Area Forecast Discussion
Issued by NWS Boston, MA

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071
FXUS61 KBOX 041740
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1240 PM EST Sun Dec 4 2016

.SYNOPSIS...

High pressure today with dry, cold conditions as winds diminish.
Light snow Monday morning will bring a minor accumulation before
tapering off to a mix of light rain and snow in the afternoon.
Mainly dry conditions expected Monday night into Tuesday with high
pressure. Low pressure off the mid Atlantic coast may bring some
mixed light rain and/or snow into Southern New England Tuesday
night into Wednesday night. A polar front crosses the region
Thursday with rain showers possibly changing to snow showers. Very
windy and cold conditions forecast for Friday and next Saturday
with wind chill indices in the single digits and teens Friday night.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

1230 pm update...no major changes to the forecast.

High pressure building in from the W as heights rise aloft. Should
continue to see a downward trend in NW winds with gusts in excess
of 20 mph over the E-half of S New England initially. Increasing
subsidence across the region, should see clouds clear out. Still a
decent cold airmass aloft given residual cyclonic flow out of the
NW ahead of aforementioned ridging. Near-seasonable conditions,
just a tad below, with highs around the upper 30s to low 40s seems
reasonable for today. Overall pleasant and dry, more tolerable with
less wind.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...

Snow Potential...

Good model agreement on snow overspreading CT and western-central
MA between 09z-12z...then into RI and eastern MA 12z-15z. Here are
a few limiting factors that will keep snowfall potential low...

1) Short wave trough is very progressive, thus short duration of
snowfall

2) Deep layer moisture is brief as well

3) PoPs from guidance are only high chance/low likely. This could
be a potential red flag. Prefer to see guidance offering category
PoPs if expecting accumulating snow.

4) Forcing for ascent is fairly weak and brief in the snow growth
region (-10c to -18c) with dry air entering this layer rapidly
from 15z-18z. Given this not expecting large snow ratios.

5) Short wave trough is deamplifying along with weakening wind
fields with time as it approaches and moves across southern New
England. This will limit forcing for ascent.

6) Warm air at 925 mb surging northward will likely change snow
to rain toward midday across CT/RI and south of the MA turnpike.
This will also limit snowfall potential. While boundary layer wet
bulb temperatures are below freezing...precipitation may be too
light to take advantage of evaporative cooling or from melting
precipitation.

Given all of the above factors will play this portion of the
forecast conservative for now. Potential snowfall looks to be from
a coating to an inch for most locations. A few locations in the
higher terrain of CT/MA will have a low risk for up to 2 inches.
Overall not a high impact event. Best chance of roads briefly
becoming snow covered will be across CT/western-central MA
including the Greater Hartford-Springfield areas before snow
shield weakens as it moves eastward. Given these factors above
will hold off on any headlines or statements for the moment.

Temperatures...

Temps will fall rapidly this evening with sunset given cold/dry
airmass in place combined with clear skies and light winds. Thus
blended some of the colder MOS guidance to account for this. Then
clouds arrive around or shortly after midnight resulting in temps
leveling off or perhaps rising a few degs toward daybreak. Temps
begin to rise late Mon morning and afternoon as blyr winds become
light SE.

Winds...

Winds will be light given weak pres gradient as feeble low pres
tracks south of New England.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...

Highlights...

* Mixed precipitation possible late Tuesday night and Wednesday
* Very strong front blasts across the region Thursday
* Windy and very cold into next weekend with strong gales over
  the waters

Monday night...High confidence.

Expecting quiet conditions as high pressure builds across the
region. Skies becoming mostly clear, except for a few lingering
clouds across the E slopes of the Berkshires. N winds will be a
bit gusty early along E coastal areas, then will diminish as the
high takes over. With the lighter winds, should see some
radiational cooling especially in the normally colder locations.
Temps will fall back to the mid and upper 20s across most
locations, ranging to the 30s along the immediate coast.

Tuesday...Moderate confidence.

High pressure ridge extending from a central across Quebec down
the eastern seaboard will lift NE during the day. Winds become
light NE-E. Weak low pressure moving NE out of the SE U.S. will
bring some clouds across the region during the afternoon.
Overrunning moisture will push NE toward the region after 18Z, so
carried just chance POPs across N CT into SW RI by sunset. Some
question on the timing and extent of the moisture into the region,
however. Expect highs in the mid and upper 30s across the higher
inland terrain, ranging to the lower-mid 40s along the coast.

Tuesday night through Wednesday night...Moderate confidence.

Fast moving weakening H5 short wave in the SW flow behind the
exiting ridge will cross the region Tuesday night, with weak low
pressure off the mid Atlantic coast. With onshore easterly flow,
will see light precip starting off as light snow, then mixing with
or changing to rain across the coastal plain with the light but
steady onshore winds.

As the short wave lifts NE into Wednesday morning, the surface
low off the mid Atlantic will shift E well south of the region
taking its moisture field offshore. So, will see precip become
rather spotty by Wednesday evening. Temperatures on Wednesday will
rise to the upper 30s to mid 40s across most areas, but remaining
a bit cooler across the higher inland terrain. Looks like precip
should change to mainly light rain by midday Wednesday.

