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Area forecast discussion
National Weather Service Taunton Massachusetts
1009 am EDT Mon Jul 24 2017

Synopsis...
widespread rain and unseasonably cool temperatures impact the
region today. Rain tapers off to light showers and areas of
drizzle later today, but it remains rather cloudy and
unseasonably cool on Tuesday. High pressure will bring a return
to dry and pleasant weather for Wednesday. Another cold front
should bring a period of showers and perhaps isolated
thunderstorms sometime late Thursday and/or Friday. Early signs
are for dry weather returning in time for most of the weekend,
but it is possible clouds/showers linger into Saturday if the
cold front slows up.

&&

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

Watching northward progress of a dry slot towards the South
Coast of New England. Strong convection well south and southeast
of Nantucket this morning has limited inflow farther north,
which in turn has reduced the threat of really heavy rainfall.
Also noted another band of higher reflectivity farther west,
which is associated with a low level frontogenetical band. This
is much more typical of a winter Nor'easter than late July.

Temperatures continue to hold nearly steady or slowly fall. Made
some adjustments to reflect observed trends. Thinking we have
already reached our maximum temperature earlier this morning.

Previous discussion...

A morning light to moderate rain event with the greatest risk of
heavy rain for southeast New England, mainly S-coast and islands.
Lead 500 mb-7 shortwave energy within the diffluent flow pattern aloft
and beneath the right rear quadrant of the upper level jet is
invoking a surface area of low pressure off the Delmarva Peninsula
which will sweep east along the quasi-stationary frontal boundary
(qsfb) immediately S of New England through afternoon.

Accompanying anomalous low-level convergent focus, per isallobaric
response, of sub-tropical moisture with precipitable waters in
excess of 2-inches advects from the S along the qsfb as a wedge of
stable, cooler air is drawn in from the North/East across the interior of
New England by a 4-5 Standard deviation low-level east jet.

A pattern more typical for the cool season, a north-S cross-sectional
analysis shows a pronounced warm frontal frontogenetical slope
associated with the qsfb with modest moisture convergence all the
way up through h4 associated with ageostropic flow signaling a good
deep-layer frontal circulation. It's along this slope a consensus of
forecast guidance is signaling two distinct areas of mesoscale
banding and upward forcing: 1.) Along the qsfb, and 2.) Beneath the
lead 500 mb-7 shortwave, mid-level deformation zone. But it remains a
challenge as to exactly where these two areas emerge and how they
evolve over time, especially with regards to magnitude and with
respect to associated spatial mesoscale features (i.E., Strongest
low-level jet).

As alluded to earlier, an anomalous event per analysis of the gefs
and sref. This as the top 5 cips analogs signal a heavy rain event
for the immediate S-coast. So there is some concern, especially in
regards to potential flooding. However not all that certain after
interrogating the ncar ensembles and evaluating the latest hrrr.
Only thing we can comfortably hang our hat on is that much of
the rainfall will occur this morning within the 12z-18z (8a-2p)
timeframe and that much of the heavy rain focus will lie S upon
the qsfb collocated with the greatest focus of sub-tropical
moisture and instability.

In all, evaluating latest radar trends and hrrr guidance as of 330a,
storms are hinging to the qsfb over the waters. The islands look to
be under the greatest threat of seeing amounts around 2 inches in a
short period of time, Nantucket especially. A flood potential, yet
during a time of high tide transitioning to low tide. Ncar ensembles
signal the potential for 1 inch in 1 hour over Nantucket, with the
potential for 2 inches out of the entire event. The remainder of the
area, probabilities are low for seeing over an inch, thinking the
result of The Wedge of stable air out of the North/East plus the storm
environment along the qsfb robbing the environment north of sub-tropical
moisture and instability. Will not be issuing any headlines for this
event. Low risk of thunder, believe it will stay mainly short wave of our
area as we find ourselves beneath the stable wedge.

Wet weather tapering for the remainder of the day as a mix of light
showers and/or drizzle under a deck of low clouds. Brisk North/East
wind with gusts up to 30 mph for the east Massachusetts coast.

&&

Short term /6 PM this evening through Tuesday night/...
cool, damp weather at first, clearing out late, mainly overnight
Tuesday night, as surface high pressure builds into the region. Bulk
of sub-tropical moisture pushed out to sea, a more Continental air-
mass resides out ahead of the deamplifying mid-level positively-
tilted shortwave trough with precipitable waters around 1.25 inches.
Weakening ascent upon lingering moisture as drier air works into
the region and heights rise. Indications of a building dry inversion
from forecast model soundings.

Expecting low clouds to linger with initial showers tapering to
drizzle tonight into Tuesday with continued North/East winds keeping a
cooler, stable airmass in place at the surface. Chance pops
accordingly with highest confidence up against high terrain with
orographic support (n/west ma). A clearing trend begins to emerge late
Tuesday with clouds becoming broken to scattered, continuing
overnight as a 1020+ high pressure builds across the area. Winds
becoming light, an opportunity may emerge for radiational cooling
allowing temperatures to drop well down into the 50s along with the
possibility of patchy ground fog given recent damp conditions.
Unseasonable temperatures with highs mainly in the 60s with lows
around the 50s.

