Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 240429
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1129 PM EST Mon Jan 23 2017

.SYNOPSIS...
A significant coastal storm will impact the region tonight into
Tuesday with a wide variety of weather from snow and/or ice across
the interior, a period of strong to damaging wind gusts along the
coast including a period of very heavy rain across RI and eastern
MA. High pressure builds in behind the departing storm and brings
dry mild weather Wednesday and Thursday. A cold front crosses New
England Thursday followed by seasonably cool temperatures and
blustery winds.

&&

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

***Moderate to heavy sleet dominates across the interior overnight
 but brief burst of moderate to heavy snow across north central MA
 near the NH border***

1130 pm update...

Mainly moderate to heavy sleet continues to fall across interior
southern New England. Some areas may see a couple inches of sleet
especially across interior MA. This a result of low level cold air
in the 850 to 900 mb layer, but a warm nose in the 700 to 800 mb
layer.

The exception to the sleet, was across a small portion of north
central/interior northeast MA. Intense vertical motions were
temporarily overcoming the warm nose allowing for moderate to
heavy snow. While this will not last long, a small area near the
New Hampshire border may receive a total of 3 to 6 inches of snow
before the change back to sleet.

Along the coastal plain may see some sleet mixed in with the rain
for a time. Otherwise, main concern will be for heavy rain. Much
of the high resolution guidance indicates 2 to 3 inches of rain
with locally higher amounts as strong easterly LLJ coupled with
abnormally high Pwats intersecting the coastal front. Localized
urban flooding remains possible.

Strongest winds expected through 10z as the easterly low level
jet around 5 standard deviations above normal sets up on the
coast. 35 to 50 mph gusts across the interior with 50 to 60 mph
gusts on the coast. Wind Headlines remain posted and have already
received reports of a few downed trees/isolated power outages.


Previous discussion...

*** Significant Multi-Facet Winter Storm will bring a wide variety
of hazards to the region tonight into Tuesday with greatest
uncertainty revolving around ptype across the interior ***

Many hazards to consider as moisture laden southern stream system
tracks up the Eastern Seaboard.  Here are the storm
details/attributes for tonight into Tuesday.

1) Timing...

Nose of increasing low level onshore jet continues to generate
numerous showers of rain and snow across much of the region at 4 pm.
Where snow showers are occurring surface temps are above freezing so
roadways remain wet. Thus not much impact here other than decorating
the landscape with light snowfall amounts.

Deep layer moisture and synoptic scale lift arrives onto the south
coast around 00z and then spreads northward later this evening. All
ready seeing the firehose of Atlantic moisture on the OKX radar
coming into NJ/NYC area. This will advect northward into our region
this evening. However with steady/heavier precip not arriving until
roughly 7 pm along the south coast...about 10 pm up to the MA pike
and then to the MA/NH border around midnight. Thus looks like the
bulk of the heavy rain will miss the evening commute. The
steady/heaviest precip should impact MA/RI and CT from 10 pm to 4 am
perhaps lingering until 7 am across northeast MA.

2) Ptypes and Snow/Ice Amounts...

This is the most challenging portion of the forecast. For northern
and western MA subfreezing temps from 850 mb and to the surface
would support lots of snow. However very strong warming aloft occurs
in the 850-700 mb layer. This will result in ptype issues. However
model guidance differs on the degree of warming in this layer. The
12z NAM/Rgem and 00z NCAR ensembles are the warmest aloft while the
12z EC/GFS are not as warm showing soundings nearly isothermal. This
combined with very strong forcing for ascent may cool this portion
of the column via diabatic and dynamical cooling processes. This is
a very delicate balance as a temp difference of 1C in this layer
could be the difference from several hours of heavy wet snow or lots
of sleet and freezing rain. In addition snow growth looks favorable
03z to 09z with strong forcing and deep layer moisture in the snow
growth region during this time. Thus increasing the risk for a
period of heavy snow. Given the model spread/uncertainty we followed
a model blend. This yielded 3-6" of snowfall possible across east
slopes of the Berkshires, 1-4" along the MA/NH border with an inch
or two of snow and sleet across the Worcester Hills. Nevertheless
big bust potential here. Therefore do not focus solely on our most
likely snowfall forecast as the snow probabilities better
communicate the full range of possibilities.

Further south across CT a mix of snow/sleet and freezing rain is
likely then all rain Tue. For RI and eastern MA, rain/snow showers
this evening transitioning to a steady rain later this evening with
some sleet mixed in at times...then all rain overnight into Tue.

3) Heavy Rain Across Eastern MA/RI...

Global guidance including the 12z EC/GFS have widespread 1-2"
rainfall amounts with 2+ across eastern MA where coastal
front/inverted trough sets up and enhances low level convergence.
Not surprising that the Hi Res guidance such as the ARW/NMM have
isolated 3-4" amounts. 00z NCAR ensembles support this as well with
probs of 3+ greater than 70% across RI into eastern MA with highest
values of 90% over Cape Ann! This seems reasonable given the
subtropical connection down into the Bahamas with this southern
stream system along with the impressive wind anomalies within this
warm conveyor belt of precip. Thus very heavy rainfall expected
tonight into early Tue morning, likely impacting the early morning
Tue commute especially in northeast MA where heavy rain will linger
until 7 am or so. Thus flood watch continues.

