River Flood Warning 50, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Boston/Norton MA
732 AM EDT Wed Apr 3 2024

.SYNOPSIS...
Another slug of rain arrives from west to east today with some
sleet especially across the high terrain north of the
Massachusetts Turnpike. A robust system Today and Tonight will
bring; locally heavy wet snow over higher elevations, strong to
damaging northeast wind gusts, heavy rainfall which could lead
to renewed river flooding concerns, and coastal flooding.
Unsettled conditions may linger into next weekend as the storm
will be slow to depart.

&&

.NEAR TERM /THROUGH TONIGHT/...
Delayed the onset of the rainfall by a few hours across eastern
MA. That said, overall trend of the forecast remained on track.
Minor tweaks to temperatures to reflect observed trends.

Previous Discussion...

Key Points

* Rain spreads east across the region today with sleet in the high
  terrain

* Locally heavy wet snow in the high terrain could result in
  power outages and impact the Thursday morning commute

* Strong to damaging winds this afternoon into Thursday morning

* Heavy rain will likely cause minor river flooding in RI and
  southeast MA

* Minor to moderate coastal flooding possible for both the
  southern and eastern coasts

Sleet and Heavy Snow:

A strong upper level closed low continues to drop from the
Great Lakes and moves east. This closed low will provide the
support to rapidly intensify a surface low pressure system over
SNE. Rain begins to overspread SNE from west to east again this
morning. A rather complex temperature profile will be set up
across the region today. Bufkit soundings show a warm nose aloft
around 750mb with temps above freezing. Temps below 800mb drop
well below freezing. This will cause precip that melts in the
warm nose to refreeze into sleet before reaching the ground.
However, with the strong April sun angle temps near the surface
will be in the low to mid 40s except in the high terrain where
temps stay in the low to mid 30s. What this means is that sleet
is most likely in the high terrain with rain mixing in with
sleet elsewhere, away from the coasts. Sleet could become heavy
enough to accumulate on surfaces for elevations above 1000 feet.

Overnight, the warm nose aloft begins to cool below freezing.
This combined with temperatures dropping below freezing in the
high terrain will allow precip to turn to heavy wet snow. Lower
elevations along the CT river valley and along the coastal plane
should see temps remain above freezing tonight, however, with
temps dropping into the mid 30s, snowflakes may mix in with
rain. Snow accumulation totals will vary greatly with elevation.
Expecting the highest totals to be in the northern berks where
6-12 inches are possible. In the southern Berks, Temps will take
longer to cool below freezing resulting in lower snow
accumulations of 3-6 inches. Further east, the northern
Worcester hills should receive 4-8 inches with localized areas
of 12 inches. Uncertainty increases significantly across NE MA,
where a shift in the storm track could bring colder temperatures
down into northern Essex and Middlesex counties. Decided to
hold off on winter weather advisories with WSSI indicating less
then 50% chance for minor winter weather impacts in NE MA. The
heaviest snow is expected to fall tonight, which combined with
strong winds, will lead to very hazardous travel and isolated
power outages. Snow lingers into Thursday, with isolated snow
showers through Thursday night.

Strong to Damaging Winds:

Winds begin to ramp up in the afternoon and a strong easterly
LLJ begins to push north. Winds just above the surface at 2000
ft will approach 75 knots. Bufkit soundings show mixing
potential up to 1500-2000ft across the waters and near the
coasts. This could allow for gusts of 55-65mph. Sustained winds
could also approach 40mph for several hours tonight. Decided to
upgrade the High Wind Watch to a High Winds Warning for all
coastal zones. Further inland, the mixing potential does not
reach as high and the 2000ft winds are only 55-65 knots. Opted
to issue a Wind Advisory for all interior zones that are not
included in the High Wind and Winter Storm Warnings. Winds
across the interior are expected to be sustained at 20-30 mph
with gusts of 50-55mph esspically across the high terrain.

Heavy Rain:

Moderate to heavy rain will fall Wednesday night as the core of
the LLJ lifts north across the region. Between Today and
Thursday, a wide spread 1-2 inches of QPF is expected region
wide. This will likely cause renewed minor river flooding for
the rivers in RI and eastern MA that have recently experienced
flooding. River flood watches remain in effect for those rivers.

&&

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
The center of the low continues to move northeast and weaken
during the day on Thursday. This will cause surface winds to
shift NNE, brining in cooler and drier air. Precip begins to
taper off Thursday morning, however, it will be cold enough by
Thursday to cause any lingering precipitation to fall as snow.
Generally not expecting major accumulations with only a trace to
half inch in the high terrain. Winds slowly diminish through
Thursday afternoon from west to east as the low pulls away.
Gusts of 15-25mph are still possible through Thursday afternoon.
Highs Thursday only top out in the upper 30s to low 40s.

