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000
FXUS61 KBOX 211757
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1257 PM EST Wed Nov 21 2018

.SYNOPSIS...
An arctic cold front may bring a few localized snow squalls
later today into this evening, followed by record cold and
bitter cold wind chills tonight into Thanksgiving. High pressure
moving off the coast will allow temperatures to moderate some
on Friday and particularly on Saturday. Dry weather should
prevail most of the day Saturday, but a fast moving low pressure
system will bring a period of rain Saturday night with a low
risk it may begin as some ice across portions of the interior.
The rain may linger into the first part of Sunday before coming
to an end. A potentially stronger low pressure system may bring
more unsettled weather by Monday and/or Tuesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
1255 PM update...
Scattered snow showers approaching from the north and west will
move across SNE this afternoon. Low risk for a heavier snow
squall but it should be isolated. Low level lapse rates are not
as favorable across SNE compared to the north which will be a
limiting factor. No changes to forecast.

Previous discussion...

Main concern for the afternoon is the risk of a few snow
showers/isolated squall assocd with the arctic front along with
brief strong wind gusts.

Arctic front located from the St Lawrence Valley to eastern
Lake Ontario with numerous snow showers developing along the
front. Some heavy squalls as well with lightning noted east of
Lake Ontario. This activity is in an area of steep low level
lapse rates and convergence along the front. Guidance suggests
the steepest 0-3km lapse rates will remain focused to the north
this afternoon along with best forcing. However, enough moisture
and convergence for isolated to sct snow showers to survive the
trip into SNE mid/late afternoon as the front moves through.
There is a low risk for an isolated heavier squall, mainly
across the Berkshires and northern MA which will be closest to
steeper low level lapse rates. The other concern is for brief
strong wind gusts right behind the front. Soundings indicate
potential for brief 40-50 mph gusts, especially over higher
terrain. Will highlight this potential in SPS.

Previous discussion...

On the heels of the low pressure system that brought us
yesterday`s rain and snow exiting to the northeast, we`re ever
so briefly under the influence of some high pressure ridging
extending from the surface high over the southeast U.S.
Satellite imagery this morning shows mostly clear skies, though
upstream we see clouds associated with mid and upper level
moisture that will bring increasing cloud cover from the
northwest today. Freezing temperatures that brought a black ice
risk overnight will warm above freezing by early to mid morning
for most locations, topping out in the low to middle 40s (upper 30s
in higher elevations). This comes ahead of our next strong
frontal system, an arctic cold front associated with the low
pressure system currently north of the Great Lakes. As this
relatively weak low tracks east along the U.S/Canada border it
will cross northern New England this afternoon/evening, dragging
with it our arctic front. We`ll see gusty winds increasing
through the day as the pressure gradient tightens and a robust
LLJ moves over. With good mixing behind the front one hazard
will be brief stronger winds in the wake of the front up to
35-45 mph. Current timing of the front would be from west to
east between about 2 PM and 7 PM. We`ll see a chance for
scattered snow showers along and ahead of the front, but the
main threat from this front will be potential for some localized
but strong snow squalls. Model soundings show marginally good
low level lapse rates and plenty of moisture in the lowest
levels of the atmosphere, though the best forcing and steepest
lapse rates of 7-8 C/km remain over northern New England. Thus,
the best chance for strong snow squalls lies to our north, but
will still be possible for us, especially over northwest MA, the
closest proximity to the more favorable environment. These
squalls would have the potential to drop quick bursts of heavy
snow along with gusty winds, dropping visibilities on area
highways during one of the busiest travel days of the year.
Motorists are encouraged to avoid driving through these if
encountered.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
Behind the front temperatures plummet under northwest wind,
as a strong surface high pressure over the Great Lakes region
moves toward SNE. It will be an very anomalous air mass with
H85 temps around -20C. This translates to low temperatures at
the surface in single digits for central and western MA with
teens elsewhere. Additionally, gusty winds will mean even colder
wind chill values in the single digits and even below zero. For
this reason a Wind Chill Advisory has been issued for the east
slopes of the Berkshires Thursday morning where wind chill
values will drop to -15 F. Most of the region will be dry, with
one possible exception being the outer Cape where we may see
some ocean effect snow showers. The combination of arctic air
temps and relatively warm SSTs in late Nov will generate
impressive instability. Much of the snow would likely form
offshore, but there is a chance for some showers over land given
the favorable NW wind trajectory and temperature profile.

Thursday will be a very cold and blustery Thanksgiving
indicated by a striking pressure rise/fall couplet and strong
LLJ. Given the good CAA we`ll be well mixed and model soundings
show that LLJ being mixed down to the surface keeping winds gusty
all day. Meanwhile high temperatures won`t make it out of the
teens and 20s. Potential for ocean affect snow continues on
Thursday, especially for areas east of HYA late in the
day/overnight as winds become more northerly.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Highlights...

* Record low temperatures possible Thanksgiving night/early Fri am

* Ocean effect snow showers/flurries may clip outer-Cape into Fri am

* Dry Fri into part of Sat with moderating temperatures

* Period of rain Sat night perhaps lingering into part of Sun with a
  low risk it may begin as a period of ice across the interior

* Another round of unsettled weather likely Mon and/or Tue with odds
  favoring ptype mainly rain but that is not set in stone

Details...

Thanksgiving night...

Arctic high pressure will build into the region from the west.  This
will allow winds to gradually diminish and with highly anomalous
cold airmass in place...an incredibly cold night is in store for the
region for this time of year.  Given the diminishing winds/mainly
clear skies this will be the night for the radiator sites to be much
colder than some of the urban centers.  Expect most of the outlying
locations to see low temperatures bottom out in the single
digits...with perhaps even some below zero readings in the normally
coldest spots of western MA. The urban heat islands of downtown
Boston/Providence will see overnight low temps in the lower to
middle teens.  Record low temperatures may be set at some of our
sites.

