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Area forecast discussion
National Weather Service Taunton Massachusetts
410 am EDT Wed may 24 2017

Synopsis...
a wave of low pressure tracks south of New England this morning
bringing a period of light rain or sprinkles to the South Coast.
Behind the departing low drier air overspreads the area this
afternoon and tonight along with late day sunshine. A more potent
low pressure center delivers a widespread rain Thu and Fri along
with cooler temperatures. The rain will likely be heavy at times Thu
night into Friday morning as the low intensifies. The Holiday
weekend gets off to a dry start Saturday followed by the risk of
showers late Sunday into Sunday night, then a drying trend possible
Monday. Expect mild days and cool nights each of the three days.

&&

Near term /until 6 PM this evening/...

Overall trend in the forecast remains on track early this morning.
Surface low off the coast of Delmarva will continue its
northeasterly track towards the 40n/70w benchmark.

Still a spread in the guidance on precipitation shield for the near
term. Currently believe that the 00z NAM is an outlier and not
accounting for the potential fgen that is starting to set-up per Storm Prediction Center
meso Page. Thus anticipate the precip will move into the region
early this morning esp across locations south and east of bdl to
bos. The cape and the islands have the best shot of steadier
rainfall, with light showers/sprinkles elsewhere as the mid-levels
are still quite dry. Precipitation chances will begin to wind down
by the mid-morning hours with a few lingering showers across the
cape/islands around 18z.

Improving conditions will occur from west to east through the day.
Consistent easterly flow will keep temperatures near or below
seasonable especially across eastern MA/RI. If clouds break out this
afternoon then temperatures could quickly warm into the mid 70s.
Highest confidence is across the CT River Valley. However, if clouds
remain steady then daytime highs will linger in the low 70s.
Overall, a damp morning will lead to a dry afternoon with near
seasonable conditions.

&&

Short term /6 PM this evening through 6 PM Thursday/...

Tonight...

Weak mid to upper level ridging will pass through the area this
evening. Dry weather to start, with precip chances increasing closer
to dawn tomorrow. Mainly cloudy conditions will keep temperatures
in the low to mid 50s.

Thursday...

Anomalously large and deep upper level trough and associated upper
low will slide eastward into the Ohio/Tennessee River valleys by
morning.

An upper level shortwave and associated mid-level warm front
associated with this trough will begin to move through the area
during the first half of the day. This frontal system will trigger
widespread showers across the area., Which could be heavy at times
as precipitable water values increase to near 1.5 inches thanks to the sub-
tropical connection. Still a spread in the guidance on where the
heaviest precip will occur. The GFS/rgem keeps the bulk of the
precip offshore while the hi-res arw and NAM pushes the heavy precip
over the I-95 corridor. Despite on where the heaviest quantitative precipitation forecast will
align, it appears a dreary morning will be on tap.

During the later half of the day, better dynamics will be north of
the region. Resulting in just showery weather as dry slot begins to
push into the area. Latest soundings indicate that a good low level
moist inversion will set-up during Thursday afternoon resulting in
areas of fog and drizzle for the remainder of the day.

Persistent easterly winds through the day will not only help increase
the low level moisture, but also keep temps from warming. Blended
the forecast with mav/met guidance which keeps temps in the mid to
upper 50s. A few sites may hit 60 but it will be a damp, dreary day
on Thursday.

&&

Long term /Thursday night through Tuesday/...

Highlights...

* widespread rain Thu night/Fri morning with locally heavy downpours

* the Holiday weekend starts dry Sat but then the risk of showers
late Sunday into Monday, but a washout not expected

* the Holiday weekend will feature mild days and cool nights

Details...

Thu night...

Good agreement among both deterministic and ensembles that potent
short wave rounding the base of the high amplitude East Coast trough
takes on a negative tilt and captures a plume of tropical moisture
from the lower latitudes northward up the eastern Seaboard into New
England. This also induces secondary low pres south of New England.
Very strong jet dynamics with mid levels closing off combined with
subtropical moisture will result in a period of heavy rain Thu night
into Fri morning, especially as comma-head forms. The 12z eps and
gefs both support modest probs of 1+ inches of rainfall with highest
probs over northeast Massachusetts. Many of the global models have 1+ inches of
rainfall in 9 hrs from about 06z-15z. It will be a wind swept rain
for eastern Massachusetts as low level onshore jet sweeps across this region
from south to north.

Friday...

A period of heavy rain is likely (esp in the morning) in response to
trough amplification as negatively tilted closed low moves across
the region. As mentioned above, very strong jet dynamics coupled
with subtropical moisture will result in comma-head rains, heavy at
times yielding nuisance poor drainage Highway/street flooding,
enough to impact/slow down the Fri morning commute. Temps will be
cooler than normal given overcast conditions and periods of rain.

