Area Forecast Discussion
Issued by NWS Boston, MA

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136
FXUS61 KBOX 210805
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
305 AM EST Sat Jan 21 2017

.SYNOPSIS...
Weak high pressure will bring tranquil weather into Sunday. A
significant coastal storm will push toward the region Sunday
night, bringing a mix of rain and/or snow, along with some icing
conditions possible inland. This storm may bring strong to
possibly damaging winds to coastal areas, and be potentially
dangerous for mariners late Monday into Tuesday. High pressure
brings dry weather for Wednesday.

&&

.NEAR TERM /THROUGH TODAY/...
Weak mid level shortwave moves to the east today as weak high pres
builds in from the west. Main forecast challenge is timing of any
clearing this afternoon as low level moisture may remain trapped
below shallow inversion. Expect low clouds to linger through at
least the morning and it could be later, but we indicated some
partial clearing developing in the afternoon from west to east as
westerly flow becomes established. We have highs in the upper 40s
to lower 50s.

&&

.SHORT TERM /TONIGHT THROUGH 6 PM SUNDAY/...
Tonight...
Weak high pres will be over the region with light to near calm
winds. Model cross sections indicate enough low level moisture
for patchy low clouds and fog to develop. Not certain of the areal
extent of low clouds but best chance will likely be along the
south coast. Lows will be mostly in the 30s.

Sunday...
Weak cold front will sag south across SNE during the day as
surface ridging noses down from northern New Eng with increasing
NE flow developing in the afternoon. The NE flow and increasing
low level moisture will result in widespread lower clouds
developing across SNE and can`t rule out some spotty light rain
or drizzle in the afternoon. High will range from lower 40s north
to upper 40s south but cooling from the north in the afternoon
with temps likely falling into the 30s north of the Pike.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Details...

Sunday...

Weak front pushes S across the region early Sunday. Should be
mainly dry across the region through midday as ridging builds down
the coast. However, leading edge of precip working NE from
strengthening low pres across the SE U.S. will start to approach
during the afternoon. Light/variable winds will shift to N-NE,
then eventually E during the day as the ridge over Quebec shifts
E. E-NE winds will start to increase along the S coast late in the
day as pres gradient increases between the exiting high to the
north and the strong low pressure over the southern Appalachians.
Gusts may approach 25-30 kt across S coastal RI and Block Island
by 00Z Mon.

Have brought low CHC POPs int S coastal areas between 21Z and 00Z
as leading edge of precip slowly approaches.

Highs will be mainly in the 40s.

Sunday night through Tuesday...

Impressive low pressure will continue to strengthen as it
approaches the mid Atlc coast, along with its H5 cutoff low
swinging across the SE U.S. Good low level moisture plume with
this system, with PWATs around 1 to 1.25 inches crossing the
region. Mid level system becomes negatively tilted as it rotates
NE during the day Monday. This will bring good amounts of QPF
across the region on the strengthening E wind flow. QPF forecast
of 1.25" across NW Mass ranging to 2-2.25" across SE Mass/RI
through 00Z Wed.

Big question with this system will be how much colder air can
filter down across the interior and, if this does occur, there
will be PTYPE issues. At this point, looks like mainly rain across
the coastal plain, with a wintry mix of RN/SN/IP mainly but could
also see some FZRA mixed in mainly across N Mass along the Route 2
corridor to the MA/NH border for a time. Partial thickness
patterns suggest mainly SN and IP at this point, though expect
rather low rain/snow ratios as temps only fall back to the upper
20s to around 30. During Tuesday, with strong easterly flow off
the ocean, will see precip change over to rain across all areas.

Best shot for precip looks to occur from mid-late morning Monday
through Monday night. This is also when heaviest rain is expected.

Another aspect of this storm will be the winds. Noting a very
strong low level jet, on order of 60-70 kt at 900 hPa, will come
close to if not over S coastal Mass. Have noted excellent mixing
from 900 hPa downward, so could see gusts up to 45-50 kt across
Cape Cod and the islands, and possibly across more areas along E
coastal Mass and possibly as far W as E RI late Monday and Monday
night. High wind warnings will likely be needed, but too early to
issue now.

Some timing and track issues for the low`s exit, but looks like it
should push across Cape Cod or Nantucket sometime Tue afternoon.
Back of precip shield should exit late Tue or Tue evening. Clouds
will linger through the night, though could break toward daybreak
across the southern CT valley.

Wednesday through Friday...

Weak high pressure will build across the region Wednesday,
bringing dry and mild conditions.

Another fast moving front will cross the region during Thursday.
This system looks rather dry as most of the moisture and energy
moves into northern New England. However, can not rule out some
patchy light rain and/or snow across the E slopes of the
Berkshires, with light rain possible through the remainder of the
CT valley.

A persistent NW wind flow will be in place by Friday, along with
the chance for some isolated light rain and/or snow showers as a
digging upper level trough approaches during the day and into
Friday night. Temperatures will be at more seasonal levels for
late January.

