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Area forecast discussion 
National Weather Service Taunton Massachusetts
959 am EDT Friday Jul 29 2016

Synopsis...

Low pressure along the mid Atlantic coast will pass offshore late
in the day as a weak cold frontal boundary sweeps New England. Dry
conditions for Saturday with high pressure. Another low will pass
south of New England bringing another round of scattered showers
with a few thunderstorms Sunday into Monday. High pressure returns
with dry and seasonable conditions for the middle and latter portion
of next week.

&&

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

10 am update...

Flash Flood Watch continues for the South-Coast of Massachusetts
which includes Cape Cod and the islands.

Focus specifically on low to mid level sub-tropical moisture
convergence. This beneath mid level forcing ahead of the weak
trough axis and accompanying vortex energy, as well as beneath the
right-rear-quadrant of the upper level jet. Objective simply is to
pinpoint regions of greatest lift / forcing, nailing down areas
where precipitation efficiency will be greatest, perhaps with some
accompanying instability.

Based on latest radar trends and high-res near-term guidance, the
area of greatest focus is developing over the S waters progressing
north into far southeast New England where the Flash Flood Watch is currently
in effect. There is a Camp of forecast guidance which keeps the
bulk of outcomes over the waters encompassing offshore islands,
but in monitoring radar trends, including the fine-line mesoscale
features, there is certainly some threat to southeast Massachusetts. This
along the leading edge of a distinct wave inferred from latest
water vapor satellite.

A general half to one-inch forecast for far S-coastal Rhode Island and Massachusetts
with the greatest amounts exceeding one-inch over the cape and
islands. Can not rule out that the batch of heavy rain moving into
southeast New England is very efficient with respect to rainfall. Rates
of 1 to 2 inches per hour of instantaneous rainfall presently being
discerned via dual-pol.

Otherwise, clearing working in from the west and have this and bulk
of rain moving offshore around midday into afternoon. Monitoring
upstream thereafter a weak cold frontal boundary sweeping behind
the departing offshore system. Boundary layer destabilization but
beneath some weak ridging aloft and warmer air around 500 mb. As model
trends would concur, any activity that gets organized seems to
diminish quickly and/or get suppressed. Will have to watch
closely. Storms possible, thinking isolated, maybe a threat of
gusty winds and heavy rain given the atmospheric setup.

Warming into the low-80s for most locations. Coolest southeast with the
heavier rain. Soupy air as dewpoints in the upper 60s to low 70s
linger into evening.

&&

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

As the offshore low pressure moves off to the east, weak high
pressure over Canada will build with somewhat drier air. Dew
points will range from the mid 50s north to mid 60s south, down
from the 70s in Rhode Island/southeast Massachusetts today but still noticeable humidity in
southern areas. The moisture and light wind will allow fog patches
especially in the more humid south.

Temps aloft of 14-16c will support Max temps in the 80s Saturday.
The light wind flow will allow sea breezes to form, keeping
coastal areas cooler.

Additional concern is potential for convection upstream in NE PA
that could move east into Connecticut late in the day Saturday. Li
values will be sub-zero across PA and the lower Hudson Valley with
SBCAPE values near 1000 j/kg. We have included chance pops for
showers/tstms in CT and greater Springfield MA, mostly 5-8 PM
Saturday.

&&

Long term /Saturday night through Thursday/...

Highlights...

* high pressure and mainly dry conditions Saturday
* showers and a few thunderstorms possible Saturday night through
Monday
* drier and seasonable weather Tuesday through Thursday

Overview and model preferences...

Fast mid and upper level flow across central and southern Canada
has left weaker steering currents across the northern tier of the
U.S. Into early next week. Noting a slow moving 500 mb short wave out
of the Great Lakes which will cause upper level winds to shift to
west-SW across New England. Weak surface low will move slowly along a
stalled front south of the region, keeping the chance for showers/
thunderstorms. Models and ensembles continue to signal that the
short wave will finally move east Monday night into Tuesday, so
should see improving conditions as mid level winds shift to northwest.
Also noting building heights by late next week, so will see temps
run near or slightly above seasonal normals.

Used a consensus blend of available model and ensemble guidance
for this portion of the forecast.

Details...

