Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 060341
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1041 PM EST Tue Dec 5 2017

.SYNOPSIS...
An approaching cold front will bring widespread showers with
embedded heavier downpours tonight along with a period of gusty
southerly winds and mild temperatures. Dry and seasonably cool
temperatures return Wednesday and Thursday behind the departing cold
front. Slightly below normal temperatures are anticipated Friday and
Saturday with colder weather Sunday into Monday. The seasons first
snowfall may occur late Friday into Saturday across southeast
Massachusetts as a series of weak lows track southeast of Cape Cod.
Only light snow accumulations are expected at this time.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...

1030 PM Update...

Heavy rainfall has entered southern New England this past hour.
The bulk of this rain appears to be associated with the 1000-900
mb frontogenesis as well as develop low level moisture. The
actually cold front which will swing through the region is still
back in upstate NY, and should be passing through BGM over the
next hour.

Ahead of all the rain, LLJ has strengthen with VWP indicating 60
kts at 925mb. This jet has mixed down to Blue Hill obs which has
gusted to 54 kts the past hour. Other places a portion of the
jet continues to mix down with gusts around 40-45 mph. As
surface temps continue to climb into the 60`s expect more mixing
down of the LLJ an anticipate more reports of 40-50 MPH.
Therefore the Wind adv appears to be on track for tonight as
highest confidence is across the I-95 corridor and portions
east.

Overall trend in the forecast remains on track as the bulk of
the rain currently will swing through over the next several
hours. Then the cold front will follow closer to the early
morning hours. Qpf values over 0.5 inches to near an inch are
still likely even though the nature of this system is very
progressive. Strong omega/lift combined with high PWAT values
will help in the higher rainfall amounts.

Continue to monitor this system tonight and will make updates as
they are needed.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Wednesday...

Frontal system quickly moves offshore Wed morning. Scattered showers
in the vicinity of the Cape and Islands come to an end by late
morning, with dry conditions and increasing sunshine to follow.
However despite the sunshine, strong cold air advection behind the
front is expected to produce nearly steady or falling temps during
the day.  High temps around daybreak for much of the region, in the
mid 40s-low 50s, except nearly steady temps in the 30s and low 40s
across the far interior. Gusty NW winds near 20 mph expected due to
strong mixing. Cold enough for ocean effect clouds though W/NW winds
will limit fetch, still may have some stratocumulus around on the
outer Cape/Nantucket.

Wednesday night...

Dry and seasonably chilly weather under a NW flow and mostly clear
to partly cloudy skies. Overnight lows in the 20s for most locales,
except some low-mid 30s along the shoreline.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
350 PM update...

Highlights...

* Dry and seasonably cool Thu
* Season`s first snowfall possible (albeit light) late Fri into Sat
* Temps a bit colder than normal Friday into early next week
* POTENTIAL for a significant coastal storm next Tue/Wed

Details...

Thursday...

Looks fairly pleasant for early December with forecast area in
between two frontal boundaries. Tonight`s front will be well
offshore by Thu and a trailing cold front remains across the eastern
Great Lakes into Ontario and Quebec. So a seasonably cool airmass in
place over southern New England with 850 mb temps around -8C and -2C
at 925 mb. Model soundings support mixing up to 850 mb or so and
this will support highs in the 40s except upper 30s high terrain,
which is near normal for this time of year. Modest CAA and pgrad
will generate west winds 10-20 mph. Model moisture fields suggest
SCT-BKN diurnal CU likely with more clouds across northwest MA/hilly
terrain and more sunshine in the coastal plain. Overall, typical
early Dec day with a mix of sun and clouds, modest west wind and
seasonably cool temps.

Friday/Saturday...

Deterministic and ensembles guidance beginning to converge on a more
positively tilted/sheared trough across the northeast for this time
period. This suggest a series of weak waves of low pressure tracking
offshore but possibly close enough to bring a period of snow to RI
and southeast MA, especially Cape Cod and the Islands from late Fri
into Sat. QPF looks to be a few tenths, so preliminary snow accums
looks like the risk for a few inches. However blyr temps are
marginal to support snow and given light qpf, snow intensity would
likely be light as well, suggesting daytime accumulations may be
confined to the colder surfaces. Thus expecting a low impact event
here. Farther inland the risk for snow accums diminish from
southeast to northwest. Although for whatever reason the GEFS
appears to far northwest with its qpf field given the positively
tilted/sheared mid level trough. Temps slightly colder than normal
given airmass, cloud cover and precip. Highs mainly in the upper 30s
and lower 40s, mid 30s northwest MA in the higher terrain. Temps
will be slightly lower while precip is falling.

Sunday/Monday...

