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

Synopsis...
a weak front will slip south of New England by midday. Low
pressure along the mid Atlantic coast will pass offshore late in
the day. This will bring showers and isolated thunderstorms today.
Dry conditions for Saturday as high pressure builds across.
Another low will slowly pass south of New England, bringing
another round of scattered showers with a few thunderstorms
possible 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/...
low pressure over eastern Maryland this morning with a primarily
zonal flow aloft. There is a faint signal of a shortwave in this
flow that could draw the surface low farther north, but most of
the push looks out to sea. Also note the 92-knot upper jet
crossing New England with our area in the right entrance region
through early afternoon. This will support lift and pcpn
generation over southern New England... if there is enough
moisture to tap.

Precipitable water values continue to show 2+ inches along the New
England South Coast with 1.5 inches north & west. Surface dew
points in Rhode Island and southeast Massachusetts are 70-74. On the other hand, convection
over the mid Atlantic is moving more toward the ocean than toward
New England. This would intercept the inflow of additional
moisture from the south. Broad area of showers over eastern PA is
showing a northeast trend and should bring US wet weather this
morning, but its trailing edge is progressing east and should
limit the time for precipitation.

Model trends early this morning have also shifted the forecast a
little farther south. This shifts the heavier pcpn over the South
Coast or offshore, so this should be our best chance of higher
rainfall amounts. We have shifted rainfall amounts south, with 1
inch or higher south of a Willimantic-Plymouth line.

Flash Flood Watch continues in Rhode Island and southeast MA, but has been
discontinued in northern CT where expected total rainfall has
diminished. Even so, remember that extremely dry soils can also
lead to rapid runoff, which could result in rapid flooding of some
small streams and rivers, again primarily due to urban runoff.

The Storm Prediction Center marginal risk for severe weather remains along the
immediate South Coast today. Sb cape is more limited with this
run, with values of 200-500 j/kg depending on the model of choice.
Storm Prediction Center rap helicity values are forecast to stay well offshore. The
farther-south track of the low will not help, nor will the low
lapse rates. But mesoscale features could still generate an
isolated damaging wind gust.

Rain should fall this morning and early afternoon but taper off
from west to east during afternoon, with most of it done toward evening
on east Massachusetts coast. Highs will top out in 70s to around 80.

&&

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 /09z Friday through Tuesday/...
forecaster confidence levels.

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

Moderate confidence today and tonight. High confidence Saturday.

Areas of IFR/LIFR along the South Coast and islands due to a broad
area of fog and low clouds along the shore.

Showers and isolated thunderstorms moving toward east-northeast from the mid
Atlantic will bring a period of MVFR vsbys and possibly cigs
during the morning and early afternoon. Locally heavy downpours
could bring vsbys briefly to IFR. The showers taper off from west
to east this afternoon and evening. Conditions will return to VFR
tonight, except for local MVFR/IFR in patchy fog.

High pressure will maintain VFR on Saturday. The light flow will
allow sea breezes late morning through afternoon. There is also
be showers developing west of CT that could move into the state
during the late afternoon. Possible MVFR vsbys in any
showers/tstms.

Kbos taf...moderate confidence. Heavier rain and lower conditions
may stay farther south today and miss the terminal. Higher
confidence tonight.

Kbdl taf...moderate confidence. Heavier rain and lower conditions
may stay farther south today and miss the terminal. Higher
confidence tonight.

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.

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 or even severe 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 through this evening for maz017>024.
Rhode Island...Flash Flood Watch through this evening for riz001>008.
Marine...none.

&&

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

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