Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 132107
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
407 PM EST Wed Dec 13 2017

.SYNOPSIS...
A fast moving low pressure will track south of New England late
tonight and early Thursday bringing a period of accumulating snow to
the south coast and especially the Islands. Cold persists into
the weekend with a chance of snow showers Friday night into
Saturday. Blustery and cold for Sunday but the beginning of a
milder trend Monday ahead of which there`s the possibility of a
mixed wintry precipitation event. Quiet and dry with an ebb and
flow pattern during the week up till late Thursday into Friday
during which time there`s the possibility of another mixed
wintry precipitation event.

&&

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

4 PM update...

This evening...

CAA continues across southern New England with SPC mesoanalysis
indicating -19C air at 850 mb upstream over northern portions of NY
and VT. Thus modified arctic air continues pour across the area on
blustery WNW winds up to 45 mph, courtesy of 975 mb low over New
Brunswick. This will result in very cold conditions this evening
along with bitterly cold wind chills.

Overnight...

Fairly robust mid level trough currently over the Great Lakes races
eastward overnight with increasing clouds and light snow/flurries
developing toward daybreak especially across CT, RI and south of the
MA pike in western part of the state. Initially airmass is very dry
so while radar may look impressive late tonight probably a few hours
of virga before snow reaches the ground toward daybreak Thu.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...

*** Accumulating Snow Likely Immediate South Coast and Islands
 Thursday Morning ***

4 PM update...

Thursday...

Mid level trough moves across the region from west to east during
the morning. Good synoptic scale lift associated with this system as
seen on Q-vector maps. This results in surface low jumping from near
Cleveland at 06z to redeveloping south of Long Island 12z Thu and
tracking east along the 40N parallel during the day. Good model
agreement that modest omega (15 -ubar/s) occurs in the DGZ (snow
growth layer of -10C to -20C) 12z-15z Thu along the immediate south
coast and Islands. Thus should see at least modest size flakes
(dendrites) which will help reduce vsby and yield an efficient snow
system with 15-1 snow ratios. In addition, models suggest some
instability in this layer too in the form of -epv. So the
combination of modest synoptic scale lift, good Fgen in snow growth
layer may result in this system over performing. However limiting
factors for more snow is very dry airmass on the front end of this
system. Will likely lose a few hours of qpf to virga as blyr slowly
saturates. Furthermore, system is very progress which leaves just a
1-3 hr window for moderate snowfall. Thus have leaned toward a 1-3
inch snowfall for immediate south coast and islands with impact to
the morning commute from Westerly to Newport to New Bedford to Cape
Cod. Given cold antecedent conditions snow will immediately stick to
all surfaces. Will have to see how 00z models trend as these systems
sometime trend northward. This would result in accumulating snow
possibly as for north as Hartford to Providence to Plymouth.
Nonetheless there will be a very sharp cutoff on the northern edge
of the snow shield.

Given trough is very progressive expecting sunshine to develop
during the afternoon. Still colder than normal with highs only in
the upper 20s and lower 30s. Not nearly as windy as today but still
a chilly day.

Thursday night...

Becoming blustery during the evening behind departing low. However
winds drop off late as weak high pres builds in. This will allow for
another cold night including the south coast given a fresh snow
cover. Lows in the teens most areas with single digits well inland.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...

*/ Highlights...

 - Light snow showers possible Friday night into Saturday
 - Cold and blustery Saturday night into Sunday
 - Possible mixed precipitation Sunday night through Monday night
 - Perhaps quiet, cold until Thursday night into Friday
   with another mixed precipitation event

*/ Overview...

Interpreting through late December. Considerable forecast warming in
the stratosphere, polar low / night-time jet displaced to the other
side of the N hemisphere subsequent of noteworthy anomalous ridging
across Northern N America. Beneath which warm air continues to surge
into the Arctic from the NW Pacific (with perhaps MJO contributions)
and the NW Atlantic displacing colder air S where it becomes bowled
up across NE Asia / NE N America. Ensemble means continue to signal
an active weather pattern with an amplified H5 ridge-trof pattern
over the CONUS, anomalous H85 T warming over the N Pacific with cold
anomalies continually reloaded over Central/E N America, however a
trend towards higher heights / surface mean sea level pressure with
stratospheric cross-polar flow shifting into NW Europe.

Making sense of it all, NW Pacific disturbances translating E across
the CONUS seemingly look to flatten the H5 pattern, lock colder air
N for a time, the NE CONUS falling into the transitional zone of air-
masses before colder air reloads and surges S again. An ebb and flow
setup however not barred from N shots of colder air, the lack of S-
stream dominance. Yet a trend of higher heights as signaled from the
ensemble means, cold shots may not be as deep, storm development may
not be so pronounced long-term. Perhaps La Nina signals are starting
to emerge across N America with a lesser pronounced H5 trof pattern
over Central/E N America downstream of stout ridging / warmth over
the NW Pacific.

Tough to get down on the details and specifics, taking a broad view
approach with the discussion above. Will hit on any threats/impacts
in the details below. With perhaps a flatter pattern evolving and
the lack of downstream traffic over the N Atlantic, the move into a
zone of transition between the airmasses as noted above, as forecast
guidance suggests, could end up in a regime of quick-moving systems
with mixed precipitation type outcomes that later deepen downstream
and yield that reloading shot of colder air, yet that colder air not
as deep, perhaps simply swiping the NE CONUS more to the N.

*/ Discussion...

Friday through Saturday...

