Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 111445
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
945 AM EST Sun Dec 11 2016

.SYNOPSIS...
Unseasonably cold conditions linger today. Two weak waves of low
pressure will bring about accumulation snow everywhere but the
islands tonight, with a change to rain and snow for some tomorrow.
Winter Weather Advisories will be issued. A brief period of snow
is possible late Wednesday or Wednesday night. Another surge of
arctic air is expected Thursday and Friday. A storm system may
bring a wintry mix of precipitation next weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
945 AM update...
Latest hi-res guidance suggests snow will hold off until after
dark with just a low prob of some light snow or flurries reaching
reaching SW zones from BAF-IJD-WST by evening. Adjusted PoPs
accordingly. Rest of forecast on track.

Previous discussion...

The bulk of the day will be cold, but generally dry with clear
skies and only late day building of high and mid clouds. In spite
of the early sunshine, H92 temps as cold as -10C won`t help much.
Therefore, expect another day where highs to struggle to reach
freezing, if they do at all. Bulk of the expected snowfall should
hold off, even across CT until after the sun sets late this
afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
*** Accumulating snowfall overnight transitions tomorrow ***

This evening into afternoon tomorrow...

Overall synopsis and models...
Still expecting double barreled system as low pres from the Great
Lakes and stalled boundary to the S allows for secondary wave
development early tomorrow. 00Z guidance continues the trend of
allowing secondary low pres to follow a slightly further NE track,
and stronger S LLJ late tonight, suggesting faster flux of warmer
air into the mid lvls. GFS is much warmer, but ECMWF is a better
thermal solution given it blends previous thinking and is a little
cooler, taking into account the early cold low-lvl temps. Thermal
profile and timing of the change from snow-rain are based mostly
on its timing.

Timing...
With mesoscale guidance beginning to capture the event well,
still appears the first signs of light snowfall will arrive in CT
between 5 and 7pm tonight, then spread into RI, SE MA and up the
MA pike between 7 and 9PM, finally reaching N MA by 11PM or so.
Timing may be better finalized as newer guidance comes in today.
Expect it to remain snow through at least 08Z, inland (while
islands could change to or even start as rain much earlier).

P-type and changeover...
The rain-snow line moves north from 08Z (about 3AM local) and
makes it to the CT/RI border and SE MA through about 11Z (6AM), it
also may sneak into the Boston metro by daybreak. It then
gradually spreads N further after 6AM, reaching the border with NH
across NE MA by 9AM. It remains somewhat uncertain as whether
portions of NW MA will change at all. Meanwhile, a period of
light freezing rain will accompany the changeover especially as it
moves through N CT/RI and into portions of central MA S of the MA
Pike. This is mainly because sfc temp warming will lag the warming
aloft initially. Could see localized tenth of an inch of ice in
this corridor.

Snow accumulations and impacts...
The bulk of the snowfall across portions of NW CT through
Worcester and into NE MA will range from 3-5 inches, with a few
locally higher amounts possible. Will be hoisting Winter Wx
Advisories in these areas as a result. Portions of CT/RI and Areas
along and SE of the I-95 Providence-Boston corridor will mainly
stay between 1-3 inches, which is sub advisory, therefore will be
holding off on headlines, allowing the day crew to get one last
look before making any changes. Near the coastlines, expect mainly
a coating before a faster changeover. Will also be hoisting
advisories for portions of N CT/RI mainly for the ice threat, as
snowfall should remain below the advisory threshold. Commuting Mon
morning will still be more difficult than normal, because even
where there is a change to rain, slushy conditions could still
impact untreated roadways. Elsewhere outside of the I-495,
snowfall will will make areas slippery as expected.

The primary areas of uncertainty here are whether Advisories may
need to be expanded. Am a bit concerned this morning with how
poorly guidance is handling the temperatures even though they
typically struggle in radiational cooling events. Although the LLJ
will be strong suggesting warm advection during the morning hours
will be robust, am concerned cold air may hold in a bit longer
than currently forecast. Also, although brief, the window of best
omega in the snow growth regime could be longer if the colder air
is allowed to hang in, leading to higher snow-liquid-ratios. Will
allow the day crew to assess these last minute concerns before
precip moves in late.

All in all, a solid advisory level event for many. Timing is of
most concern given some areas could still see accumulating snow or
slush during the time of the morning commute.

Tomorrow afternoon into evening...
Noting very rapid influence of dry slot given the NE track of the
secondary low pres. Therefore expect rapid end to precip through
the afternoon hours from NW to SE (lingering longest near the S
coast, although it will be rain by this point). Pops rapidly
decline through the evening and it`s not even out of the question
a few spots break out early enough to see the sun set, otherwise
still cloudy for most with highs topping out in the upper 30s and
low 40s primarily.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Highlights...

* Another blast of arctic air Thursday and Friday
* A wintry mix possible next weekend

Overview...

Mid level flow remains fairly zonal through mid week, then becomes
more amplified late this week. Main feature during this period will
be the blast of arctic air at the end of this week. With the lack of
downstream blocking over the favorable location in the north
Atlantic, pattern is transitory. So, arctic air will be relatively
short-lived. There is a potential for another winter weather event
next Sat, as the arctic air departs, and SW flow aloft becomes
re-established.

