Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 181203
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
703 AM EST Wed Jan 18 2017

.SYNOPSIS...
A mixed bag of wintry weather will continue to impact S New England
this morning, spotty showers lingering through the day into the
overnight hours. High pressure builds over the region beginning
Thursday and through the weekend. A weak cold front slips through
the high late Saturday, followed by Canadian high pressure and
slightly colder temperatures for Sunday. Developing low pressure
over the Southern Plains will bring rain and possibly some initial
ice to New England Monday, with the rain lingering into Tuesday.

&&

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

7 AM update...

One area of rain showers moving through RI and Southeast MA...
additional showers of rain and snow moving from NY State into
Western MA. Back edge of the showers lines up with axis of
northern shortwave moving east toward New England. This shortwave
crosses our area during the afternoon/evening. Expect pcpn to
diminish by that time.

No significant changes made to the forecast with this update.


Previous discussion...

Cold, dreary, overcast day. Breezy N/NE low-level fetch of moisture
on the backside of the immediate offshore deepening surface low.
Undergoing weak orographic ascent beneath a measure of mid-level
forcing brought upon by a sweeping H5 positive-tilted trough axis.
Precipitation output trending moderate to light likely continues,
especially over E/NE MA and up against the E-facing slopes of high
terrain.

There is, however, uncertainty on precipitation type, dependent on
surface temperatures and whether ice is present in the column. No
longer is there an issue of a warm-layer aloft as the column cools
within the low levels as colder air is drawn S behind the departing
low. It`s either rain, snow, or freezing rain.

Water vapor satellite presently shows the wedge of drier air moving
into S New England, however mid to upper level cloud decks, likely
composed of ice nuclei, are apparent out of the Great Lakes region
in association with a positively tilted trough sweeping into the
area. Also considering onshore E flow of marine influence and CCN
which could aid in the presence of ice and development of snow
wherever the column and surface temperatures are sub-freezing,
rather than freezing rain / freezing drizzle.

Overall, going to keep it simple. Mainly across N/NE MA it`ll be a
mix of rain or snow changing over to rain as we warm at the surface
and the low-levels cool aloft. Higher confidence in the presence of
ice nuclei. Over N/W MA, especially the Berkshires, will go with
freezing rain or snow combination. This approach given the overall
uncertainty of ice present in the column.

Precipitation becomes light throughout the day, so weather-types
will alter becoming drizzle, freezing drizzle, or light snow.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
Tonight...

Continued ascent of low level moist profiles ahead of weak mid-level
shortwave energy and associated positively tilted trough axis across
the region by morning. Stronger signal of drier mid to upper levels
and thus the lack of ice nuclei favorable towards snow development.
But still a good onshore flow and maritime influence. Temperatures
dropping overnight, there is the expectation over a good chunk of S
New England of transitioning back over to wintry precipitation types.

Precipitation still rather light given the weak ascent, will keep it
simply and go with freezing drizzle OR light snow for those areas
where temperatures are at or below-freezing. Confidence that the
window of freezing drizzle will be from sundown to roughly around
midnight. After midnight, another mid-level shortwave impulse should
yield the right ingredients to make the dominant precipitation type
as snow for any sub-freezing locales.

Will see the light precipitation move offshore towards morning with
winds reverting northwesterly. The drying process begins as cloud
decks become broken W to E.

Thursday...

High pressure and drier air working into the region. Will see clouds
become scattered W to E through the day allowing for sunshine prior
to sundown over a good portion of S New England. Continued NW flow.
Will see temperatures warm into the low 40s as heights rise across
the region and warmer air moves in from the W. H85 temperatures
nudge above freezing towards the end of the day.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Big Picture...

Upper ridge remains in control of the flow over the Eastern USA
Thursday night through the weekend. A couple of weak shortwaves move
through New England on Saturday. A more prominent shortwave currently
south of the Aleutian Islands will reach the Pacific coast Friday
and re-form over the Southern Plains Saturday night. The associated
surface storm moves to the Southeast Atlantic coast Monday, then
up the coast Tuesday.

Upper height contours remain higher/warmer than normal much of the
period, with heights dropping Monday night/Tuesday as the Southeast
storm moves up the coast.

Model mass fields are similar through Sunday but then diverge. Early
next week the Canadian GGEM attempts to develop not one but two lows
along the East Coast ahead of the GFS and ECMWF Sunday night, and
the GGEM shows a much more intense storm than the other models for
off our coast Tuesday.  The differences limit forecast confidence,
however impressive the storm is depicted.  With no clear favorite,
we will use a blend of model data.

Details...

Thursday night-Friday...

