Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 090314
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1014 PM EST Thu Dec 8 2016

.SYNOPSIS...
A strong front moves offshore overnight. Mainly dry, but
unseasonably cold temperatures are on tap for our region Friday
into Sunday. Some mixed wintry precipitation is looking more
likely late Sunday evening into the day on Monday, but some
uncertainty remains. Cooler conditions return by mid next week.

&&

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

Mainly dry, and clear, weather continues across southern New
England this evening. The exception was toward the Berkshires,
where moisture from the Great Lakes was pooling along the western
slopes. Some of these clouds were spilling over the crest of the
Berkshires, but little indication of any precipitation doing so.

Ongoing forecast has this handled fairly well. Only made some
adjustments to bring the forecast back in line with observed
trends.

Previous discussion...

Turning colder. Cold air advection proceeding behind the initial
cold frontal boundary with H85 temperatures down around -10C by
Friday morning. Lapse rates becoming well-mixed nearly as high as
H5, will see the mix down of faster momentum and drier air. Just
some question as to the strength of the winds within and at the
top of the mixed layer and how effectively they`ll mix down,
whether there will be some stability right at the surface that`ll
limit mixing. Considering this along with the trends upstream,
given the magnitude of cold air advection, believe we`ll see
westerly winds sustained around 10 mph with gusts 15 to 20 mph,
strongest across the high terrain and over the waters with gusts
up to 30 mph. Maybe perhaps shallow interior valleys will be
spared strong winds over- night. With lows dropping down into the
20s looking at wind chills down into the teens in spots.

Scattered to broken cloud decks subsequent of cyclonic flow and
fetch off the warmer lakes. Decent signal of snow squalls possibly
into the Berkshires. Depends on availability of moisture within
the first 2 km above the surface. Will go ahead with chance PoPs
for the E slopes of the Berkshires with slighter chances to nil
elsewhere with downsloping wind off the high terrain. Light snow
accumulations of around a tenth or two possible along with
restrictions to visibility of a mile or two.

&&

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

Cold conditions continue. Cold air advection continuing aloft with
lapse rates remaining well-mixed to at least H85. Will see weak
ridging and drier air around H8-3 aloft move across the region
limiting deep layer mixing, but with well-mixed low levels, expect
scattered to broken cloud decks to continue especially with the
reinforcing arctic front around the morning hours. Continued cyclonic
flow and fetch off the warmer lakes, maybe a flurry over the
Berks.

Been a challenge to forecast the magnitude of the winds, what will
effectively mix-down to the surface and what will the winds be
aloft. NAM seems to be the most robust guidance. Overall anticipate
NW blustery flow 10 to 15 kts sustained with gusts up to 35 kts
immediately behind the arctic front, relaxing towards Saturday
morning as high pressure builds in from the W and the pressure
gradient begins to relax. Going to feel some 10 degrees colder
than actual. Looking at wind chills overnight in the single digits.

Looking at highs around the low to mid 30s during the day, lows
dropping down into the teens.

&&

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

 * Cool and dry Sat into early Sun.
 * Mixed wintry precip with some impact late Sun into Mon.
 * Some uncertainty remains with p-types.
 * Cooler, but unsettled possible later next week.

*/ Overview and model preferences...

The long term looks to favor very broad, unamplified, and nearly
zonal W-E mean jet across the N tier of the CONUS through the long
term. While this might suggest relatively persistent conditions,
there are a couple of waves that will need to be watched as go
through next week. While there remains fairly good agreement on
the large scale, the smaller scale features and thermal profiles
remain somewhat in flux, and these will have everything to do with
the final sensible wx details.

The first wave of note is currently wrapped within the vortex
sitting off the coast of British Columbia coastline at this time,
it will weaken and open as it moves across the CONUS into Sun, but
maintains enough energy to provide a focal point for a two phased
mixed-precip event that should begin by late Sun and continue into
Mon. The second wave is actually allowed access to the northeast
via the height falls provided by the initial late weekend/early
week wave. It is a stronger vortex associated with an arctic
influenced airmass from Nunavut and the Northwest Territories.
This could provide a focal point for a much more seasonably cold
late week period. Given there are good agreements here in the
broad term, and the uncertainty of small scale features and
thermal profiles, a consensus blend of 08.12Z operational models
will provide the baseline for this long term forecast update.

