Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 090547
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
147 AM EDT Tue Apr 9 2024

.SYNOPSIS...
High pressure will build over the Maritimes Tuesday,
bringing another mild day, but onshore winds will keep it much
cooler near the coast. Low pressure tracking through the Great
Lakes Thursday and Friday. Heavy rain and renewed river flooding
are possible. On top of this we could have strong winds, and
possible coastal flooding along the South Coast. Though there is
more uncertainty in the strong winds and coastal flooding at
this point. Drier weather follows for the weekend.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
1015 PM Update:

Mid to high clouds continue to stream in from NW to SE
across interior Southern New England associated with a warm
front aloft, though it is otherwise a pretty tranquil evening
with rising 500 mb heights. Despite their high-level nature,
these clouds still appear to be blunting good radiational
cooling, and current temps are still quite mild with values
ranging from the upper 40s to mid/upper 50s, though are in the
mid 40s over the Cape and Islands. This stream of cloud cover is
still expected to progress SE through the rest of the evening.
Brought hourly temps and lows up a few degrees with this update
given the cloud cover trends.

Previous discussion...

Partial eclipse has begun and sky cover forecast is favorable with
just some high clouds around SNE, although there are some thicker
patches evident on satellite across western CT.

Fairly quiet weather tonight as we see more in way of mid and high
clouds associated with weak short wave crossing region. This
will keep overnight lows on mild side, mainly in upper 30s to
mid 40s.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/...
Upper ridge remains west of New England Tue and Tue night which
will maintain dry weather. Meanwhile, high pressure will build
south across the Maritimes, which will produce an onshore wind
(E/NE), keeping coastal areas some 15 degrees cooler than inland
areas. For example, Boston`s high should be around 53 while
Hartford should reach 70. We should still see some high
cloudiness around but not enough to limit warming in afternoon.

Cloudiness will once again become more noticeable Tue night
ahead of next system, but with ridge axis over SNE by daybreak
Wed, we should remain dry.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Highlights

* Warm front lifts toward our region on Wed bringing isolated to
  scattered rain showers. Temperatures around seasonable levels.

* Frontal system begins to move in bringing more showers on Thu.
  Widespread rain Thu Night through a good portion of Fri, which
  could be heavy at times. There is potential for strong southerly
  winds and coastal flooding. Still uncertain on the exact details
  at this point.

* Appearing drier for the weekend, but there still could be some
  hit/miss showers.

Wednesday through Wednesday Night...

A ridge axis will initially be in place over New England early on
Wed. The ridge gets shunted further to the northeast/offshore later
on Wed. A shortwave will lift in behind the ridge from the central
Great Lakes up into southern Quebec. High pressure building near
Nova Scotia will push further offshore, while a warm front lifts
toward our region and perhaps through Wed Night.

Dialed back precipitation chances a bit during the first half of the
day as the NBM appears a bit too quick for the onset. Thinking the
better opportunity for us to get into the shower activity is during
the afternoon when the shortwave makes it closest pass to our
region. This is also when the best low/mid level moisture increases
along with increased southerly flow. At this point have scattered
light rain showers. Not anticipating much QPF with totals less than
a tenth of an inch. High temps expected to be around seasonable
levels with readings in the 50s.

Appears that there could be a brief lull in precip activity Wed
Night as drier air punches in at the mid levels. Some signals that
this lull could last into part of Thu before our next surge of
moisture. May actually bring our precip back in a bit too quickly,
but there is some discrepancy on the warm front swinging through.
Lows in the 40s.

Thursday through Friday...

A deeper trough will initially be situated over the Mississippi
River Valley early on Thu. The trough will lift into the TN/OH
Valley and Great Lakes Region by late Thu. The trough cuts off over
the Great Lakes Thu Night and rotates into the eastern Great
Lakes/Mid Atlantic/New England by late Fri. A warm front may still
be in the process of lifting through on Thu. A frontal system moves
into the region later on Thu and through on Fri.

This will be our next opportunity for more widespread potentially
impactful weather. At this point confidence highest in rainfall,
which could be heavy at times. Given how wet we`ve been this will
lead to renewed river flooding for portions of our region. There
will also be the risk for strong wind gusts and given the high astro
tides there could also be some coastal flooding.

