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Area forecast discussion 
National Weather Service Taunton Massachusetts
344 PM EDT Thursday Aug 21 2014

Synopsis...

Showers continuing across western New England overnight into
Friday...dissolving southwest as high pressure builds in from the
north. High pressure out of the Maritimes will bring drier and
cooler conditions this weekend. Milder temperatures will return
early next week. Unsettled conditions may return late Wednesday or
next Thursday as a front may approach out of Quebec.



&&

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

Into the evening...

Moisture loading continuing within the low-levels beneath middle-
level ascent forced by vorticity advection round the broad area of
low pressure beginning to slide southeast and weaken. Challenging to say
the least to Pin-Point showers and be specific. Have gone with a
broad-brush consensus emphasizing forecast confidence of where
likely low-level convergence and f-general forcing will be greatest
beneath middle-level lift. Aside from the immediate east...most locales
could see wet weather into the evening hours with the highest probability of precipitation
presently forecast across the west.

Tonight...

Low transitions as an open-wave low towards the southeast. Accompanying
main crux of energy yields ascent of available moisture fueled by a
SW-plume of higher Theta-E air wrapping ahead of the low /1.5 precipitable water
airmass/ converging against high pressure and dry air building SW
below 800 mb. This yields northwest-southeast oriented areas of f-general forcing within
the low-levels that will likely parent with middle-level forcing.

A consensus weighting of model guidance puts the likelihood of
showers towards the west/SW forecast zone which would be expected but
can not rule out of chance of showers elsewhere. Only the
immediate east/southeast may be spared of wet weather...but am not holding
my breath. Low confidence of thunder with the stable environment.

Will need to monitor for fog and low clouds. Highest confidence
is in areas of strong low-level convergence and showers...
especially across the higher terrain. Yet with east-onshore flow
driving in higher dewpoint air...along with the greater capacity
for areas to radiate efficiently the last couple of nights...fog
and low clouds may be possible for east/southeast portions of New England.
Increasing clouds from the west may complicate the matter.

Will go with lows around the upper-50s to low-60s with the cool
spots to the north/west which barely saw any sun...and the east/southeast immediate
interior.

&&

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

Friday...

With the low swung southeast of the region...cutoff energy wraps rearward
and southeast into the region invoking a hanging trough. Rising heights in
wake of the trough allow for enhanced ridging of high pressure SW
ahead of which f-general forcing is maximized within the low-levels.
Parenting with middle-level ascent per cutoff energy...there is the
likelihood for continued showery weather over SW portions of New
England.

Will hold with likely probability of precipitation across far west/SW portions of the
forecast area beneath an abundance of clouds. Am not going to get
specific with the forecast probability of precipitation but rather broad brush an area
where there is highest confidence. Chance to slight chance probability of precipitation east.

Combination of clouds and rain could make for a very chill day
especially for middle- to late-August. The dreary weather could yield
highs around the low- to middle-60s for the short wave...warmer east where the
greater chance of seeing some sunshine yet only into the upper-60s
with the brisk onshore east-flow surging into interior New England.

Friday night...

Activity should diminish with subsidence and dry air building SW as
heights rise in response to the hanging trough swinging S/E. Low-
level f-general forcing and middle-level forcing should continue to push SW
into New York/PA ahead of continued ridge enhancement.

While consensus weighting of model guidance yields a slight chance
of showers for short wave portions of the forecast area...the long east-fetch
off the waters likely resulting in moisture pooling within the low-
levels could result in low clouds and fog...perhaps even drizzle as
the flow undergoes orographic lift along the E-shoreline.

A tough forecast and a likely cool and dreary night with lows
getting down around the mid-50s. With dewpoints at similar values...
anticipating a plethora of patchy dense fog where the east-flow is
light.

&&

Long term /Saturday through Thursday/...

Highlights...

* quiet weather expected through much of this period
* milder temperatures will return Monday and Tuesday as ridge tries
to build east
* timing of approaching front by late next week remains uncertain

Blocking upper level pattern looks to remain in place across North
America as 500 mb cutoff low pressure remains persistent S of Greenland
through most if not all of the long term period. Some questions do
come into play as the northern stream upper flow tries to
reconfigure early next week. Northwest flow tries to flatten out later this
weekend...then upper level ridging appears to build in briefly
around Monday or Tuesday. Operational model run differences may be
resolving...but still some timing issues continue late next week.
This lends to lower forecast confidence beyond Tuesday.

