Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 072356
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
655 PM EST Wed Dec 7 2016

.SYNOPSIS...

Dry weather expected tonight but residual ground moisture may result
in patchy black ice developing. A strong front crosses the region
late tomorrow. Mainly dry but unseasonably cold temperatures are on
tap for the region Friday into Saturday. Low pressure will likely
bring at least some rain/snow and/or even ice sometime Sunday into
Monday. A shot of arctic air is then expected by mid to late next
week.

&&

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

655 pm update...

An abundance of low clouds lingered across southern New England
early this evening, but there were a few breaks developing.  Winds
will be quite light tonight, so while low clouds will gradually lift
this has been a slower process than guidance has indicated.  In
addition, model cross sections indicate an abundance of mid level
cloudiness that will work in from the west. So expect considerable
cloudiness persisting overnight, but with some breaks at times.

The other issue is that residual ground moisture/light winds and
temps dropping below freezing across much of the region, may lead to
both patchy fog/black ice.  We have gone ahead and issued a special
weather statement to highlight these concerns.  Low temps will
bottom out mainly in the upper 20s to the lower 30s.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
Tomorrow...
Unsettled conditions as arctic front approaches from the NW,
arriving by evening across the N tier. W flow will allow for some
influence of Great Lake enhanced moisture into the region. This
combine with continued cold advection through the day could allow
for a few isolates flurries to impact the region, mainly out of a
mid deck of virga as the dwpt depressions continue to increase.
Accums will likely be little to none, as even the BTV squall
parameter is not too high given the overall lack of moisture
through the lower column, and relatively weak W winds until the
FROPA. Highs tomorrow once again remain mostly in the upper 30s
and low 40s despite a few breaks of sun, mid lvl temps will be
cooling with the approaching arctic front.

The primary caveat could be the E slopes of the Berkshires,
although shadowed under W flow, the enhanced upslope on the
windward side of the Hudson valley could work with enough of this
moisture to promote light, actual snowfall of around an inch or
less. As one descends into the CT valley, this threat will drop
quickly.

Tomorrow night...
Arctic fropa is mainly dry outside of a few isolated flurries or
light snow continuing in the E slopes of the Berkshires as
mentioned above. Sfc dwpt depressions continue to increase and H92
temps drop as low as -6C by around 12Z on Fri. Therefore,
advective cold (rather than radiative given the increase pres
gradient following the frontal passage) should allow more
widespread 20s across the region, even with continued SCT-BKN cold
advection cloudiness.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...

Highlights...

* Unseasonably cold but mainly dry Fri into Sat

* Period of light snow possible Sun into Sun Eve

* Uncertain late Sun night/Mon whether a secondary low brings more
  significant rain/ice/snow or misses us to the south

* Shot of arctic air likely mid to late next week

Details...

Friday...

Colder air will continue to work into the region on Friday
accompanied by northwest wind gusts of 25 to 35 mph.  Given low sun
angle and 850T dropping to around -14c by late afternoon, expect
high temps to only reach the 30s.  Model cross sections indicate a
fair amount of strato-cumulus clouds.  While dry weather will
dominate the vast majority of the day, a brief flurry/snow shower or
two is certainly possible particularly along the east slopes of the
Berkshires.

Friday night and Saturday...

Still can not rule out a spot flurry/snow shower or two Friday
evening.  Otherwise, expect low temps to bottom out in the teens to
around 20 in many locations.  Winds will not decouple which will
allow wind chill values to drop into the single digits.  Despite
partial sunshine, high temps will only recover into the upper 20s
and lower 30s.

Saturday night...

Large high pressure settles across southern New England.  This will
result in mainly clear skies/light winds and an ideal night of
radiational cooling.  Low temps should drop well down into the teens
for most locations, with some single digits likely in the normally
coldest outlying locations.

Sunday into Sunday evening...

High pressure moves off the coast as a weak wave of low pressure
moves up towards the Great Lakes.  Enough of a pressure gradient may
allow the development of a southwest low level jet of 30 to 40
knots. This will over ride the cold dome at the surface and may
allow light snow to break out from west to east Sunday into Sunday
evening.  If this occurs, antecedent airmass is quite cold so mainly
light snow would be expected near and northwest of the Boston to
Providence corridor. Southeast of that line, ptype probably should
start as light snow but warming boundary layer should allow for a
change to light rain. One interesting thing to note, 850T are still
below -10C at along the south coast at 18z Sunday with southerly
winds developing. Ocean SST between 50 and 55 may even allow
scattered ocean effect snow showers to develop along the south
coast, before any change over to light rain.