Light snow showers could linger into Friday morning as the
center of the low slowly pulls away. Winds continue to diminish
and overnight lows drop near or below freezing.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Highlights:

* Cloudy and cool Fri through Sat with intermittent showers
  (flurries or light snow showers in the higher terrain).

* Decreasing cloud cover Sunday with moderating temps.

* Drying out for Monday but another storm system from the Great
  Lakes could arrive later Tue.

Details:

Amplified, blocky 500 mb height pattern governs conditions
across the Northeast states. Deep, vertically-stacked closed
cyclone and its associated cold pool of air aloft will be very
slow to progress eastward in this blocky weather pattern. Latest
models and ensemble means point to greater eastward/southeast
progression by later in the weekend. The 500 mb height pattern
remains amplified into the early week forecast period, but with
more of a WSWly flow offering a gradual warming trend as we move
into Monday and Tuesday.

Friday through Saturday Night:

The low pressure area responsible for the active weather through
Thurs very slowly fills with time and moves little. Really, more of
a slow southeastward meander.

Conditions through this portion of the forecast should be pretty
similar both days. Though the bulk of the heaviest precipitation
will have ended before Fri, cold-conveyor-belt wraparound moisture
will favor stubborn overcast with more showery/intermittent precip
chances; rain showers more favored in the lower elevations and
flurries or light snow showers in the hilly terrain. Little if any
additional accumulation is anticipated. Will also mention that
depending on how much snow falls in interior New England, some of
that snow could melt in this period and lead to gradual rises on
rivers and streams. It is too uncertain on how much rise we see
and/or which rivers or streams would be most at risk (CT and
Merrimack basins likely to have the best chance given snow
forecasts).

Overall, expect this period to be a pretty raw one with lots of
cloud cover, below normal high temps in the lower to mid 40s and a
northerly breeze to boot.

Sunday:

Our area of low pressure makes greater southeastward progress into
Atlantic Canada, with high pressure starting to ridge in from the
W/WNW. Expect decreasing cloud cover trends, reduced northerly
windspeeds and temps moderating back into the mid 40s to the lower
50s for Sunday.

Monday and Tuesday:

Medium-range ensembles continue to indicate a ridge of high pressure
extending into Southern New England for Mon and Tue. This would
favor dry weather and a gradual moderating trend to temps. With the
solar eclipse set to take place on Monday, this could portend
favorable viewing conditions with less cloud cover, however we do
have to watch for any moisture moving in from an upper level low
progged over the Great Lakes. Will need to monitor these conditions
carefully over the coming days.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

Today... Moderate confidence.

Another slug of moisture moves in from SW to NE. This brings
rain for most along with MVFR/IFR conditions. The only exception
is across the higher elevations, such as ORH/BAF where sleet
mixes in with rain or becomes all sleet during the afternoon.
Could even see sleet mixing in at BDL and perhaps BED/BOS toward
the very end of the TAF. For now have held off at BOS, but may
be needed in future updates. Winds out of the ENE/E and continue
to increase. Will be quite strong with gusts 25 to 50 kts
during the afternoon. Will have the highest gusts across the
higher terrain and coastal areas.

Tonight...Moderate confidence.

IFR to LIFR conditions. Wintry mix across the higher elevations
gradually transitioning to all snow. Some uncertainty with the
extent of wintry mix/snow across some lower elevation terminals,
such as BED/BDL. Elsewhere am anticipating a cold rain at this
point. Strong E to NE winds with gusts 30-55 kts. The strongest
gusts are anticipated across the higher terrain and coastal
areas.

Thursday...Moderate confidence.

IFR to LIFR conditions continue. Mainly snow across the higher
elevations, with rain at lower elevations. Precipitation ending
from south to north during the day.

KBOS TAF...Moderate Confidence

Rain spreads in 14-17Z today with MVFR conditions. Should have
IFR by this afternoon, and perhaps some RAPL toward the end of
the TAF. Strong ENE to E winds tomorrow at 15-25 kts with gusts
of 25-35 kts.

KBDL TAF...High confidence in TAF trends, moderate in timing.

Rain returns for the morning push with ENE to E winds increasing
throughout the day and a return to MVFR ceilings. May have some
RAPL  mixing in during this afternoon.

Outlook /Friday through Sunday/...

Friday: Mainly VFR, with local IFR possible. Breezy. Chance
SHRA.

Friday Night: Mainly VFR, with areas IFR possible. Breezy.
Chance SHRA.

