Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 122112
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
412 PM EST Tue Dec 12 2017

.SYNOPSIS...
Strengthening low pressure over upstate NY redevelops over
northeast Massachusetts this evening, then intensifies into a
gale center over New Brunswick Wednesday. A shot of arctic air
wraps around this exiting low into Thursday, with a period of
strong winds and bitterly cold wind chills, ahead of which a
period of light snow is possible. Another low pressure may bring
some light snow around Friday night. Cold and dry once again
for Saturday. A wintry mix possible by Sunday. A milder weather
pattern on the horizon into next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Continuing to monitor temperatures this afternoon. So far,
timing of the ongoing forecast looks good. Extended a Winter
Weather Advisory for Franklin County and western Hampshire
county, where the risk for accumulating snow continues through
7 PM.

Potent negatively-tilted mid level shortwave crosses our region
later tonight. This feature will help to drive much colder air
into southern New England by Wednesday morning. Expecting plenty
of clouds with this shortwave. Thinking it might just a bit too
dry for widespread snow showers, but that remains a
possibility. Most of the moisture looks to be trapped below 800
mb, which should keep any showers mainly on the western side of
the Berkshires with a west wind.

Below normal temperatures, well below freezing, tonight. Any
standing water and/or slush will freeze. Thus this afternoon is
your chance to remove any leftover snow/ice.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Strong low pressure will spin across the Maritimes into
Labrador through Wednesday night, continuing a strong surge of
arctic air into our region. While the core of this arctic air
remains north of our region, will see increasing W-NW winds
with the sharp pressure gradient. 0-3 km lapse rates are near
dry adiabatic, meaning excellent mixing and momentum transfer.
Will probably need a Wind Advisory for portions of southern New
England, especially across the higher terrain and towards the
coasts. Still trying to refine the area and timing a bit more.

Some leftover snow showers may linger across the E slopes of the
Berkshires through midday Wednesday, with a few lingering along
the S coast early. Otherwise, mainly dry across much of southern
New England. Below normal temperatures with highs below
freezing. Wind chill values later in the day remaining in the
single digits and teens above zero. A few locations towards the
Berkshires could see subzero wind chills Wednesday afternoon.

Winds diminish some Wednesday night as better mixing moves
offshore. Another mid level shortwave should pass by to our
south. While this should mean some clouds for our region,
temperatures will still fall into the teens away from the
immediate coast. Lows in the 20s for there. Still very low wind
chills in the single digits and teens for much of the region,
with readings as low as -5 toward daybreak across the E slopes
of the Berkshires.

Any snow showers would be closer to the south coast of new
England. Limited moisture would also limit any accumulations,
which should be less than 1 inch, where it snows at all.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...

*/ Highlights...

 - Skirting clipper low with light snow Thursday
 - Cold and dry Thursday night into Friday
 - Another shot of snow shower activity going into Friday night
 - Cold and dry again for Saturday
 - Potential wintry mix event for Sunday
 - Early on cold, light snow followed by a warmer trend next week

*/ Overview...

Cold, active weather pattern persists into the weekend,
beginning to relax into the following week. H5 trof pattern
maintains over NE N America round which disturbances rotate, at
times capturing S-stream energy, quickly escaping E with lesser
downstream traffic. Looking at a series of clipper systems which
will bring periods of wintry weather followed by gusty winds
that aid in preserving the below- average temperature pattern we
have seen as of late. Strength of individual disturbances is in
question which will be broken down below, however their
forecast quick movement and short residency, looking at brief
impacts and leaning towards less impressive / deep storm
development in vicinity of New England. Certain they`ll keep on
reloading negative H85 temperature anomalies across the region.

But perhaps a warming trend towards late December? The wave
breaking over the NW Pacific as a stout jet of milder air
pushes E into W N America, colder air is shunted back N as the
H5 pattern across the CONUS relaxes, becoming more zonal. While
still within the periphery of the polar jet, seemingly looking
at a return of an ebb and flow pattern with a nod towards shots
of cooler air out of the N rather than S-stream dominance.

Continued confidence in operational forecast guidance out
through 72-96 hours, thereafter preference to ensemble means.
Wintry weather followed by cold and blustery themes into the
weekend then a gradual warm-up into next week with potential for
mixed precip events early on.

*/ Discussion...

Thursday...

Skirting clipper low, potential for wintry weather. Consensus of
forecast guidance has a near-miss, the crux of the forcing
offshore associated with positive differential vorticity
advection, stronger ascent of greater moisture availability.
However, evaluation along isentropes and the environment within
the snow growth region, there are indications of weak lift and
available moisture along decent SW to NE frontogenetical banding
around H7-5. Can not rule out light snow chances spreading as
far N/W of the MA/VT/NH border. Not sold on just saying zero
based on synoptics. Consensus of guidance has the dry, subsiding
wedge further N over N New England. Thus like to keep chance
PoPs for all of S New England, likely along the S coast. Amounts
light, thinking little impact. The EPS probabilities paint a
decent picture of outcomes with respect to 24-hour 0.01 liquid.
A mainly early-half of the day event, tapering into
afternoon/eve. Nantucket having the best shot of seeing
appreciable accumulation of precipitation.

