Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 210850
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
350 AM EST Wed Nov 21 2018

.SYNOPSIS...
A strong cold front will move through the region this afternoon and
evening. Cold high pressure will follow for Thanksgiving and Friday.
A surface low will move near or south of the region on Saturday and
Saturday night, followed quickly by another one on Monday. High
pressure may nose into the region by the middle of next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Cold front currently moving through the Great Lakes and upper
Midwest will track to the south and east today. Strong high pressure
over central Canada will build to the south and east and will be
over the upper Great Lakes by this evening. That cold front works
its way south through the region later this afternoon.

The passage of that front will be accompanied with a 90-100 kt jet
streak along a 40-45 kt low level jet. In addition, some mid-level
shortwave energy will pass through the region later this afternoon,
but the strongest shortwaves will stay to the north and east of the
local forecast area. However, there should be enough instability to
touch off some snow showers.

Coverage should be limited to mainly the Poconos and northern NJ,
and if instability is stronger than models are suggesting, there
could even be a few isolated snow squalls in Carbon and Monroe
counties. For now, will cap PoPs at slight chance for the Lehigh
Valley, southern Poconos, and northern NJ, and low end chance at
northern potions of Carbon and Monroe counties. Not expecting much
in the way of widespread snowfall, generally less than 1/4", but
parts of the southern Poconos could get 1/2-1" or so, possibly
higher in those isolated snow squalls.

Behind the passage of the cold front, west winds increase to 15-20
mph with 20-30 mph gusts.

Highs today will top off in the 30s in the southern Poconos and
northern NJ, and otherwise in the low to mid 40s for most of NJ and
southeast PA. Southern NJ and southern Delmarva will get into the
mid and upper 40s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
Cold front works its way south through the region tonight. The bulk
of the upper level energy will be north and east of the region, so
any isolated snow showers will move into the Delaware Valley and up
to the I-95 corridor in NJ to Philly. Although the bulk of the
models keep the region dry, but the WRF-ARW forecasts reflectivity
down through central NJ and southeast PA. Will only carry slight
chance PoPs. Again, not expecting much in the way of snow
accumulation, perhaps a trace or so. But this will arrive around the
evening commute.

Temps begin to fall off behind the passage of the cold front, but
the true Arctic air will arrive after midnight with the passage of a
secondary cold front. Winds diminish to 10-15 mph with gusts up to
20 mph in the evening, then winds shift to the northwest and
diminish to 5-10 mph after midnight.

Lows tonight drop into the single digits in the Poconos, and
otherwise in the teens and low 20s for most of the region, and into
the mid 20s in southern Delmarva.

Wind chills will be in the single digits/teens for most of NJ and
Delmarva, and as low as -10 in the southern Poconos.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
The medium-range forecast features a very cold end to the week, a
couple of lows affecting the region this weekend into early next
week (accompanied by a warmup), and a colder/drier finish.

Thursday and Friday will be mostly sunny but very cold, with some
record low maximum and minimum temperatures likely Thursday and
Thursday night. Winds will remain breezy on Thanksgiving Day, so the
wind chills will be low (likely near zero in the Poconos; mainly in
the single digits and lower teens north of I-78 and teens to the
south). Winds will diminish Thursday night, but this will only allow
temps to crash. Forecast highs are 20 to 30 degrees below seasonal
averages, with lows around 15 to 25 degrees below seasonal averages.
Friday should be warmer as the surface high associated with the
arctic cold begins to move offshore, establishing some semblance of
return flow. However, max temps will still likely be around or below
freezing north of I-78 and just above freezing to the south.

Midlevel ridging moves through the region Friday night and Saturday,
and this will allow for a substantial warmup (with temps on Friday
night nearly steady and back near seasonal averages for highs
Saturday). However, a surface low in the Southeast will develop and
move off the East Coast by Saturday afternoon, with warm-advection
precipitation breaking out in much of the eastern U.S. 00z
operational runs remain in fairly large disagreement, with the ECMWF
remaining much farther north with the track of the low than the GFS,
with the CMC splitting the difference to some degree. Think a
consensus approach is best, with the ECMWF likely too weak with the
polar blocking downstream and the GFS likely exhibiting its
progressive/south bias with these systems. Temperatures will be
warming on Saturday, with models continuing to suggest the
precipitation holds off until most/all of the CWA is above freezing.
Cannot rule out a brief period of messy/wintry precipitation at the
onset (Saturday morning), but this is looking less and less likely
as the event approaches. Confidence is not high enough to remove
this potential from the forecast, but this is increasingly looking
like a low-probability scenario.

How much rain falls is dependent on the proximity of the low`s
track, with the ECMWF/CMC suggesting a rather decent 1-2+ inch QPF
event for most of the area. The GFS is considerably lower with
amounts. Recent 1-2 inch rainfall events have led to a few flooding
issues, so if the heavier QPF materializes, some localized flooding
should be anticipated.

