Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 102345
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
645 PM EST Sat Dec 10 2016

.SYNOPSIS...
Dry but unseasonably cold weather continues into Sunday afternoon.
Low pressure and its attending warm front will bring accumulating
snow to much of the region Sunday night into early Monday morning,
which will likely change to rain across much of the area Monday
morning, except mainly all snow for northwest Massachusetts. A
brief period of snow is possible late Wednesday or Wednesday
night in advance of a surge of arctic air invading southern New
England Thursday and Friday. Another storm system may bring snow
and mixed precip next Saturday.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
645 PM update...
Other than some ocean effect clouds clipping the outer Cape,
skies are clear across SNE. As high pres builds in from the west
and NW flow weakens clearing trend will develop over the outer
Cape toward midnight. Adjusted sky grids for current trends,
otherwise forecast on track. Coldest night of the season with
ideal radiational cooling as high pres builds into SNE.

Previous discussion...
Very cold and dry airmass over the region late this afternoon
with dew pts in the single digits! This combined with mostly clear
skies overnight along with high pres building into the area from
the OH/TN valleys will result in diminishing winds and ideal
radiational cooling conditions. Thus have derived tonight`s min
temps from the coldest datasets...MAV and MET mos guidance. This
yields low temps in the teens for most of the region and single
digits in the valleys and typically colder spots of eastern
interior MA including Marthas Vineyard.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY/...
Sunday...

Very cold start to the day, although with neutral thermal advection
eventually transitioning to weak warm advection will result in temps
climbing a few degs higher than today with highs Sunday 30-
35...upper 20s high terrain.

Morning sunshine fades behind increasing afternoon clouds. The onset
of warm advection snows/flurries may enter western portions of CT/MA
between 4 pm and 7 pm. Elsewhere snow holds off until Sunday evening.

*** Accumulating snows Sunday night into early Mon morning ***

Sunday night and Monday...

Synopsis and Model Trends...00z models last night started the trend
toward warmer solutions...holding onto a parent low longer over the
Great Lakes Monday and delaying secondary low formation along with a
more northeast position. All of this suggest warm air advancing
quicker into southern New England faster than earlier model runs.

Timing...good model agreement on light snow/flurries breaking out
across western CT/MA 4 pm to 7 pm Sunday. Farther east dry air
lingers and takes column longer to saturate. Thus snow begins 7 pm
to 10 pm remainder of CT/MA and RI. A period of steady snow
overspreads the region from about midnight to sunrise/daybreak
Monday.

Ptype/Changeover...the rain-snow line should begin lifting south to
north across CT/RI and eastern MA between 4 am and 7 am Monday. Then
the remainder of the region between 7 am and 10 am. However precip
may remain all snow across northwest MA.

Snow Accumulations and Impacts...most likely snowfall projections
are 3-6" across northwest MA...2-4" for interior CT into central MA
(including Worcester Hills) and Merrimack Valley...then tapering
down to 1-2" for northeast MA (including Boston) into northern RI.
Then a coating to an inch for the south coast of MA and RI. Snow
will be changing to rain during the morning commute across RI and
southeastern MA...probably into the Greater Boston area too. Thus
snow covered roads in this area will transition to slushy conditions
which will offer some improvement vs north and west of I495 in
northeast MA. This is where the Monday morning commute will be most
treacherous.

Uncertainty...still 36-48 hrs out in model time so still some
uncertainty. However model solutions have been converging the past
two runs. Held off on issuing Winter Storm Watches for northwest MA
for the following reasons...modest forcing for ascent/limited
instability aloft/poor snow growth-ratios and very progressive
system limiting snow duration. GEFS and SREF plumes both support sub-
watch/warning criteria. In addition a superblend of all guidance
sources only yields 20-30% risk of 6+ inches across northwest MA.

Monday afternoon...dry slot overspreads the region rapidly from west
to east...so expecting some partial clearing during the afternoon.
However clouds and rain showers will linger longest over the south
coast including Cape Cod and islands. With secondary low forming
northeast of the region over the Gulf of ME...west-northwest winds
develop after 18z and any leftover shallow cool air mixes out. This
results in surface temps rising into the low and mid 40s across much
of CT/RI and eastern MA...and near 50 over the islands. Cooler
northwest MA with highs only mid to upper 30s. However these above
temps above freezing will help snow covered roads across the
interior transition to bare pavement.

