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Area forecast discussion
National Weather Service Taunton Massachusetts
459 PM EDT Wed Jul 26 2017

Synopsis...
high pressure remains over the region tonight providing dry
cool weather. The high moves offshore Thursday giving way to
slightly more humid conditions along with the risk for a few
showers and an isolated thunderstorm, but by no means a washout.
A drying trend likely develops Thu night into Fri as a cold
front moves through the region and then stalls near the South
Coast Fri. A coastal storm will push off the mid Atlantic coast,
which will bring showers and a few thunderstorms, along with
gusty onshore winds with cool temperatures Friday into early
this weekend. Dry and seasonable conditions return by early next
week.

&&

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

4 PM update...

1020 mb high over the region late this afternoon providing
splendid weather with temps mainly 75-80 along with comfortable
humidity. Stable/cool marine airmass has come onshore via
seabreeze circulation and has resulted in a lack of diurnal
clouds along the immediate coast per latest GOES-16 imagery. All
of these attributes will set the stage for very pleasant
weather this evening.

The only issue will be with dry airmass /dew pts in the 50s -
lowest inland/ in place coupled with light winds, temps will
fall quickly with sunset. So expect cool conditions with sunset.
Otherwise tranquil weather prevails.

Later tonight mid and high clouds will overspread the region
from northwest to southeast and this will stabilize temps from falling much
more. However given the ideal radiational cooling conditions the
first half of the night sided with the cooler MOS temps for
mins. Thus another cool night with lows in the 50s except around
60 in the urban areas.

&&

Short term /6 am Thursday morning through Thursday night/...

4 PM update...

Thursday...

Increasing clouds ahead of next approaching short wave. Weak
low level warm air advection will result in surface dew pts climbing into the
low and mid 60s, so it will feel more humid than today. Airmass
is slightly warmer than today but with increasing clouds highs
tomorrow will be close to values of today, 75-80. It will be
slightly warmer along the eastern Massachusetts coast/beaches than today
given south-southwest winds tomorrow. Overall a typical Summer day, perhaps
a few degs below average.

As for the risk of showers/T-storms, approaching short wave
provides modest mid level moisture advection (k index low 30s)
along with weak forcing/height falls for ascent. This should be
sufficient for isolated to widely scattered showers. However
T-storms especially strong to severe appears very low as northwest flow
precludes true warm sector airmass from advecting northeast
into southern New England. This is evident on all model guidance
offering less than 500 j/kg of MUCAPE. This combined with weak
mid level lapse rates (h5 temps only cool from -9c to -10c) will
result in weak updrafts and stronger storms likely remaining in
the warm sector across NYC/NJ/eastern PA. After collaboration
with Storm Prediction Center the slight risk was removed from southwest CT for Thu.

So isolated to widely scattered showers and T-storms Thu
afternoon, however many hours of dry weather too, so by no means
a wash out.

Thu night...

Good drying thru the column per decreasing k index values as
short wave trough moves east of the area and mid level
anticyclonic flow and associated subsidence take over. This will
result in a drying trend from north to south. So any evening
showers/T-storms will shift south and then offshore. However
front will not reach the South Coast until Fri morning. So
expect warm and humid conditions to persist much of the night.
This moist low level airmass will increase the risk of patchy
fog as well.

&&

Long term /Friday through Wednesday/...

Highlights...

* risk continues for showers and a few thunderstorms possible Fri
night and Sat, best chance S coast
* gusty east-NE winds possible along coastal areas, highest along S
coast, Cape Cod and the islands late Fri night into Sat evening
* dry conditions likely Sunday into early next week

Overview and model preferences...
continue to see rather wide model solution spread for track,
strength and timing of low pressure moving off the mid Atlantic
coast and S of New England. The big question is how far north the
precip shield moves into the region and how strong the east-NE
winds will be especially late Fri night through into Saturday
night, especially along the S coast, Cape Cod and the islands.
Rather strong 500 mb short wave associated with this system,
especially pronounced on the European model (ecmwf) across S New Jersey during Sat. Could
also see gusty winds along the coast, especially the S coastal
areas late Fri night into early Sat night, but timing is still
an issue.

Models showing a bit better agreement after the low exits Sat
night or even Sun morning, with dry conditions as high pressure
builds across. Will see leftover cool temps on Sunday, then
becoming more seasonable by the beginning of the work week.
However, still noting developing 500 mb long wave troughing across
the eastern U.S. While another high amplitude ridge builds
across the western states. Some question as to timing of another
cold front moving southeast out of southern Quebec/Ontario, but
appears not a lot of upper support with this system. Timing and
movement still very much in question.

Details...

Friday...
will start off dry and seasonable early Friday, then some
showers may start to push NE by midday well ahead of low pres
moving off the S New Jersey coast along a stationary front S of New
England. Clouds will increase, along with the chance for
showers. May see some marginal instability for a few
thunderstorms Fri afternoon. Light/variable winds become light
easterly, though may start to pick up along the S coast by
evening. Expect highs to run a few degrees below seasonal
normals due to the developing onshore wind.

Friday night-Saturday...
best chance for showers especially from central mass southward.
Have likely pops mentioned there for most of this period.
Marginal instability remains across the region, so have kept in
slight chance for thunderstorms as well.

