Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 150202
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
902 PM EST Thu Dec 14 2017

.SYNOPSIS...
High pressure over the central U.S. builds E into New England into
early Friday, providing dry but cold conditions. Two areas of low
pressure, one tracking across southeast Canada and another well
southeast of Nantucket may bring a period of light snow or flurries
Friday evening to the area. Blustery and cold Saturday and Sunday.
Potential for light freezing rain Sunday night into Monday, followed
by additional disturbed weather through Tuesday. Cold and blustery
Wednesday into Thursday. Monitoring a potential storm system for
Friday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
9 PM update...
Winds remain gusty, especially along the coast, but will
gradually diminish after midnight as high pres builds in from
the west. Lowered temps a bit in the colder outlying locations
with expectation of good radiational cooling. Expect lows in the
single numbers, except teens along the coast.

Previous discussion...
Cold advection continues to pour into southern New England
behind departing ocean low with -18C air at 850 mb upstream
across NY state per latest SPC mesoanalysis. Dry weather tonight
in response to mid level confluent flow across the region.
Beneath this confluent flow high pres builds across the area,
resulting in gusty WNW winds this evening diminishing overnight.
So cold/dry airmass combined with diminishing wind and a fresh
snow cover from south-central CT-RI into southeast MA will yield
a cold night ahead. Given the radiational cooling conditions
derived mins from the colder MOS and bias corrected MOS
datasets. This supports lows in the single digits and teens
regionwide.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Friday...

Cold start to the day but fortunately not much wind. Some dim
morning sunshine possible behind increasing mid/high clouds. This
combined with cold airmass remaining in place over the area with 850
mb temps around -12C, not expecting much of a temp recovery. Highs
only 25-30 with warmest readings along the south coast and coldest
across the high terrain.

Friday evening and overnight...

Then toward evening another vigorous northern stream short wave
approaches southern New England from the west. Parent low tracks
from Ontario to Quebec. However this short wave induces cyclogenesis
off the Mid Atlc coast but races ENE south of the benchmark given
the fast/progressive upper air pattern. Will have to watch and see
if this short wave trends stronger and backs the mid level flow more
along the eastern seaboard. This would result in deep moisture and
lift tracking closer to the coast into southeast MA Friday evening.
All model guid indicate trough amplification just east of Cape Cod
with increasing mid/upper level Q-vector convergence. In addition
mid level lapse rates begin to steepen too. If later model runs are
6 hrs quicker with trough amplification a few inches of snow would
be possible over southeast MA, possibly eastern MA. However for now
will follow a model blend which offers a period of light snow/flurries
with snow accums less than an inch.

Drying trend by midnight along with a reinforcing surge of cold air
advection via blustery WNW winds on the backside of this departing
wave.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
*/ Highlights...

 - Cold and blustery Saturday into Sunday, snow showers possible
 - Sunday night into Monday, possible light mixed wintry precip
 - Monday night through Tuesday night, warmer first, then cold
 - Wednesday into Thursday, return cold and blustery
 - Friday onward, monitoring for potential mixed wintry precip events

*/ Overview...

Transitional, ebb and flow pattern. Into the following week, strong
onshore Pacific push of mild air as the H5 ridge breaks over the NW
Pacific, warming over the CONUS, heights rise and bowled colder air
over E Canada that has continually wrapped S over the NE CONUS this
past week is shunted back N. H5 heights less pronounced, more flat.
Impulses riding along the transitional zone, warmer air riding N/E
along lingering cold air presents the opportunity of light mixed
precip events, exiting quickly with absence of downstream traffic
though wrapping up and deepening over offshore regions of better
baroclinicity, colder air is drawn S, less pronounced given flatter
flow. Again, ebb and flow.

Then interpreting the pattern into the Christmas holiday. As noted
yesterday, there`s considerable warming / height rises over N Canada
withing the stratosphere as the polar low / night-time jet displace
to the other side of the N Hemisphere. This concurrent with a strong
push of mild air from the NW Pacific into the Arctic, stout H5 ridge
into Alaska, pattern reloads as it buckles downstream over the CONUS
into the N Atlantic. But the main polar low over E Canada seemingly
pushes E, nudged by the aforementioned positive height anomalies to
the W all the way up through the stratosphere. The N Atlantic wide
open, looking at a SW-NE transitional zone towards regions favorable
for storm development over the N Atlantic into NW Europe. Ejecting
energy sheared S over the SW CONUS from the favorable H5 ridge over
the NW Pacific, along the SW-NE transitional zone, H85 temperature
and parent anomalies from ensemble means showing this well, looking
at a pattern favorable over the E CONUS of mixed precip events. With
storm development as noted over the N Atlantic, expecting colder air
to remain bowled over E/NE Canada, held N up against southwesterlies
and positive height / mean sea level pressure anomalies, presumably
parent with the positive height anomalies / warmer air surging into
the Arctic over Northern N America. Notably N Atlantic surface high
builds back W towards the SE CONUS.

Got awhile to make sense of it all. Low confidence forecast by the
end of the week into the following weekend. Will hit upon threats /
impacts in the discussion below, noting any particular details and
touching upon any specifics.

*/ Discussion...

Saturday into Sunday...