&&

.AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...

Through 12z...
Widespread MVFR with areas of IFR cigs and patchy fog. Spotty
light rain possible, mainly south of the Mass Pike.

Today...
Moderate confidence. MVFR/IFR cigs in the morning with patchy
fog, improving to VFR in the afternoon.

Tonight...
Moderate confidence in patchy MVFR/IFR fog developing, but low
confidence in potential for IFR cigs. Guidance is ranging from
VFR to IFR/LIFR. If IFR cigs develop, highest prob likely along
the south coast.

Sunday...
Moderate confidence. Widespread MVFR cigs developing with
potential for IFR in the afternoon. Patchy light rain or drizzle
possible in the afternoon. Increasing NE winds along the coast in
the afternoon with gusts to 20 kt developing.

KBOS TAF...Moderate confidence in TAF.

KBDL TAF...Moderate confidence in TAF.

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

Sunday night through Monday night...
MVFR-IFR CIGS/VSBYS across the region Sunday night in rain and/or
snow, with some IP developing after midnight and continuing
through Monday. May see LIFR conditions Monday afternoon into
early Tuesday morning in patchy moderate rain and/or snow and
patchy fog. E wind gusts up to 40-50 kt likely along portions of
the immediate S coast, especially the outer Cape and Nantucket Mon
night. LLWS likely as E winds at around 2000 ft increases to 50-70
kt Mon night.

Tuesday...
Areas of IFR CIGS/VSBYS in RA/SN/FZRA early, changing over to RA
during the day. Precip mixes to FZRA/IP/SN over western areas
early Tue night before ending. IFR-LIFR CIGS linger Tue night
across eastern areas through midnight or so, then should improve
toward daybreak Wed.

Wednesday...
May see local MVFR-IFR conditions across the E slopes of the
Berkshires into the CT valley and central areas in SN/IP/FZRA as
cold front approaches. Most of the energy remains N and W of the
region, so looks like mainly VFR conditions elsewhere. NW winds
around 10 kt shift to SW during the day. Conditions should improve
Wed night.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...High confidence.

Today...Southerly winds becoming W/SW in the afternoon with gusts
15-20 kt at times. Easterly swell of 4 to 5 ft develolping over
the outer waters.

Tonight...W winds gradually becoming N after midnight as weak cold
front moves into the waters from the north. Some lingering 5 ft
swell over the eastern waters.

Sunday...Increasing NE winds, epecially in the afternoon with
gusts to 20 kt developing. G25 kt over southern waters by evening.

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

Sunday night through Tuesday...
Strengthening low pressure of the mid Atlantic coast Sunday night
will push NE. Expect E winds to rapidly increase as strong winds
will mix down from about 3000-5000 ft. Gusts will likely reach
50-60 kt across a good portion of the waters. Have already issued
storm watches across most of the waters due to the potential
threat of prolonged storm force winds. Seas will also build up to
15-20 ft. Expect the strongest winds and highest seas during
Monday afternoon and night. This is a potentially life threatening
storm for mariners. Visibility restrictions in rain and patchy fog
likely through most of Tuesday.

As the low passes across SE Mass toward the Gulf of Maine,
leftover gale force winds will push out of the eastern open waters
Tuesday morning. Winds will briefly diminish around midday and
through the afternoon Tuesday, then NW gusts could reach 25-30 kt
Tue night. Seas will remain at or above 5 ft over the open waters
through Tuesday night. Some visibility restrictions continue
early, then should improve from S-N.

Wednesday...
Winds will diminish as they back to W-SW during Wed. May see some
gusts up to 25 kt on the southern waters. Seas remain at or above
5 ft over the open waters. May see some visibility restrictions
on the waters S of RI in patchy light rain Wed afternoon and
night.

&&

.TIDES/COASTAL FLOODING...
Given potent easterly LLJ and period of strong wind gusts, expect
seas to probably build to 20 feet or higher 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...None.
RI...None.
MARINE...Storm Watch from Monday afternoon through late Monday night
     for ANZ232.
     Gale Watch from Sunday evening through late Monday night for
     ANZ233-234.
     Gale Watch from Monday morning through late Monday night for
     ANZ230.
     Storm Watch from Monday afternoon through Monday evening for
     ANZ231.
     Gale Watch from late Sunday night through late Monday night
     for ANZ236.
     Storm Watch from Monday morning through Monday evening for
     ANZ235-237.
     Storm Watch from Monday morning through late Monday night for
     ANZ250-254.
     Gale Watch from Sunday evening through Tuesday morning for
     ANZ251.
     Storm Watch from late Sunday night through Monday evening for
     ANZ255-256.

&&

$$
SYNOPSIS...KJC
NEAR TERM...KJC
SHORT TERM...KJC
LONG TERM...EVT/JWD
AVIATION...KJC/EVT/JWD
MARINE...KJC/EVT/JWD
TIDES/COASTAL FLOODING...



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