Details...

Monday night through Tuesday night...Quiet period with mainly dry
and seasonable conditions. High pressure moves to the mid Atlantic
coast with dry westerly flow. Seasonable temperatures expected.

Wednesday into Wednesday night...This still appears to be a
transition period before the arctic air arrives. Models trends still
keep a coastal waver well to our south. Still the possibility of
some light rain or snow in southern New England, with the greatest
risk toward the south coast of New England. It`s possible this low
pressure shifts farther south, and leaves our region mainly dry
during this period.

Thursday and Friday...There is high confidence for arctic intrusion,
with coldest air so far this season. Bitter cold wind chills well
below zero possible late Thursday night and early Friday.  Mainly
dry during this time, with the possibility of ocean-effect showers
toward the east coast, Cape and islands. Wind direction will be
crucial. To much of a west winds, and the showers stay offshore.
While a north wind would impact Cape Ann and outer Cape Cod. Still a
bit early to get too focused on the details.

Saturday...Low confidence forecast in the details. Model consensus
brings a low pressure from the northern plains into Quebec. Still
some question on whether or not a secondary low pressure develops,
as well as where it would develop. Given the presence of arctic air
right before it arrives, we should be cold enough for at least
another round of snow. The high pressure off to the east suggests
portions of the coastal plain should also eventually mix, or change
over to plain rain, with water temperatures still in the 40s. Again,
too early to get too caught up in the details.

&&

.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Through this evening...High confidence.
VFR. Gradually increasing high and mid clouds through the day.
Otherwise, dry. Winds back to the south.

Tonight...moderate confidence.
Low clouds and light snowfall overspread the region from SW at
around 00Z through 04Z. This will yield MVFR transitioning to IFR
vsbys and CIGS. Some light accumulation on area runways likely.
Otherwise, rain spreads from S-N after 06Z, reaching at least the
MA pike by 12Z. Some light freezing rain could impact the region
as the snow changes to rain.

Monday...moderate confidence.
IFR/LIFR in snow interior Monday morning...rain across RI and
eastern MA but improving to MVFR and VFR Monday afternoon.
Probably only improving to MVFR across the higher terrain.

KBOS TAF...High confidence in TAF. Snow begins 00z-03z this
evening with steadiest snow 06z-09z Mon before changing to rain
09z-12z Mon.

KBDL TAF...High confidence in TAF. Snow begins 22z-01z Mon with
steadiest snow 03z-09z then changing to rain 09z-12z but possibly
a brief period of freezing rain 09z-12z.

Outlook /Monday Night through Thursday/...

Monday night through Tuesday night...High confidence. VFR.

Wednesday into Wednesday night...Moderate confidence. Mainly VFR. A
period of MVFR possible sometime late Wed or Wed night with some
light snow, especially near the coast.

Thursday...Moderate confidence. Mainly VFR cigs.

&&

.MARINE...
Forecaster Confidence Levels.

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

Short Term /through Monday/...

Through most of tonight...High confidence.
W winds will back to the S late today and overnight...but
generally remain less than 20 kt. Seas too, should remain below
5 ft. This suggests no need for headlines during this period.
Light rainfall overspreads the waters through the overnight hours,
which could be accompanied by some fog and lower vsbys.

Early tomorrow morning through the remainder of the day....
Moderate confidence. Winds start out of the S but will attempt a
shift E then back to the W. At times gusts to 35+ kt are possible
suggesting a risk for Gales on the outer waters. A Gale Watch has
been issued with small craft advisories elsewhere. Seas reach
nearly 7-9ft on the outer SE waters by tomorrow evening.

Outlook /Monday Night through Thursday/...

Monday night...High confidence. West gusts to 25 kt likely, with
rough seas across the outer coastal waters. Small Craft Advisories
likely.

Tuesday into Wednesday...Moderate confidence. Diminishing westerly
winds below advisory thresholds by Tuesday, and continuing into
Wednesday. Seas gradually subsiding.

Thursday...Moderate confidence. Increasing W/NW winds with gusts to
30 kt likely. A moderate probability for gales up to 40 kt over
outer waters.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory from 7 PM this evening to 1 PM EST
     Monday for CTZ002>004.
MA...Winter Weather Advisory from 7 PM this evening to 1 PM EST
     Monday for MAZ002>014-026.
RI...Winter Weather Advisory from 7 PM this evening to 1 PM EST
     Monday for RIZ001.
MARINE...Small Craft Advisory from 6 AM to 7 PM EST Monday for
     ANZ231>234-251.
     Gale Watch from Monday morning through Monday evening for
     ANZ235-237.
     Gale Watch from Monday morning through Monday evening for
     ANZ250-254>256.

&&

$$
SYNOPSIS...Belk/Doody
NEAR TERM...KJC/Doody
SHORT TERM...Doody
LONG TERM...Belk
AVIATION...Belk/Doody
MARINE...Belk/Doody



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