Area Forecast Discussion
Issued by NWS Boston, MA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS61 KBOX 060820
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
320 AM EST Tue Dec 6 2016

.SYNOPSIS...
Low pressure off the mid Atlantic coast may bring some mixed light
rain and/or snow into Southern New England Tuesday night into early
Wednesday. A cold front crosses the region late Thursday or Thursday
evening, with another cold front Friday. This will be followed by
blustery and unseasonably cold conditions Friday night into
Saturday. Low pressure may bring some snow or rain Sunday night
into Monday.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
1 AM Update...
Prior forecast still looking good overall. Dropped overnight
temperatures include forecast lows a little across north central
MA given current observation trends. Mostly clear skies and fresh
snow cover, even though only an inch or two, is helping to drop
temperatures rather sharply in that region. There continues to be
clouds from moisture picked up over the eastern Great Lakes
spilling over the Berkshire crest, but these should diminish
during the early morning hours as surface high pressure edges
toward New England.

1015 PM Update...
Some lake effect clouds have been spilling over the Berkshires
this evening and are expected to continue to do so until early AM
when surface high pressure and shift to more of a NW or even NNW
flow cuts off the moisture feed. Made some modifications to sky
grids and only very minor tweaks to temperatures as forecast
remains generally on track for the overnight.

Will freshen up the black ice SPS shortly.

630 PM update...
Lake moisture extending into western New Eng has resulted in some
lower clouds across western MA and northern CT. Otherwise, mostly
clear skies prevailed. Expect lake effect clouds to persist into
the early morning hours across western MA, then clouds should
erode as high pres builds in and flow weakens. Adjusted short
term sky grids, otherwise forecast on track.

Previous discussion...
Expect less radiational cooling tonight as pres gradient
increases with the approaching high and weak low pres development
associated with the trof. Most of this will be felt offshore, but
winds may remain 5-10 mph and mitigate temperature drop through
the early portion of the overnight.

Still temps will fall back into the 20s by the early AM hours
thanks to weak CAA. Will be issuing a special statement for black
ice, mainly for the interior as area roadways are unlikely to dry
completely before temps cool once again below freezing. This may
need to be expanded for areas that received less snow or all rain
should the roads be observed as remaining wet through the evening
hours.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/...
Today...
Mainly clear day to start although some CAA induced SC is
possible, lingering from the overnight hours. Temps should
moderate given the sunshine, thanks mainly to H92 temps still
between 0C and -2C. Expecting highs then to rebound into the low-
mid 40s. Winds will be light as they begin to veer from the NW to
the E by evening, in advance of another 2-wave system approaching
from the W and SW.

Tonight...Similar setup to last night as double-barreled system
approaches with dominant shortwave remaining in the N stream and
to the NW of S New England. Meanwhile, S stream wave initiates
strong cyclogenesis offshore, such that coastal low pres
strengthens and gradually shifts E, well S of the area by Wed
morning. Therefore, most of the impact comes from an inverted trof
and modest convergence between these two systems, rather than
either one themselves. PWATs increase to about 130-150 percent of
normal and modest lift is noted within the snow growth region as
the column moistens from the top down. Therefore we are monitoring
for the development of light snowfall late Tue night and
continuing into Wed AM, similar to this past event. More detail to
follow.

Temps...
Went lower than most guidance with temps as even with increased
upper lvl, then lower lvl clouds, some modest radiational cooling
during the evening could allow mins to drop into the upper 20s and
low 30s before precipitation onset. Afternoon dwpts are lower,
suggesting some wetbulb effect cooling is likely as the precip
shield approaches from SW to NE.

Snowfall/Rainfall...
With this combination of cool temps and wetbulb, combined with
modest moisture and some lift within the snow-growth regime, could
see the development of another round of light accumulating
snowfall across the region particularly during the AM hours Wed
morning. Lift/temps suggest snow ratios between 10-12 per inch
liquid, but rates will once again be light enough that only a
widespread coating to an inch can be expected by sunrise. This may
once again impact commuting traffic as some untreated roads could
become slippery.

Areas across W Ma and the higher terrain may relieve more along
the line of 1-3 inches, mainly due to better moisture convergence
under E flow. This flow also will keep areas near shore, likely
all rain thanks to warm SSTs. More information will come, but stay
tuned due to the possible impact this may have on the Wed morning
commute.

Wednesday...
Southern stream shortwave and attending coastal low pass well south
of New Eng. However, inverted trof extending to the north combined
with deep moisture axis will result in some light snow/rain
lingering into the morning. Much of any accum will occur before 12z
Wed. Then good mid level drying moves in from the west during the
afternoon resulting in dry conditions with partial clearing possible
toward evening.

