Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 172112
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
412 PM EST Fri Nov 17 2017

.SYNOPSIS...
High pressure crests across our area tonight, providing dry but
cold weather. Strengthening low pressure will push across the
St. Lawrence River Valley this weekend with gusty southwest
winds developing Saturday afternoon and night. Strong to
damaging wind threats continue behind this departing system
Sunday, potentially lingering into Monday. The ebb and flow
pattern of southerly warm air surges followed by northerly cold
air drainage continues through the week into the Thanksgiving
weekend.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
High pressure with dry weather and diminishing winds. Below
normal min temperatures mainly in the 20s. Slightly higher near
the coast.

&&

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

An intensifying low pressure moving through the Great Lakes into
the Saint Lawrence River valley Saturday into Saturday night
will mean an increasing southwest wind for southern New England.
These southwest winds will also mean higher temperatures, likely
slowly rising Saturday night instead of falling.

Lots of dry air to overcome in the lower levels. Thinking the
NAM might be a bit too fast bringing precipitation into our
region. Stayed with the slower timing presented by the GFS,
ECMWF and CMC solutions. This would bring a chance of showers to
much of our region during the afternoon. The brunt of the light
rainfall should be Saturday night across our region.

Will need to contemplate a Wind Advisory for the south coast of
RI and MA late Saturday afternoon into Saturday night. Will
mention in the Hazardous Weather Outlook, and let the next shift
get a closer look at the details.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
*/ Highlights...

 - Widespread strong to damaging W/NW winds Sunday into Sunday night
 - Winds will linger into Monday with perhaps less of an impact
 - Ebb and flow pattern of S warm air surges followed by N cold air
   continues

*/ Overview...

Ebb and flow continues. Without greater amplification and subsequent
transfer between the equator and the poles, the flow remains mainly
zonal. The MJO largely subdued as the present -WPO/-EPO/-PNA all
trend positive into December per ensemble means, counter-productive,
lending seemingly to a signal of persistent W to E flow. The only
consistencies it seems from analysis of potential vorticity and
isentropic surfaces is a persistence of poleward transfer in the N
Pacific and the NE Atlantic that lends to S shearing of energy into
the Gulf of Alaska and S across Europe, respectively. Otherwise it`s
zonal in-between. Any energy across the CONUS is quick moving with
little residency such that forces acting to invoke greater curvature
and storm development reach their full potential E of our region
across E/SE Canada into the NW Atlantic. With racing storm systems
over our region, warm-surge in advance followed by cool shots, both
with attributes of gusty winds, a chance of wet-weather in-between
along associated frontal boundaries, cold fronts with more emphasis.
Attendant threats and impacts highlighted in the discussion below.

*/ Discussion...

Sunday into Monday...

Winds shifting NW ushering colder air, remaining blustery. Consensus
of forecast guidance, cold front sweeping 7-10a Sunday, undercutting
colder air, lapse rates mixing out to H85, dry adiabatic, the top of
the mix layer averaging W/NW 45 to 50 mph (40 to 45 kts). Widespread
WIND ADVISORY likely, more likely daytime Sunday with boundary layer
heating and contributions from quick pressure rises. Ensemble means,
strongest winds likely across the high terrain and along the coast
adjacent to 50-degree ocean waters, also N/E closer to the storm
center, so N/E portions of MA at greatest threat. CIPS analogs with
a high probability of at least 35 mph widespread gusts, lower for 40
to 45 mph, however considering local climatology, believe there is
the potential to see 40+ mph gusts.

Headlines may drop overnight with the potential of a shallow surface
inversion, however model guidance consensus signals continued cold
air advection right into Monday with additional energy rotating
through the cyclonic flow across the region lending to an enhancement
of the wind profile. Something will have to watch closely.

Will highlight the wind threat in the HAZARDOUS WEATHER OUTLOOK. No
headlines at this time associated with W/NW winds.

Otherwise, drier air surging in aloft behind the quick sweeping cold
front, will see an end to shower activity rather abruptly during the
morning hours. With the blustery W/NW winds and cold airmass pouring
in, across the warmer Great Lakes, the lake effect machine will be
ongoing and could see some of that energy reach far enough E with
the mean flow to impact N/W portions of MA and CT with some snow
and/or rain shower activity, 2m temperature dependent. If snow, some
light accumulations are possible, mainly for the high terrain.