*/ Details...

Sat into early Sun...Chilly conditions in spite of slight
warming at H92. The issue at hand is the warming at this level is
from -14C to -12C, still almost a full standard deviation below
climatological normal for mid December. Full mixing will still
only feature highs reaching the upper 20s and low 30s, as several
locations are unlikely to even reach the freezing mark. By Sun
night with weak high pres ridge at crest, the combination of
clearing and weak flow should yield good radiational cooling in
spite of the lack of an established snowpack. Looking at lows
falling into the low teens and possibly even a few single digits.
Highs on Sun before the loss of sunshine should reach once again
near or even slightly above freezing as the H92 temps continue to
increase.

Sun night into Mon...A two phased event as inland low pres
combines with weak wave development along a stalled frontal
boundary to the S. Moisture increases to nearly 0.75 in PWATs at
their peak on Mon, so moisture is generally not lacking. Initial
overrunning setup under return flow N of the stalled front could
yield an initial round of light snowfall, although noting that
within the column, omega is rather light within the snow growth
regime. Still increased LLJ near 40 kt could help this. Initial
thermal profiles through the Sun evening hours are supportive of
snow nearly universally across S New England, so the initial start
could feature measurable snowfall even as thermal profiles warm
the overnight hours. QPF totals are generally 0.1 inches or less,
so the final totals should generally remain around an inch or less
assuming this holds.

The secondary and potentially higher impact feature arrives by early
Mon AM in the form of a frontal wave developing near the Delmarva,
ahead of the parent low pres in the Great Lakes. This increases
deformation in the mid lvls N of the secondary wave, adds moisture
(this is when the aforementioned PWATs peak). It also looks to
increase omega within the snow growth region although the bulk of
the moisture is actually below it per latest profiles (this is
likely a result of the warming expected, raising the height of the
snow growth regime). Therefore, final precip types and totals are
a uncertain.

Precipitation/Snowfall...The initial overrunning during the
overnight is generally cold enough for all snow, but the dynamics
(omega, deformation, moisture) are lacking in comparison to the
Mon time-frame. The second wave for Mon, features better dynamics
but much more marginal thermal profiles as H92 increases above 0C
across CT/RI/SE MA by Mon afternoon on several ensembles and
operational guidance. Even with more favorable high pres setup to
the N yielding stronger Nly isallobaric flow, the warm advection
supplied by the 40 kt LLJ will be able to counter. Therefore, S to
start, but gradual transition to maybe light icing before a change
to rain for some is certainly possible. Counting the initial
overrunning snow potential, have moderate confidence that some
portions of interior S New England could see plowable snowfall
especially if the thermal profiles remain cold enough through the
day given the Nly isallobaric flow mentioned early. Further S,
totals could be less and feature more of a mix of P-types. In any
case, the early AM commute is likely to be impacted with wintry
precip across much of the region. For what it`s worth, ensemble 24
hour probabilities of 3 inches or more of snow are as high as
60-80 percent in NW MA, dropping to as low as 10-20 percent across
RI/SE MA and CT due to uncertainty in the timing of warming in the
lowest levels and amount of moisture/lift in the snow growth
regime, increasing snow to liquid ratios.

Timing...Lightest snowfall begins Sun evening from S-N overspreading
into the region through midnight. Heaviest should begin through
the early AM hours and continue into the commute. Some precip
changing would begin mainly after sunrise and continue to attempt
to lift N through the daylight hours. Note, more specific timing
of the start/end and changeover will be coming over the next 48
hours.

Tue...Transitional period with arctic frontal passage and drying
behind the exiting low pres centers. Wind chills could be quite
low during this period, but there are some differences in the pres
gradient that will need to be resolved. In any case, colder wx is
expected as H92 temps settle back to nearly -6C. Some ocean effect
snows possible, if winds are more N oriented given nearly 18C SST-
H85 delta-Ts. Unsettled conditions under cold advection could also
allow for occasional flurries, but these would be hit or miss.
Highs return to the mid 30s to near 40.