Late Thursday into Thursday night...

Cold and dry. Initially blustery, storm system quickly steering
out as high pressure builds in. Gales briefly on the waters,
all else being good. Flirting with noteworthy wind chills yet
seemingly prior to the crux of cold air with single digit lows
into Friday morning. That being said any liquid from melt along
roads will easily freeze.

Friday into Friday night...

Possible snow shower activity, otherwise maintained cold.
Positively-tilted H5 trof axis quickly sweeping across the
region yields little threat and impact to the region. Moisture
present but lacking deep synoptic lift. Agree with the bulk of
development across the better baroclinic zone well offshore as
the H5 trof undergoes neutral tilt out across the Atlantic. Lack
of mid-upper level curvature until further downstream. Chance
PoPs with energy transfer from N-stream clipper to S-stream wave
along the better baroclinic zone. Mainly snow showers and
looking at light accumulations.

Saturday into Saturday night...

Blustery and cold. High pressure settling in behind exiting
storm, would expect ocean-effect stratus / showers near the
coast, perhaps some Lake effect snow shower activity getting
into N/W MA before deeper subsidence and drier air builds in.

Sunday into Sunday night...

Potential mixed precip storm. Monitoring cold air damming as
there is the possibility of over-running precipitation.
Gradually warming as the warm front lifts N but also as the dry
slot seemingly wraps in, looking at potential change overs
before all is said and done. Too early to get on particulars but
something to watch.

Into next week...

Some final clipper systems, shots of cold, then the warming
trend begins as we get back into an ebb and flow pattern. With
clipper and cold, the usual shot of snow shower activity
enhanced by the Lakes, followed by blustery winds and the
typical offshore ocean-effect stratus / shower activity. The
H5 trof relaxing, we get back into S flow and a warmer trend.

&&

.AVIATION /21Z TUESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Wednesday Night/...

Through 00Z...High confidence on trends but lower on exact
details, particularly timing of precipitation type changes.

MVFR CIGS across central and western areas will push E, with
local IFR-LIFR at times across N central and western areas.

Tonight...High confidence on trends, lower on details.

MVFR CIGS/MVFR-IFR VSBYS improve to VFR by around 05Z-07Z,
except lingering lower CIGS across higher terrain. Gusty W-NW
winds develop.

Tomorrow...Mainly VFR, with local MVFR possible in scattered
SHSN. Strong winds with gusts to 40 kt.

Tomorrow Night...VFR. Strong winds with gusts to 40 kt.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Thursday through Sunday/...

Thursday: VFR. Breezy. Chance SN.

Thursday Night: VFR. Breezy.

Friday: VFR.

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

Saturday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHSN.

Saturday Night: VFR. Breezy.

Sunday: VFR. Breezy. Chance SHSN.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...

Tonight...Arctic cold front sweeps across the area with SE
winds shifting to W-SW up to 20-30 kt. Vsby improves after
evening rain/snow showers.

Wednesday and Wednesday night...Gale force winds with gusts up
to 45 kt. Rough seas up to 15 ft. Slight chance of snow showers
Wednesday.

Outlook /Thursday through Sunday/...

Thursday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas. Chance of snow.

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas.

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.

Friday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of snow showers. Local visibility 1 to 3 nm.

Saturday: Moderate risk for Small Craft Advisory winds with
local gusts up to 30 kt. Local rough seas. Slight chance of
snow showers.

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

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Wind Advisory from noon Wednesday to midnight EST Wednesday
     night for MAZ007-019-022>024.
     Winter Weather Advisory until 7 PM EST this evening for MAZ002-
     003-008.
     Wind Advisory from 10 AM to 8 PM EST Wednesday for MAZ002-008-
     009.
RI...Wind Advisory from noon Wednesday to midnight EST Wednesday
     night for RIZ008.
MARINE...Gale Warning from 8 AM Wednesday to 6 AM EST Thursday for
     ANZ232-251.
     Gale Warning from 8 AM Wednesday to 4 AM EST Thursday for
     ANZ233-234.
     Gale Warning from 1 PM Wednesday to 1 AM EST Thursday for
     ANZ230-236.
     Gale Warning from 10 AM Wednesday to 6 AM EST Thursday for
     ANZ231.
     Gale Warning from 6 AM Wednesday to 6 AM EST Thursday for
     ANZ235-237-250-254.
     Gale Warning from 5 AM Wednesday to 6 AM EST Thursday for
     ANZ255-256.

&&

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



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