR ceilings,
VFR to IFR visby moderate to heavy rains, strong winds and low-
level wind shear/turbulence.

Ceilings trend IFR to LIFR overnight into early Mon AM as rain
moves northward. Visbys in rain thru about 10z should stay 3-6
SM with intervals of mist at times. Rain ramps up to steady
moderate to heavy-intensity with IFR visbys in western MA/CT
airports, then around 12-16z for eastern airports. There may be
rumble or two of thunder near the southeast MA/RI coast.

SE winds to continue to increase in the 09-18z timeframe, with
peak gusts 35-45 kt in the interior, 45-50 kt in the I-95
BOS/PVD corridor airports, and 55 kt Cape and Islands. Low level
jet of 60-80 kt could support areas of turbulence, although
surface winds are likely to be gusty. Winds then shift to SW to
WSW 19-00z with a rapid decrease in gusts to 25-35 kt and
improving ceilings/visbys to MVFR with that windshift.

Tonight: Moderate to high confidence.

SCT-BKN VFR to MVFR ceilings with better chance at MVFR bases
near the higher terrain. There may be returning MVFR bases over
Cape Cod and Islands as passing secondary wave of low pressure
brushes our southeast outer waters. WSW gusts 25-35 kt
initially, strongest over SE MA/RI/Cape and Islands although
gust speeds decrease a bit to around 20-25 kt thru overnight.

Tuesday: Moderate to high confidence.

Continued SCT-BKN VFR to MVFR bases, though may trend OVC in the
western higher terrain. Could see isolated -SHRA, or -SHSN in
the higher terrain, although drier weather should prevail most
of the time. W to WNW speeds 10-14 kt with gusts 22-23 kt.

KBOS TAF...High confidence in TAF.

MVFR trending to IFR early this morning. 4-6 SM rain showers
trend to an IFR moderate to heavy rain after 13z. Moderate to
heavy rain begins late overnight and lasts through tomorrow
morning. Wind shear of 40-60 knots is possible starting this
evening through tomorrow afternoon. SE winds increase into the
morning, peaking between 15-19z with gusts up to 50 knots.
After 19z Monday, winds begin to diminish and ceilings improve
to MVFR.

KBDL TAF...High confidence in TAF.

MVFR to IFR early this morning. Steady light rain becomes heavy
rain with gusty winds to 45 knots through 18z tomorrow. After
18z, should see improvements to MVFR and gusty WSW winds.

Outlook /Tuesday Night through Friday/...

Tuesday Night through Wednesday: VFR. Breezy.

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

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

&&

.MARINE...
Forecaster Confidence Levels:

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

Overall high confidence.

** Dangerous Marine Conditions Today **

SE winds have begun to increase to SCA levels on most waters,
with continued increases expected. Storm warnings remain in
effect for all waters for southerly gusts 50-55 kt, up to 60 kt
over outer waters. Seas will build up to 25 ft over southern
outer waters and 20 ft over eastern outer waters. Rain will
become heavy at times and reduce visibility through this
afternoon, with rapid clearing expected late today.

Winds will shift to WSW late this afternoon and tonight, with
decreasing gusts into the Gale and SCA levels. Will need to add
marine headlines at least for the tonight period, with seas
still well above SCA criteria into Tues.

Outlook /Tuesday Night through Friday/...

Tuesday Night: Winds less than 25 kt. Rough seas up to 12 ft.

Wednesday: Winds less than 25 kt. Rough seas up to 10 ft.
Slight chance of rain.

Wednesday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 12 ft. Slight chance of rain.

Thursday: Moderate risk for gale force winds with gusts up to
35 kt. Rough seas up to 13 ft. Slight chance of rain.

Thursday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 12 ft. Slight chance of rain.

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 11 ft.

&&

.TIDES/COASTAL FLOODING...
400 AM Update:

Observed storm surge levels overnight were running about a half
foot above the prior forecast and the PETSS mean surge guidance
in Narragansett Bay. Water levels have also been slow to
evacuate out of Narragansett Bay in the face of sustained SE
winds. These observations, and that offshore seas of 20 to 25 ft
may yield greater impact through wave action, resulted in an
increase in total water levels in Narragansett Bay. We expect
peak storm surge to take place a couple of hours before noon
high tide of at least 4 ft at Providence/Fox Point with 5 ft
surge a distinct possibility. This is supported by latest PETSS
guidance. Latest forecast now calls for total water levels in
Narragansett Bay to reach Moderate flood stage, and for Fox
Point is at a level not seen since December 26th, 2022.

Coastal flooding of at least minor flooding is expected further
eastward into Buzzards Bay (e.g. New Bedford, Fall River,
Westport area). It is possible but less likely we see moderate
coastal flooding. Thus maintained the Coastal Flood Advisory for
Buzzards Bay.

Still could areas of minor flooding along the North Shore,
especially at Gloucester during the mid afternoon high tide.
Coastal Flood Advisory was maintained for the North Shore. This
area is more exposed to S/SE flow so we issued a coastal flood
advisory. A surge up to 2 ft is forecast which is enough for
minor flooding given wave action. Less of a risk for Boston and
the south shore but can't rule out minor splashover in a worst
case scenario. Would need a 2 ft surge for minor flooding which
may be tough to get with a S wind.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch through this evening for CTZ002>004.
     Wind Advisory until 4 PM EST this afternoon for CTZ002>004.
MA...Flood Watch through this evening for MAZ002>005-008>013-017-
     026.
     Wind Advisory until 4 PM EST this afternoon for MAZ002-003-
     008>011.
     Wind Advisory until 7 PM EST this evening for MAZ004>006-012-
     026.
     High Wind Warning until 7 PM EST this evening for MAZ007-
     013>024.
     Coastal Flood Advisory from 1 PM to 5 PM EST this afternoon
     for MAZ007.
     Coastal Flood Advisory from 10 AM this morning to 2 PM EST
     this afternoon for MAZ020.
RI...Flood Watch through this evening for RIZ001>004.
     High Wind Warning until 7 PM EST this evening for RIZ001>008.
     Coastal Flood Warning from 10 AM this morning to 2 PM EST this
     afternoon for RIZ002-004>007.
MARINE...Storm Warning until 7 PM EST this evening for ANZ230>237-250-
     251-254>256.

&&

SYNOPSIS...Loconto/RM
NEAR TERM...Loconto
SHORT TERM...Loconto
LONG TERM...RM
AVIATION...Loconto/RM
MARINE...Loconto/RM
TIDES/COASTAL FLOODING...KJC/Loconto/RM