Saturday night through Monday...with relative weak steering
currents between the active northern stream flow across southern
and central Canada and the normal Summer ridging across the
southern tier states, expect low pressure to move slowly along a
stalled front south of New England keeping low and mid level
moisture in place along with marginal instability during this
timeframe. Pwats of 1.8 to 2 inches will also linger mainly near
and south of the Mass Pike, which could lead to some locally
brief, heavy precip mainly Sunday into Sunday night. With the low
passing S of the region, winds will generally be east-southeast off the
cooler ocean, which tend to stabilize any surface based convection
but could see some spotty elevated activity.

As the 500 mb short wave slowly pushes across the region on Monday,
any showers should push east. However, some may linger across inland
areas mainly during the afternoon, though there is individual
model solution spread so not a lot of confidence with this.

Expect temps to run close to or slightly below seasonal normals
especially on Sunday with the steadiest precip.

Tuesday through Thursday...500 mb high amplitude ridge moving east
across Hudson Bay on Tuesday will help push the trough offshore.
This will bring generally dry conditions as northwest winds aloft take
over, albeit on the light side. Noting the 00z European model (ecmwf) is trying to
keep some troughing lingering across interior central and southern
New England for Wed-Thu, which could mean some isold diurnal
convection. With somewhat higher 500 mb heights, temps should return
to near or slightly above normal levels by late next week.

&&

Aviation /14z Friday through Tuesday/...

Forecaster confidence levels.

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

Short-term /through Saturday/...high confidence.

15z update...

Today...

IFR-LIFR for southeast terminals with rain/+ra and thunderstorms in the vicinity. VFR elsewhere
with north/NE winds. Conditions clearing from the west presently. There
is the moderate risk of additional rain showers/thunderstorms and rain development N/W.

Tonight...

Improving VFR over the southeast. MVFR-IFR patchy fog may develop over
the interior, especially thinking more so in the CT River Valley
as winds become light and conditions become scattered to sky clear.

Saturday...

Scattered-broken cigs especially towards late in the day. Light winds.
Sea-breezes developing along the shores late morning into
afternoon. Risk of rain showers/thunderstorms and rain into short wave New England late.



Kbos taf...high confidence. Will push the clearing line into the
terminal around 16-18z. Prior to which conditions should remain
VFR but can not rule out MVFR with any rain.

Kbdl taf...high confidence. Improving by 16z. Thereafter
monitoring north/west of the terminal for the possibility of additional
rain showers/thunderstorms and rain development.

Outlook /Saturday night through Tuesday/...

Saturday night through Monday...moderate confidence.

Scattered showers and a few thunderstorms possible through the
period, with the best chance during Sunday into Sunday night.
Occasional MVFR conditions, but VFR likely dominates. Patchy late
night/early morning fog with IFR ceilings/visibilities each night.

Monday night and Tuesday...high confidence.

Mainly VFR. May see brief IFR ceilings/visibilities in patchy fog after 06z
through 12z-13z in the normally prone areas as well as along the
coast.

&&

Marine...

Forecaster confidence levels.

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

Short term /through Saturday/...high confidence.

10 am update...

Areas of low vsbys in fog along the southern waters, especially
the nearshore bays and sounds. Locally dense fog will have vsbys
less than 1/4 mile.

Main concern is potential for strong storms over south coastal
waters today, mainly south of islands and east of Nantucket where
brief waterspouts are possible this afternoon. A marine weather
statement continues to highlight this potential, especially for
recreational boaters.

Otherwise winds will remain below 25 knots and seas below 5 feet.
Rain and fog will reduce visibility today and this evening.

Outlook /Saturday night through Tuesday/...high confidence.

Saturday night through Monday...expect winds and seas below small
craft criteria. Seas 4 ft or less. Locally reduced visibilities in
scattered showers through the period, with patchy late night/early
morning fog. A few thunderstorms possible.

Monday night and Tuesday...NE winds gusting up to 20 kt mainly
across the eastern outer waters. Seas may brief build up to 5 ft
Tuesday.

&&

Box watches/warnings/advisories...

CT...none.
Massachusetts...Flash Flood Watch until 5 PM EDT this afternoon for maz020>024.
Rhode Island...none.
Marine...none.

&&

$$

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

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