Precipitation...

There is some uncertainty during the day Thu as it appears the dry
slot could still be over the area. There is some discrepancy amongst
guidance if a warm front will still be working its way through the
region. Fortunately it appears that precip should be fairly light
through the day as PWATs drop a bit. Better shot for more widespread
rains late on Thu through Friday as another slug of moisture pushes
in. At this point both the NAEFS and EPS situational awareness
tables showing values of 2.5 to 3.5 STD above model climo in the
late Thu int Fri timeframe. This is also when an anomalously strong
southerly LLJ impinges on the region (more details on this below).
At the moment values ranging from 1 to roughly 1.5 inches, which
also lines up well with the latest deterministic guidance. Given the
southerly warmer air advection it is also looking like we will have
the potential for efficient rain processes with warm cloud layer
depths of 2.5 to 3.5 km. There may be some instability present, but
for now have only kept an isolated thunder mention.

Does appear that the precip amounts have backed off slightly from a
couple of days ago per probabilistic guidance. At this point
probabilities range from low/mod (10-50 percent) to mod/high (50-80+
percent) for total QPF AOA 1 inch. The GEFS is much more muted than
the EPS/GEPS at this point, but all indicating the risk the highest
across western/central MA and CT. The GEPS/EPS essentially has a 100
percent chance of 1 inch of rainfall, whereas the GEFS is around 50-
70 percent. Since a couple of days ago guidance backing away from
those 2+ inch amounts with only low (10-30 percent) to nil probs
mainly confined to western/central MA and CT. For now have stuck
with WPC QPF, which paints roughly 0.75 to 1.75 inches of rainfall
with the highest amounts across the CT River Valley and lower totals
in the east. Any convective elements could bump up those amounts
locally significantly. Though should note that it does appear we
could see another dry slot punch in on Fri. This tends to end precip
a bit faster from experience and result in lower QPF, so will be
something to hone in on as we get closer. Regardless with these
precip amounts and probs it does appear that area rivers could see
renewed flooding. May also have to deal with potential snowmelt
aiding in river flooding across our slower responders.

Strong Winds and Coastal Flooding...

Does appear there will be a risk for strong to perhaps damaging
winds. This could also coincide with astronomically high tides,
which could result in coastal flood impacts. Definitely much more
uncertain when it comes to this risk as this time of year it tends
to be much more difficult to mix down the southerly winds with a
colder ocean. The NAEFS/EPS guidance shows winds being 3-4 STD above
model climo with the EPS actually peaking out at near 4.5 STD above
model climo for the V component. This is all centered at the 850 hPa
level, where we should see a 50-70+ kt southerly low level jet
impinging on the region. Dialed back our wind speeds/gusts a bit
with the NBM10th percentile despite the signals from the situational
awareness table. Given the ocean temps we tend to struggle to mix
down these stronger southerly LLJ for this time of year per
experience. Can already start to see it per GFS Bufkit soundings,
where indicates we could potentially mix down perhaps half the LLJ.
Any convective elements would increase the risk of higher winds
being brought down, but hard to tell those details at this point.

The strong winds in combination with high astronomical tides could
result in coastal flooding. Risk at this point would be greatest
across the south coast where Stevens Institute guidance indicates
anywhere from a 1-3 ft surge possible. As is typically the case we
need the timing of the stronger winds to coincide with high tide,
which is just too difficult to pin point 84+ hrs out. Though think
that the risk would be greatest during the high tides late Thu Night
into Fri.

Given the southerly warm air advection should see temps trending
near to above seasonable levels through this timeframe.

Saturday and Sunday...

Will be caught in cyclonic flow with the cutoff over our area
perhaps lifting into Quebec. Definitely uncertain on how thing
exactly evolve this far out. For now stuck with the NBM, but should
be drier with hit/miss showers possible through the weekend.
Definitely not looking like a washout at this point. Temperatures
near to above seasonable levels.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
Forecast Confidence Levels:

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

06z TAF Update:

Through 12z Tuesday: High confidence.

VFR. Light W winds to trend light NW to N by daybreak.

Today: High confidence overall, moderate to high on timing
of windshift to ENE.