All threat diminish into Monday night, and it is during this period
that winds may become light enough beneath clear conditions to allow
for radiational cooling and night-time lows to drop well down into
the 20s, possibly teens. Hard to say right now. If W/NW winds remain
brisk, then the mechanical mixing in the environment would limit the
radiational cooling potential. Will hold it conservative with wide-
spread lows in the 20s. Highs Sunday into the upper 40s to low 50s
right before the cold front punches through in the morning, upper
30s to low 40s for Monday.

Tuesday...

Quiet. High pressure. Return S flow. Turning mild. Still believe
that warmest temperatures will be over interior SE New England, that
an onshore will develop in response to an approaching storm system
for the overnight period into Wednesday. Highs in the upper 40s to
low 50s.

Tuesday night into Wednesday...

Possible showers. Keeping with slight chance PoPs, the EC on again
off again on the N and S stream energy beginning to interact with a
draw of S-moisture ahead of the N sweeping cold front before both
systems push offshore quickly and phase downstream. The GFS keeps
the dry-streak going. Given the emphasis on the cold front agreed
upon within all available guidance, would expect that the associated
lift on available moisture should get something going, especially
considering the warmer ocean waters that line the shore.

Thanksgiving onward...

Thanksgiving looks quiet and seasonable, while into the weekend it
appears another quick moving storm system sweeps the region. The ebb
and flow pattern continues with milder shots and SW flow ahead of
any disturbance followed by colder air and NW winds as the system
departs.

&&

.AVIATION /21Z FRIDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Saturday Night/...

Tonight...High confidence.

VFR. Dry weather along with diminishing winds.

Saturday...High confidence.

VFR, light winds and dry weather through the morning. MVFR and
rain begin to increase from west to east, with current timing
18/20z-19/00z. Also S-SW winds increase during the afternoon
with gusts approaching 35 kt by late in the day along the south
coast including Cape Cod and the Islands.

Saturday Night...High confidence. Mainly MVFR, with local IFR
possible. Strong winds with local gusts to 40 kt. RA, patchy BR.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday through Wednesday/...High confidence.

Sunday: Mainly VFR, with areas MVFR possible. Strong winds with
gusts to 40 kt. SHRA likely.

Sunday Night: VFR. Windy with gusts to 35 kt.

Monday: VFR. Strong winds with local gusts to 40 kt.

Monday Night: VFR. Windy with gusts to 35 kt.

Tuesday: VFR. Breezy.

Tuesday Night: VFR. Windy with local gusts to 30 kt. Slight
chance SHRA, slight chance FZRA.

Wednesday: VFR. Breezy. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Tonight...Winds will continue to diminish this evening. As
such, dropped the Small Craft Advisory headlines which would be
expiring over the next several hours, and issued new Gale
Warnings for the greater wind threat Saturday into Saturday
night. The exceptions were the coastal waters east of MA, where
buoys still reporting gale force gusts. Even there, expecting
winds to diminish this evening.

Saturday and Saturday Night...Increasing SW winds as a low
pressure moves through the Great Lakes into the Saint Lawrence
Valley. Showers most likely to arrive over the waters late
Saturday afternoon, and continue into Saturday night.

Some risk for storm force gusts Saturday night, but have greater
confidence in gale force winds.

Outlook /Sunday through Wednesday/...Moderate confidence.

Sunday: Moderate risk for gale force winds with local gusts up
to 45 kt. Rough seas up to 14 ft. Rain showers likely.

Sunday Night: Strong winds with gusts up to 35 kt. Rough seas
up to 14 ft.

Monday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 11 ft.

Monday Night: Low risk for gale force winds with gusts up to
35 kt. Areas of rough seas.

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

Tuesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas. Slight chance of
rain showers.

Wednesday: Winds less than 25 kt. Local rough seas. Slight
chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning from 4 PM Saturday to 4 AM EST Monday for
     ANZ232>234.
     Gale Watch from Sunday morning through Sunday evening for
     ANZ230.
     Small Craft Advisory until 7 PM EST this evening for ANZ230.
     Gale Warning from midnight Saturday night to 4 AM EST Monday
     for ANZ231.
     Gale Warning from 4 PM Saturday to 10 PM EST Sunday for ANZ236.
     Gale Warning from 2 PM Saturday to 4 AM EST Monday for ANZ235-
     237-255-256.
     Gale Warning until 9 PM EST this evening for ANZ250-251-254.

&&

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


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