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-- Discussion containing changed information from previous version are highlighted. --
971
FXUS61 KBOX 180730
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
330 AM EDT Wed Apr 18 2018

.SYNOPSIS...
An upper level system moving into the Maritimes will bring dry
but cool weather today. A low pressure area from the Midwest will
produce a period of chilly rain potentially mixing with wet
snow during Thursday . Lingering snow showers Thursday night.
The weekend looks to be dry but with more below normal
temperatures with milder weather early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Upper low pressure moves from Maine into the Maritimes, but with
several shortwaves moving through the cyclonic flow. The most
significant shortwave moves off through the Gulf of Maine this
morning, with drier air moving in behind the system. Deep
moisture from overnight diminishes, and skies did clear over
much of Southern New England during the early morning. Sunshine
and lingering cold air aloft will allow clouds to redevelop
later this morning, but expect more sunshine than Tuesday. Can`t
rule out a stray shower, but the diminished moisture aloft will
work against that.

Mixing extends through 850 mb, which will bring gusts of 20-25
knots to the surface. Temperatures at that level -3C to -5C
support max sfc temps in the low to mid 50s...except mid to
upper 40s in the Worcester Hills and Berkshire East Slopes.

&&

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

Another shortwave moves across the northern tier of the USA,
crossing the Great Lakes Wednesday and approaching New England
early Thursday. This system redevelops south of New England
Thursday morning, and with a progressive axis to the upper
system we expect the surface system to steadily move past.

The upper system will be supported by a 100 knot jet with upper
divergence reaching New England between 06Z and 12Z. This will
enhance lift over our region, supporting precip through the day.
Strongest lift should focus around midday and early afternoon.

As to precip type: thermal profiles mostly suggest rain in our
area. Profiles are more marginal in northern Mass and over the
Berkshires, so a wet snow or rain/snow mix is more likely in
these areas. Once the surface system moves past late Thursday
and winds turn more from the northwest, colder air may move in
and change the rain to snow especially in Western/Central Mass
and Northern CT.

Snow accumulations would mainly be in the Berkshire East slopes
where 1-3 inches is possible in spots near the Berkshire County
line. The northern Worcester Hills would be another area of
concern.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
*/ Highlights...

 - Potential N/W snows Thursday night
 - Lingering showers for Friday
 - Gradual warm-up weekend into early next week, dry weather

Overview and model preferences...
New England finally gets a lobe of the S stream through early
next week. This is thanks to a merger of cutoffs between a
stalled cutoff near Labrador and the wave/cutoff associated with
low pres for Thu. While this final phased cutoff shifts E, New
England remains under cyclonic/cold airmass through most of the
weekend. However, reinforced ridging, fed in part by warm
advection from a cutoff across the SW CONUS continues to build,
forcing the cutoff E and allowing the ridge to take control into
the middle of next week. While the airmass is warmer, it has
been moderated in part by the N stream as well, given a strong
vortex near Baffin Island. Therefore, while temps through early
next week will warm, they are also likely to remain near normal
only going slightly above. Synoptic patterns are well agreed
upon, so a blend of deterministic guidance will be used as a
baseline for this forecast.

Details...

Thu night...
Low pres continues to deepen through the Gulf of Maine as it
shifts toward Nova Scotia. By Thu evening the strongest low-mid
lvl forcing will be shifting E, along with the peak moisture
(PWATS drop back below 0.50in through the overnight hours).
However, the merger of waves aloft continues into Fri morning
and may provide enough forcing to translate into continued
precip even as the column dries somewhat. Therefore, will likely
see at least some light precip linger into the early morning
hours especially across the E half of the region. Meanwhile,
lower lvl temps continue to drop as drier air filters in,
yielding wetbulbs below 0C. Rain will likely change to a period
of wet snow, first in the higher terrain, then even possibly
closer to the coastline. Accumulations will likely be relegated
to grassy surfaces, and highest in the higher terrain where the
change occurs earliest. Lingering QPF through early Fri is
generally below 0.2in, accumulations of SN should generally
remain around an inch or less.

Fri and Sat...
Cutoff continues to slowly shift E. Coldest air lags somewhat,
with H92 temps remaining above 0C through the day. Lingering
risk for clouds/showers with the cyclonic curvature aloft,
limited mainly by a continually drying column. Low risk for some
ocean enhancement mainly across SE MA. However, the clouds will
likely limit mixing somewhat, yielding highs in the mid 40s to
upper 40s. By Sat, a gradual transition to more anticyclonic
flow is evident, which should limit additional cloud
cover/shower risk. However, temps will still be limited as the
colder air remains locked in aloft. Highs mainly in the upper
40s and low 50s, still below seasonal normal values.

Sun into Wed...
High pres exceeding 1030+ settles across the NE. Each day,
temps continue do warm, mid to upper 50s Sun, followed by 60s
Mon through Tue as subsidence inversion may continue to limit
mixing. Also, with sfc high pres cresting at the sfc, weak flow
will allow for the development of coastal sea breezes.

&&

.AVIATION /08Z 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/...

Today... High confidence. VFR. West winds gusting to 25 knots,
possibly near 30 knots around Nantucket.

Tonight... High confidence through midnight, moderate confidence
after midnight. VFR through 5 AM with increasing/lowering sky
cover. MVFR cigs/vsbys develop in Western/Central Mass and
Northern CT as the morning rush starts. Rain develops, with snow
possible over the Berkshires.

Thursday... Moderate confidence. MVFR cigs lowering to IFR in
the morning. Vsbys lowering to IFR in rain and snow. Some
freezing rain is possible in Northwest Mass early Thursday
morning.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

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

Thursday Night: Mainly MVFR, with local IFR possible. Breezy.
Chance RA, chance SN.

Friday: Mainly VFR. Breezy.

Friday Night through Saturday: VFR. Breezy.

Saturday Night through Sunday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Thursday/...High confidence.

Today...West winds gusting 25-30 knots from mid morning through
evening. Seas 5 to 8 feet, mainly over the outer waters and RI
Sound. Small Craft Advisory remains in place for all except
Boston Harbor, Narragansett Bay, and Cape Cod Bay.

Tonight...Diminishing winds and seas. Lingering rough seas over
the southern and southeast outer waters. Small Craft Advisory
lingers in those areas.

Thursday... A coastal weather system brings rain to the waters,
especially the southern waters, with visibility of 1 to 3
miles. Winds remain less than 25 knots. Seas remain at 5 feet or
higher along the southern outer waters, but lower values
farther north.

Outlook /Thursday Night through Sunday/...Moderate

Thursday Night: Small Craft Advisory winds with gusts up to 25
kt. Areas of rough seas. Chance of rain.

Friday: Winds less than 25 kt. Local rough seas. Small Craft
Advisories linger.

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas locally approaching 5 ft.

Saturday through Sunday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 10 PM EDT this evening for
     ANZ232>235-237-250.
     Small Craft Advisory until 6 AM EDT Thursday for ANZ254>256.

&&

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