VFR. An abrupt windshift from NW/N around 3-6 kt to ENE/E around
8-12 kt is the main operational concern. This windshift takes
place around 13-16z along the immediate eastern MA coast, then
gradually advancing westward thru the aftn.

Tonight: High confidence.

VFR, though lowering cloud bases ~6-10 kft. E/ESE winds around
5-10 kt.

Wednesday: Moderate confidence.

Initial OVC VFR to trend MVFR/IFR ceilings with SCT SHRA
associated with warm front, however visbys in SHRA should be in
VFR range. Possible LLWS after 15z. SE winds around 5-10 kt.

KBOS TAF...High confidence in TAF. VFR, though windshift from
NW/N around 5 kt to ENE around 8-12 kt by ~14-16z. Winds become
ESE later aftn/tonight.

KBDL TAF...High confidence in TAF. VFR. Light W winds become
light N this morning, then become ESE around 5 kt.

Outlook /Wednesday Night through Saturday/...

Wednesday: Mainly VFR, with local MVFR possible. Breezy. Chance
SHRA.

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

Thursday: Mainly VFR, with areas IFR possible. Breezy. Chance
SHRA, isolated TSRA.

Thursday Night: MVFR/IFR conditions possible. Strong winds with
local gusts up to 40 kt. RA, isolated TSRA.

Friday: Mainly VFR, with local IFR possible. Strong winds with
gusts up to 40 kt. RA.

Friday Night: Mainly VFR, with local MVFR possible. Windy with
gusts up to 35 kt. Slight chance SHRA.

Saturday: Mainly VFR, with local MVFR possible. Windy with
gusts up to 30 kt. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

High confidence through Tue night.

High pressure brings light W winds and diminishing seas tonight.
SCA remains posted for lingering rough seas on outer waters.
Winds turn E/NE Tue as another high pressure system builds S to
Maritimes but winds and seas should remain below SCA.

As a heads up later in the week, deepening low pressure is forecast
to track through eastern Great Lakes Thu-Fri. Confidence is
increasing for Gale force S-SW winds and building seas offshore.

Outlook /Wednesday Night through Saturday/...

Wednesday: Winds less than 25 kt. Chance of rain showers.

Wednesday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers.

Thursday: Winds less than 25 kt. Seas up to 5 ft. Chance of
rain showers.

Thursday Night: Low risk for gale force winds with gusts up to
40 kt. Rough seas up to 11 ft. Rain.

Friday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 13 ft. Rain.

Friday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 13 ft. Slight chance of
rain showers.

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

&&

.FIRE WEATHER...
Afternoon RH values dropped much lower than forecast, into the
20-25 percent range inland and even to 25-35 percent near the
coast. This is a typical bias of forecast guidance in spring
where dewpoint forecasts are much too high.

Another dry day expected Tue but RH values will be somewhat higher
than today thanks to onshore winds (E/NE). This should result in
lowest afternoon RH values across interior, especially in western MA
and much of CT, where RH will drop to 30-40 percent. Closer to the
coast, RH will be higher, averaging 40-50 percent. Wind speeds will
average 5-15 mph, strongest near coast.

&&

.TIDES/COASTAL FLOODING...
Key Points:
 - Several high tide cycles along E MA coast approach minor
   flood stage through Wed.
 - Minor flooding possible along South Coast Wed night and/or
   Thu night.

High astronomical tides will result in marginal coastal flood
impacts during some of the high tide cycles through Wed along E MA
coast. Coastal Flood Statement issued for high tide around midnight
tonight, mainly from Gloucester to Boston and the bay side of Cape
Cod. We will issue additional statements as needed since any impacts
will be minimal and limited to most vulnerable low lying areas.

Later in the week, low pressure is forecast to track through eastern
Great Lakes Thu-Fri, putting our area under increasing S/SW winds.
This could bring minor coastal flooding to South Coast, including
Narragansett Bay, during high tide Wed night and/or Thu night.
Something we`ll be keeping tabs on this week as well given continued
high astro tides.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$

SYNOPSIS...BL/JWD
NEAR TERM...Loconto/BL/JWD
SHORT TERM...JWD
LONG TERM...BL
AVIATION...Loconto/KS
MARINE...Loconto/KS
FIRE WEATHER...JWD
TIDES/COASTAL FLOODING...JWD


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