Colder air arrives Sunday with trailing/secondary short wave trough.
Could be some snow showers with this trough given amplitude, some
mid level moisture and instability from falling H5 temps. Core of
cold air arrives Sun night with 850 temps falling to about -14C
across southern New England.

Tuesday/Wednesday...

Deterministic and ensemble guidance all agree on a very amplified
northern stream this time period as a piece of the polar vortex
dives southward into Ontario/NY state and finally New England. This
results in the POTENTIAL for strong cyclogenesis/coastal storm
somewhere along or off the New England coast. Obviously lots of
uncertainty/model spread on track and intensity (esp latitude of
intensification) ranging from an inside runner, coastal hugger to
benchmark - Gulf of Maine storm. However what is interesting at this
time range is that many of the 50 members from the EPS have
pressures down to the 970s and 980s at our latitude Tue ngt/Wed!
Again the focus here is on the POTENTIAL and not the specifics, thus
way too early to say much more.

&&

.AVIATION /04Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Wednesday Night/...

This evening and Tonight...Moderate confidence.

0030Z Update...

Not much change from previous TAFs. Heaviest rain and strongest
wind 03z-09z from west to east overnight. Earlier discussion
below.

=================================================================

IFR/MVFR conditions remain in place thru this evening with
periods of showers. Improvements from W to E during 06Z to 12Z.
LLWS into tonight with strong low level jet. Increasing winds
thru this evening and into the overnight ahead of approaching
cold front. Highest gusts will occur across eastern MA and RI
where gusts of 30 to 40 knots are anticipated tonight. Winds
diminishing towards 12Z.

Wednesday...High confidence.

VFR to start across the interior. Conditions improving to VFR
12Z-15Z along BOS-PVD corridor and points east. VFR conditions
for the remainder of the day. Westerly winds gusting around 20
kts.

KBOS Terminal...Moderate confidence in TAF. MVFR conditions
expected thru much of the overnight in showers. Chance for
periods of IFR tonight especially 06Z-10Z. Improvements around
12z Wed. South winds increasing to 30-40 kts overnight, then
diminishing towards 12Z. LLWS 22Z-06Z.

KBDL Terminal...Moderate confidence in TAF. Expect mainly MVFR
conditions with showers, with a period of IFR conditions in
showers tonight. Improvements by 09-11Z Wed. LLWS thru 06Z
tonight.

Outlook /Thursday through Sunday/...

350 PM update...

Thursday: VFR, dry and modest west winds. High confidence.

Friday: Mainly VFR with low risk of light rain/snow south coast
late. High confidence.

Friday Night: VFR except MVFR/IFR conditions possible with light
snow across southeast MA possibly into RI. Moderate confidence
with some uncertainty on ptype.

Saturday: VFR except MVFR and IFR southeast MA in snow/rain into
early afternoon. Moderate confidence with some uncertainty on
ptype and precip departure.

Sunday: Marginal VFR/MVFR with scattered snow showers possible.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Tonight and Wednesday...

SE/S winds this evening increase due to a strong LLJ. This will
help build seas to over 10 feet across the ocean waters
tonight. Gusts of 30-40 kts are likely this evening and into
the overnight hours, continuing Gale Warning. Southerly winds
shift to the west by Wednesday morning and begin to decrease,
with gradually subsiding seas. SCA will be needed for Wednesday.

Wednesday Night...SCA headlines may be needed. Winds less than
25 kts. Seas 5 to 7 ft on the outer coastal waters and RI/BI
Sounds, gradually subsiding.

Outlook /Thursday through Sunday/...Moderate to high
confidence.

Thursday...west winds 10-20 kt, dry weather and good vsby.

Friday...light winds with weak high pressure over the area.

Friday night/Saturday...a series of weak lows passing near or
southeast of the 40N/70W benchmark yielding light to modest N-NE
winds. Vsby lowering in rain and snow.

Sunday...increasing NW winds behind secondary cold front. Scattered
snow showers possible.

&&

.TIDES/COASTAL FLOODING...
Astronomical high tides will occur once again during
Wednesday. Winds leading up to and during the time of high tide
will be from the W/NW, offshore. Minor splashover may occur
during the midday high tide.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Wind Advisory until 7 AM EST Wednesday for MAZ006-007-013>024.
RI...Wind Advisory until 7 AM EST Wednesday for RIZ001>008.
MARINE...Gale Warning until 7 AM EST Wednesday for ANZ230>237-250-251-
     256.
     Gale Warning until 9 AM EST Wednesday for ANZ254-255.

&&

$$
SYNOPSIS...Nocera/NMB
NEAR TERM...Nocera/Dunten
SHORT TERM...NMB
LONG TERM...Nocera
AVIATION...Nocera/NMB
MARINE...Nocera/NMB
TIDES/COASTAL FLOODING...Staff



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