Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 101510
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1010 AM EST Sat Dec 10 2016

.SYNOPSIS...
Mainly dry, but unseasonably cold temperatures are on tap for our
region through Sunday. Low pressure will bring accumulating snow
to much of the region Sunday night and Monday morning, which will
likely change to rain on the coastal plain. Unsettled weather
continues through much next week including a surge of arctic air
invading southern New England late next week.

&&

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

10 AM Update...

Changes to previous forecast include adding ocean effect clouds
and snow showers to the outer Cape Cod as cold air streams on NW
winds across the relatively warm ocean waters of still near 50
degs across the eastern MA waters. This yields steep low level
lapse rates with 12z CHH sounding having -14C at the top of the
blyr. With SST around +9C this yields at delta-T of 23C. Given
NW surface winds will remain unchanged today along with CAA
expecting scattered snow showers to continue impacting the outer
Cape. Although diurnal heating today will disrupt/fragment snow
bands. Not expecting much impact here.

Otherwise forecast remains on track with cold temps today. 12z ALY
sounding has -15C air at H850 advecting into our region this
afternoon. Despite sunshine (ineffective) and downslope NW winds
will only yield highs in the 20s inland (esp high terrain) and low
30s along the coast including Boston to Providence. Previous
discussion below.

=====================================================================

Cold temperatures out there with most in the teens to lower 20s.
Wind chills are in the single digits to teens with light winds for
most areas and up to 10 to 15 mph along the coast. Made a few
minor updates to the temperatures for the latest trends but
otherwise the forecast is on track.

Southern New England will remain between two surface systems, a
low pressure over the Maritimes, and a high pressure over the
central USA, for one more day. Expecting another day of gusty
west-northwest winds, but not quite as strong as Friday.

One more mid level shortwave will also pass us by. However, there
is not a lot of moisture for it to work with. Expecting another
day of diurnal clouds, but not much more than that. The greatest
risk for any showers will be offshore, where the humidity will be
higher.

Max temperatures should be 5-10 degrees lower than Friday.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
High pressure reaches the mid Atlantic coast late tonight, then
moves offshore Sunday. Winds will diminish, setting the stage for
another cold night. Winds become light southwest during Sunday,
which will help to boost temperatures slightly. Dry weather
prevails.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...

*** Accumulating snow Sunday night into Monday will pose a
significant impact to the Monday morning commute north and west of
the Boston/Providence corridor ***

* Snowfall estimates unchanged from previous forecasts
* Unsettled weather continues through the middle of the week
* Another round of arctic air late in the week

Sunday night and Monday...High confidence in snow event occurring,
lower confidence in details.

Models are in much better agreement than in previous days on this
event. That being said, there is still a decent amount of
uncertainty. Minor changes in the path and timing of both the main
low and the secondary low could result in significant changes to the
timing and amount of snow.

Low pressure in the Great Lakes will move through Quebec and into
the Maritimes.  One of the things models differ on is the
development of a secondary low and its timing and location.  The GFS
and its ensembles are in excellent agreement, developing a secondary
low somewhere in the tri-state region early Monday morning.  The
ECMWF and its ensembles are also in great agreement, developing the
secondary low in the Gulf of Maine Monday afternoon.

This system will bring a period of accumulating snow to much of
southern New England. Light snow will begin Sunday evening as warm
air starts moving in over the arctic air at the surface. With very
cold air in place over the region, everyone should start as snow. As
the low level jet increases in response to the low pressure system
in the Great Lakes late Sunday night, snow will intensify. At this
point, it appears the heaviest snow will coincide with the Monday
morning commute.

While everyone will start as snow, expect a quick change to rain
particularly along the south coast, Cape, and Islands.  Elsewhere,
snow will gradually change to rain through Monday morning.  The
speed at which this occurs and the extent of the rain will depend on
when and where the secondary low develops and how quickly the warmer
air moves into the region.

Tuesday and Wednesday...Low confidence.

While there continue to be discrepancies in the models, this looks
to be the driest, quietest period of the long term forecast with
temperatures starting to cool but still near normal.  Previous
models have indicated another storm system affecting southern New
England.  The 00Z model runs show a coastal storm, but keeps it far
enough offshore to not affect southern New England.

Thursday and Friday...Low confidence.

Another shot of arctic air moves into southern New England as high
pressure builds into the region.  Temperatures will be well below
normal.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

10 am update...

Not much change from previous 12z TAFs. Modest NW winds up to 20
kt or so will continue into this afternoon. MVFR cigs will be
confined to the outer Cape Cod area along with scattered light
snow showers/flurries. Otherwise VFR and dry weather.

Short Term /through Sunday/...High confidence.

VFR through this period. Low risk of MVFR cigs across the outer
Cape today. Northwest wind gusts of 20 to 25 knots at times today,
especially across the higher elevations.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Sunday Night through Wednesday/...

Sunday night through Monday...Moderate confidence.  MVFR/IFR
conditions develop in snow Sunday night.  Snow may change to rain
across portions of the coastal plain early Monday.  Accumulating
snow is likely across the interior.  Some improvement is likely late
Monday as precipitation comes to an end.

Monday night through Wednesday...Low confidence.  Improving
conditions. Mainly VFR. Low probability of a precip event with
MVFR/IFR conditions sometime late Tuesday or Wednesday.

&&

.MARINE...
Forecaster Confidence Levels.

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

10 AM update...

NW winds 20-25 kt continue into this afternoon along with
scattered light snow showers. Previous discussion below.

=============================================================

Short Term /through Sunday/...High confidence.

Gusty northwest winds will diminish from west to east this
afternoon into this evening. Rough seas will also subside
overnight. Will stagger the expiration of Small Craft Advisories
into tonight.

Outlook /Sunday Night through Wednesday/...

Sunday night and Monday...Moderate confidence.  Seas below 5 feet
will increase from the south as low pressure approaches the waters.
Winds increase as well and small craft advisories will be necessary.
There is a moderate probability of a period of westerly gales.

Tuesday...Moderate confidence.  Westerly to southwesterly winds and
seas slowly diminish. Small craft advisories will likely continue
through much of Tuesday.

Wednesday...Low confidence.  Southwesterly winds shift to the
northwest and increase, along with the seas.  Small craft
advisories are likely.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 6 PM EST this evening for
     ANZ232>235-237-256.
     Small Craft Advisory until 8 PM EST this evening for ANZ231.
     Small Craft Advisory until midnight EST tonight for ANZ250-255.
     Small Craft Advisory until 11 AM EST this morning for ANZ251.
     Small Craft Advisory until 3 AM EST Sunday for ANZ254.

&&

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



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