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Area forecast discussion 
National Weather Service Mount Holly New Jersey
409 am EDT Tuesday Apr 21 2015

Synopsis...
a cold front will move rapidly offshore this morning. Reinforcing
cold fronts will move through the Middle Atlantic States on Wednesday
and Thursday. A low pressure system should then slide south of
the area passing off the Carolina coast on Saturday. A high
pressure will gradually build in from the west through the
remainder of the weekend into Monday.

&&

Near term /until 6 PM this evening/...
the cold front continues to push across the area early this morning
and will make its way off the coast by sunrise. There remains
widespread showers and a few isolated thunderstorms across much of
the area. This activity will lift out of the area through sunrise as
a short wave approaches and lifts north of the area. The showers may
linger for a couple of hours across the northern tier of the area,
but should end by mid-morning. Skies should begin to clear out
through the day, and a fair amount of sunshine should develop across
much of the area.

Another short wave/vorticity impulse is forecast to approach the
area later in the day. With steep low/middle level lapse rates and some
low/middle level moisture forecast across our northern areas, there
could be some isolated showers develop later in the day. So we
include a slight chance of showers this afternoon across the
northern tier of the area.

A blend of mav/met MOS, mosguide, and mixed 925 mb European model (ecmwf)
temperatures was used for highs today, which brings everyone pretty
close to normal.

&&

Short term /6 PM this evening through 6 am Wednesday/...
any showers that develop during the day are expected to have
diminished by sunset as we lose daytime heating. We do expect an
increase in middle-level cloud cover as a short wave/vorticity impulse
slides across the area. Right now it does not look like there will
be deep enough moisture to create any showers, so we keep the
forecast dry.

A blend of mav/met MOS and mosguide was used for overnight lows,
which brings everyone pretty close to normal.

&&

Long term /Wednesday through Monday/...
the long term will feature an all too familiar pattern of closed
lows lingering across southeast Canada and normal to cooler than
normal weather pattern prevailing. The GFS and WRF 500mb
initialization looked good. They both were intermittently too cold
at 850mb and 925mb at upstream sites.

On Wednesday, a reinforcing cold front is predicted to move
through our County Warning Area. This has decent middle and upper level support (qvec
convergence, exit region of the 250mb jet) as well as predicted
low level Theta-E ridging. Coming quickly on the heels of the
current system there will not be much air mass recovery for
heavier precipitation. But forecast soundings are showing the inverted v
look and there could be some gusty showers or thunderstorms as precipitation moves
across the area. Both the NAM and GFS are fairly bullish with wind
gusts reaching or exceeding 35 miles per hour. Given decent modeling
consensus about showers, we upped probability of precipitation to likely. There are
differences as to whether or not a ribbon of instability capable
of producing thunder will be sufficient. We added low chances
during the afternoon over the southeastern two thirds of our County Warning Area
where daytime heating should have more time to generate
instability. Oddly a faster cold front passage did not Transfer into lower GFS
MOS maximum temperatures and we prefer their higher number based on forecast
925mb temperatures.

Unlike now where precipitation is lingering and lingering and lingering,
showers/thunderstorms Wednesday should be more diurnally driven and decrease
rapidly with sunset. The one exception is far northwest where a
short wave rounding the base of the 500mb trough could keep precipitation
chances going into the night and Thursday morning. Higher terrain
locations would be cold enough for possibly some snow showers.

Between the channelized vorticity maximum and convective temperatures within reach,
self destruct sunshine should be occurring on Thursday. Once
again GFS MOS agrees more with the nam's thermal profiles than its
colder own. Not much change from continuity, a cooler step down
of maximum temperatures.

With the surface and upper low lingering over southeastern Canada, there
should be too much of a gradient for radiational cooling in places where
the growing season has made strides the last week or so. Sub freezing
mins were kept far north.

Another breezy but dry day Friday. Possibly less self destruct
sunshine than Thursday as predicted moisture is less. Maximum temperatures
though are forecast to be similar to Thursday, cooler than normal.
Friday night has a greater chance to be a frosty one as weak high
pressure comes closer to our County Warning Area.

On Saturday (this sounds like a winter scenario), a southern slider
low pressure system is predicted to pass off the Carolina coast. As
long as the antecedent upper closed/surface low remain lingering over
the Canadian Maritimes, this new low may not have the ability to
turn the corner and affect US. The latest European model (ecmwf) backed off on precipitation
extent and we kept the low chance far south because of its closer
proximity to the exiting low.

Then not much change for Sunday and Monday as the air mass should
recover slightly as the maritime system's cold air supply becomes
staler.

&&

Aviation /08z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

The cold front continues to push across the area early this morning
and will make its way off the coast by sunrise. There remains
widespread showers and a few isolated thunderstorms across the taf
sites. Where the heaviest rain has fallen, MVFR or IFR conditions
have been occurring. Fog has also developed at ridge. As the showers
end and west-northwest winds develop across the area, conditions are
expected to become VFR everywhere. Once VFR conditions return, they
will remain through the taf period.

Winds will increase out of the west-northwest, especially after
sunrise and we being to mix strong winds from aloft. Wind gusts of
25-30 knots are expected through the day. The gusts will drop off
around or after sunset tonight and become more southwest overnight.

Outlook...
Wednesday...mainly VFR conditions expected with sub-VFR showers and
thunderstorms possible as a cold front crosses the region. Gusty southwest
to west winds should exceed 30 knots, especially in proximity to
any showers.

Wednesday night through Friday night...mainly VFR conditions
expected. Some gusty daytime northwest winds expected.

Saturday...mainly VFR conditions...lowest confidence
southern airports as showers might occur there.

&&

Marine...
the Small Craft Advisory remains on the ocean as wind gusts of 25
knots should increase today behind the cold front and seas will
remain elevated. The advisory remains in place on the lower Delaware
Bay and we have expanded the advisory into the northern Delaware Bay
as winds will increase today behind the front on the Bay. All of the
advisories end at 6 PM tonight, although it is possible the advisory
on the ocean may need an extension in time for a few hours this
evening if seas take longer to subside.

Outlook...
Wednesday...an increasing southwest gradient preceding the
next cold front could bring Small Craft Advisory conditions
back to all of the area waters.

Wednesday night through Thursday...borderline Small Craft
Advisory conditions are outlooked as a northwest gradient
wind will persist.

Thursday night and Friday...near Small Craft Advisory conditions
are outlooked. The persistent northwest flow might weaken slightly.

Friday night and Saturday...greater confidence about sub Small
Craft Advisory conditions.

&&

Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...Small Craft Advisory until 6 PM EDT this evening for anz430-
431-450>455.

&&

$$
Synopsis...gigi/meola
near term...Gaines/gigi/Robertson
short term...Robertson
long term...gigi
aviation...gigi/Robertson/meola
marine...gigi/Robertson/meola

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