&&

Long term /Wednesday through Sunday/...
highlights...

* dry and pleasant weather Wednesday
* some showers/isolated T-storms likely late Thu and/or Fri
* early signs are for pleasant weather for most of the weekend

Details...

Wednesday...

High pressure remains in control but slowly slides east of the
region. This should result in dry/pleasant weather Wed with partly
sunny skies. May see some more clouds develop as the day wears on
across the interior with weak warm advection beginning. Regardless,
high temps should recover into the upper 70s to around 80 along
with comfortable humidity levels.

Thursday and Friday...

An impressive shortwave for July standards will drop southeast from
the Great Lakes, but still quite a bit of timing differences amongst
the models. It is possible that some showers/isolated T-storms arrive
during Thu afternoon, but the greater risk for more widespread activity
may end up sometime Thu night/Fri. High temps should be in the upper
70s to lower 80s Thu and perhaps some middle 80s if more sunshine
is realized. Friday will probably be a bit cooler with a greater
risk for showers at this time.

Saturday and Sunday...

Still a lot of uncertainty this far out in the forecast, but most
current model guidance indicates trough axis moves to our east. This
would allow for dry and pleasant weather with high temps well up
into the 70s to lower 80s Sat and sun. However, if the shortwave
trough ends up moving slower clouds and showers could linger into
Saturday, especially along the coast.

&&

Aviation /13z Monday through Friday/...
forecaster confidence levels...

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

Short term /through Tuesday night/...

Today...
MVFR to IFR conditions develop as widespread rain lowers
cigs/vsbys this morning. Heaviest rain tapers to lighter
showers/drizzle later today, but low end MVFR-IFR conditions
continue. East winds becoming north over a majority of terminals, vrb
along the S coast. Strongest winds across east Massachusetts sustained around
20 kts with gusts up to 30 kts.

Tonight...
mostly IFR cigs, lowest across high terrain terminals, with shra/dz.
Winds turning N, strongest along east Massachusetts coast sustained around 15 kts
with gusts up to 25 kts.

Tuesday into Tuesday night...
improving trends. MVFR-IFR mix gradually lifting, eroding into the
overnight hours Tuesday night. North winds diminishing becoming lighter
towards morning, still a bit breezy along the East Coast with sustained
winds up around 10 kts, gusts potentially to 20 kts.

Kbos terminal...east winds becoming strong through the day with the
potential for gusts up around 30 kts. Winds back late in the day
out of the NE remaining breezy during which time IFR cigs are
possible.

Kbdl terminal...cigs lowering with areal coverage of -ra. Low
conditions persisting through roughly midday Tuesday as east winds
becoming north.

Outlook /Wednesday through Friday/...

Wednesday...moderate confidence. VFR conditions.

Wednesday night and Thursday...moderate confidence.
VFR conditions probably dominate, but will have to watch for localized
MVFR-IFR conditions in some low clouds/fog patches especially during
the late night/early morning hours.

Thursday night and Friday...moderate confidence.
A period of MVFR- IFR conditions likely in showers and perhaps isolated
thunderstorms, but timing is uncertain.

&&

Marine...
forecaster confidence levels...

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

Short term /through Tuesday night/...

A more winter-like low pressure is expected to travel south of
New England this morning. This will result in an increasing
easterly flow with gusts up to 30 kt at times. Its possible for
a brief period of gale force gusts today, but have low
confidence it will persist long enough to warrant a gale watch
or warning at this time. Small Craft Advisory for all waters
continues, concluding late Tuesday into Tuesday night as winds
subside and seas diminish.

Outlook /Wednesday through Friday/...

Wednesday...high confidence.
High pressure slowly moves east of the waters, but weak pressure
gradient will keep winds/seas below Small Craft Advisory thresholds.

Thursday and Friday...moderate confidence.
Winds and seas will generally remain below Small Craft Advisory
thresholds. However, a period of southwest wind gusts of 20 to
25 knots and choppy seas are expected Thursday afternoon.

&&

Tides/coastal flooding...
continued strong east/NE flow continue today into tonight with
sustained winds around 10 to 15 mph with gusts upwards of
30 mph. This yields a surge upwards of a foot for locations
along the east Massachusetts coast north of a low sweeping west to east along the waters
south of New England.

Another coastal flood advisory has been issued for the evening
high tide accordingly, given surge on top of high astronomical
high tides.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...coastal flood advisory from 11 PM this evening to 3 am EDT
Tuesday for maz007-015-016-019-022-024.
Rhode Island...none.
Marine...Small Craft Advisory until 5 PM EDT this afternoon for
anz232>234.
Small Craft Advisory until 2 am EDT Tuesday for anz230.
Small Craft Advisory until 2 PM EDT Tuesday for anz231-251.
Small Craft Advisory until 2 PM EDT this afternoon for anz236.
Small Craft Advisory until 5 PM EDT Tuesday for anz235-237.
Small Craft Advisory until 2 am EDT Wednesday for anz250-
254>256.

&&

$$
Synopsis...Frank/sipprell
near term...Belk/Frank/sipprell
short term...sipprell

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