Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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063
FXUS61 KPHI 070905
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
405 AM EST Wed Dec 7 2016

.SYNOPSIS...
A cold front will cross the area early today as an area of low
pressure slowly drifts through southeast Canada. This low will
continue to slowly drift eastward through the end of the week and
move offshore of the Canadian maritimes by Friday and Saturday.
With high pressure to our west, a strong west to northwest flow
will remain through the end of the week. High pressure briefly
affects the area Saturday night. An area of low pressure is
forecast to move north of the area Sunday into Sunday night,
bringing a cold front across the area.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Low pressure was positioned about 150 miles off the lower
Delmarva coast early this morning. The coastal low will move
farther out to sea today. Meanwhile, a weak cold front that was
currently located on the eastern side of the Appalachians will
progress eastward through the region later this morning and off
the coast this afternoon.

Low clouds and drizzle have persisted across the forecast area for
much of the night. RAP/HRRR forecast soundings show a saturated
layer in the lowest 10 kft before 09Z. The column will dry from top
down over the next few hours as mid-level subsidence behind last
night`s disturbance arrives and winds aloft veer out of the NW. As a
result, the drizzle will become more patchy over the next few hours.
With temperatures still hovering around 31-32F in the higher terrain
(roughly above 1000 ft in elevation) and lingering patchy drizzle
still be reported at MPO-FWN, we have decided to play it
conservatively and keep the Winter Weather Advisory in effect
through 6 AM. Do not expect much more than a trace of ice with the
freezing drizzle in these colder spots. Do not anticipate the need
to extend the headline as the drizzle should taper off for the
morning rush. The low clouds should hang around most of the morning
but there will be some breaks that start to develop in our far
western zones late this morning behind the cold front. Skies should
clear out faster from west to east this afternoon though there will
be some lingering mid-level clouds across the area as well as
stratocu in the coastal plain.

High temperatures will generally be in the 45-50F range with cooler
spots in the higher elevations of NE PA and NW NJ (mid 30s to lower
40s) and warmer spots in S DE and E MD (low 50s).

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
An incredible 1054 mb surface high over the northern Rockies is
forecast to build southward into the Great Plains tonight. An
extension from this high will ridge eastward toward the Mid-Atlantic
region tonight. Meanwhile, an area of low pressure will move across
James Bay and into northern Quebec tonight. This system`s cold front
advances southeastward into the Great Lakes region.

The pressure gradient is modest but it may still be strong enough to
produce a light W-NW across most of the area. However, the sheltered
valleys will likely see the boundary layer decouple and temperatures
dropping rather fast after sunset. Accordingly, we`ll likely see a
decent spread in temperatures tonight depending on location. High
clouds associated with a strong upper-level jet streak are expected
to move into the region from SW to NE overnight, but the impact on
radiational cooling is questionable. A blend of the MAV/MET was used
to make the min temperature forecast, but also used some of the
mesoscale model guidance (RGEM, NCEP WRF ARW/NMM) to highlight the
cooler valleys.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
On Thursday, an area of low pressure will slowly be moving across
eastern Canada, before reaching the Canadian maritimes Friday into
Saturday, while high pressure remains across the central part of
the country. This will keep a steady west to northwest flow across
the area. A broad trough aloft will remain across the east coast,
with an elongated vorticity impulse moving across the area
Thursday and a weaker impulse moving across the area Friday. There
is not a lot of moisture across the area Thursday as it`s forecast
to remain northwest of the area, but the strength of the vort max
and the surface winds may help carry some flurries/sprinkles
across the area during the day. On Friday, the short wave is
weaker, but there is more enhanced low level moisture and steep
low-mid level lapse rates during the day, so there could be even
more widespread showers or flurries/sprinkles across the area.
Especially across our northwestern areas. By Saturday, another
short wave/vorticity impulse is forecast to move across the area
as the northwest flow begins to weaken as the low lifts farther
away and high pressure begins to approach from the west. There
will continue to be steep low-mid level lapse rates during the
daytime combined with some enhanced moisture, especially across
the northern areas. Therefore there will continue to be a chance
for some isolated showers/flurries across the northern half of the
area. High pressure briefly affects the area Saturday night,
providing dry conditions. Thursday through Saturday will be a cold
and windy period, with Friday being the windiest, and Saturday
being the coolest of the 3 day period.