Saturday: MVFR/IFR conditions possible. Breezy. Chance SHRA.

Saturday Night: Mainly VFR, with local IFR possible. Breezy.
Chance SHRA.

Sunday: 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.

Through Thursday...

Upgraded to storm warnings for all waters, except storm watches for
Boston Harbor and Narragansett Bay.

Winds will be ramping up during Wed with the peak of the wind
occurring Wed night into Thu morning as 60-70 kt easterly low level
jet lifts north across the waters Wed night into Thu. The core of
the jet will be over the south coastal waters Wed evening, then lift
into the NE MA waters late Wed night and Thu morning. Wind gusts to
45-55 kt expected. The strongest of the gusts will likely be focused
over NE MA waters Thu morning. Seas will build to 20-25 ft over
the eastern MA outer waters late Wed night and Thu. This will be
a dangerous storm for mariners.
Outlook /Friday through Sunday/...

Friday: Winds less than 25 kt. Rough seas up to 10 ft. Chance
of rain showers.

Friday Night: Winds less than 25 kt. Areas of rough seas.
Chance of rain showers.

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. Chance of rain showers.

Saturday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Rough seas up to 9 ft. Chance of rain
showers.

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Areas of rough seas.

&&

.TIDES/COASTAL FLOODING...
Widespread minor flooding is expected along both the south and
east coasts during the Thu morning high tide, with a risk for
moderate flooding for portions of the eastern MA coast. We
issued a coastal flood watch for the entire coast.

Along the east coast, the greatest risk is from Scituate to the
north shore as the core of the low level jet will coincide with
the high tide around 12z. PETSS guidance is suggesting a 3-3.5
ft surge which combined with 20+ ft seas just offshore will be
enough for widespread minor flooding with pockets of moderate
flooding. The greatest risk for moderate will be along the north
shore including Plum Island. Significant beach erosion is also
possible for vulnerable areas from the ocean side of the Cape to
northeast MA.

Along the south coast, there is more uncertainty with the
magnitude of the storm surge. Stevens Institute is indicating
a surge around 2.5 ft while PETSS guidance is 3.5 to 4 ft. We
think pressure falls will play a big role along the south coast
as max pressure falls are occurring near high tide with 25+ mb
falls in 24 hours which equates to about a 1 ft sea level rise.
We blended the guidance which would support minor flooding.
However, if the surge approaches 4 ft we could see pockets of
moderate flooding so we decided to issue a coastal flood watch
for the south coast as well.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Wind Advisory from 4 PM this afternoon to 2 PM EDT Thursday
     for CTZ002>004.
MA...Wind Advisory from 4 PM this afternoon to 2 PM EDT Thursday
     for MAZ005-006-010>014-017-018.
     Winter Storm Warning from 2 PM this afternoon to 2 AM EDT
     Friday for MAZ002-008.
     Winter Weather Advisory from 5 PM this afternoon to 5 AM EDT
     Friday for MAZ003.
     Winter Storm Warning from 8 PM this evening to 8 AM EDT Friday
     for MAZ004-026.
     High Wind Warning from 8 PM this evening to 2 PM EDT Thursday
     for MAZ007-015-016-019-022.
     Coastal Flood Watch Thursday morning for MAZ007-015-016-019-
     022>024.
     Winter Weather Advisory from 2 PM this afternoon to 2 AM EDT
     Friday for MAZ009.
     High Wind Warning from 4 PM this afternoon to 5 AM EDT
     Thursday for MAZ020-021-023-024.
     Coastal Flood Watch from late tonight through Thursday morning
     for MAZ020-021.
RI...Wind Advisory from 4 PM this afternoon to 2 PM EDT Thursday
     for RIZ001>005.
     High Wind Warning from 4 PM this afternoon to 5 AM EDT
     Thursday for RIZ006>008.
     Coastal Flood Watch from late tonight through Thursday morning
     for RIZ002-004>007.
MARINE...Gale Warning from 2 PM this afternoon to 2 PM EDT Thursday for
     ANZ230.
     Storm Warning from 8 PM this evening to 11 AM EDT Thursday for
     ANZ231.
     Storm Warning from 2 PM this afternoon to 8 AM EDT Thursday
     for ANZ232-255.
     Storm Warning from 2 PM this afternoon to 4 AM EDT Thursday
     for ANZ233>235-237-256.
     Gale Warning from 11 AM this morning to 8 AM EDT Thursday for
     ANZ236.
     Storm Warning from 8 PM this evening to 2 PM EDT Thursday for
     ANZ250-251.
     Storm Warning from 5 PM this afternoon to 11 AM EDT Thursday
     for ANZ254.

&&

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