Holiday weekend...

Occluded low exits into the Maritimes yielding rising heights and
anticylonic flow across New England Saturday. This supports dry
weather and seasonably warm conditions (70-75), although cooler
across eastern Massachusetts with afternoon seabreezes. By late sun next short
wave trough approaches. This will result in clouds increasing sun
esp in the afternoon along with the chance of a few showers late sun
and Sun night, possibly lingering into Monday but by no means a
washout. Temperatures will be near normal featuring mild days and
cool nights.

Tuesday...

Much of the ensemble and deterministic guidance suggest deep
southwest flow into New England with mid level trough over the Great
Lakes. This warm advection pattern should support temps warmer than
normal and possibly dry weather given how far west position of mean
trough axis.

&&

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/...high confidence.

Today...showers south and east of bdl to bos line will push
eastward through the day. Any MVFR/IFR cigs will quickly improve
to VFR by the afternoon.

Tonight...VFR with easterly winds. Fog could develop across low
lying terminals.

Thursday...IFR cigs and vsbys in rain and fog with east wind
gusting to 20 knots along the shoreline.

Kbos taf...high confidence in taf. Easterly flow through the
period.

Kbdl taf...high confidence in taf.

Outlook /Thursday night through Sunday/...moderate to high forecast
confidence.

Thursday night... IFR cigs and vsbys in rain and fog with east wind
gusting to 20 knots along the shoreline. Rain heavy at times late.

Friday...IFR/LIFR in heavy rain during the morning, then conditions
improve to mixed VFR/MVFR in showers as winds shift from the
northwest.

Saturday... VFR. Diminishing northwest wind.

Sunday... VFR early, trending to MVFR/IFR in showers and fog by
Sunday night. Winds from the southeast.

&&

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 and tonight...coastal low will move northeastward across
the benchmark this morning followed by high pressure during
afternoon lasting into the overnight hours. This gradient
between the high and low will increase wind gusts to 20 kts by
the late morning hours. Seas will begin to build in response.
Passing showers this morning will limit vsbys.

Thursday...approaching system from the west, will help increase
winds and seas through the day. Small Craft Advisory have been extended where
confidence is highest for seas above 5 feet. Good inversion
through the day will limit gusts but can't rule out a few
reaching 25 kts.

Outlook /Thursday night through Sunday/...

Thursday night - Friday...

Poor vsbys in rain and fog Thursday night. Easterly winds around 25
knots with strongest winds during the evening. These winds then lift
north of the waters tonight. Seas building up to 10 feet across the
Cape Ann waters, less elsewhere. Low pressure moves past the waters
Friday morning, at which point the winds shift from the north-northwest with
speeds 20 knots or less. Expect seas 5-7 feet on the outer waters
and Rhode Island Sound. Small Craft Advisory may be needed on some of the
waters both days.

Saturday-Sunday...

Winds remain below 25 knots through the period. Seas subside below
5 feet early Saturday. Seas may return to 5 feet on the outer waters
late Sunday night. Poor vsbys possible in showers and fog late
Sunday and Sunday night.

&&

Tides/coastal flooding...

High astronomical tides will occur over several tide cycles along
eastern Massachusetts coast from Thursday into Memorial Day, with
tides around 12 feet in Boston and 4 feet on Nantucket.

At the very least, minor nuisance flooding will occur in the most
vulnerable locations such as morrissey Blvd in Boston. Any surge on
top of these tides would lead to more widespread, but minor, coastal
flooding, including on Nantucket. Right now, the greatest risk for
widespread minor coastal flooding is the Thu night/early Fri
midnight high tide, which is 12/26 ft at Boston, due to expected
onshore winds and surge. Model surge guidance (estofs) shows a
potential 0.7 ft surge, which would give a storm tide near 13 feet
Thursday night in Boston and just under 5 ft on Nantucket. In
addition waves up to 10 ft are possible just offshore with highest
seas and greatest surge along the Cape Ann coastline to the New Hampshire
border. This should be sufficient for widespread minor coastal
flooding with a low risk of isolated areas of approaching low end
moderate flooding across Cape Ann coastline to Massachusetts/New Hampshire border.
Elsewhere minor coastal flooding is likely however the severity is
expected to be much less.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory from 8 PM this evening to 9 PM EDT
Thursday for anz235-237.
Small Craft Advisory from 6 am to 9 PM EDT Thursday for anz250.
Small Craft Advisory from 11 am to 9 PM EDT Thursday for
anz251.
Small Craft Advisory from 2 PM this afternoon to 9 PM EDT
Thursday for anz254>256.

&&

$$
Synopsis...nocera/dunten
near term...dunten
short term...dunten

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