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000 
FXUS61 KBOX 211457
AFDBOX

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

.SYNOPSIS...
Weak high pressure will bring tranquil weather Today and Sunday.
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.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

10 AM UPDATE...

Only change to previous forecast is to be more aggressive with
clouds lifting and thinning this afternoon...giving way to at
least partial sunshine from west to east. Reason for the optimismlatest
visible satellite imagery already showing large breaks in the
overcast across the lower Hudson valley of NY and western CT.
Expecting this trend to spread eastward into MA and RI as deep
layer ridge west of New England advects eastward. This will
promote more anticyclonic flow and associated subsidence to erode
low level moisture. In fact southwest surface winds this morning
should veer to the west this afternoon, providing some downslope
component helping to erode low level moisture. Current VWP already
showing winds just off the deck at 1KFT and 2KFT from the WNW.
Once sun breaks out these WNW winds aloft will mix to the surface
and help the clearing trend. In addition new 12z NAM low level RH
and ceiling grids support this clearing trend. Thus have trended
the forecast toward decreasing clouds/increasing sunshine this
afternoon.

Other issue will be how high temps climb today. Mild start with
temps already in the low to mid 40s many locations. 925 mb temps
on SPC mesoanalysis has +5c to +7c already over MA/RI and CT.
Given these temps and WNW winds just off the deck low to mid 50s
very achievable. Previous forecast captures this well. However if
enough sunshine develops this afternoon and blyr can deepen to 925
mb could see high into the upper 50s! Earlier discussion below.

====================================================================== 

Previous discussion...

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 /6 PM THIS EVENING 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/...
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...

Sunday night...

High pressure builds south into Northern New England. This will
create an ENE wind coming off the Gulf of Maine into our area. Dry
air aloft in the -10C to -15C nucleation zone suggests any precip
before midnight will be limited to drizzle or freezing drizzle and
perhaps some light rain that is just above trace level. 

The moisture starts filling in aloft during the night, which will
bring a better chance of measurable pcpn after midnight. Even so,
the focus of upper venting and lower level transport of moisture
will remain to our southwest along the Mid Atlantic coast through
the night with New England on the fringe.

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 /15Z 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/...

15z update...

Only change from previous TAFs is to speed up clearing trend as
currently seen on latest observations. Earlier discussion below.

=================================================================

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...MVFR cigs through the night. Some drizzle possible in
the evening but vsbys should remain no lower than MVFR for that.
Light rain and snow develop after midnight but with vsbys roughly 5
miles or greater. Increasing ENE winds along the coasts with gusts
30 to 35 knots, but lighter inland.

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...

VFR. An approaching cold front may bring MVFR cigs/vsbys in rain
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.

10 am update...

Tranquil weather by late Jan standards. Only issue will be patchy
fog this morning limiting vsby to 1-3 miles but improving this
afternoon as winds shift from SW to W. Earlier discussion below.

====================================================================

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 developing 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, especially 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 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... Winds diminish and back from the WSW. Lingering gusts
to 25 knots. Seas will subside through the day, but with lingering
5 to 6 foot seas on the exposed waters.

&&

.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...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...WTB/KJC
NEAR TERM...WTB/KJC/Nocera

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