Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 212100
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
400 PM EST Sat Jan 21 2017

.SYNOPSIS...
Weak high pressure will bring tranquil weather through tonight.
Developing onshore winds Sunday will result in an increase in clouds
as well as spotty light rain or drizzle as the day progresses. A
significant storm will reach the East Coast Sunday night and Monday
and move up the coast Monday night and Tuesday. This will bring a
mix of rain and/or snow along with a period of icing well inland.
The storm may also bring damaging winds to the coast and the coastal
waters. High pressure then brings dry and mild weather Wednesday
into Thursday. A cold front crosses New England late week followed
by seasonably cool temperatures and blustery winds.

&&

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

4 PM UPDATE...

Quite mild late this afternoon with temps in the mid 50s across much
of the region. Not record breaking (60s) but nonetheless impressive
for late Jan. Mostly clear to partly cloudy skies will prevail this
evening and along with light winds temps will fall fairly quickly
with sunset. However with dew pts in the low 40s (well above normal
for late Jan) this will support at least the formation of patchy
fog. This becomes tricky later tonight as mid/high clouds overspread
the area. This may result in vsbys improving toward morning.

Nonetheless temperatures above normal tonight with just about all
locations above freezing. Low risk of some patchy freezing fog
across the higher terrain but this is a very low prob.

&&

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

4 PM update...

Sunday...

High pressure over Quebec will combine with weakening low pres east
of DE to yield an increasing moist onshore flow into southern New
England. Thus any dim sunshine thru mid/high clouds at sunrise will
give way to overcast conditions. This increasing onshore flow will
result in lowering clouds and spotty light rain/drizzle especially
across the east slopes of the Worcester Hills and Berkshires.

Given the clouds and onshore winds temps will be much cooler than
today with highs in the 40s...still above normal for late Jan.

Sunday night...

Tricky forecast as northern stream short wave exits southeast Quebec
into New Brunswick. This advects a cooler/drier airmass into the
region. This erodes the leading edge of the precip shield as it
approaches from the south. However at the same time it provides
lowering wet bulb temps increasing the risk for wintry precip...that
is if precip makes it this far north. Despite the synoptic scale
lift and deep layer moisture possibly remaining south of New England
thru 12z Monday...should be enough low level moisture given onshore
flow and combined with upslope component will likely see spotty
light rain. The problem will be across the higher terrain where the
hi res guidance lowers temps into the upper 20s and lower 30s. Thus
at least a moderate risk (30-60%) of freezing rain/drizzle across
the high terrain of the Worcester Hills and east slopes of the
Berks...possibly into northern CT as well.

Given the shallow nature of the cold air followed the colder 4 km
NAM temps for Sunday night.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...

Big Picture...

Deep trough digs over the Western USA while a ridge builds over the
West Atlantic.  In between the two, a Pacific shortwave rides the
flow and deepens to a closed low as it crosses the mid-South. By
this point, the developing storm taps moisture from the Gulf of
Mexico and draws it north. The storm runs into the West Atlantic
ridge and slows, taking on a negative tilt as it moves across the
Eastern States. The Pacific trough then shifts east across the
nation, reaching the Northeast USA by late week.

The GFS and ECMWF mass fields are similar until early Wednesday, and
agree on the broad scale pattern later in the week while differing
on details. The GGEM shows some differences such as moving the
coastal storm farther offshore, but bears some similarity to the
other two solutions. Temperature fields at 850 mb look similar
through the long term period. The resulting forecast now that the
parent shortwave is moving ashore from the Pacific is sufficiently
similar to previous solutions to improve confidence in the forecast.
An overall model blend should work, especially with some focus on
the GFS and ECMWF.

The Monday-Tuesday storm will lower 500 mb heights to near normal
over the region Monday-Tuesday.  Height may recover to above normal
Wednesday, but then the advancing trough from the West will bring
below normal heights for the latter part of the week.

Details...

Monday-Tuesday...

