Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 210846
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
346 AM EST Mon Jan 21 2019

.SYNOPSIS...
Arctic air pours into our region today behind the departing storm,
bringing dangerously cold wind chills. Moderating temperatures
Tuesday into Wednesday, then another storm brings a soaking rain
late Wednesday through Thursday night. Return of cold and mainly dry
conditions Friday into this weekend.

&&

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

A much drier morning on tap than 24 hours ago. The low center
is exiting off the coast of Maine this morning while the 500 mb
center lags behind over northern New York. Skies have cleared
out for much of southern New England overnight as drier air
wraps around the low. That won`t last long, though, as much of
the region will see a good deal of cloudcover on Monday. The 500
mb low will bring a deep mid level cold pool and a good plume of
low and mid level moisture with it from Canada. Clouds fill in
through the morning and expand from the coast where ocean effect
clouds are already evident on IR satellite. Dangerous icy
conditions remain a risk for the morning commute where rain from
yesterday`s storm remains frozen. Untreated roads especially
will be very slick and there is no thaw forecast for the next
48 hours. Take it slow.

With a truly arctic airmass in place (H85 temps down to -25 C)
SSTs in the low 40s, and NW winds, the atmosphere is ripe for
ocean effect snow showers as well. Delta T between the sea
surface and the top of the mixed layer still look to be around
25-30C producing >800 J/kg of CAPE. Given the wind trajectory
snow showers should remain over the outer Cape. Before its all
said an done Monday night we`re looking at a possible 1-2" of
snow.

The biggest story of the day for most will be the absolutely
frigid temperatures and wind chills. Arctic air spilling south
will only allow temperatures to climb into the single digits,
below zero for the highest elevations. If the cold wasn`t harsh
enough, the pressure gradient between the low exiting to our
northeast and the high building in from the west will mean very
gusty north winds and CAA efficiently mixing down gusts 35-40
mph. Together they`ll make it feel more like -10 to -25 F. A
Wind Chill Advisory is in effect for most of southern New
England (besides the Cape and islands) while a Wind Chill
Warning covers western and north central MA including the
Berkshires and Worcester Hills. Today will not be the nicest day
to spend outdoors.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...

Overnight and Tuesday mid level heights are on the rise as a
high pressure ridge builds into the northeast. This will relax
our pressure gradient and bring down the winds by Tuesday as the
surface low moves over head. Overnight, skies clear rapidly
from the west as dry air moves in and plenty of sun is in store
for Tuesday. Don`t let the sunshine fool you, though, since
we`ll remain locked in a very cold pattern until Wednesday. Lows
Tuesday morning in the single digits to below zero will rebound
to the mid 20s by Tuesday. It`s not warm but compared to the
single digit highs of Monday, plus the lack of wind, Tuesday
will feel practically balmy.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...

*/ Highlights...

 - Soaking rains late Wednesday through Thursday night
 - Return cold air Friday into the weekend
 - Watching a following early week storm system

*/ Discussion...

21/00Z guidance remains in rather good overall agreement through
Wednesday, then differences start to amplify through early next
week. Will prefer a consensus approach to this forecast to smooth
over the less predictable details through Wednesday, and to minimize
the timing and amplitude differences for the second half of this
forecast period.

In general, broad mid level ridge over the eastern USA shifts over
the North Atlantic Wednesday, while a trough dominates much of the
central USA. This configuration should result in a potent SW flow.
This will not only track a low pressure our way toward mid week, but
also raise temperatures where mainly rainfall is expected. Decent
signal for low level temperatures to be about 1 standard deviation
above normal. Also looking like another moisture-rich system, with
precipitable water values about 3 standard deviations above normal.

Have high confidence in a period of wet weather sometime Wednesday
into Thursday. Depending upon timing, looking like snow to start
with a transition to all rain by late Wednesday afternoon. Small
risk for precipitation to end as snow towards NW MA Thursday
afternoon.

Once this low pressure departs our region, we should have another
period of very cold weather heading into this weekend. This could
set the stage for another winter storm early next week. Still lots
of moving parts with that storm, so not much confidence in the
details at this time.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...High confidence

This morning...
VFR conditions. NW gusts 25 to 35 kts, isolated 40 kts.
Residual water along runways will have frozen.

Today...
Mainly VFR cigs, but areas of MVFR across eastern MA
and Cape / Islands. SCT flurries or snow showers possible. NW
gusts to 30 to 35 kt with isolated 40 kts across SE coastal
terminals.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF

Outlook /Tuesday Night through Friday/...

Tuesday Night: VFR. Breezy.

Wednesday: Mainly VFR, with local IFR possible. Windy with local
gusts to 30 kt. RA or FZRA, with a chance of SN or PL.

Wednesday Night: Mainly VFR, with areas IFR possible. Windy
with gusts to 30 kt. RA or FZRA.

Thursday: Mainly VFR, with local IFR possible. Strong winds
with areas gusts to 40 kt. Chance RA.

Thursday Night: VFR. Windy with gusts to 35 kt. Slight chance
RA.

Friday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

NW gales persist through today in the strong cold advection.
With the arctic air pouring into the region and rough seas,
widespread moderate freezing spray will develop on all open
waters, with pockets heavy freezing spray.

Tuesday NW winds come down through the day, gusting 20-25 kts by
early afternoon. Seas start 5-7 ft with 9-11 ft on the outer
waters. By evening 2-4 ft for coastal waters and 5-8 ft outer.

Outlook /Tuesday Night through Friday/...

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

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

Wednesday Night: Low risk for gale force winds with gusts up to
30 kt. Areas of rough seas. Rain.

Thursday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 14 ft. Rain.

Thursday Night: Low risk for gale force winds with gusts up to
40 kt. Rough seas up to 13 ft. Chance of rain.

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Rough seas up to 11 ft.

&&

.TIDES/COASTAL FLOODING...
Monday morning astronomical high tides are a few tenths higher along
the east coast.  While both seas and surge will have subsided, there
is a lingering easterly swell and wave action offshore to lead to an
additional 0.1-0.3 foot surge.

With an astronomical tide around 11.9 at Boston at 11AM, there is
the risk for some spotty minor coastal flooding.  Overall, the
flooding risk is lower, and will likely be limited to the typically
prone locations.

A Coastal Flood Statement has been issued.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Wind Chill Advisory until 8 AM EST Tuesday for CTZ002>004.
MA...Wind Chill Advisory until 8 AM EST Tuesday for MAZ005>007-
     010>021.
     Wind Chill Warning until 5 AM EST Tuesday for MAZ002-008-009.
     Wind Chill Warning until 8 AM EST Tuesday for MAZ003-004-026.
RI...Wind Chill Advisory until 8 AM EST Tuesday for RIZ001>007.
MARINE...Heavy Freezing Spray Warning until 6 AM EST Tuesday for ANZ232.
     Gale Warning until 7 AM EST Tuesday for ANZ232>234.
     Freezing Spray Advisory until 11 AM EST Tuesday for ANZ231-
     233>235-237-250-251-254>256.
     Gale Warning until 9 PM EST this evening for ANZ230.
     Gale Warning until 6 AM EST Tuesday for ANZ231-251.
     Gale Warning until 10 PM EST this evening for ANZ236.
     Gale Warning until 5 AM EST Tuesday for ANZ235-237.
     Gale Warning until 10 AM EST Tuesday for ANZ250-254>256.

&&

$$
SYNOPSIS...Sipprell/BW
NEAR TERM...BW
SHORT TERM...BW
LONG TERM...Sipprell
AVIATION...Sipprell/BW
MARINE...Sipprell/BW
TIDES/COASTAL FLOODING...


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