By Wednesday night...most models and ensembles start to bring more
moisture to the region after midnight. So, brought chance POPs
back in for now. QPF amounts remain low with lack to deep moisture
field. As temps fall back, any mixed rain and/or snow across the
interior will change back to light snow, with the best chance
across the E slopes of the Berkshires.

Thursday and Thursday night...Moderate confidence.

Timing and track differences continue as a polar front wraps
around the cutoff low pressure across Hudson Bay. A digging H5
long wave trough, bringing surge of very cold air from western
Canada and Alaska, will work eastward. Will see southerly winds
ahead of the front Thursday, but exact timing of the frontal
passage in question. GFS tending to be faster with this feature
than the ECMWF, which was similar to the previous forecast.
Continued with a model blend this forecast.

The front should pass through Thursday night, accompanied by a
chance of snow showers across most of the region, except rain in
southern RI, Cape Cod, and the Islands. Lows 25 to 30 except mid
to upper 30s Cape Cod and Islands.

Friday and Saturday...Moderate confidence.

Colder air will sweep in across the region Friday as W-NW winds
quickly increase. Models differ with the development of strong low
pressure over the Maritimes during this timeframe. However, with
the strong pressure gradient, expect winds to gust up to 25-35 kt,
highest along the east coast, which looks to continue through
Saturday. May see gusts around 40 kt along the coastal waters
Friday night and Saturday. May also see lingering snow showers
across the E slopes of the Berkshires, as well as ocean effect
snow showers off the coast.

With the strong cold air advection moving in, expect highs on
Saturday to run up to 10 degrees below seasonal normals. Will also
see wind chill values Friday night into early Saturday morning down
to the single digits and teens.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...

Forecaster Confidence Levels...

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

18z update...

Short Term /through Monday/...High confidence.

Rest of Today...
VFR. NW winds diminishing.

Tonight into Monday morning...
Cigs lowering to MVFR with onset of -SN around 9-12z over W and
Central MA and all of CT, around 12-15z for E MA and all of RI.
IFR vsbys at 2SM with -SN. -RA mixing with -SN for coastal
terminals with lesser impacts to visibility. Snow accumulations
anticipated on non-coastal runways, around 1-2 inches forecast for
the high terrain, around 0.5-1 inch for W and Central, coating to
0.5 inches for coastal plain terminals S/E.

Monday afternoon into Monday night...
Improving to VFR as conditions clear. MVFR cigs lingering across
the Berkshires and developing across the Outer Cape as winds turn
N.

KBOS TAF...
MVFR with -SN around 12z-15z Monday mixing with and changing over
to -RA. Snow accumulations possible, expecting mainly a coating
but possibly up to half an inch.

KBDL TAF...
MVFR with -SN 9z-12z Monday. IFR visibilities with snow, 2SM. Snow
accumulations possible around half an inch to 1 inch. Lower risk
of snowfall amounts in excess of 2 inches.

Outlook /Tuesday through Thursday/...

Tuesday...High confidence. VFR.

Tuesday night through Wednesday night...Low to moderate
confidence. MVFR to local IFR ceilings in light rain RI/SE Mass,
a mix of rain/snow northern CT, central and northeast Mass, and
light snow northwest MA. May briefly change over to all rain
across most areas late Wednesday morning into the afternoon.

Thursday...Low to moderate confidence. Mainly MVFR ceilings in
areas of light rain showers ahead of approaching polar front.
Depending on timing, rain showers could change to snow showers
with local IFR conditions in western MA late in the afternoon.

&&

.MARINE...

Forecaster Confidence Levels.

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

Short Term /through Monday/...

Today...
Gusty NW winds diminish as the day progresses allowing small craft
advisories to conclude through the day, lingering the longest over
the E outer waters.

Tonight...
Light winds as high pressure crest over the waters. Dry weather
and good visibility.

Monday...
Light SE winds as weak low pressure tracks near the Southern New
England coast.

Outlook /Monday Night through Thursday/...Moderate confidence.

Monday night through Wednesday night...Winds and seas remain
below small craft criteria for most of the waters. NW winds Mon
night will shift to E, gusting up to 20 knots at times. Seas may
build to near 5 ft over the southern outer waters by Wednesday
night.

Thursday...Small craft advisories will likely be needed. Winds
will shift to the S-SE and may gust up to 25 to 30 kt by late
Thursday afternoon. Winds may shift to SW toward Thu evening. Seas
will be building to 5-7 ft over the outer waters.

Attention mariners...Expect strong W-NW gales with some potential
for storm force gusts are forecast to develop Friday into Saturday.
Stay tuned for the latest forecast updates.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 4 PM EST this afternoon for ANZ250-
     254-255.

&&

$$

SYNOPSIS...Nocera/EVT
NEAR TERM...Nocera/Sipprell/EVT
SHORT TERM...Nocera
LONG TERM...EVT
AVIATION...Nocera/Sipprell/EVT
MARINE...Nocera/EVT



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