Wed and Thu...In spite of the good agreement synoptically,
uncertainty continues in the sfc pattern as cold vortex continues
to shift E-SE. Weak energy from this vortex looks to create a
second system developing near the OH valley and shifting offshore.
Ranges in track between ensembles and operational models are from
well to the S (far enough for a complete miss and much colder
temperatures) to near or even within the 40/70 benchmark
(potential for another round of wintry precipitation). Given the
zonal flow, timing/track very uncertain and depends on how the
early week system ultimately develops. In any case, looking at
back to seasonably colder conditions.

&&

.AVIATION /03Z 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/...

No major changes with the 03Z update.

Tonight...High confidence.
VFR. Blustery W/NW winds especially along the immediate coast and
high terrain with gusts up around 25 kts. Sustained around 5 to
10 kt. Interior low-lying terminals within the valleys are
expected to see less wind. -SN possible for the Berkshires, with
accompanying IFR visibilities.

Friday into Friday night...High confidence.
VFR. NW winds continuing. Sustained around 10 to 15 kts, gusts up
to 35 kts, diminishing late towards Saturday morning. SCT-BKN
low-end VFR cigs.

KBOS TAF...W/NW flow increases overnight. Gusts up to 35 kts
possible going into and through Friday.

KBDL TAF...NW winds gusting up to 20 kts tonight, increasing on
Friday before diminishing late.

Outlook /Saturday through Tuesday/...

Saturday into Sunday afternoon...High confidence.
VFR. Winds will shift from the W Sat, to the S on Sun. Some light
flurries possible during the day on Sat, but clearing and cold is
expected Sat night into early Sun.

Late Sunday through Monday...Moderate confidence.
Mix of IFR/LIFR at times possible as light snow spreads over the
region from S-N during the overnight and early Mon morning period.
Some of this changes to a mix of light ice and rain, mainly along
the CT/RI/SE MA borders and points S, while northern areas could
remain all SN into the day on Mon. Ice/Snowfall amounts are
somewhat uncertain, but some area runways may need to be plowed.
Winds mainly NE.

Tuesday...Moderate confidence.
Expect improvement Mon night into Tue as winds shift from the NE to
N to NNW. Some gusts on tuesday may range between 25-35 kt at
times especially along coastal locations. More widespread VFR as
mixed precipitation ends.

&&

.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.

W/NW winds on the increase with the likelihood of gales on the E
waters but can not rule out gales on the S waters as well. May
need to update headlines in the near-term. Looking at winds
increasing into this evening, becoming sustained around 10 to 15
kts beginning tonight with gusts 30 to 35 kts. Wave heights
building 5 to 8 feet on the outer waters. Will begin to see a
diminishing trend into Saturday morning.

Outlook /Saturday through Tuesday/...

Saturday into Sunday...High confidence.
W winds will continue to gust to around 25 kt int Sat afternoon
while seas also recede. Therefore, small craft advisories will
continue much of the day, but be dropped by evening and overnight.
On Sun, generally quiet boating weather is expected with winds
shifting out of the S.

Monday...Moderate confidence.
Winds shift to the northeast through the day, but should generally
remain between 10 and 20 kt. Seas will gradually build as low
pres develops well to the S, but generally should remain at or
below 5 ft. Snow/changing to rain is expected Sun night into the
day on Mon, with some vsby restrictions likely.

Tuesday...Moderate confidence.
Rainfall will be ending and winds gradually shift from NE, to N, to
NNW early Tue morning into the daylight hours. Wind gusts could
approach gale force, between 35-40 kt with seas building to 7-9
ft. Small craft advisories are likely, but Gales may also be needed.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 7 AM EST Saturday for ANZ231>235-
     237-255-256.
     Small Craft Advisory until 1 AM EST Saturday for ANZ230.
     Small Craft Advisory until 4 AM EST Saturday for ANZ236.
     Gale Warning from 10 AM to 10 PM EST Friday for ANZ250-251-254.

&&

$$
SYNOPSIS...Doody/Sipprell
NEAR TERM...Belk/Doody/Sipprell
SHORT TERM...Sipprell
LONG TERM...Doody
AVIATION...Belk/Doody/Sipprell
MARINE...Doody/Sipprell


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