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Area forecast discussion 
National Weather Service Taunton Massachusetts
803 PM EDT Tuesday Apr 21 2015

Synopsis...
a cold front will bring a round of scattered to numerous
showers...and perhaps a few thunderstorms Wednesday afternoon and
evening. Unseasonably cool weather follows Thursday through this
weekend with spotty showers from time to time.

&&

Near term /until 6 am Wednesday morning/...

745 PM update...
diurnal clouds tending to melt away with sunset...though some
broken clouds lingering across central Massachusetts/NE CT into Rhode Island at 23z.

Monitoring clouds and scattered showers across New York state...which are
shifting slowly east while moving NE in the SW flow aloft ahead of
approaching 500 mb cutoff low. This may delay the leading edge of
precipitation a few hours overnight...so kept forecast mainly dry through
midnight before bringing in slight chance probability of precipitation into the east slopes
of the Berkshires. Also noting wide temperature/dewpt spreads through
most of the night so will be tough to get light precipitation to make it
down unless temperatures drop a bit faster than currently forecast.

Have updated grids to bring conditions current and incorporate
latest thinking for this evening/S update.

Previous discussion...
a shortwave moves across the region overnight. However...moisture
and instability is quite limited in the wake of today/S cold
frontal passage. Therefore mainly dry weather expected...but can/T
rule out a low risk of an isolated spot shower.

Otherwise...skies should be partly cloudy with low temperatures mainly in
the 40s...with some middle to upper 30s possible across the
normally coolest outlying locations...especially in northwest Massachusetts.

&&

Short term /6 am Wednesday morning through Wednesday night/...
***high confidence in scattered to numerous showers Wednesday
afternoon/evening***

***low confidence on whether we see any thunder/small hail/gusty
winds Wednesday afternoon/evening***

Wednesday and Wednesday night...

While a few showers are possible Wednesday morning...much of the
morning probably ends up dry as forcing will still be to our west.
Things get a bit more interesting Wednesday afternoon and evening as
a potent shortwave/cold front approach the region. High confidence
that scattered to numerous showers develop Wednesday afternoon and
continue into the evening. In fact...feel most locations will see a
period of rain showers so will continue with likely probability of precipitation.

The biggest uncertainty is whether or not will be able to muster
enough instability for any thunder/small hail or gusty winds. Its
quite cold aloft with very steep middle level lapse rates on the order
of 7 to 8 c/km. Wind fields aloft are quite impressive as well.
However...there is one big and key ingredient missing which is
instability. If we can manage a few hundred j/kg of cape it might
be enough to result in isolated thunderstorms. Given cold air aloft
would not be surprised to see some graupel/small hail and gusty
winds with heavier showers even if we do not end up seeing thunder.
Also could end up with just rain showers...given very marginal setup.

For now we went with a slight chance of thunder with the risk of
small hail/gusty winds in the forecast. Regardless...does not look
like will have enough instability for severe weather. Bulk of the
activity should end Wednesday evening...but a few showers may linger
after midnight given strong shortwave.

As for temperatures...it will be breezy Wednesday afternoon but
highs should reach from near 60 across the higher terrain to mainly
lower to middle 60s elsewhere.

&&

Long term /Thursday through Tuesday/...
highlights...

* unseasonably cool Thursday-sun with widely scattered showers at times
* uncertainty in overall weather pattern early next week

Overview and model preferences...

12z model suite and ensembles continue to signal cooler than normal
temperatures through this timeframe. Blocking pattern across the
lower 48 seen...with cutoff 500 mb low pressure across the northeastern
U.S. As well as near or just off the Pacific northwest coast. With cold
pool overhead through this weekend...looks like there will be spotty
showers mainly during the daylight into the early evening hours each
day. Pinning down locations of any precipitation will be tough though due
to track of any short wave that moves along in the upper west-northwest flow.

Models try to slowly push the upper low east toward the Maritimes
around the late sun or Monday timeframe...but model timing is in
question. Used a blend of available guidance for this forecast...
which showed good continuity with the previous forecast.

Details...

Thursday through Sunday...moderate confidence.

The combination of the cold pool sitting across the region...any
weak short waves that move around the base of the upper low to the
north as well as diurnal heating allowing for increasing lapse low
level lapse rates...scattered showers should develop mainly in the late
morning through the evening hours on most days through this
weekend. While there is a decent shot of shower development...a
lot of questions where the light rain will fall. Looks like best
shot of precipitation will be both Thursday and Friday especially away from the
coast.

Some question as to whether the upper low will start to migrate east
late sun or Sun night. Timing is still in question...but have
lowered probability of precipitation though did keep slight chance going mainly near and north
of the Mass Pike.

Temperatures will continue to run on the cold side...on the order of 5 to
10 degrees below seasonal norms for late April. Highs will mainly be
in the 50s...with a few upper 40s seen across the higher inland
terrain.

Monday-Tuesday...low confidence.

Wide variance with model solutions as well as the ensembles on
timing the upper low far enough to the east to bring drier conditions.
It does look like...for now...will see a mainly dry day on Monday
but still can not rule out a stray shower or two mainly across north Massachusetts.
As for Tuesday...a lot of questions as to whether another cutoff
upper low may work into the southeast u... may try to work NE. Models
having a lot of trouble tracking this system so very low confidence
during this timeframe.

&&

Aviation /00z 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/...

Overnight...high confidence. Mainly VFR conditions.

Wednesday and Wednesday night...high confidence that VFR conditions
dominate. However...a cold front will bring a band of scattered to
numerous showers Wednesday afternoon into Wednesday evening
resulting in briefly lower cigs/vsbys. A few isolated thunderstorms
can not be ruled out...which may contain localized gusty winds/small
hail graupel but confidence in that is low.

Kbos taf...high confidence in mainly VFR through Wednesday morning.

Kbdl taf...high confidence in mainly VFR through Wednesday morning.

Outlook...Thursday through Sunday...

Thursday through Saturday...high confidence. Mainly VFR. Conditions
may lower to MVFR quickly in a brief diurnally driven widely scattered
showers mainly during the late morning into the evening hours each
day.

Sunday...high confidence. Mainly VFR. May see a brief shower or two
across northern and eastern Massachusetts during the midday and afternoon
hours. Low probability of brief MVFR conditions in any showers.

&&

Marine...
forecaster confidence levels...

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

Short term /tonight through Wednesday night/...

Tonight through Wednesday night...moderate to high confidence. Left
over southeast swell will continue to generate small craft seas
across most of our open waters into Wednesday night. Will also hoist a
near shore Small Craft Advisory late Wednesday morning and
afternoon. BUFKIT supports near shore southwest 25 to 30 knot wind
gusts Wednesday afternoon with good mixing ahead of an approaching
cold front.

The other concern will be for a band of showers late Wednesday
afternoon and night. If enough instability can be realized...a few
of the heavier showers may contain gusty winds/small hail. Low risk
of a few thunderstorms as well.

Outlook...Thursday through Sunday...

Thursday through Saturday...high confidence.

Expect west winds to gust up to 25-30 knots through most of the period
mainly over the open waters. Seas on the outer waters up to 5-7 feet
through the daylight hours Sat. Winds and seas should diminish Sat
night. Generally good visibilities.

Sunday...high confidence.

West winds diminish below small craft criteria as weak high pressure builds
across. Seas below 5 feet.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory from 11 am to 7 PM EDT Wednesday for
anz230>234-236.
Small Craft Advisory until 8 am EDT Thursday for anz235-237-
250-254>256.
Small Craft Advisory until 3 am EDT Thursday for anz251.

&&

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

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