4) Strong To Potentially Damaging Wind Gusts...

Not much change from previous forecasts. Models locked in on a 70+
kt low level easterly jet moving across coastal RI and MA between 1
am and 7 am Tue. Despite warming aloft strong pressure falls and
some enhancement from very heavy rainfall dragging strong winds
aloft to the surface will yield wind gusts up to 60 mph or so along
the coast. Sustained winds will be very strong as well with speeds
up to 45 mph or so. These winds may result in a few downed
trees/large limbs and isolated power outages. Less wind away from
the coast but still close to wind advisory criteria (sustained 31
mph and higher and/or gusts 46 mph and higher). Thus will keep wind
headlines as is. Strongest winds over Cape Cod and islands around 1
am to 5 am.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
Tuesday...

Precip mainly rain except possibly a wintry mix leftover cross
northwest MA.  Heavy rain will begin exiting northeast MA as low
level jet exits. However as dry slot approaches from the south and
comma-head begins to crank-up there will be embedded periods of
moderate to perhaps briefly heavy rain especially as some dry air
aloft on the nose of the dry slot begins to steepen mid level lapse
rates. This could be sufficient for isolated thunder. Mild over
southeast MA with temps rising to 45-50 but north-northeast winds
will keep the interior in the 30s with 40-45 for the Boston to
Providence corridor. Remaining windy across northeast MA otherwise
winds diminish elsewhere.

Tuesday night...

Models have trended slower with mid level closed low still not thru
the region Tue night. This will result in comma-head rains lingering
with possibly some sleet northwest MA as cooler/drier air begins to
advect in. Could have some standing water/slush refreeze across the
interior with temps falling into the upper 20s and lower 30s.
However enough mixing via WNW winds to keep temps above freezing
across much of RI and eastern MA.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Overview...

Looks like a relatively quiet period from Wednesday through next
Monday. The main feature at the surface will be a cold front that
sweeps through the area late Thursday/Thursday night. Aloft, there
are weak short wave troughs moving through Thursday, Saturday, and
late Monday. Temperatures will be quite mild ahead of the cold front
Wednesday and Thursday...returning to near normal wintertime
readings this weekend into early next week.

Details...

Wednesday...

High pressure off the southeast U.S. coast will ridge northward into
southern New England. Clouds will exit the coast early in the
morning, followed by abundant sunshine throughout the area. Highs
in the mid to upper 40s except lower 40s northwest MA.

Wednesday night and Thursday...

As a cold front approaches the region, winds turn to the southwest
and clouds increase with some warm advection. There is a chance of a
few showers late Wednesday night. With some sunshine in eastern MA
and RI Thursday, highs should climb to 50 or even the lower 50s.
Highs will be in the mid 40s in western MA, where a cold front will
move through in the early afternoon. The cold front will move
through the rest of the region in the mid to late afternoon and
there should be some good mixing as winds shift to the west-
northwest. Am forecasting gusts to 25-35 mph for a time late Thu.
Cannot rule out a snow shower or flurry in the slopes of the
Berkshires late in the afternoon.

Thursday night through Monday...

Much ado about partly to mostly cloudy skies during the daytime due
to broad cyclonic flow with cool air aloft...and clear to partly
cloudy skies at night. Can`t rule out a stray flurry especially in
northwest MA each day, but probabilities are too low to include in
the forecast. Highs Friday mid 30s to lower 40s...and highs in the
30s each day from Saturday through Monday. Lows in the 20s Friday
night...then teens and lower 20s Saturday night through Monday night.

Previous GEFS ensembles showed development of a coastal storm late
Monday, passing to our southeast. New runs are not very bullish on
this. For now, have chance PoPs over the coastal waters late Monday
to the southeast of our area.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

0030z Update...

Tonight...high confidence except low confidence on ptypes

Mainly MVFR to start but lowering to IFR across eastern MA later
tonight. Rain/snow showers transitions to a steadier/heavier precip
00z -03z except toward 05z for the MA/NH border. Precip mainly rain
with possibly some sleet at times but a wintry mix across northern
and western MA. Snow and sleet accumulations of 3-6" possible here
with an inch or two Worcester Hills. Some freezing rain possible too
across the high terrain. Very strong winds overnight from 03z-09z
south coast to 06z-12z northeast MA including Boston/Logan.

Tuesday...high confidence.

MVFR-IFR in periods of light rain and drizzle but improving trend to
VFR toward Wed morning.

KBOS TAF...Moderate to high confidence in TAF. Strongest winds 08z-
12z Tue.

KBDL TAF...Moderate to high confidence in TAF.

Outlook /Wednesday through Saturday/...Moderate confidence.

Wednesday...VFR.

