000
FXUS61 KPHI 141711
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1211 PM EST Thu Dec 14 2017

.SYNOPSIS...
Low pressure will move quickly to the north and east of the
area this morning. High pressure will build in from the west for
the rest of today through Friday. A low pressure system will
move up the Mid Atlantic coast on Friday, remaining well
offshore. High pressure then returns for the weekend. A weak low
pressure system will move through Sunday night into Monday with
another system expected around midweek.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Midday update...Changes to the hourly temperatures were needed
to account for the more pronounced drop in temperatures that
occurred behind the cold front late this morning. Expect
temperatures to not change a whole lot at current readings
through mid afternoon.


Previous discussion...Mid morning surface analysis shows an
area of low pressure located about 250 miles east of the NJ
coast or 100 miles south of Cape Cod. A cold front with this
system was moving southeastward across Delmarva and southern NJ.

The low pressure system and associated mid-level shortwave
trough that brought light snowfall to most of the area last
night and early this morning will move farther away from the
area today while high pressure builds in from the west. Subsidence
in its wake will lead to a sunny afternoon after we deal with a
few hours of CAA stratocu late this morning immediately in wake
of the fropa.

West-northwest winds will become breezy late this morning and
afternoon in response to a tightening pressure gradient between
the downstream low and upstream high pressure. Hourly
temperatures were updated for the 930 AM ESTF to take into
account a non-diurnal temperature curve: Temperatures should
struggle to rise much during the afternoon when CAA behind the
front strengthens enough to predominately offset diurnal
heating. Forecast highs are in the low to mid 20s for the
Poconos, around freezing in the Lehigh Valley, upper 30s to
lower 40s for the I-95 corridor and mid 40s for far southern NJ,
southern DE and adjacent eastern MD (with these highs occurring
by around midday in many spots).

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
High pressure moves into the Mid-Atlantic tonight, and this will
cause the winds to subside after dark. Mostly clear skies to start
the night will set the stage for a sharp drop in temperatures during
the evening, likely exacerbated in areas where remaining snow exists
(i.e., especially in the north). Generally went below guidance for
lows, though with increasing cloudiness likely to occur late, there
is some uncertainty here. The increasing cloud cover will be aided
by a 250-mb jet streak encompassing much of the eastern U.S.
downstream of another midlevel vort max entering the Great Lakes
region. The main question is timing, with faster approach of the
clouds likely resulting in warmer lows. Current forecasts are for an
increase rapidly after midnight, which should be ample time for
temperatures to plummet in the evening.

Speaking of lows, they are forecast to range from single digits in
the Poconos to around 20 in the Philly metro area. Bundle up.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Friday through Friday night...High pressure will make its way
into the area overnight and cross the region on Friday. A
coastal low develops along the Carolina coast and makes its way
up the Mid Atlantic coast. The models continue to show this
system remaining offshore and out of our area. However, some
moisture may reach into our area and we could see some snow/rain
fall towards the afternoon and into the evening. Additionally,
a couple of shortwaves will rotate through the area on Friday
with a another clipper system staying to the north of the area.
Guidance shows the moisture from the clipper drying out before
it reaches the region but with a strong northwest flow, we could
see some streamers make it from the lakes to our area.

Saturday through Sunday morning...High pressure slides across
the eastern United States Saturday, bringing some beautiful late
fall weather. The weather should remain quiet through at least
early Sunday before the next system arrives. Saturday may be a
bit windy, especially along the coast. Warmer air filters in
from the southwest but we won`t really start to feel the
difference until Sunday. Temperatures are expected to be in the
30s to lower 40s on Saturday and in the upper 30s to upper 40s
on Sunday.

Sunday afternoon through Monday...A weak low pressure system
will approach the region sometime around Sunday night into
Monday. While the models are not too robust with moisture, it
seems more likely underdone and we expect a period of rain or
mixed precipitation through the region. However, with
differences in timing and strength of the low, confidence
remains low.

Tuesday through Wednesday...The models diverge with how to
handle a strong low pressure system to our north and the
attendant cold front. Indications are we will see some
precipitation occur sometime in the Tuesday/Wednesday timeframe
but confidence is low with respect to coverage and timing.

&&

.AVIATION /17Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Today...SN had ended across the area earlier this morning. With
a W-NW winds increasing late this morning, BLSN could lead to
localized/brief reductions in visibility to MVFR where fresh
snow fell early this morning. This has been occurring so far
this morning at the terminals north of PHL.

Gusty west or northwest winds will occur from 15Z to 22Z for
most of the area (speeds 10-15 kts with gusts 20-25 kts or so).
High confidence.

Tonight...VFR with northwest winds around or below 10 kts.
Increasing mid and high clouds late. High confidence.

Outlook...

Friday...Mainly VFR conditions expected. MVFR or lower possible
in snow showers Friday afternoon and evening. Light winds.
Confidence: Moderate

Saturday...Mainly VFR conditions expected. West winds around 10
to 15 knots possible. Gusts up to 20 knots possible, mainly at
KACY and KMIV. Confidence: Moderate

Sunday through Monday...Mainly VFR conditions expected. MVFR
conditions in snow/rain showers possible Sunday afternoon
through Monday. Southwest winds around 5 to 10 knots.
Confidence: Low

&&

.MARINE...
GLW for ANZ454-455 was allowed to expire at noon EST and then
replaced with a SCA for the afternoon and evening.

Elsewhere, a SCA is in effect thru the afternoon for the DE Bay
waters and thru this evening for the coastal NJ waters.

Seas should remain elevated in the 4-7 ft range for the coastal
waters this afternoon, but as northwest flow develops this
afternoon, there should be a diminishing trend this evening.

Outlook...

Friday...Sub-advisory conditions are expected.

Friday night through Saturday night...Small Craft Advisory
conditions possible. West to northwest winds with gusts around
25 to 30 knots. Gale force gusts possible but confidence is low
at this time. Seas will increase and near 5 feet on Saturday.

Sunday through Monday...Sub-advisory conditions expected.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 10 PM EST this evening for
     ANZ450>455.
     Small Craft Advisory until 6 PM EST this evening for ANZ430-
     431.

&&

$$
Synopsis...Meola
Near Term...Klein
Short Term...CMS
Long Term...Meola
Aviation...CMS/Klein/Meola
Marine...CMS/Klein/Meola

NWS PHI Office Area Forecast Discussion