Details...

Saturday and Sunday...

Expect high pressure ridging to continue to nose in from the Maritimes.
The orientation of this ridge will keep cool onshore winds. Several
operational model solutions...mainly the 12z GFS/ECMWF...try to work
clouds and some light precipitation off the ocean during Sat with long
fetch off the Gulf of Maine. Can not rule out isolated showers that may
move onshore Saturday morning then push west. Could even see some light
drizzle around sunrise along the immediate coast. Have mentioned
only slight chance probability of precipitation at this point...though...because of
uncertainty of how much moisture will be realized. One certainty
though is that cooler than normal temperatures will remain with the onshore
winds through the weekend.

Monday and Tuesday...

Looks like the Maritimes ridge axis tries to push offshore as 500 mb
ridge pushes southeast out of Ontario and the Great Lakes and actually
builds into Quebec...a set up that has been rare this Summer. Will
remain dry but looks like milder temperatures on the horizon as winds back
around to west and SW. Temperatures look be close to seasonal norms for late
August...though will remain a bit cooler along the S coast. Humidity
will also start to increase during Tuesday.

Wednesday and Thursday...

While the last several European model (ecmwf) op runs have been much faster than the
counterpart GFS...the 12z run looks much closer with its solution in
keeping ridging across most of the eastern Seaboard. One big fly in
the ointment is the possibility of a system that will try to make
the turn around the subtropical ridge offshore...but that is highly
uncertain. Weak cold front may still try to work out of Quebec last
Wednesday or Thursday...then could stall somewhere across the region. Still a
tough call. Also...depending upon available moisture and
instability...might see some showers and/or thunderstorms in
vicinity of the front. Leaned toward a blend of the 12z GFS run
along with GFS/ec ensembles for this portion of the forecast.

&&

Aviation /20z Thursday through Tuesday/...

Forecaster confidence levels...

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

Short term...through Friday night...moderate confidence.

VFR into this evening. But as east-onshore flow increases and showers
build over the short wave New England terminals...greater propensity for
a mix of MVFR-IFR conditions during the evening hours. Unsure as
to outcomes E/se. Its possible the region remains VFR albeit some
patchy dense fog and low clouds across the interior with evening
cooling.

Into the Friday into Friday night period...anticipating low-end
VFR clouds mixed in with MVFR and showers to the short wave especially
across the high terrain. East-winds continuing with a long fetch off
the waters. Transitioning into evening...while showers conclude...
a majority of the terminals may become socked in with low-clouds
and/or dense fog. Likely to see -dz along the East-Shore terminals
towards Saturday morning.

Kbos terminal...moderate confidence. Will hold low-end VFR for a
majority of the forecast. Could see conditions gradually lower
Friday into Friday night. East-flow persisting throughout and
increasing towards the end of the period...thus lending to
confidence more so of MVFR-IFR low-clouds over dense fog with -dz.

Kbdl terminal...moderate confidence. Conditions anticipated to
lower into evening with increasing -shra chances. May remain
socked in around MVFR through a majority of the forecast period
all the way into Saturday morning.

Outlook...Saturday through Tuesday...

High confidence for VFR. Low probability of patchy fog with brief
local MVFR-IFR conditions each night.

&&

Marine...

Forecaster confidence levels...

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

Short-term...through Friday...moderate confidence.

NE-winds increasing in response to interior low pressure shifting
southeast across the waters with some mild strengthening as high pressure
builds in from the north. Subsequent pressure-gradient may yield some
decent fetch with gusts up to 20 kts to cause seas to rise...but
will hold at 4 feet. Feel brisk winds will diminish fog potential
yet low clouds and showers may be an issue for areas towards the
SW.

Outlook...Saturday through Tuesday...

High confidence.

Expect winds and seas below small craft criteria through the period.
There is a chance that east swells could build close to 5 feet on the
outer waters this weekend with long east fetch...even with wind speeds
less than 15 knots. May see patchy overnight fog reducing visibilities
each night.

&&

Box watches/warnings/advisories...

CT...none.
Massachusetts...none.
New Hampshire...none.
Rhode Island...none.
Marine...none.

&&

$$

Synopsis...sipprell/evt
near term...sipprell
short term...sipprell
long term...evt
aviation...sipprell/evt
marine...sipprell/evt

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