&&

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

* Blustery, and unseasonably cold Friday night into Saturday
* Some snow/rain possible Sunday night into Monday

Thursday...A mid level longwave trough will pass by Thursday,
along with a surface cold front. There appears to be enough
moisture to result in abundant clouds, but perhaps not enough to
generate widespread rain and snow showers. Still looking like
steady west winds, which are generally not favorable for either
lake-effect or ocean-effect showers across southern New England.
As such, will keep this portion of the forecast mainly dry.
Seasonable temperatures expected.

Friday and Saturday...
This will be the most noticeable change during this portion of the
forecast. Unseasonably cold air should arrive Friday afternoon,
and really take hold Friday night into Saturday. Expecting
temperatures Friday night to be about 10 degrees below normal,
with high temperatures Saturday generally below our normal low
temperature, some 10-15 degrees below normal. Dry weather should
prevail across southern New England.

Gusty NW winds both days will result in wind chills in the single
digits and teens Friday night into Saturday. Saturday night lows
remain tricky, and will ultimately depend on extent of radiational
cooling. There remains potential for temperatures to be
significantly lower than the current forecast if it ends up clear
with winds decoupling.

Sunday into Monday...

Low confidence in the details of this portion of the forecast.
Significant spread in the track and timing of a low pressure
moving through the Great Lakes. GFS develops a stronger mid level
shortwave, while the ECMWF retains a nearly zonal flow over the
USA. Canadian model develops a weaker, more progressive,
shortwave than the GFS. Will maintain a chance of some rain and
snow across southern New England. However, these details are
likely to change over the next several days.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Overnight...High confidence. VFR most locations with a couple of
exceptions. MVFR ceilings will persist near the Berkshire crest
through 08Z or 09Z. Also, there will be some patchy dense fog in
some of the interior valleys.

Tuesday...High confidence. VFR. Winds NW in morning becoming N and
then NE during the afternoon.

Tuesday night...High confidence in trends, lower confidence in
timing. Similar setup to last night/this morning. VFR early gives
way to light snow/rain mix (mainly snow first with light
accumulations generally an inch or less). This transitions to rain
during the morning. With the arrival of precip, expect category to
drop to widespread IFR/MVFR. Winds mainly E.

Wednesday...Moderate confidence. Areas of MVFR/IFR in lingering
snow/rain Wed morning, then gradual improvement in the afternoon.

KBOS TAF...High confidence. VFR. Winds becoming N or NNW by around
daybreak.

KBDL TAF...High confidence. VFR.

Outlook /Wednesday Night through Saturday/...

Thursday...Moderate confidence. Mainly VFR cigs. Areas of MVFR
possible across the interior with a few snow/rain showers.

Friday and Saturday...Moderate confidence. Mainly VFR cigs.
Localized MVFR in isolated snow showers far interior MA Fri.

&&

.MARINE...
Forecaster Confidence Levels.

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

Short Term /through Wednesday/...

1 AM Update...
Have opted to keep Small Craft Advisories in place. MAREP
suggested that winds had eased across Vineyard Sound but later
reports from MVY showed some increase in gustiness. Hence, have
opted to keep the Small Craft Advisory hoisted there. Noting that
we are still getting gusts to around 30 knots just east of Cape
Cod.

Prior Discussion...

Overnight into early Tue...High confidence. W-NW winds increase
overnight as low pres shifts E of the waters and high pres moves
in to take it`s place. Gusts around 25 kt can be expected across
much of the waters as a result. Otherwise, this will lead to a
slight building of seas, reaching 5-6 ft on the ocean waters which
will linger into the early AM hours. Current SCAs seemed good, but
will extend outer waters as seas may take a bit longer to
diminish.

Rest of tomorrow and tomorrow night...High confidence.
High pres will slowly build from W-E across the waters leading to
diminished winds and seas below small craft advisory thresholds.
Some early morning rain is possible, mainly across the S and SE
waters.

Outlook /Wednesday Night through Saturday/...

Wednesday through Thursday...High confidence. Light winds and
seas.

Thursday Night...High confidence. Increasing WNW winds, with gusts
up to 30 kt possible. Small Craft Advisories may be needed for
most waters.

Friday into Saturday...High confidence. NW gusts to 30 kt likely.
There is a moderate risk for some gale force gusts, too. Winds
gradually diminishing Saturday. However, rough seas will take
longer to subside across the outer coastal waters.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 7 AM EST this morning for
     ANZ231>235-237-251.
     Small Craft Advisory until 10 AM EST this morning for ANZ250-
     254>256.

&&

$$
SYNOPSIS...Belk/Thompson
NEAR TERM...Thompson
SHORT TERM...Thompson
LONG TERM...Belk
AVIATION...Belk/Thompson
MARINE...Belk/Thompson


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.