Area Forecast Discussion
Issued by NWS Boston, MA

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689
FXUS61 KBOX 041158
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
658 AM EST Sun Dec 4 2016

.SYNOPSIS...
High pressure will build across the area today, resulting in
diminishing winds along with dry but cold conditions. A period of
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 as high
pressure moves across. 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/...

645 AM Update...

Bands of stratocumulus clouds moving across most of the region
early this morning as seen on latest IR satellite imagery and
surface OBS. General trend is for the clouds to dissipate this
morning, though will linger across outer Cape Cod and
occasionally into Nantucket off and on through midday or so. NW
winds sustained at 10-20 mph, with gusts up to 25-30 mph across
the outer Cape and Nantucket.

High pressure centered over western Quebec, extending across W
NY/W PA to central VA at 11Z. The ridge will slowly build E this
morning, so expect winds to diminish. Pretty much same trend from
the previous forecast.

With the clouds around, temperatures a bit milder than previous
forecast, ranging from 30-35 for most locations at 11Z, but to the
upper 30s at KPVC and KCQX.

Near term forecast pretty much on track except for the cloudiness,
so have updated to bring conditions current and incorporate into
today`s trends.

Previous Discussion...

As clouds dissipate with rising mid level heights, will see more
sunshine this afternoon than during the morning.

Sunshine will be ineffective today given the low Dec sun angle
and cold airmass in place with H85 temps down to about -9C this
morning and -5C at 925 mb. Model soundings indicate blyr mixing
will be limited to about 875 mb due to subsidence inversion. This
will yield highs only in the upper 30s to lower 40s...about 5 degs
colder than normal. However less wind than yesterday will make it
more tolerable.

Ocean effect snow showers early this morning affecting the outer
Cape and possibly into Nantucket will come to an end later this
morning as subsidence inversion lowers and squashes updraft
potential as high pres crest over the region.

&&

.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 guid 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 blyr wet bulb temps are
below freezing...precip may be too light to take advantage of
evaporative cooling or from melting precip.

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 /12Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

1 am update...

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

Today...VFR with CIGS from 3500-4500 ft. The exception will be
over the outer Cape and possibly into Nantucket where snow showers
at times will yield MVFR conditions through mid morning. Any lower
conditions should improve to VFR by midday.

Light NW winds, except gusty N-NW winds over Cape Cod and the
islands with gusts up to 30 kt. Winds will diminish during the day

Tonight...VFR and light winds. MVFR in snow overspreading CT and
western-central MA between 09z-12z...holding off til 12z-15z
across RI and eastern MA.

Monday...a period of light snow in the morning then then tapering
off during the afternoon. Light snow will mix with or change to
rain along the south coast and possibly north to PVD-BOS. Snow
accumulations will be light...mainly a coating to an inch. Low
risk for a few spots across CT receiving up to 2 inches. MVFR
conditions will trend to VFR toward sunset across CT and western-
central MA...MVFR likely persisting across RI and eastern MA.

KBOS TAF...high confidence for VFR/dry runways and NW winds not
nearly as gusty as Sat. MVFR in light snow overspreads Logan
12z-15z Monday. Snow accumulations from a coating to an inch
possible.

KBDL TAF...high confidence for VFR/dry runways and NW winds not
nearly as gusty as Sat. MVFR in light snow overspreads BDL 09z-
12z Monday. Snow accumulations of a coating to 2 inches possible.

Outlook /Monday Night through Thursday/...

Monday night...VFR. Mainly clear skies, except lingering clouds
early across the E slopes of the Berkshires and across E coastal
Mass through around midnight. Light NW winds.

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. Ocean effect snow
showers east of Cape Cod will dissipate by midday. Small crafts
should end by mid to late morning most of the waters, but will
linger through the day over the eastern outer waters.

Tonight...
Light winds as high pres crest over the waters. Dry weather and
good vsby.

Monday...
Light SE winds as weak low pres 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 10 AM EST this morning for
     ANZ230>237-251-256.
     Small Craft Advisory until 4 PM EST this afternoon for ANZ250-
     254-255.

&&

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



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