NWS Technical Forecast Discussion

287
FXUS61 KBOX 250032
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
732 PM EST Tue Nov 24 2020

.SYNOPSIS...
Dry but chilly weather will persist tonight and into Wed morning, as
high pressure builds in from the northwest. This high pressure
system moves offshore Wednesday afternoon, giving way to milder
conditions. A frontal system moves through on Thanksgiving, bringing
periods of rain to the area. Some of the rain could fall heavy at
times. A drying trends develops Friday, although clouds may persist.
Other than an isolated shower Saturday, mainly dry weather will be
featured this weekend along with seasonably cool conditions.
Unsettled weather likely returns early next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...

7 PM update...
As a high pressure system continues to push towards Nova Scotia
early this evening, clear conditions remain dominant over New
England. Diminishing winds this evening, accompanied by low
dewpoints and clear skies, will allow for rapid radiational
cooling through about 02 Z. Mid to high level clouds begin to
build in around this time, preventing temperatures from falling
even further through sunrise. Warm air advection along a
passing front may supply just enough moisture to allow for a few
flurries to fall along the Massachusetts/Vermont border around
daybreak. The column remains dry however, limiting the potential
for any substantial precipitation.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
3 PM update...

This high pressure moves farther offshore Wednesday, leading to
more warm air advection, meaning milder conditions despite the
abundant clouds across the coastal plain. Farther inland, it
still appears like the colder air near the surface will take
longer to scour out. So not expecting much of a warmup there
when compared to today.

Low risk for showers Wednesday. Thinking the greatest risk will
be towards the SE coast of MA, where a coastal front is more
likely to become established.

Increasing risk for showers develops Wednesday night as a
stronger push of warm air advection arrives late. Dynamics are a
bit better after midnight Thursday, so expecting rainfall to be
be at least likely during this time. Later forecasts will likely
refine this timing.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
350 PM update...

Highlights...

* Wet Thanksgiving with periods of rain, possibly heavy at times
  along with isolated thunder

* Drying trend develops Fri but clouds may be stubborn to break

* Other than a spot shower Sat/Sat night, mainly dry weather &
  sunshine this weekend along with a cooling trend

* Unsettled weather likely returns early next week

Precipitation...

A closed mid level low approaches the region from the southwest late
Wed night into Thanksgiving. Despite this trough deamplifying with
time (rising heights & weakening wind fields aloft), abundant
moisture associated with this system as PWATs rise to about 1.25
inches, which is approximately +2 standard deviations above climo.
This combined with frontal convergence & secondary low pressure
developing over the area, along with elevated instability from -20C
air at 500 mb, expecting rain, heavy at times with isolated
thunderstorms possible much of Thanksgiving Day and into the
evening. Not as much rain as our previous event Monday, but
preliminary rainfall projections for Thu range from about 0.50 to
1.00 inches region-wide. Higher amounts possible if convection
develops. This system exits later Thu night with a drying trend Fri.
Clearing appears slow on Fri especially across eastern MA as low
level WNW flow remains weak. Thus more sunshine west across CT into
western-central MA and clouds linger longer for eastern MA, possibly
into RI. For the weekend, moisture starved northern stream short
wave and attending cold front zip across the area late Sat/Sat
evening, Other than an isolated shower, mainly dry weather prevails
this weekend with more sunshine Sunday. Active weather pattern
setting up for early next with split flow across the CONUS. Good
agreement among deterministic and ensemble guidance that streams
merge yielding potential for a heavy qpf event here early next week.
Model differences appear in exactly where the streams merge, which
will impact the track of the surface cyclone and any potential ptype
issues.

Temperatures...

Surface boundary draped across the region on Thanksgiving Day will
yield fairly large temp gradient across the region, with highs in
the 50s to perhaps near 60 across CT/RI and southeast MA, and 40s to
near 50 over northern MA. Mild Friday with highs in the 50s to near
60, especially across CT into western-central MA where probability
is higher for clouds to give way to breaks of sunshine. More clouds
east. Seasonable Saturday ahead of an approaching cold front with
highs 50-55, upper 40s high terrain. Cooler behind the front Sunday
with highs 45-50, but light winds and lots of sunshine should help
offset the cool temps. Early next week temps will hinge on the exact
storm track. As of now Monday looks warmer than normal favoring a
storm track west of New England. Thereafter, Tue into middle of next
week, post frontal environment favors trending toward cooler
weather.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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


Tonight...00Z update: High Confidence. VFR conditions expected to persist
through the overnight hours. As stated in previous discussion,
winds will diminish as the evening progresses.

Tonight...High Confidence. VFR with winds becoming calm/light.

Wednesday...Moderate Confidence. A broken deck of marginal
MVFR/VFR ceilings. Dry weather dominates, but brief very spotty
sprinkles/light showers possible with even a touch of
spotty/very light frozen precipitation not out of the question
across portions of the interior. Winds becoming S to SE at 5 to
15 knots, but they may remain light and variable across
portions of the interior.

Wednesday Night...Moderate confidence. VFR to start, trending to
MVFR/IFR after midnight.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Thursday through Sunday/...

Thanksgiving Day: Mainly IFR, with local MVFR possible. Breezy.
RA likely, isolated TSRA.

Thursday Night: Mainly MVFR, with areas IFR possible. Chance
RA, isolated TSRA.

Friday: Mainly VFR, with areas MVFR possible.

Friday Night: Mainly VFR, with local MVFR possible.

Saturday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHRA.

Saturday Night: VFR. Breezy.

Sunday: VFR.

&&

.MARINE...
Gusty winds diminish and shift through north to east to
southeast by daybreak Wednesday. Expecting Small Craft
Advisories to be able to be let go at some point overnight.
South to southwest winds increase Wednesday night. At this time,
thinking wind speeds stay under 25 kt with seas less than 5
feet.

Outlook /Thursday through Sunday/...

Thanksgiving Day: Winds less than 25 kt. Seas up to 5 ft. Rain
likely, isolated thunderstorms. Local visibility 1 nm or less.

Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Chance
of rain, isolated thunderstorms. Local visibility 1 to 3 nm.

Friday through Friday Night: Winds less than 25 kt. Seas up to
5 ft.

Saturday: Winds less than 25 kt. Seas up to 5 ft. Slight chance
of rain showers.

Saturday Night through Sunday: Winds less than 25 kt. Seas up
to 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 4 AM EST Wednesday for ANZ250-254.
     Small Craft Advisory until 1 AM EST Wednesday for ANZ255.

&&

$$
SYNOPSIS...Belk/Nocera
NEAR TERM...Belk/KS
SHORT TERM...Belk
LONG TERM...Nocera
AVIATION...Belk/Nocera/KS
MARINE...Belk/Nocera

NWS BOX Office Area Forecast Discussion

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