Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 182115
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
415 PM EST Mon Dec 18 2017

.SYNOPSIS...
Light snow with the possibility of occasional freezing drizzle
across the north this evening, drying everywhere thereafter.
Seasonably mild conditions becoming breezy tomorrow. Blustery
and colder conditions will move in Wednesday and continue into
Thursday. Low pressure will like passes west of the region
Friday night and Saturday, which could bring mainly rain and a
period of rather mild temperatures along with gusty winds.
Another period of unsettled conditions may approach late
Saturday night into Christmas Eve, but timing and precipitation
types remain uncertain.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Weak warm front continues to struggle scouring out the cold air
which has remained entrenched across S New England, although
recently, mainly in the past few hours, have noted a marked
increase in sfc dwpts particularly within about 20nm of the S
coast suggesting it is starting to shift N. Modest drainage
flow, partially thanks to a weak meso-low pres on the new
GOES-E is helping to draw some of the colder air across the MA
NH/VT border. This has combined with a slight increase in lift
through the lower column to enhance saturation in the low lvls
and produce some light precip, starting as snow initially.

Through about 03Z (10PM local), as the warm front attempts
advancement, a modest warm nose is apparent around H95, which
allows temps at this lvl to reach +1C to +2C. This should be
enough, even as dry air over tops, to allow for a change to
light RA/DZ. Across the N tier, where dwpt depressions remain
the largest, temps should remain a few degrees below freezing
yielding some light FZRA/FZDZ this evening. This will be most
likely along and west of the I-495 corridor in MA, particularly
portions of N Worcester, Middlesex and Essex counties away from
the coast. Will be continuing an SPS for a light icing risk
during the evening commute until dry air infiltrates the warm
nose.

The dry air aloft will slowly make its way toward the sfc during
the overnight hours, but given trapped near sfc moisture as the
sfc reaches near wet-bulb, could be pockets of fog or even
freezing fog which may require an overnight expansion of SPS for
black ice.

Temps near to slightly above freezing this evening, based in the
warm frontal approach. In essence little diurnal cooling this
evening.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
Tomorrow...
Cold front linked to N stream shortwave will be sliding across
the region tomorrow. Although drier/cooler air will be gradually
filtering in, there remains enough lower lvl moisture for SCT-
BKN clouds on occasion through the day. With a slight
W-component to the wind, also could see some light SHRA/SHSN in
the E slopes of the Berkshires with the frontal passage, but the
risk for precip further E remains low thanks to
downsloping/drying. By late afternoon, LLJ reaching 35-40 kt at
H92 and near 50 kt at H85 should be tapped somewhat, suggesting
gusty W winds following the fropa. Temps will be warm
comparatively speaking thanks to H92 temps reaching +2C to +3C
before the late afternoon fropa. Looking at highs reaching the
low-mid 40s.

Tomorrow night...
W winds remain elevated thanks to an evening increase in sfc
pres gradient. This should keep winds elevated enough during the
overnight hours, that, in spite of clearing skies with a drying
column, full decoupling looks unlikely. Min temps drop into the
low-mid 30s.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Highlights...

* Blustery and colder Wed into Thu with dry weather
* Mainly rain Fri night/Sat with some wind and mild temps favored
* Unsettled weather possible sometime Christmas Eve into Christmas

Pattern Overview/Confidence...

12z guidance is in agreement with the overall pattern. Still some
disagreement in exact timing and location of each wave that
approaches over the weekend into early next week. Models are locked
on to a significant amplification pattern with stout ridge building
from the West Coast all the way up into Alaska. Which will result in
a mean trough from the Desert Southwest all the way to the Hudson
Bay. This set-up will result in a sharp thermal gradient across the
southern tier states up into the Northeast, where several waves will
ride along. Biggest concern is if southern New England reside on the
cold side or the warm side. Right now with the subtropical ridge
strongly building, appears the region will be along the warm side
of the gradient resulting in above average temperatures for the
weekend. Still a lot of uncertainty so stay tuned to the forecast.

Details...

Wednesday into Thursday...High Confidence.

Exiting upper level jet will move offshore as high pressure from the
north builds into the region. Strong NW winds will result in CAA to
move into the region during the day on Wednesday. Expect highs to be
near of slightly below average as temps reach into the mid 30s to
low 40s. Blustery northwest winds will continue through much of the
day thanks to strong mixing, so it may feel a lot cooler.

High pressure will continue to build north of the region resulting
in clearing skies and winds becoming relax during the overnight.
This will aid in radiational cooling with low temps dropping back
into the teens to single digits.