The first system moves out by Sunday morning, with a brief lull
Sunday afternoon. However, a strong surface low to the west will
lift northeastward to the Great Lakes by Sunday evening and
slow/stall as it approaches the blocky ridge to the northeast. This
leads to triple-point low development downstream near the Mid-
Atlantic/New England coast on Monday. The ECMWF remains rather
progressive, though the GFS/CMC have trended faster as well. This
may very well lead to a rather quick-hitting second (mostly) rain
event on Monday, but the blockiness to the north has me worried that
the effects could last longer than model consensus is suggesting. It
may also turn windy as the coastal manifestation of the surface low
moves northeastward away from the area. Will need to watch this
system closely, as the pattern does not need to change too much for
this to be a fairly impactful system. For now, temperatures look
rather warm for most of the event, so aside from some snow showers
on the cold side of the system as the northwest flow sets in, rain
is primarily forecast.

Trended PoPs downward and more showery on Tuesday as the low lifts
northeastward into New England and beyond. Temperatures should be
colder as the eastern North America trough reloads through
midweek.

&&

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

Today...VFR. BKN CIGs develop this afternoon at 8000-10000 feet. A
few snow showers are possible at KABE late this afternoon, but
confidence is too low to include in the TAF at this time. West winds
5-10 kt, becoming NW 15-20 kt with 20-25 kt gusts this afternoon.

Tonight...A few isolated snow showers may move through KRDG/KABE and
the I-95 corridor terminals this evening with brief sub-VFR
conditions with the passage of the cold front, but confidence is too
low to include in the TAF at this time. NW winds 15-20 kt with 25 kt
gusts this evening veer closer to the north at 10-15 kt with gusts
up to 20 kt after midnight tonight.

OUTLOOK...
Thursday: VFR with northwest winds 10 to 15 kts with gusts to 25+
kts. High confidence.

Thursday night and Friday: VFR with light winds. High confidence.

Friday night: Increasing cloudiness, with sub-VFR conditions
possible, especially late. Light easterly winds. Moderate confidence.

Saturday and Saturday night: Periods of sub-VFR likely with a good
chance of rain. Primarily east winds 5 to 15 kts, with potentially
higher gusts near the coast. Winds possibly switching to a more
northerly direction Saturday night. Moderate confidence.

Sunday: May see a period of improvement to VFR with north winds 5 to
15 kts, possibly with a few higher gusts. Moderate confidence.

&&

.MARINE...
Will go ahead and carry a Small Craft Advisory for all waters for
today and tonight. There will be a brief lull in SCA conditions this
morning, but then winds ramp back up in the afternoon. A few gale
gusts are possible on the ocean waters late this afternoon and this
evening, but at this time, it does not look as if gale gusts will be
widespread enough or last long enough to warrant issuing a Gale
Warning. NW winds 15-20 kt with 25-30 kt gusts will continue through
tonight. Seas on the ocean will build to 3 to 5 feet tonight.

A few snow showers are possible on northern NJ ocean waters late
this afternoon and this evening.

OUTLOOK...
Thursday: Small craft advisory in effect. Gales cannot be ruled out,
especially in the morning.

Thursday night: Lingering advisory-level west-northwest winds
diminishing to sub-advisory levels with time.

Friday and Friday night: Sub-advisory winds/seas expected.

Saturday: East to southeast winds will likely increase to advisory
levels, with seas approaching/exceeding 5 feet. Rain likely.

Sunday: Winds switch to northwest and may hover near advisory
levels.

&&

.CLIMATE...
There is the potential for near record low maximum AND low
minimum temperatures for the Thanksgiving holiday. Here are the
record coldest high and low temperatures across our climate
sites for Thanksgiving day, accounting for the change in date.
(Note, this is not records for November 22 itself):

Thanksgiving record low minimum:
Atlantic City, NJ: 10 (1989)
Allentown, PA: 16 (2000)
Georgetown, DE: 20 (1996)
Mount Pocono, PA: 1 (1903)
Philadelphia, PA: 20 (1901)
Reading, PA: 15 (1903)
Trenton, NJ: 20 (1930)
Wilmington , DE: 18 (1996)

Thanksgiving record low maximum:
Atlantic City, NJ: 31 (1996)
Allentown, PA: 27 (1996)
Georgetown, DE: 27 (1989)
Mount Pocono, PA: 20 (1901)
Philadelphia, PA: 27 (1901)
Reading, PA: 30 (1989)
Trenton, NJ: 30 (1930)
Wilmington , DE: 29 (1996)

Here is a list of the record low temperatures for November
22 and November 23:

Site            November 22         November 23

Allentown       15/2014             16/2000, 1964
Atlantic City   16/2014, 1964       10/1989
Georgetown      16/1987, 1969       16/1984, 1964
Mount Pocono    5/1969              6/2000, 1964
Philadelphia    14/1980             10/1880
Reading         16/1969             16/1972
Trenton         15/1880             16/1880
Wilmington      17/1964             17/1964

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 PM EST Thursday for ANZ430-431-
     450>455.

&&

$$
Synopsis...CMS
Near Term...MPS
Short Term...MPS
Long Term...CMS
Aviation...CMS/MPS
Marine...CMS/MPS
Climate...MPS


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