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Area forecast discussion 
National Weather Service New York New York
544 am EDT sun Jul 31 2016

Synopsis...
low pressure slowly approaches from the Ohio Valley today. This
low slowly tracks along a frontal boundary that becomes nearly
stationary near the tri-state area with waves of low pressure
tracking along it through Monday night. High pressure begins to
build toward the region Tuesday, and moves over the area
Wednesday. The high then slides off the northeast coast Thursday.
A cold front approaches Friday.

&&

Near term /until 6 PM this evening/...
broad upper trough with embedded shortwaves move through the
region today. Area of heavy showers which prompted a Flash Flood
Warning currently exiting Putnam County. A Flood Advisory remains
in effect for Putnam until 6:45am.

Potential for two rounds of heavy rain today...mainly from Nassau
County New York and Fairfield County CT westward. The first is expected
until around mid morning. Current water vapor imagery is
indicating a shortwave moving into an increased area of Theta E
resulting in showers currently developing across New Jersey and southeast PA. Hi
res models has been signaling redevelopment across this area
between 09z and 12z. There hasn't been any thunder all night...but
a moisture laden atmosphere remains in place with a waa regime
continuing and is resulting in pw's of around 2 inches. Despite
the lack of thunder...1 to 3 inches of rainfall has occurred
across portions of the lower Hudson Valley. This area as well as
portions of NE New Jersey have received 2 to 4 inches of rainfall over the
past 2 days and with more expected today have issued a Flash Flood
Watch.

Several hi res models are indicating another round of showers and
tstms this afternoon with the potential to produce several more
inches of rain. There is some uncertainty in how far north the
warm front currently located near Delaware Bay and extending into
central PA will get...but if it does partially lift into the
area...additional instability will be realized. If convergent
inflow can develop...there will be a potential focus for heavy
rain and subsequent flash flooding. The threat also exists for
training thunderstorms with the storm motion nearly parallel to
the boundary.

Additionally...areas in central New Jersey and eastern Maryland experienced
damaging flash flooding yesterday...therefore with the threat now
lifting northward...feel the Flash Flood Watch is warranted. Wpc
has also placed most of this area in a moderate risk for excessive
rainfall.

With ridging to the east...expect the pcpn to be less intense as
it moves across Long Island and CT...thus have kept areas east of
Fairfield and Nassau out of the watch.

&&

Short term /6 PM this evening through Tuesday/...
eastern US upper trough sharpens as cutoff low tracks through
western Canada and ridging strengthens across the plains.
Additional shortwaves and waves of low pressure track along the
frontal boundary near the area through Monday night with chances
for showers and thunderstorms continuing. Difficult to pinpoint
exact timing and location of additional rainfall...but there could
be additional threats for flash flooding on Monday. Have kept chc
pops through the period for now...until things can be better
defined.

Upper trough finally passes through on Tue with rain coming to an
end.

&&

Long term /Tuesday night through Saturday/...
upper ridging then builds into the east through Thursday.

A rather vigorous shortwave will be coming onshore of the Pacific
northwest early next week and will track along the northern US,
digging a trough into the upper Midwest and Great Lakes by the end
of the week. Deepening surface low pressure will send a cold front
toward the area Friday into Saturday as the ridge moves offshore.
Humidity will be increasing for Friday and Saturday as a deep
southwesterly flow sets up.

&&

Aviation /09z Sunday through Thursday/...
multiple waves of low pressure will ride along a frontal boundary
situated south of the area through Monday. The front works north
as a warm front this morning before stalling near or just south of
the region this afternoon.

Showers will be possible at almost anytime, but are forecast to
be more widespread from 09z to 18z today, developing from SW to
NE. Thereafter, the activity should be more scattered in nature.
Isolated thunderstorms are also a possibility, but confidence of
occurrence and in timing is too low to mention in the tafs.

Widespread MVFR/IFR conditions throughout the period.

Southeast flow expected through today, then backing to the east tonight,
generally 10 kt or less. Coastal locations may be slightly
stronger.

Outlook for 06z Monday through Thursday...
Sunday night-Monday night...chance of showers and possibly a
tstm. MVFR or lower conditions possible.
Tuesday...mainly VFR, although isolated showers are possible.
Wednesday-Thursday...VFR.

&&

Marine...
a warm front approaches from the south this morning and then stalls
near or just south of the waters this afternoon. Multiple waves of
low pressure will track along the boundary through Monday night,
while high pressure off the New England coast retreats into the
western Atlantic. This will result in a mainly east/southeast flow which will
be strongest today into early this evening with possible gusts up to
20 kt. High pressure will then follow for the mid week. Sub-sca
conditions are forecast during the period.

&&

Hydrology...
a Flash Flood Watch is in effect through 11pm tonight for areas
west of and including Fairfield CT and Nassau New York. Basin average
rainfall of between 1 inch and 2 1/4 inches is expected across the
area through tonight...with the highest amounts in and around the
NYC Metro. There is the potential for heavy rain with embedded
tstms this morning...however a bigger threat is possible this
afternoon with increasing instability and heavy rain tracking
across the same areas. Thus there is the potential for several
inches of rain to fall in a short amount of time.

An additional 1/4 to 3/4 of an inch of rain is then expected from
Monday into Tue...highest totals in the lower Hudson Valley.

Dry weather returns later Tuesday and continues through Friday.

&&

Okx watches/warnings/advisories...
CT...Flash Flood Watch through this evening for ctz005-009.
New York...Flash Flood Watch through this evening for nyz067>075-176>179.
New Jersey...Flash Flood Watch through this evening for njz002-004-006-
103>108.
Marine...none.

&&

$$

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