&&

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

* Some light snow possible late Wed and Wed night
* Blast of arctic air Thu/Fri
* More snow and mixed precip possible next Sat

Overview...
Large scale pattern features an anomalous ridge across Alaska and
blocking ridge over northern reaches of Europe. This results in
anomalously low heights across Canada as polar vortex rotates south
and east from north central Canada. Main feature in the extended
will be the blast of arctic air at the end of the week as the vortex
moves to eastern Canada. Ahead of the arctic intrusion we will have
to watch for a potential coastal wave which may spread some light
snow to coastal New Eng late Wed and Wed night. With lack of
downstream blocking over the favorable location in the north
Atlantic, pattern is transitory so arctic air will be relatively
short lived, and there is a potential for another winter weather
event next Sat as the arctic air departs and SW flow aloft becomes
re-established.

Details...

Monday night through Tuesday night...
Quiet period with mainly dry and seasonable conditions. High pres
moves to the mid atlc coast with dry westerly flow. Some mid/high
clouds will move through Tue. Seasonable temps expected.

Wednesday into Wednesday night...
This is expected to be the transition period ahead of the arctic
intrusion during the end of the week. Mid level shortwave diving SE
from the Gt Lakes will sharpen the trof somewhat leading to
potential offshore wave developing. Consensus of the guidance favors
this to remain far enough offshore, but northern extent of the
moisture may at least clip coastal areas with some light snow
sometime late Wed and Wed night. There are a modest number of
GEFS/EPS members that have some snow with low probs for 0.10" QPF.
So we will continue to advertise chc pops, mainly for Wed night
ahead of the arctic air.

Thursday and Friday...
There is high confidence for arctic intrusion with coldest air of
the season. Ensemble mean 850 mb temps down to -20C late Thu into
Fri with deterministic ECMWF even several degrees colder. Expect highs
in the teens and 20s both days with Fri the coldest day, and
potential for near zero mins in the interior high terrain Fri
morning, and single numbers for much of the interior. Bitter cold
wind chills well below zero possible late Thu night and early Fri.
Looks mainly dry during this period, with low prob for some ocean
effect snow showers over the outer Cape if winds veer more than
forecast. Low level wind forecast is W/NW which would keep best
chance of ocean effect snow offshore but this is still several days
away.

Saturday...
Low confidence forecast at this time range but pattern setting up
for some overrunning snow as very cold air in place at 850 mb before
arctic air departs. With high pres positioned to the east, may also
see an eventual changeover to rain/mixed precip. Still lots of
uncertainty with the details so stay tuned.

&&

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

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

Tonight...High confidence.

VFR...dry and diminishing winds.

Sunday...high forecast confidence

VFR...dry weather and light winds. Exception will be across
western MA/CT where cigs may lower to MVFR in light snow 21z-22z.

Sunday night...moderate confidence.

MVFR in snow trending toward IFR toward morning along with areas
of LIFR. Snow changing to rain 09z-12z from south to north across
RI and eastern MA.

Monday...moderate confidence.

IFR/LIFR in snow interior Monday morning...rain across RI and
eastern MA but improving to MVFR and VFR Monday afternoon.
Probably only improving to MVFR across the higher terrain.

KBOS TAF...High confidence in TAF. Snow begins 00z-03z Mon with
steadiest snow 06z-09z Mon before changing to rain 09z-12z Mon.

KBDL TAF...High confidence in TAF. Snow begins 22z-01z Mon with
steadiest snow 03z-09z then changing to rain 09z-12z but possibly
a brief period of freezing rain 09z-12z.

Outlook /Monday Night through Thursday/...

Monday night through Tuesday night...Moderate confidence. VFR.

Wednesday into Wednesday night...Moderate confidence. A period of
MVFR possible sometime late Wed or Wed night with some light snow,
especially near the coast.

Thursday...Moderate confidence. Mainly VFR cigs.

&&

.MARINE...
Forecaster Confidence Levels.

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

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

Tonight...leftover NW winds 20-25 kt and 5+ seas will warrant SCA
for the outer waters. Downward trend later this evening and
overnight. Dry weather and good vsby.

Sunday...light wind with high pres overhead. Dry weather and good
vsby continue.

Sunday night...south to southeast winds increasing to 20 to 30 kt
toward Monday morning. Vsby lowers to 1 mile or less at times in
snow.

Monday...southeast winds 20 to 30 kt in the morning shift to WSW in
the afternoon. Vsby 1 mile or less in morning snow changing to rain
from south to north. Improving and drying in the afternoon but
slowest improvement on vsbys will be southern waters as rain
lingers.

Outlook /Monday Night through Thursday/...

Monday night...High confidence. West gusts to 25 kt likely with SCA
conditions.

Tuesday into Wednesday...Moderate confidence. Diminishing westerly
winds below SCA by Tue and remaining below SCA into Wed. Seas
subsiding.

Thursday...Moderate confidence. Increasing W/NW winds with gusts to
30 kt likely and moderate prob of gales and G35-40kt over outer
waters.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 6 AM EST Sunday for ANZ250-254>256.

&&

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



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