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Area forecast discussion 
National Weather Service Taunton Massachusetts
525 PM EDT Thursday Mar 26 2015

Synopsis...
an approaching cold front will bring periods of showers into Friday
morning. Weak low pressure may bring scattered snow showers Friday
night and Saturday. High pressure will push south of New England
Sunday. Another batch of precipitation is possible late Sunday
night into Monday ahead of a cold front.

&&

Near term /until 6 am Friday morning/...
***periods of showers with brief localized heavy rainfall tonight***

Numerous showers with brief locally heavy rainfall will continue
into early this evening in association with a middle level shortwave.
Marginal elevated instability will result in an embedded
thunderstorm or two. There then may be a bit of a lull in the
activity...but cold front dropping down into southern New England
will allow additional showers to re-develop. Brief locally heavy
rainfall may result in typical poor drainage nuisance street
flooding...but no significant flooding problems are expected.

There is a pretty good band of middle level frontogenesis behind the
actual surface cold front. Therefore...expect rain to persist right
through the overnight hours across much of the region. In
fact...colder air will begin to work into the region from the
northwest. If precipitation intensity is enough...dynamic cooling
may allow rain to change to a brief period of wet snow across the
high terrain of interior mass toward daybreak. Most of the guidance
suggests just a dusting to less than 1 inch of snow...which is in
line with our current thinking. However... the NAM model is most
robust with it actually showing the potential for a narrow swath of
2 to 3 inches of snow across the Worcester Hills. Given that
boundary layer is marginal...feel that is a low probability but we
can not completely rule it.

Finally...we issued a dense fog advisory for the South Coast/cape
and islands through midnight. South to southwest winds and
relatively high dewpoint air blowing over the cold ocean will allow
for areas of dense fog at times across this region. The loss of
daytime heating may also allow this to expand a bit...so felt it was
Worth an advisory. Winds shift to the north behind the cold front
after midnight...so visibilities should then improve.

&&

Short term /6 am Friday morning through 6 PM Friday/...
Friday...

Middle level frontogenesis behind the actual cold front will allow for
rain to continue across eastern New England Friday morning. As we
mentioned in the previous section...it may become just cold enough
across the higher terrain of mass for a change to wet snow before
precipitation ends. Things should dry out for most of the region by
afternoon as best forcing departs...but some lingering showers will
remain possible over the southeast New England coast. Across the
rest of the region...plenty of clouds will linger even though the
rain will have come to an end. High temperatures mainly in the 40s.

&&

Long term /Friday night through Thursday/...

Highlights...

* scattered snow/rain showers Friday night into Saturday evening
* weak cold front will move through on Monday bringing scattered showers
* moderating temperatures by the middle of next week

Overview and model preferences...

Long term amplified upper level pattern continues into the early
portion of the long term forecast. Beyond this weekend...medium
range models and ensembles continue to signal the long wave
troughing that has been nearly a constant over the last couple of
months across the eastern U.S. Will shift slowly east...but will
bring a fast northwest flow aloft early next week. This will bring fast
moving weak systems across New England. Timing is in question due
to fast movement lending to wide operational model variance along
with lowered confidence from late Tuesday through Thursday.

Used a blend of available guidance through Tuesday...then leaned
toward the superblend along with some blending with the ensemble
means to try and resolve timing of systems toward the end of the
forecast period.

Models and ensembles continue to signal fast flow aloft with more
waves moving along into fast flow aloft into Easter weekend which
will need to be monitored.

Details...

Friday night through Saturday night...moderate confidence. Cold
front works through early Friday night as front weakens.
However...still good lift and forcing as upper trough works east out
of New York state overnight. This...along with cold pool aloft...will
cause light rain showers to change to snow showers overnight.
Generally a tenth of a inch or less of quantitative precipitation forecast expected...so not
expecting much in the way of accumulations at this point.
However...

Most models and ensembles continue to signal development of
inverted trough late Friday night through midday Saturday as another
low passes well S of the 40n/70w benchmark. With the low level
moisture in place along with steep lapse rates will allow for
enhancement of rain/snow showers. Expect precipitation to remain mainly
as light snow... though temperatures might rise enough /mid-upper 30s/ to
mix/change to rain around midday but change back snow before
tapering off from north-S late in the day or Sat evening.

Big question at this point will be whether and...if so where...
the inverted trough will set up to focus the instability and
precipitation. Generally...expect less than an inch. But...if this does
set up...some locations might see a slushy 1-2 inch accumulation.
Low confidence with this aspect but something to definitely keep
an eye on.

Overnight lows Friday night will be drop to the Lower-Middle 30s across
the higher inland terrain...ranging to around freezing along the S
coast. However...with cold air advection along with precipitation...temperatures
will not rise much on Sat. Highs will only rise to the middle and
upper 30s...more typical of January rather than the end of March.

Conditions will improve Saturday night for most areas...though
some leftover light snow showers might linger along the S coast. North
winds will be gusty...especially across the cape and islands.
Overnight lows will fall to the middle and upper teens inland ranging
to the upper 20s on the Outer Cape and Nantucket.

Sunday-Sunday night...high confidence.
As upper trough finally moves offshore...expect high pressure ridging
at the surface and aloft will bring sunshine across the region. As
fast northwest flow aloft sets up...ridge will build east during the
afternoon and clouds will begin to filter in. Will see some warm
air advection work in as winds back to W-SW...but not in time for
temperatures to recover so highs will only be in the upper 30s to middle
40s.

Temperatures will drop a few degrees Sun night as clouds continue to
increase. Next fast moving short wave in the upper flow
approaches. Timing is an issue...though looks like most of the
model suite holds precipitation off until Monday except for possibly a
few isolated snow showers toward daybreak across the east slopes of the
Berkshires.