On Sunday night, an area of low pressure is forecast to move
north of the area, pulling a cold front across the region. Another
low pressure is forecast to develop along the boundary, although
the GFS and ECMWF differ on where they develop the low. The GFS
develops this secondary low north of the area, while the ECMWF
develops it to our south. Sunday night looks like the most likely
period for precipitation, but it could linger into Monday if the
low does develop around or south of the area as the ECMWF
indicates, so we`ll keep a chance of precipitation into Monday.
Temperatures will be cold enough for portions of the area to
experience wintry precipitation. The exact timing and placement of
wintry precipitation is still to be determined, but as usual, the
most likely areas for snow would be the northern half of the area.

Dry conditions are currently expected for Monday night into
Tuesday as high pressure is forecast to briefly build to our south
and nose its way into our area. The GFS and ECMWF are showing the
potential for a coastal low to develop on the old frontal boundary
to our south by next Wednesday, which could bring some
precipitation to the southern half of the area. With cold
temperatures, it`s possible that a period of snow could develop
for the southern portions of the area.

&&

.AVIATION /09Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

VSBYs have improved over the past few hours with RA and DZ tapering
off. CIGs vary between 600 and 1500 ft AGL across the area early
this morning. Expect low end MVFR and IFR CIGs to persist until mid
morning. Then gradual improvement from SW to NE to 1500-2500 ft CIGS
is expected to occur between about 15-18Z. The low clouds should
scatter out later in the afternoon, giving way to VFR mid to late
afternoon. Wind direction varies between NE and NW early this
morning with speeds between 5-10 kt. A preferred wind direction out
of the NW will develop by mid morning as a cold front moves thru.
Winds speeds will be around 10 kt.

VFR is expected tonight. W-NW winds will be light (under 10 kt).

OUTLOOK...

Thursday-Saturday...MVFR CIGS may be possible each morning before
lifting to VFR during the day. Scattered showers or
flurries/sprinkles are possible during the daytime of each day which
may temporarily lower conditions. Gusty west to northwest winds 15-
25 knots, strongest winds Thursday and Friday.

Sunday...VFR early, lowering to MVFR then IFR by the afternoon and
overnight. Rain or snow possible late in the day and overnight.

&&

.MARINE...
GLW was downgraded to a SCA for the southern coastal waters and
lower DE Bay. This SCA goes through the mid evening. Farther north
along the NJ coast, the SCA was extended through the afternoon. We
should still see wind gusts to 25 kt through about mid morning, then
seas will be the hazard. Seas will eventually subside to below 5 ft
late today.

Also, the SCA was canceled for the upper DE Bay.

OUTLOOK...

Thursday-Saturday...Small Craft Advisory levels winds expected
through the periods with west to northwest winds gusting 25-30
knots.

Saturday night-Sunday...Winds expected to drop below Small Craft
Advisory levels, but pick up again late Sunday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory until 6 AM EST early this morning for
     PAZ054-055.
NJ...Winter Weather Advisory until 6 AM EST early this morning for
     NJZ001.
DE...None.
MD...None.
MARINE...Gale Warning until 6 AM EST early this morning for ANZ452>455.
     Small Craft Advisory until 6 AM EST early this morning for
     ANZ450-451.
     Small Craft Advisory until 4 AM EST early this morning for
     ANZ430.
     Gale Warning until 4 AM EST early this morning for ANZ431.

&&

$$

Synopsis...Robertson
Near Term...Klein
Short Term...Klein
Long Term...Robertson
Aviation...Klein/Robertson
Marine...Klein/Robertson



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