Cold, blustery at times, chance of snow showers Friday night into
Saturday. A nod to ensemble means for 24-hour rainfall probabilities
in excess of 0.01 inches as N-stream clipper energy translates to a
dominant S-stream disturbance across the offshore baroclinic front.
Yet leaning very light outcomes. Lack of ascent within moist snow
growth regions parent with the positively-tilted H5 trof undergoing
neutral transition along the surface transition boundary. It isn`t
till better mid-level curvature and ejecting vortmax above the S-
stream can get things going, in this instance further downstream.
Leaning snow accumulations along the N/W slopes of high terrain
given orographic support, and along the SE coast as energy / ascent
begins to come together before ejecting out to sea. Highest snow
accumulations possibly over an inch for S/SE coast, especially over
the Islands and Outer Cape. Visibility impacts, perhaps some slick
road conditions. Behind the disturbance, winds kick up, colder air
drives back in, wind chills drop back down into the single digits
Sunday morning, lows in the teens.

Sunday into Monday...

Potential mixed precipitation event. Ascent overall light, seemingly
quasi-parallel flow along the lifting warm front. Above a cold air
damming signature with an indication of northerly ageostrophic flow,
the surface high to the N however a weaker low over the Great Lakes,
could see a period of freezing rain / drizzle centered around late
Sunday night into early Monday morning, especially in the interior
sheltered valleys where cold will be more difficult to dislodge. A
nod again to ensemble mean for 24-hour rainfall probabilities in
excess of 0.01 inches which is roughly 70 percent or greater. Again
light outcomes but enough to potentially make for hazardous travel
and could yield WINTER WEATHER ADVISORIES for a trace of ice. Some
question as to how quickly the warm front will lift N and how long
threats / impacts may linger. Could hold through Monday evening. A
low confidence forecast given poor handling via operational guidance
on individual upstream waves.

Tuesday onward...

Leaning a flatter, progressive flow regime with transitioning air-
masses. With any additional waves will be looking for potential
mixed precipitation events with the ebb and flow of 2m temperatures.
Weaker energy ejecting E and deepening, expecting behind each system
the return of colder conditions ushered by breezy N/W winds. Lack of
confidence in the forecast beyond 96 hours (which does include the
Sunday into Monday timeframe). In leaning with ensemble means, after
the early week disturbance, our next event may not occur until late
Thursday into Friday, an initial cold blustery pattern transitioning
warmer, more seasonable in-between.

&&

.AVIATION /21Z WEDNESDAY 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/...

1815z update...

Thru 00z...high confidence.

VFR but occasionally marginal MVFR in scattered light snow
showers/flurries. Strong WNW winds with gusts up to 35-40 kt
continue.

After 00z...high confidence.

VFR and winds slowly subside. Then after 06z and especially
after 09z MVFR in light snow overspreads CT to the south coast
of RI/MA. Elsewhere VFR cigs with just very light snow or
flurries. Light west winds.

Thursday...moderate confidence with some uncertainty how far
north accumulating snows track.

MVFR along the south coast of RI/MA in snow from 12z-15z. Low
prob of brief IFR from BID-MVY-ACK. Snowfall of 1-3" along
immediate south coast with highest totals on the islands.
Conditions improving to VFR in the afternoon along with dry
weather. Elsewhere, MVFR in very light snow or flurries
improving to VFR around midday and turning dry.

Thursday night...high confidence.

VFR and dry with west winds becoming gusty over Cape Cod and
islands.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Friday through Monday/...

Thursday Night: VFR. Breezy.

Friday: VFR.

Friday Night: Mainly VFR, with areas MVFR possible. Breezy.
Chance SHSN.

Saturday: VFR. Windy with gusts to 30 kt.

Saturday Night: VFR. Windy with areas gusts to 30 kt.

Sunday: Mainly VFR, with local MVFR possible. Slight chance SN.

Sunday Night: Mainly VFR, with areas MVFR possible. Chance SN,
chance RA, slight chance FZRA.

Monday: Mainly VFR, with local MVFR possible. Breezy. Chance
RA, chance SN, slight chance FZRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

4 PM update...

Tonight...

WNW gales this evening then diminishing toward midnight as 975 low
over New Brunswick lifts farther north into Canada.

Thursday...

Weak low moves east across 40N south of New England with a period of
morning snow lowering vsbys especially near south coast of MA/RI.
Low races out to sea with improving conditions during the afternoon
and winds becoming NW.

Thursday night...

Modest NW winds in the evening becoming light late as weak high pres
builds in from the west. Dry weather and good vsby.

Outlook /Friday through Monday/...

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas.

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.

Friday Night: Low risk for Small Craft Advisory winds with
local gusts up to 30 kt. Areas of seas approaching 5 ft. Chance
of rain showers, chance of snow showers.

Saturday: Low risk for gale force winds with gusts up to 35 kt.
Areas of rough seas. Slight chance of rain showers, slight
chance of snow showers.

Saturday Night: Low risk for gale force winds with gusts up to
35 kt. Local rough seas.

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Wind Advisory until 10 PM EST this evening for MAZ007-019-
     022>024.
     Wind Advisory until 8 PM EST this evening for MAZ002>006-
     008>018-020-021-026.
RI...Wind Advisory until 10 PM EST this evening for RIZ008.
     Wind Advisory until 8 PM EST this evening for RIZ001>007.
MARINE...Gale Warning until 1 AM EST Thursday for ANZ230>235-237-251.
     Gale Warning until 9 PM EST this evening for ANZ236.
     Gale Warning until 3 AM EST Thursday for ANZ250-254>256.

&&

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



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