Dry weather and mainly clear skies are anticipated for Thanksgiving
night.  The exception will mainly be the outer-half of Cape
Cod...where the highly anomalous airmass will result in an extreme
850 to SST differential of 30C.  Northwest wind trajectories and dry
low level air will limit potential...but given the extreme nature
of the airmass and over 1000 J/KG of ocean induced Cape will
probably see some ocean effect snow showers/flurries.  A coating to
1 inch of snow is certainly possible across portions of the outer-
Cape with even the low risk of localized 2 inch amounts.

Friday into Saturday afternoon...

High pressure will move east of the region along with some upper
level ridging moving in from the west.  The result will be in dry
weather continuing but moderating temperatures Fri and especially
by Saturday afternoon.

High temperatures on Friday should recover into the upper 20s to the
lower 30s...which is still well below normal for this time of year.
However...a big improvement from Thanksgiving Day and much less
wind.  While clouds will be on the increase during the day Saturday
ahead of a wave of low pressure...dry weather should prevail most of
the day.  Milder air should continue to work into the region ahead
of a shortwave trough...allowing many locations to see highs recover
well into the 40s.

Saturday night into Sunday...

A fast moving wave of low pressure should bring a period of mainly
rain to southern New England Saturday night...perhaps lingering into
the first part of Sunday.  However...the system looks fairly
progressive so dry weather may dominate by Sunday afternoon/night.
Ptype should mainly be rain given lack of Canadian high pressure and
current thermal profiles.  Still we will need to watch for at least
the low risk for a period of ice across the interior at the
onset...dependent on how cold the boundary layer is at that start.

Monday and Tuesday...

Appears another low pressure system...perhaps an intensifying storm
will impact the region sometime Mon into Tue.  Appears a fairly
stout -NAO will be in place...but the antecedent airmass will not be
that cold.  Therefore...odds favor this being mainly a rain event.
However...if we can get rapid intensification off the coast can not
rule out some wintry weather getting involved.  Too early to say
much more other than it looks unsettled.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short Term /through Thursday night/...High Confidence.

Through 00z...
Mainly VFR cigs, but brief lower conditions in scattered snow
showers. Low risk of brief IFR in a squall. W/SW gusts 20-30 kt
with brief 40 kt NW gust possible late day with the arctic
front, especially higher terrain.

Tonight through Thursday night...
Mainly VFR, but areas of MVFR cigs developing over the outer
Cape late tonight into Thu evening. A few flurries possible at
times over the outer Cape. NW gusts to 25-35 kt at times
through Thursday, diminishing Thu night.

KBOS Terminal... High confidence.

KBDL Terminal... High confidence.

Outlook /Thursday Night through Sunday/...

Friday through Friday Night: VFR. Breezy.

Saturday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance FZRA.

Saturday Night: Mainly MVFR, with areas VFR possible. Breezy.
RA, chance FZRA.

Sunday: Mainly MVFR, with areas VFR possible. Breezy. Chance
RA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Thursday/...High confidence.

Winds will temporarily diminish this morning while shifting to
W/SW, but will increase this afternoon and especially tonight
while shifting back to W then NW behind arctic front. Gusts to
35-40 kt likely, peaking tonight. Gale warnings have been
issued. Vsbys may be reduced in snow showers late Wed night
across waters east and NE of Cape Cod.

Outlook /Thursday Night through Sunday/...

Thursday Night: Moderate risk for Gale force wind gusts up to
35 kt during the evening. Areas of rough seas. A chance of snow
showers. Areas of visibility 1 to 3 nm.

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.

Friday Night through Saturday: Low risk for Small Craft
Advisory winds with gusts up to 25 kt.

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

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Areas of rough seas. Chance of rain.

&&

.CLIMATE...
An early season shot of arctic air will likely result in some records
being broken Thanksgiving into early Friday morning.

November 22nd
Location / Record Low / Record Minimum High

Boston..........9 (1879) / 24 (1880)
Hartford.......14 (1969) / 27 (1978)
Providence.....16 (1987) / 30 (2008)
Worcester......11 (1987) / 24 (2008)

Thanksgiving
Location / Record Low / Record Minimum High

Boston.........11 (Nov 27, 1873) / 24 (Nov 28, 1901)
Hartford.......12 (Nov 28, 2002) / 27 (Nov 23, 1989)
Providence.....14 (Nov 23, 1972) / 30 (Nov 28, 1996)
Worcester.......9 (Nov 23, 1989) / 22 (Nov 23, 1989)

November 23rd
Location / Record Low

Boston.........11 (1880)
Hartford.......12 (1972)
Providence.....14 (1972)
Worcester.......9 (1989)

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Wind Chill Advisory from 4 AM to 10 AM EST Thursday for MAZ002-
     008-009.
RI...None.
MARINE...Gale Warning until 7 PM EST Thursday for ANZ231>234-251.
     Gale Warning from 6 PM to 9 PM EST this evening for ANZ230.
     Small Craft Advisory until 7 PM EST Thursday for ANZ236.
     Gale Warning until 7 PM EST Thursday for ANZ235-237-250-
     254>256.

&&

$$
SYNOPSIS...Frank/BW
NEAR TERM...KJC
SHORT TERM...BW
LONG TERM...Frank
AVIATION...KJC/Frank
MARINE...Frank/BW
CLIMATE...