Overall, there really is not any organized system Sunday into Sunday
evening.  We are mainly just looking at southwest low level jet,
over riding the cold dome at the surface.  So while snow
accumulations are certainly possible, amounts look to be on the
light side at this time.

Late Sunday night and Monday...

Forecast confidence is quite low at this time.  A stronger wave or
two of low pressure may develop and threaten the area with more
significant precipitation.  Model guidance and ensembles are all
over the place on the location/track and potential strength of any
low pressure system.  There are 3 main scenarios on the table right
now.  The first solution would be a stronger inside runner which
would bring mainly rain to the coastal plain and a mixture of
rain/ice/snow to the interior.  The second would be a weaker wave
passing to our southeast, bringing mainly accumulating snow from the
I-95 corridor points northwest.  The third scenario would have the
wave sliding so far southeast that we end up mainly dry late Sunday
night and Monday.

All options remain on the table at this point, but certainly is the
potential for some wintry weather Sunday into Monday.

Tuesday and Wednesday... Specific timing remains uncertain, but
cross polar flow will result in a shot of very cold air by the
middle of next week.  In fact, a period of arctic air appears likely
some time in the mid to late next week time frame.  Specific timing
though is uncertain and some models hint at a wave of low pressure,
slowing its delivery until late next week.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short term...through Thursday night.

Tonight...Moderate confidence. Remnant MVFR CIGS should improve to
VFR by 03z.  However, light winds/wet ground will lead to patchy
ground fog and localized MVFR to perhaps even IFR conditions in the
typically prone locations.

Tomorrow and tomorrow night...High confidence.
Mainly VFR. Some MVFR possible in higher terrain due to lower
CIGS and even a low risk for light snowfall late in the day. W
winds increase, with gusts 15-20 kt at times by late afternoon,
continuing into the overnight. Some gusts +20kt along the
immediate coast.

KBOS TAF...High confidence in TAF.  Left over MVFR CIGS should
improve to VFR by 02 or 03z.

KBDL TAF...Moderate confidence in TAF, but risk for some patchy
ground fog developing tonight with light winds/residual ground
moisture.

Outlook /Friday through Monday/...

Friday...High confidence.  Mainly VFR, but brief marginal MVFR CIGS
possible at times mainly across the interior.  Northwest wind gusts
of 20 to 30 knots during the daylight hours.

Friday night and Saturday...High confidence in VFR conditions.

Sunday and Monday...Low to moderate confidence.  A period or
extended period of MVFR-IFR conditions are likely later Sunday and
possibly into Monday in some rain/snow and/or ice.

&&

.MARINE...
Forecaster Confidence Levels.

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

Short Term /through Thursday Night/...

Through tomorrow morning...High confidence.
Mainly quiet boating weather under light W flow. Only caveat is
the outer waters E of Long Island, where lingering swell from low
pres passage hovers near 5 ft through the overnight hours. Small
craft advisory will be issued for this risk.

Tomorrow afternoon and overnight...High confidence.
With cold frontal passage late tomorrow, expect an increase in
W-NW winds late in the day and continuing into the overnight.
25-30 kt is likely at times. Seas gradually build as well reaching
about 6 ft by early Fri AM on the outer waters. Small craft
advisories will be issued for this threat.

Outlook /Friday through Monday/...

Friday and Friday night...High confidence.  Strong cold advection
will result in widespread northwest wind gusts of 25 to 35 knots.
Opted to issue a Gale Watch across our eastern outer-waters, where
the winds look to be the strongest.  At the very least strong small
craft winds are expected elsewhere, with even the risk of brief
marginal gale force gusts.

Saturday...High confidence.  Despite weakening pressure gradient
enough cold advection over the relatively mild ocean will allow
northwest 20 to 30 knot wind gusts to continue.  Small craft
headlines will be needed.

Sunday...High confidence.  High pressure builds overhead and finally
allows winds and seas to drop below small craft advisory thresholds.

Sunday night and Monday...Low confidence.  Winds and seas are quite
uncertain and will depend on location/track and strength of a
potential wave of low pressure.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 7 PM Thursday to 9 AM EST Friday
for      ANZ230>234-236-251.
     Small Craft Advisory from 3 PM Thursday to 9 AM EST Friday for
     ANZ235-237.
     Gale Watch from Friday morning through late Friday night for
     ANZ250-251-254.
     Small Craft Advisory from 1 PM Thursday to 9 AM EST Friday for
     ANZ250-254-255.
     Small Craft Advisory until 9 AM EST Friday for ANZ256.

&&

$$
SYNOPSIS...Doody/Frank
NEAR TERM...Frank
SHORT TERM...Doody
LONG TERM...Frank
AVIATION...Doody/Frank
MARINE...Doody/Frank



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