Big question will be the track of the low as it deepens to a
rather anomalous system as it passes S of New England Friday
night to mid day Saturday. Very strong h925-h9 jet associated
with this system, with east-NE winds up to 40-50 kt S of eastern
Long Island and Block Island by around midday Saturday. How much
mixes down is in question, but can not rule out gusts to 30-40
kt especially over the coastal waters. Could see gusts that high
across Cape Cod and the islands during Saturday as well.
Something to keep a close eye on.

Still noting convective feedback issues, but rather good slug
of precip moves across. Noting pwats up to 2 to 2.2 inches
moving across Cape Cod and the islands Sat, so could see some
heavier rain there Fri night/Sat. Will be a rather sharp line
where heaviest precip falls, mainly along S coast, with about
0.25 inches or less from about north CT/Rhode Island to S of Boston northward.

At this point, could see some improving conditions Saturday
afternoon/evening from west-east as the low shifts offshore, but
timing is still in question.

Sat will be another chilly day by July standards, with highs
only in the lower-mid 70s.

Saturday night through Monday...
mid level long wave shifts offshore during Sat night, with northwest
flow in place aloft into early next week which may shift into
Quebec. Meanwhile, at the surface, high pres builds eastward out
of the Ohio Valley to the Appalachians. Good ridging and dry
conditions build in this period.

Will remain on the cool side Sunday, with highs running around
5 degrees below seasonal normals, but will warm on Monday
especially across the interior.

Tuesday-Wednesday...low confidence.
Another cold front starts to approach during Tuesday, though
timing and strength in question. Not a lot of moisture
associated with the front with the general west-northwest flow aloft in
place. Broad long wave trough sets up across the eastern U.S.,
But not much energy to work with. At this Point, May see a few
showers Tue, then should be dry on Wed but timing of features in
question.



&&

Aviation /21z Wednesday through Monday/...
forecaster confidence levels...

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

18z/2pm update...

Thru 00z...VFR with any bkn cigs 4-5kft. Light S-southeast winds
continue.

After 00z...VFR with winds becoming light and variable, then south-southwest
toward morning.

Thursday...VFR with bkn040-050 cigs in the afternoon along with
isolated to widely scattered T-storms.

Thursday night...VFR but trending toward MVFR especially across
CT/Rhode Island and southeast Massachusetts including Cape Cod and the islands where
patchy fog will form. Scattered showers and isolated T-storm
possible in the evening hours.

Kbos terminal...high confidence. Isolated to widely scattered
showers/T-storms Thu afternoon/early evening. Strongest storms
will likely remain well southwest of the region across NYC/NJ/PA
area.

Kbdl terminal...high confidence. Isolated to widely scattered
showers/T-storms Thu afternoon/early evening. Strongest storms
will likely remain well southwest of the region across NYC/NJ/PA
area.

Outlook /Friday through Monday/...

Fri...high confidence.
A few showers and thundershowers, mainly Thu evening. Then
mainly near the S coast on Fri. Otherwise, predominantly VFR
outside of some localized ground fog.

Fri night and Sat...moderate confidence.
Potential for IFR-MVFR conditions in showers. Patchy fog during
the nighttime hours. Slight chance for thunderstorms, best
chance S coastal terminals. East-NE winds which could gust 25-35 kt
mainly near the coast from kghg southward Sat. Rainfall,
especially along S coast may be heavy at times, though still
uncertain how close the heavy rain shield will come to the
coast. Conditions should improve from west-east Sat afternoon/evening.

Saturday night through Monday...high confidence.
VFR. North-NE wind gusts to around 30 kt across Cape Cod and the
islands early Sat night, then diminishing.

&&

Marine...
forecaster confidence levels...

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

4 PM update...

Short term /through Thursday night/...high confidence on wind and
waves but lower confidence on probability of showers/T-storms.

Tonight...light onshore winds continue. Small east swell of 2-4 ft
continues across the ocean waters. Otherwise tranquil waves near
shore. Dry weather and good vsby prevail.

Thursday...south-southwest winds 10-15 kt possibly increasing to 20 kt in gusts
late in the day. Isolated shower/T-storm possible. Good vsby
continues.

Thursday night...south-southwest winds 10-15 kt shifting to west-northwest toward daybreak
Friday. Isolated shower/T-storm however fog may become fairly
widespread but improving toward morning across the northern waters
given the wind shift from south-southwest to west-northwest.

Outlook /Friday through Monday/...

Friday...high confidence.
Light/variable winds shift to east-southeast but remain light. Seas 3 ft
or less.

Friday night into Saturday night...moderate confidence.
Low pres passes S of New England while it intensifies. Still
questioning how close the low will approach, but winds shift to
east-NE and pick up after 06z Sat. Could see sustained wind
reaching 20-25 kt by 12z Sat across the SW waters. East-NE winds
could gust to 30-40 kt during Sat, likely highest across the
southern waters to north and east of Cape Cod, with small craft winds
elsewhere. Seas may reach as high as 8-12 ft on the open ocean
waters. Showers with isolated thunderstorms, with periods of
heavy rain on the southern outer waters. Winds slowly diminish
Sat night, down to small craft levels. Seas will also slowly
subside but remain at or above 5 ft.

Sunday and Monday...high confidence.
Improving conditions as leftover small craft winds/seas
diminish by Sunday evening, giving way to quiet boating
conditions.

&&

Equipment...
kbox radar is has returned to service.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

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

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