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Area forecast discussion 
National Weather Service Taunton Massachusetts
703 PM EDT Wednesday Aug 20 2014

Synopsis...
wet weather likely impacting western and central portions of New
England beginning tonight and continuing through Friday. Drier and
cooler weather follows into the weekend. A warm-up is possible
into next week prior to an unsettled pattern ahead of a frontal
boundary sweeping south out of Canada.

&&

Near term /until 6 am Thursday morning/...
7 PM update...
diurnal cumulus dissipating in eastern new eng...but expect an
expansion of SC in the west tonight as low level moisture
increases from the west. Also monitoring low clouds southeast of ack
which is expected to move across the islands and Outer Cape
overnight. Still uncertainty as to how far north and west ocean
showers will get associated with area of convergence of higher thetae
air. 18z NAM/GFS have backed off and so we lowered probability of precipitation to chance
category for Outer Cape and ack as this is a low confidence
forecast.

Previous discussion...
heights fall as the Great Lakes low shifts into the NE Continental U.S....
troughing towards the west-east axis of low pressure and instability off
the mid-Atlantic. Labrador ridge of high pressure along with dry
air and subsidence will persist into New England...ahead of which
the collision towards low pressure and higher Theta-E air will
result in a defined area of convergence across PA/NY...and
possibly southeast Massachusetts.

To the west...increasing Theta-E moisture loading the middle- to upper-
levels will likely result in increasing and thickening clouds west to
east. But still a lot of dry air to overcome below h6-7 especially over
east New England with the inverted ridge. Noting the warm front well west
across PA/New York along with parent dynamics and low-level convergence...
best chances for showers within areas of deep-layer ascent should
be no further east of the CT-River Valley. Expecting sprinkles. Its
likely better activity will be well north/west parent with better moisture
convergent along the nose of the h925-85 jet into the Adirondacks
on up into Ontario/Quebec as destabilization concludes. Prefer
chance probability of precipitation only into the CT-valley.

To the southeast...troughing southeast backs low pressure and higher Theta-E air
off the east-west stalled front off the mid-Atlantic. The convergence of
air masses along an inverted-trough to the aforementioned low will
be the focus for potential heavy rain...even thunderstorms. Mainly a
warm-rain process with precipitable waters up to 1.75 inches. While a consistent
signal among forecast solutions...outcomes vary slightly likely due
to the strength of inverted-ridge. Will go with likely probability of precipitation for
Nantucket with chance probability of precipitation into the cape and southeast Massachusetts. The setup is
already coming together seen per infrared/visible satellite...but a lot
of uncertainty between the strength of the ridge and the zone
setup of convergence. WRF/NAM the most robust with outcomes. 17z
hrrr/18z rap has activity missing Nantucket. Not enough confidence
to go high categorical probability of precipitation.

Thicker clouds across the region will limit radiational cooling.
Anticipating a more mild night over much of the region with lows
around the upper-50s. Coolest conditions expected across east/southeast Massachusetts
under light and variable winds and the better chance of radiating
out...lows around the mid-50s. For such areas there is also the
possibility of patchy ground fog.

&&

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

Closed low out of the Great Lakes gradually wobbles into New England
through the forecast period. The system weakens as it transitions SW
re-emerging along the west-east frontal boundary off the middle-Atlantic
towards the better thermal-instability axis.

Subsequent rising heights allows ridging to build SW Thursday
night. While higher Theta-E air streams across the region within
the mid-levels...E-flow within low-levels from the Labrador high
undercuts moisture loading aloft keeping conditions fairly stable
and dry...likely to erode activity S and west.

As the low transitions and weakens...precipitation across the cape
and islands concludes with the strengthening influence of the
ridge. Energy and moisture parent to the low is anticipated to
sweep mainly across SW New England...perhaps SW of a line from
Boston to Keene New Hampshire. Will keep these areas under chance probability of precipitation
accordingly with ascent and likely convergence against the SW
building ridge.

With the undercutting dry and stable airmass off the ocean with the
strengthening ridge late...and clouds through the period limiting
destabilization...not expecting any thunder. Highs around the middle-
to upper-70s during the day with lows around the upper-50s.

&&

Long term /Friday through Wednesday/...
highlights...

* quiet weather expected through much of the long term
* uncertainty increases middle of next week with an approaching
front.

Models are in good agreement through much of the long term. A
persistent upper level trough over the northeast will slowly make
its way east...moving just offshore this weekend as an upper level
ridge tries to push northeastward from the southern U.S. The upper
trough is rather persistent though and each model run keeps the
trough just offshore longer and longer.

Cooler...less seasonable weather is expected while high pressure in
Quebec will keep things relatively dry. Models are indicating some
potential for quantitative precipitation forecast each day through the weekend but given the lack of
any forcing mechanism and very little moisture to speak of...
thinking this could be convective feedback rather than a depiction
of anything likely to occur.

The high will sink south through the weekend into early next
week...eventually dropping south and east of southern New England
and returning US to a more westerly flow. Low pressure in Ontario
may eventually move eastward into Quebec bringing a cold front
through southern New England Wednesday or Thursday. This is where
models start to diverge a bit with the European model (ecmwf) a bit quicker than the
GFS with the approach of the front. Depending on available moisture
this may initiate a few showers and thunderstorms ahead of the front.

&&

Aviation /00z Thursday through Monday/...
forecaster confidence levels...

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

Short term...through Thursday night...moderate confidence.

VFR. Light/vrb winds overnight before prevailing east/southeast Thursday
into Thursday night. Low-end VFR ceilings slowly building east along with
-ra chances towards ijd-orh-afn. Will also need to watch southeast Massachusetts for
potential +ra and MVFR-IFR impacts beginning after midnight into
Thursday morning. Conditions begin to improve closer to Friday
morning.

Kbos terminal...high confidence in taf. East/southeast flow prevails. Expect
conditions no lower than low-end VFR ceilings. -Ra chances minimal.

Kbdl terminal...high confidence in taf. -Ra chances begin towards
Thursday morning along with low-end VFR ceilings. MVFR ceilings possible
though low-confidence at this time.

Outlook...Friday through Monday...

Moderate confidence. Mainly VFR. Low probability of patchy
MVFR/IFR fog each night.

&&

Marine...
forecaster confidence levels...

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

Short-term...through Thursday night...moderate confidence.

Closely monitoring the southeast waters. May see a surgance of rain along
with visibility impacts during the overnight hours into Thursday
along with an increase in east-NE flow as an area of low pressure
develops well out to sea. Quite possible wind gust up to 25
kts...but will hold around 20 kts for now. Will keep seas around 3
to 4 feet. Conditions relax into Friday morning.

Outlook...Friday through Monday...

Moderate to high confidence. Quiet boating weather is expected
through much of the period with high pressure across the region.
Low probability of patchy overnight fog limiting visibilities.

&&

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

&&

$$
Synopsis...rlg/sipprell
near term...kjc/sipprell
short term...sipprell
long term...rlg
aviation...kjc/rlg/sipprell
marine...rlg/sipprell

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