Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 182106
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
406 PM EST Wed Jan 18 2017

.SYNOPSIS...
High pressure builds over the region Thursday through Saturday.
A weak cold front slips through the region late Saturday, followed
by Canadian high pressure and slightly colder temperatures for
Sunday. A significant storm system may impact New England Monday
into Tuesday with heavy rain and coastal wind but some interior
snow and ice is possible.

&&

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

Have kept the freezing drizzle advisories going for now. Temps in
those areas (Worcester, interior Essex, and the east slopes of the
Berkshires) are still hovering around the 32 degree mark, though
it appears mainly across the higher terrain. Also see some spotty
freezing rain, but precip is light so will likely see
predominately drizzle or freezing drizzle through the afternoon.

Have some scattered showers that have developed across E Mass
into N RI over the last hour or so on KBOX 88D radar imagery,
which will continue to move slowly E-SE as low pressure passes
offshore well SE of Nantucket. Also note the cold air damming
across NE/Central and W Mass on the 18Z surface map with high
pressure across the Maritimes. However, appears this is rather
shallow, hence the reported temps near or just below freezing were
mainly across the higher terrain into interior NE Mass, somewhat
closer to the better damming. Temps across the coastal plain
mainly around 40, with readings in the mid and upper 30s across N
CT.

Will see temps start to slow fall over the next few hours as we
approach sunset. Suspect more -FZDZ will develop as temps fall to
freezing or below inland. Also see more scattered precip moving
out of the Hudson Valley which will help in bringing a light
wintry mix into the CT valley through the remainder of the
afternoon.

Have updated near term grids to bring conditions current and
incorporated into the remainder of the afternoon forecast.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY NIGHT/...
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 /FRIDAY THROUGH WEDNESDAY/...
Highlights...

* Mainly dry with above normal temps Fri/Sat
* Low risk of some spotty light rain or freezing rain Fri night
* Heavy rainfall and strong coastal wind possible Mon into Tue with
  some snow/ice possible in the interior

Overview...

Mid level ridging will build north along the east coast into the
weekend resulting in mainly dry conditions with above normal temps.
However, piece of energy breaking off from central CONUS trof will
weaken as it moves into New Eng Fri night and may bring some light
precip. Attention will then focus on a significant southern stream
system moving across southern and SE CONUS then lifting north along
the eastern US. Still lots of uncertainty given the time range but
potential exists for a significant QPF event with strong coastal
winds sometime Mon/Tue timeframe. Ptype mainly rain along the coast
but interior ice/snow is possible depending on the track of the
sfc low and positioning of the downstream ridge.

Details...

Friday into Friday night...

High pressure will bring dry weather and light winds on Fri. Model
cross sections indicate enough low level moisture to support a
decent amount of cloud cover and it is possible clouds could become
trapped under a low level inversion as mixing will be limited. Temps
slightly above normal.

Then Fri night, a weakening shortwave will move into the ridge
across New Eng and may bring some light precip to mainly SW New Eng
as the column moistens. Low level temps will be critical and some
light freezing rain is possible in the interior if there is precip
around, otherwise ptype mainly rain. It appears best chance of any
freezing rain will be across W MA into the hills in N CT where
moisture is deepest and temps cold enough.

Saturday into Sunday...

High pressure remains in control. Mild day Sat with temps in the 40s
then cooling Sun as Maritimes sfc high ridges south into New Eng.
Cross sections indicate abundant low level moisture both days which
suggests mostly cloudy skies. There is a low risk deeper overrunning
moisture to the south could approach SNE late Sun but odds favor dry
weather holding on through Sun.

Light precip may move into the region Sun night and ptype would be
an issue with sfc high to the N/NE providing a source of cold air
with some snow and/or mixed wintry precip, especially interior.

Monday into Tuesday...

Low confidence forecast in details as there is considerable
uncertainty with the track of southern stream storm and positioning
of downstream sfc high. Highest confidence in a significant QPF and
possible coastal wind event as 850 mb easterly jet anomalies near 3-
4SD and anomalous PWATs which is quite impressive at this time
range. Also, GEFS ensembles and EPS both show modest probs of
greater than 2 inches QPF with individual members far exceeding
that. Also, high probs of strong coastal easterly winds. The
greatest uncertainty is in ptype, especially interior, and timing of
heaviest precip. Along the coast, it will be difficult to lock in
deep cold air given long fetch of onshore flow so precip likely
mainly rain. However, ptype more problematic in the interior as some
of the deterministic and ensemble guidance showing potential for
significant ice and/or snow. Variability in model solutions is
likely until energy reaches the west coast this weekend so all
scenarios remain in play.

Astronomical tides are quite low early next week which will limit
coastal flood potential. However, given potential duration and fetch
of easterly flow, there could be enough surge and wave action for
some minor coastal flooding if highest surge and waves can coincide
with high tide.

Wednesday...

GFS/ECMWF indicating mainly dry weather but low confidence forecast.
Timing of Mon/Tue event will determine sensible weather next Wed.

&&

.AVIATION /21Z WEDNESDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

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

1820Z update...

Through 00Z...

Mainly IFR-LIFR CIGS/VSBYS across central and eastern areas as
well as the CT Valley in areas of fog, drizzle and patchy freezing
drizzle, with some spotty -SN across interior NE Mass. Some
improvement to MVFR across portions of N Central and W Central
Mass. Light north flow through the CT River Valley into N central
and NE Mass, while E-NE winds along the immediate E MA coast with
gusts up to 25 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 /Friday through Monday/... Moderate confidence.

Friday...Moderate confidence. Mainly VFR, but areas of MVFR cigs
possible.

Friday night into Sunday...Moderate confidence. Areas of MVFR/IFR
cigs possible at times.

Monday...Moderate confidence. MVFR/IFR in coastal rain and
mixed precip interior.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

120 PM Update...

This afternoon...

Small craft advisories remain across the waters. Report from pilot
boat at 1815Z reported seas of 7-10 ft in Mass Bay, with 3-6 feet
on Nantucket Sound. Will continue to see E-NE winds gusting to
25-30 kt, with the higher seas across eastern open waters as well
as the northern portion of Cape Cod Bay and eastern Nantucket
Sound. Scattered showers and areas of fog, with some 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 /Friday through Monday/...Moderate confidence.

Friday into Saturday...High confidence. Light winds as high pres
builds over the waters. Lingering 5 ft seas outer waters early Fri,
then subsiding.

Sunday...Moderate confidence. Increasing NE winds with gusts to 20
kt in the afternoon increasing to 25-30 kt Sun night. Vsbys lowering
Sun night in rain.

Monday...Low confidence. Potential for strong easterly gales. Poor
vsbys in rain and fog.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Freezing Rain Advisory until midnight EST tonight for
     MAZ002>006-008>010-012-026.
RI...None.
MARINE...Small Craft Advisory until 1 AM EST Thursday for ANZ231-232-
     235-237.
     Small Craft Advisory until 10 PM EST this evening for ANZ230.
     Small Craft Advisory until 7 PM EST this evening for ANZ236.
     Small Craft Advisory until 6 AM EST Friday for ANZ250-251-
     254>256.

&&

$$
SYNOPSIS...KJC
NEAR TERM...EVT/GAF
SHORT TERM...Sipprell
LONG TERM...KJC
AVIATION...KJC/EVT/GAF
MARINE...KJC/EVT/GAF



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