Southern stream shortwave and low pressure system will remain well
to the south of the area by early Thursday morning thanks to high
pressure over the region. Regardless, cold airmass will remain
overhead as high temps will be in the upper 20s to low 30s. Repeat
performance and for chilly lows during the overnight thanks to
radiational cooling.


Friday into Saturday...Moderate Confidence.

Warming trend will begin during this period as upper level trough
begins to take shape out west and subtropical high strengthens to
our southeast. Developing low pressure system across the Midwest
into the Great Lakes will bring a weak warm front through on Friday
with passing cold front on Saturday. Thermal temperatures will be
the main question as this system continues to trend a bit west
keeping the region in the warm sector. However with high pressure to
the north and weak flow, cold air could remain trapped at the onset
of precip leading to some wintry mix/ice for late Friday into
Saturday. The thermal profile will warm by Saturday thanks to stout
LLJ at 925mb. In fact, both GFS and EC are in agreement with 850mb
temps warming to 8-9C for Saturday. Thus expect a transition day on
Friday as temps begin to warm into the 40s, with upper 50s to
possible 60s on Saturday.

Christmas Eve into Christmas Day...High confidence in trends, lower
confidence on exact details.

After the first wave on Friday into Saturday pushes through, a break
in the precip Saturday night into Sunday morning. Baroclinic zone
will be set-up over the region as Arctic airmass remains across much
of the Plains into the Midwest with subtropical ridge to our
southeast. Guidance continues to show a few waves of low pressure
moving along this boundary resulting in the risk for precip during
the this period into early next week. Timing/ptypes and will depend
on exactly where the baroclinic zone aligns. Current trends in the
guidance as well as ensembles and CIPS analog`s is less snow and
more of a rain/ice scenario as thermals will be on the warmer side.
Given the strength of southeast upper level ridge. If we do have
enough cold air for wintry precipitation, it might end up coming
down shallow given rather high height fields which could possibly
result in more of an ice threat than snow. Although we all would
like a White Christmas, appears that the potential is quickly
decreasing.

&&

.AVIATION /21Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Tuesday Night/...

Through tonight...High confidence.
Mainly MVFR due to CIGS through this evening. Occasional light
SHSN and even some light FZRA possible, especially SE MA this
evening. Ice accums around a T where it occurs. Otherwise, some
fog/freezing fog possible overnight away from the coast,
dissipating during the morning hours as winds shift to the SW
and CIGS lift to VFR.  Timing may be off by a few hours in TAF.

Tomorrow and tomorrow night...High confidence.
Mainly VFR, although with some occasional FEW-SCT MVFR clouds
during the day tomorrow. Main issue will be W-SW breezes which
could gust to around 20-25 kt especially late day and overnight,
peaking near shore where gusts may approach 30 kt tomorrow
night.

KBOS Terminal...High confidence. MVFR CIGS return through this
evening. FZ DZ should stay mainly NW of the terminal, but there
is a low risk this evening, mainly between 23Z and 04Z if it
were to happen.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Wednesday through Saturday/...

Wednesday: VFR. Windy with gusts to 35 kt.

Wednesday Night: VFR. Windy with local gusts to 30 kt.

Thursday: VFR. Breezy.

Thursday Night: VFR.

Friday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance RA.

Friday Night: Mainly MVFR, with local IFR possible. Breezy. RA
likely, FZRA possible.

Saturday: Mainly MVFR, with local IFR possible. Windy with
gusts to 30 kt. RA likely.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...High confidence.

Tonight through mid day tomorrow...
Winds and seas remain below small craft advisory thresholds out
of the SW.  Light rain showers possible this evening.

Tomorrow afternoon and overnight...
Gale force winds pick up during the afternoon out of the west
with gusts 35-40 mainly on the outer waters, 25-34 kt near shore
and within the sheltered bays/harbors. Gale warnings will be
issued, along with Small Craft Advisories for more sheltered
locations, although these may need to be added to Gale warnings
with later updates.

Outlook /Wednesday through Saturday/...

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

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

Thursday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft.

Thursday Night through Friday: Winds less than 25 kt.

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

Saturday: Low risk for gale force winds with gusts up to 35 kt.
Rough seas up to 16 ft. Rain likely.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning from 4 PM Tuesday to 4 AM EST Wednesday for
     ANZ231>235-237-251-255-256.
     Small Craft Advisory from 1 PM Tuesday to 7 AM EST Wednesday
     for ANZ230-236.
     Gale Warning from 4 PM Tuesday to 7 AM EST Wednesday for
     ANZ250-254.

&&

$$
SYNOPSIS...Doody/Dunten
NEAR TERM...Doody
SHORT TERM...Doody
LONG TERM...Dunten
AVIATION...Doody/Dunten
MARINE...Doody/Dunten



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