Monday-Monday night...moderate confidence.
Short wave moves across bringing a period of snow showers
early...changing to rain showers by midday as the front crosses
the region. Temperatures will moderate ahead of the front. Expect highs
mainly in the middle 40s to around 50.

Some leftover rain showers will linger across central and eastern
areas Monday night before pushing offshore as front clears the coast.
Upper level short wave also pushes east with northwest flow kicking back in.
However...core of 850 mb cold air remains north of the region. Will still
see cooler temperatures...but lows will only fall back to the upper 20s
to middle 30s as skies become partly cloudy after midnight.

Tuesday through Thursday...low confidence.
Fast flow aloft continues...bringing one system across. With wide
timing issues amongst individual models...have rather low
confidence. For now...expect dry conditions Tuesday...then possible
showers Tuesday night into Wednesday. Looks to dry out again by midday Wednesday.
Warm front may bring clouds and the chance for precipitation Wednesday
night/Thu.

&&

Aviation /21z Thursday through Tuesday/...
forecaster confidence levels...

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

Short term...this afternoon through Friday...

This afternoon and tonight...moderate confidence. Periods of
showers along with an embedded thunderstorm or two expected through
tonight. Mainly IFR conditions in lower clouds along with areas of
fog...especially along the South Coast where it may be locally
dense. Low level wind shear is also expected given low level jet around 60 knots.

Friday...moderate to high confidence. Rain showers should come to
an end for most of the region during the morning...although may
linger a bit longer across the far southeast Massachusetts coast. Ptype may
actually change to a bit of wet snow before ending across the higher
terrain...but odds of any real accumulations on runways are rather
low. Otherwise...conditions should improve to mainly VFR by
afternoon although some marginal MVFR ceilings may persist at times.

Kbos taf...moderate confidence in taf.

Kbdl taf...moderate confidence in taf.

Outlook...Friday night through Tuesday...moderate confidence.

Friday night and Saturday...mainly MVFR ceilings/local MVFR-IFR visibilities
in rain and/or snow showers Friday night. Expect light snow Friday night
after midnight into Saturday morning. Less than an inch of slushy
snow accumulation expected...best chance at korh as well as
remainder of Worcester Hills. Precipitation briefly changes to -shra
around midday...then back to -shsn toward nightfall.

Saturday night...local MVFR ceilings/visibilities across north CT/RI/se Massachusetts early
in scattered snow showers before ending from north-S by midnight.

Sunday and Sunday night...VFR conditions. Clouds increase from
west-east Sun night...may briefly drop to MVFR across higher terrain of
west Massachusetts.

Monday-Tuesday...mainly VFR. Local MVFR ceilings/visibilities in scattered rain
showers. Chance of -shsn across central-west Massachusetts/north CT through midday
Monday. Conditions improve from west-east Monday night...VFR on Tuesday.

&&

Marine...
forecaster confidence levels...

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

Short term...tonight and Friday...

Tonight...moderate confidence. South to southwest wind gusts of
20 to 30 knots expected this evening ahead of an approaching cold
front. High dewpoint air will also result in areas of dense fog.
Seas will build to between 5 and 9 feet across the outer-waters.
Winds shift to the north behind the cold front between 6 and 12z.

Friday...moderate to high confidence. Northerly winds of 10 to 15
knots with gusts to 20 knots expected behind the cold front. Left
over south to southeast swell will keep seas above small craft
thresholds across most open waters through Friday afternoon.

Outlook...Friday night through Tuesday...

Moderate confidence.

Friday night....N-NW winds gusting up to around 25 knots behind
frontal passage. Seas also build up to 6-8 feet on the outer
waters. Small crafts needed.

Saturday and Saturday night...northwest wind gusts may approach 30 knots on
the eastern outer waters during Sat...then slowly diminish Sat
night. Seas may approach 10 feet on the eastern outer waters
Sat...then will slowly subside but remain at or above 5 feet. Brief light
freezing spray possible on portions of eastern and southern outer
waters Sat night.

Sunday...expect winds and seas to briefly diminish below small
craft criteria as winds back from northwest to SW Sunday night. Seas
also subside below 5 feet.

Monday-Tuesday...SW winds pick up during Monday...gusting to 25
knots with seas building to 5-7 feet. Winds shift to west Monday night-Tuesday
with local gusts to 25 knots on the outer waters. Seas remain at or above 5
feet on the southern waters as well as portions of the eastern
waters.

&&

Hydrology...
basin average rainfall amounts will range from 0.5 to 1.25 inches
across the region through Friday morning. Nohrsc modeling adds 1
to 2 inches of water from melting snow due to dewpoints in the 40s
to near 50.

This will bring rises on rivers and streams through Friday...
especially smaller ones which may approach bankfull. However...
given rainfall has been spread out over a long duration with many
dry periods do not expect any significant flooding problems. Brief
heavy rainfall in showers may result in typical nuisance poor
drainage street flooding tonight...but again nothing significant
expected.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...dense fog advisory until midnight EDT tonight for maz020>024.
Rhode Island...dense fog advisory until midnight EDT tonight for riz005>008.
Marine...Small Craft Advisory until 6 am EDT Friday for anz232>234.
Small Craft Advisory until 2 am EDT Friday for anz231-236.
Small Craft Advisory until 8 PM EDT Friday for anz235-237-250-
254>256.

&&

$$
Synopsis...Frank/evt
near term...Frank
short term...Frank
long term...evt
aviation...Frank/evt
marine...Frank/evt
hydrology...Frank

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