Cold, blustery, snow shower activity off the Lakes. Storm system
departing, deepening downstream Friday night into Saturday, with
additional mid-level energy, moisture off the Lakes / over ocean
waters, expect blustery winds and cold air advection to contribute
to snow shower, potential snow squall activity. Highest confidence
of impacts over the Berkshires and across the Outer Cape / Islands.
Light accumulations anticipated along with reductions in visibility
through Saturday prior to high pressure and colder air into Sunday.
Wind gusts Saturday up around 30 to 35 mph forecast, holding below
WIND ADVISORY criteria. Still cold wind chills going into Sunday
morning with lows in the single digits to teens.

Sunday night into Monday...

Mixed precipitation event. Lifting warm front along which moisture
ascends above lingering cold air, albeit weak lift. Not a clear cut
scenario. Cold air damming signal via ageostrophic flow light with
absence of stronger parent low S/W. Ascent / lift along the warm
front being weak and quasi-parallel along isentropes. Ensemble means
continue to signal decent 0.01 inch probabilities but now more N/W
closer to the parent low collocated with strong SW ascending branch
of warm, moist air. Aside from signals of light outcomes and mixed
precipitation (snow to sleet to freezing rain), only takes a trace
of ice to require WINTER WEATHER ADVISORIES. Operational guidance
continues to poorly handle the weak wave event yet EC has remained
consistent. GFS drier and flatter, little indication of a weak wave
disturbance / H85 trof axis.

Monday night into Tuesday night...

Seasonable, dry, SW flow at first prior to a sweeping cold front,
perhaps anafrontal, during the overnight period. A flat mid-level
feature to accompany, looking at a lack of impacts associated with a
potential rain to snow transition. However, operational guidance
having difficulties in handling S-stream energy being swept up ahead
of the N-stream trof. Separated flow regime or a combination of the
sub-tropical with the polar jet. The 14.12z GFS more robust over the
concurrent EC. Most GEFS members clustered lower than deterministic
guidance. EC ensemble probabilities low with respect to 0.01 inches
of liquid.

Wednesday into Thursday...

Lean cold, blustery and dry for Wednesday behind the departing storm
system, deepening over SE Canada, drawing S cold air. High pressure
for Thursday, a rebound of S flow during the evening ahead of an
approaching warm front.

Friday onward...

Signals of a strong synoptic system sweeping the region. Following
closely to EC ensemble means, and inside runner, could be dealing
with mixed precipitation types out ahead of a lifting warm front,
the follow-up sweeping cold front overnight putting us back into
colder air as the storm system deepens over SE Canada. From there,
the Atlantic high build back W, the amplified pattern emerging over
the W CONUS, we get to a SW-NE transitional zone pattern welcoming
mixed precipitation / ice events over the E CONUS if ensemble means
are correct, gone over in detail in the OVERVIEW section above.

&&

.AVIATION /02Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Friday Night/...High confidence.

Tonight...
VFR. Gusty WNW winds 15-20 kts diminishing after midnight.

Friday into Friday night...
Monitoring for potential -SN late day into evening over SE New
England terminals. Potential MVFR/IFR impacts, light accums
potentially up to 1 to 2 inches over the Cape and Islands if the
associated storm system hugs closer to the coast. Elsewhere VFR
prevails with light winds.

KBOS Terminal...
VFR. Keep snow late Friday into evening S/E. Can`t rule out some
potential flurry activity.

KBDL Terminal...
VFR. WNW winds diminishing, will be light. Increasing clouds
into Friday and Friday night but should remain dry, any -SN
activity remaining mostly S/E.

Outlook /Saturday through Tuesday/...

Saturday through Saturday Night: VFR. Windy with local gusts to
30 kt.

Sunday: VFR. Slight chance FZRA, slight chance SN.

Sunday Night: VFR. Chance SN, slight chance FZRA.

Monday: VFR. Chance SN, slight chance RA, slight chance FZRA.

Monday Night: Mainly VFR, with local MVFR possible. Slight
chance RA, slight chance SN.

Tuesday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance RA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...High confidence.

Tonight...
Departing low over eastern Georges Bank yields WNW winds 15-20
kt with gusts up to 25 kt this evening across the MA/RI waters,
then decreasing overnight as high pres builds in from the west.
Dry weather and good vsby.

Friday...
Light winds with high pressure over the area. Dry weather and
good vsby.

Friday night...
Light winds during the evening, then low pres over Ontario into
Quebec along with low pres tracking well southeast of the
40N/70W benchmark, both move east and intensify. This yields
increasing WNW winds after midnight with speeds of 15-20 kt,
gusts possibly up to 25 kt toward daybreak Sat. Vsby may lower
in snow across the eastern MA outer waters, east and southeast
of Cape Cod.

Outlook /Saturday through Tuesday/...

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Slight chance of rain,
slight chance of snow.

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas.

Sunday: Winds less than 25 kt. Slight chance of rain.

Sunday Night through Monday: Winds less than 25 kt. Chance of
rain, chance of snow.

Monday Night: Winds less than 25 kt. Slight chance of rain.

Tuesday: Winds less than 25 kt. Chance of rain.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 1 AM EST Friday for ANZ232>234.
     Small Craft Advisory until 3 AM EST Friday for ANZ230.
     Small Craft Advisory until 4 AM EST Friday for ANZ231-235-237-
     251.
     Small Craft Advisory until 7 AM EST Friday for ANZ250-254>256.

&&

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



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