Wednesday night and Thursday... VFR but areas of MVFR cigs/vsbys
possible in scattered showers especially southern sections Wed.
night and a chance of a snow shower in far northwest MA late Thu.
afternoon. Winds behind a cold front turn to the WNW and gust to
25-30 kts Thu. afternoon and evening.

Friday and Saturday...VFR. Areas of ceilings 3000-4000 ft. Local
MVFR possible with a passing flurry northwest MA.

&&

.MARINE...
Forecaster Confidence Levels...

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

*** Storm Force Wind Gusts and 20 to 30 foot seas across our outer-
eastern MA waters tonight into early Tuesday will result in a
dangerous storm for mariners ***

7 PM Update...

Tonight...

Not much change with strong low pres moving up the Eastern Seaboard.
Strongest onshore winds 10 pm to 4 am southern waters of MA/RI...
then midnight to 8 am for eastern MA waters. Heavy rain and fog will
reduce vsby. By daybreak Tue seas of 20-30 ft likely across eastern
MA waters!

Tuesday...

Low pres moves from DE waters to near Nantucket by dark. Strongest
winds exit into ME waters however very rough/dangerous seas will
linger. Vsby reduced in rain and fog.

Tuesday night...

Low pres near Nantucket Tue evening lifts NE to Nova Scotia by Wed
morning. Winds become WNW as the night progresses. This results in
vsbys improving.

Outlook /Wednesday through Saturday/...Moderate confidence.

Wednesday through Friday...

NW winds gusting to 25 kts early...diminish somewhat during the day
then turn to the SW. Rough seas slowly subside to below small craft
advisory levels late Wednesday night.

A cold front sweeps across the waters late Thursday, followed by
colder air and gusty west winds up to 30 knots for a time early
Thursday evening. Colder air and gusty winds will continue into
Friday and Saturday. Seas will diminish through the period, to lower
than WNAWAVE guidance, but still have 5 to 6 foot seas lingering on
the outer waters. Small Craft Advisories will likely be needed for
some or all of this period.

&&

.TIDES/COASTAL FLOODING...
7 pm update...

Timing of strongest winds looks to be 10 pm to 4 am south coast
of MA and RI...then midnight to 8 am for the east coast of MA.

Psurge performing best when compared to current observations with
ETSS and ESTOFS running too low. Thus have followed the more robust
Psurge given strength of the low level jet, wind fetch and strong
pressure falls from approaching storm.

Low to moderate risk for minor coastal flooding this evening for
both south and east coasts. Thus have expanded advisory to south
coats of RI and MA.

More significant high tide is Tue morning. Psurge continues to
advertise up to a 3.5 surge. SWANgfs and SWANnam generating seas up
to 25 ft across eastern MA waters. Given strength of low level jet
and wind fetch would not be surprised to see seas approach 30 ft!
Thus the combination of surge and strong wave action ontop of these
elevated water levels support widespread minor coastal flooding with
pockets of moderate. Thus will issue a Coastal Flood Warning for the
entire east coast of MA for Tue morning tide from the NH/MA border
to Nantucket.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Wind Advisory until 7 AM EST Tuesday for CTZ002>004.
     Winter Weather Advisory until 7 AM EST Tuesday for CTZ002-003.
MA...High Wind Warning until 7 AM EST Tuesday for MAZ007-019-
     022>024.
     Coastal Flood Warning from 5 AM to 11 AM EST Tuesday for
     MAZ007-015-016-019.
     Flood Watch until 10 AM EST Tuesday for MAZ005>007-013>021.
     Wind Advisory until 7 AM EST Tuesday for MAZ004>006-009-
     011>018-020-021-026.
     Winter Weather Advisory until 7 AM EST Tuesday for MAZ002>006-
     008>012-026.
     Coastal Flood Warning from 6 AM to noon EST Tuesday for MAZ022-
     024.
     Coastal Flood Advisory from 6 AM to noon EST Tuesday for
     MAZ023.
RI...High Wind Warning until 7 AM EST Tuesday for RIZ008.
     Flood Watch until 10 AM EST Tuesday for RIZ001>007.
     Wind Advisory until 7 AM EST Tuesday for RIZ001>007.
     Coastal Flood Advisory from 3 AM to 7 AM EST Tuesday for
     RIZ006-007.
MARINE...Storm Warning until 4 AM EST Tuesday for ANZ231-232-255-256.
     Storm Warning until 3 AM EST Tuesday for ANZ233-234.
     Gale Warning until 7 AM EST Tuesday for ANZ230.
     Gale Warning until 4 AM EST Tuesday for ANZ236.
     Storm Warning until 2 AM EST Tuesday for ANZ235-237.
     Storm Warning until 7 AM EST Tuesday for ANZ250-254.
     Storm Warning until 6 AM EST Tuesday for ANZ251.

&&

$$
SYNOPSIS...Nocera/GAF
NEAR TERM...Frank/Nocera
SHORT TERM...Nocera
LONG TERM...GAF
AVIATION...Nocera/GAF
MARINE...Nocera/GAF
TIDES/COASTAL FLOODING...


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