Low pressure moves up the coast with the focus of venting and
moisture inflow shifting over New England by the afternoon. This
will mean an increase in precipitation. Temperatures in the coastal
plain will be warm enough for mainly rain, while the interior will
be cold enough for a mix of snow/sleet/freezing rain.  There is a
mix of cold air damming signals...the surface pressure pattern
indicates damming while the low level ageostropic flow is more
northwest than the typical north wind for drainage. Locations north
of the Mass Pike may be able to hold onto the cold air much of
Monday into Monday night creating a messy mix that could make travel
hazardous.

Meanwhile, the Maritime High and coastal low move closer to each
other and build the pressure gradient, leading to strengthening wind
surface and low level aloft.  Winds at 950 mb increase from 60 kts
aimed at New Jersey in the morning to 70 knots aimed at the Cape and
Islands Monday evening. This core of winds shifts north across our
area from later Monday afternoon through much of Monday night, then
moves north of us Tuesday morning. In anticipation of this, we have
issued a High Wind Watch for Cape Cod and all the islands. This
headline could be expanded later to much of the East Mass coast.
Most areas that aren/t in the watch will likely have a wind advisory
as we get closer to the event. Winds should diminish Tuesday as the
coastal low moves past.

Fair amount of moisture moves north with this coastal system with
precipitable water values above an inch over RI/SE Mass. Continue to
expect between 1 and 3 inches of water over the two day period.

The storm center is expected to cross Cape Cod Tuesday afternoon and
move off toward Maine late in the day.  Expect precipitation to
diminish at that time, followed by slow clearing overnight.

Wednesday through Friday...

Generally dry weather through this period. Weak high pressure builds
over the region Wednesday. A weak cold front moves across on
Thursday, but starved of enough moisture for precip most places.
There remains a chance of patchy light rain or snow in Northwest MA.
Dry and cooler weather moves in behind the front for Friday.

&&

.AVIATION /21Z SATURDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...

18z update...

This evening and overnight...moderate confidence.

Trending toward VFR. This will lead to the formation of fog later
this evening...likely lowering vsbys into the MVFR category.
However with mid and high clouds arriving toward morning this may
result in vsbys improving toward daybreak. Dry weather prevails.

Sunday...moderate confidence.

Mainly VFR to start but lowering to MVFR as the day progresses
along with spotty light rain/drizzle and patchy fog. NE winds
increasing up to 20 kt along the coast by days end.

Sunday night...moderate confidence...

MVFR likely in spotty light rain/drizzle. Concerned about freezing
rain/drizzle across the high terrain including KORH.

KBOS TAF...Moderate confidence in TAF.

KBDL TAF...Moderate confidence in TAF.

Outlook /Monday through Thursday/... Moderate confidence.

Monday-Tuesday...

MVFR cigs and vsbys in rain and snow Monday will lower to IFR/LIFR
cigs late in the day or Monday night, especially in RI and Eastern
MA as well as parts of the Worcester Hills. Best chance for snow and
sleet/freezing rain will be north of the Mass Pike, although
Northern CT has a smaller chance. Rain and fog farther south and
east.

Increasing winds above the surface Monday, with speeds reaching
their maximum Monday night and then diminishing Tuesday. Winds at
1000-2000 feet should reach 40 to 60 knots Monday afternoon and
continue Monday night. East winds of 70 knots possible over Cape
Cod and Islands. Expect low level wind shear during this time
along with strong wind gusts at the surface. Winds then diminish
during Tuesday as the low level jet shifts north into Maine.
Surface winds start from the east Monday, shift from the Northeast
Monday night, and then from the Northwest on Tuesday. CIGS and
Vsbys improve to VFR Tuesday night.

Wednesday and Thursday...

VFR most likely. An approaching cold front may bring MVFR
cigs/vsbys Thu in isolated rain/snow showers for parts of
Northwest Mass during the afternoon/evening.

&&

.MARINE...
Forecaster Confidence Levels...

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

4 pm update...