High pressure over the region will maintain dry weather. Lingering
moisture at 850 mb suggests some sky cover. Thursday night dewpoints
of 25-35 will make room for min temps in that same range. Mixing
reaches to 950 mb, with temps in the layer that will support max sfc
temps in the upper 30s and lower 40s.

Saturday-Sunday...

High pressure remains in control. One southern stream shortwave
slips through the ridge Saturday, while a northern stream shortwave
slide through the Maritimes Saturday night. These bring an increase
in moisture below 750 mb Saturday and Sat night, but little jet
dynamics to promote lift. A slightly colder high pressure area moves
in for Sunday. Expect partly sunny skies each day. Temps aloft
support max sfc temps in the 40s Saturday and mid 30s to low 40s
Sunday. Nightime temps continue in the mid 20s to mid 30s.

Monday-Tuesday...

The main concern for the long-term forecast period is, as noted
above, a system south of the Aleutians that may bring precip to
our area to start the week. The presence of Sunday`s high pressure
parked over the Maritimes suggests cold air lingering in the
interior. The thermal fields do show warming at all levels with
time, so any initial ice should eventually change to rain. We will
show chance pops for precip...based on the model differences and
the current distance of the parent system from the North American
upper-air network.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

Short Term /through Thursday/...Moderate Confidence.

Today...

MVFR-IFR mix with mostly RA/DZ across the terminals. FZRA/SN for
terminals N/W of the Mass Pike and I-495 corridor. N flow through
the CT River Valley while blustery E/NE winds along the immediate
E MA coast. Gusts up to 30 kts, especially across the Cape and
Islands.

Tonight...

Precipitation tapering off as DZ for most terminals, though FZDZ
or -SN is forecast for interior terminals, again the focus being N
of the Mass Pike and W of the I-495 beltway. Roughly speaking W
and Central MA. Winds turning more N/NW and diminishing into the
morning period. Will begin to see cigs become BKN W to E, lifting
low-end VFR. But most of the overnight period is characterized as
a mix of MVFR-IFR.

Thursday...

Conditions improving as cigs become BKN to SCT, lifting to low-end
VFR. NW winds continue.

Specific Terminals...

KBOS TAF...will hold rain across the terminal throughout the
duration of the TAF. With the E/NE onshore flow will keep cigs IFR
down around 800 ft agl into Thursday morning. Can not rule out
cigs lifting briefly at times up to 1500.

KBDL TAF...RA/DZ for the most part. Will be watching the overnight
period as to whether sub-freezing temperatures impact the terminal
thereby creating a risk of icing with FZDZ. Low confidence at this
time as the expectation is that NW winds will proceeding during
the overnight period with drier air allowing conditions to begin
to improve into morning.

Outlook /Thursday Night through Sunday/... Moderate confidence.

VFR through the period. Patchy MVFR cigs possible especially along
the South Coast.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Thursday/...Moderate Confidence.

Today...

Blustery E/NE winds which will push seas up to 8 to 10 feet on the
outer waters, especially SE. Gusts up around 30 kts. Can not rule
out gales for the SE waters so will continue with gale headlines,
though the threat seems lesser for the inner waters, so have
reverted inner water areas around Cape Cod back to small craft
advisories. Rain overall and visibility reductions at times.

Tonight into Thursday...

Winds turn out of the NW and taper. Drizzle will linger into
Thursday yielding some visibility restrictions out on the waters.
Otherwise waves dampen as conditions improve going into Thursday.
Will see see small craft advisories persist throughout the period
over the outer waters.

Outlook /Thursday Night through Sunday/... Moderate confidence.

North to northeast winds 15 knots or less. These become variable
Friday as high pressure moves overhead, then west Saturday and north-
northeast again Saturday night as a weak cold front sweeps through.
Seas of 5-6 feet Thursday night will subside as high pressure builds
overhead Friday.  Seas will be mostly below 5 feet during the
weekend, although areas of 5 foot seas may briefly return on the
outer waters Saturday and Sunday.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Winter Weather Advisory until 10 AM EST this morning for
     MAZ002>004-008-009-026.
RI...None.
MARINE...Small Craft Advisory until 7 PM EST this evening for
     ANZ230>234-236.
     Small Craft Advisory until 1 AM EST Thursday for ANZ235-237.
     Small Craft Advisory until 7 PM EST Thursday for
     ANZ250-251-256.
     Gale Warning until 4 PM EST this afternoon for ANZ254-255.

&&

$$
SYNOPSIS...WTB/Sipprell
NEAR TERM...WTB/Sipprell
SHORT TERM...Sipprell
LONG TERM...WTB
AVIATION...WTB/Sipprell
MARINE...WTB/Sipprell


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