Tonight...tranquil weather this evening with light winds. Vsby may
lower to 1-3 miles in patchy fog.

Sunday...Increasing NE winds, especially in the afternoon with gusts
to 20 kt developing. G25 kt over southern waters by evening. Vsby
may lower in patchy drizzle/rain.

Sunday night...northeast winds continue to increase to gale force
toward daybreak Monday. Seas increase rapidly to 10-15 ft by 12z Mon
across the southern RI/MA waters. Vsby may lower in rain and fog esp
southern waters.

Outlook /Monday through Thursday/... Moderate confidence.

Monday through Tuesday...

Strengthening low pressure off the Mid Atlantic coast will move up
the coast Monday and cross near Cape Cod Tuesday afternoon. Expect
increasing winds Sunday night and Monday as the low runs up against
Maritime high pressure creating an increased pressure gradient over
the New England waters, as well as 1000-4000 feet above the surface.
The strongest winds will move across the waters from Monday
afternoon to Tuesday morning. East winds above the surface will
reach 60 to 70 knots during this time, and have the potential to
produce storm force gusts at the surface. The exception may be
Boston Harbor and Narragansett Bay, where somewhat lesser east gales
may be the rule.

The Storm Watch has been expanded to include Mass & Ipswich Bays as
well as Buzzards Bay and Vineyard Sound. Only Boston harbor and
Narragansett Bay remain in a Gale Watch.

These east winds will build seas and push them toward the eastern
shoreline. Expect highest values of 15 to 20 feet over the most
exposed waters during Monday evening and night.

Winds will shift on Tuesday, becoming North during the afternoon and
Northwest Tuesday evening as the storm center moves past.

Wednesday and Thursday ... improving conditions as weak high pres
builds over the waters Wed. Then increasing WSW winds ahead of an
approaching cold front Thu with its passage late Thu/Thu night.

&&

.TIDES/COASTAL FLOODING...

Given potent easterly LLJ and period of strong wind gusts, expect
seas to probably build to 20-25 feet across the eastern waters late
Mon into Tue. Fortunately, astronomical high tides are about as low
as they get, and the strongest winds Monday evening will coincide
with an astro high tide of only 8.2 feet in Boston.
Thus the risk of any significant coastal flooding is low. In fact,
it is hard to imagine a scenario of much worse than some
splashover for the Monday evening high tide, thanks to the lowness
of the astro high tide. The Tuesday morning high tide is higher at
9.4 feet. If winds are still blowing strong out of the northeast
along with 20+ foot seas just offshore, minor coastal flooding
could occur along the eastern MA coast. It would take a storm
surge of nearly 3.5 feet and waves greater than 20 feet to even
approach a moderate level of coastal flooding. Unless this system
progresses much more slowly than the current consensus of models
indicate, it is unlikely that we will experience anything worse
than minor coastal flooding and some beach erosion for the Tuesday
morning high tide in spite of such dangerous marine conditions
just offshore.


&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Wind Watch from Monday morning through late Monday night
     for MAZ022>024.
RI...High Wind Watch from Monday morning through late Monday night
     for RIZ008.
MARINE...Storm Watch from Monday morning through late Monday night for
     ANZ232.
     Storm Watch from Monday afternoon through late Monday night
     for ANZ233-234.
     Gale Watch from Monday morning through late Monday night for
     ANZ230.
     Storm Watch from Monday afternoon through late Monday night
     for ANZ231-251.
     Gale Watch from late Sunday night through late Monday night
     for ANZ236.
     Storm Watch from Monday morning through late Monday night for
     ANZ235-237.
     Storm Watch from Monday morning through late Monday night for
     ANZ250-254.
     Storm Watch from late Sunday night through late Monday night
     for ANZ255-256.

&&

$$
SYNOPSIS...Nocera
NEAR TERM...Nocera
SHORT TERM...Nocera
LONG TERM...WTB/Nocera
AVIATION...WTB/Nocera
MARINE...WTB/KJC/Nocera
TIDES/COASTAL FLOODING...staff



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