555
FXUS61 KPHI 210821
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
321 AM EST Tue Jan 21 2025

.SYNOPSIS...
Arctic high pressure centered in the Plains extends into our area
through today, then it settles right over our region Wednesday
before moving offshore into Thursday. A system tracks well to our
south and east later today and tonight, with another one tracking
offshore during Thursday. A cold front moves through later Friday
with high pressure sliding across our area Saturday. The next cold
front may arrive later Sunday with a possible area of low pressure
developing along it Monday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Dangerous cold continues through tonight. No changes made to the
cold weather headlines. A Cold Weather Advisory is in place for a
large majority of the region. The exception being the Pocono
Plateau, where an Extreme Cold Warning is in place. All headlines
are in place through noon Wednesday.

Clouds continue to overspread the region this morning. Temperatures
range from the low teens to negative single digits with wind chills
generally around 0 to -15. Other than that, a quiet morning overall.

For today, clouds increase due to an approaching mid-level trough
and strengthening upper-level jet over the region. Arctic high
pressure remains over the region though, resulting in a dry day
overall. Cold airmass remains relatively unchanged, though winds
will be lighter today. Highs will only get into the mid to upper
teens/low 20s.

Another bitterly cold night expected. High pressure weakens, with an
area of low pressure passing by to the south and east. The
precipitation shield should get up into lower Delmarva and South
Jersey, resulting in a brief period of snow showers down that way.
Accumulations up to half an inch are possible, but not much to write
home about. A rather tricky low temperature forecast given the
timing of skies clearing out remains uncertain. Winds will be light
and a faster transition to clear skies will result in colder
temperatures with an ideal radiational cooling setting up. If
cloud cover is slow to move out, temperatures will remain a
touch warmer. Regardless, it will be dangerously cold. Single
digit lows are expected, with some spots in the northern Lehigh
Valley, Poconos, and northern New Jersey falling below zero.
Wind chills of -5 to 5 are expected, with values as low as -10
to -15 expected in the Pocono Plateau.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Bitterly cold conditions continuing through Wednesday night, then
not as harsh Thursday afternoon. Cold Weather Advisory/Warning
remain in effect through Wednesday morning.

Arctic air remains firmly entreched across the region through
Wednesday night. While the wind will not be all that strong, it will
not take much wind given the extent of the cold to drive down the
wind chill values. As a result, wind chills early Wednesday morning
are forecast to range from about 0F to minus 15F and then in the
teens to single digits in the afternoon. Given this extended stretch
of bitterly cold conditions (cumulative impacts), the Cold Weather
Advisory remains in effect for the entire area through Noon on
Wednesday (Extreme Cold Warning for the southern Poconos, with wind
chills as low as minus 20F at times especially across the Pocono
plateau).

The passage of an upper-level trough axis early Wednesday morning
results in the flow aloft turning more zonal for a time. This then
may back more to the southwest later Wednesday night into Thursday
as an upper-level trough starts to approach from the west during
Thursday. The airmass will modify some ahead of this trough and
therefore temperatures, after starting out very cold early Thursday
morning, climb into the lower to mid 30s for parts of the coastal
plain and 20s elsewhere. In association with the aforementioned
trough axis exiting to start Wednesday, an offshore system moves
farther away with the western extent of a snow shield also to our
south and east. Surface high pressure then crests over our area
Wednesday before shifting offshore into Thursday.

Ahead of the the next upper-level trough, which looks to have a
positive tilt, approaches Thursday night. A weak cold front should
approach our region from the northwest Thursday night, however
higher pressures linger across our area overall. Some guidance
though hinting at a quick but mainly small system spinning up off
the Mid-Atlantic coast initially focused along an inverted trough.
This feature overall should remain offshore, however will have to
monitor this in the event that some snow showers brush some of the
coastal areas.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
Summary...The cold eases with temperatures forecast to get to about
average Sunday and Monday. A few systems should move through as the
air mass continues to transition out of the bitter cold.

Synoptic Overview...An upper-level trough is forecast to be sliding
eastward across eastern Canada to the Great Lakes Friday with a
stronger shortwave extending this trough across the Gulf Coast
states. These features look to quickly shift off the East Coast
Friday night into Saturday. The next upper-level trough across
central Canada shifts south, however it may amplify southward some
from the Midwest and central Plains over the weekend. This trough
then moves eastward into early next week, however its positive tilt
would suggest stronger energy hangs back longer and this may develop
a wave of low pressure along a cold front that initially moves into
or crosses our area late Sunday and Monday.

For Friday and Saturday...An upper-level trough shifting eastward
across eastern Canada Friday may remain somewhat separated from a
strong tailend trough across the Ohio and Tennessee valley`s to the
Gulf Coast. A surface cold front may arrive into our area later
Friday associated with the northern part of this larger trough,
however this front looks to be on the weak side with limited
moisture. The upper-level trough looks to be progressive and as a
surface low develops off the Southeast U.S. coast, it should tend to
track out to sea. While temperatures will not be as harsh as earlier
in the week, at least some guidance has trended a bit colder and the
01z NBM has reflected this. Some areas especially across the coastal
plain are forecast to get to or a few degrees above freezing Friday
afternoon, and then this expands a bit more Saturday afternoon. Low
temperatures Friday night drop into the teens to single digits, then
teens to low 20s Saturday night.

For Sunday and Monday...The upper-level flow looks to be split as
northern energy continues to cruise across Canada and into the
northern tier of the U.S. Meanwhile, southern stream energy slides
out of the four corners region during the second half of the
weekend. This all results in a system tracking well to our north
with a trailing cold front arriving into our area later Sunday. This
front however may tend to stall in our vicinity as the southern
stream energy results in a surface low developing on the tailend of
the front in the vicinity of the southern Plains. This feature may
then may make a run at our area early next week, however the track,
timing and strength of it remains uncertain. This is especially the
case as the positioning of the troughs themselves will likely change
over the coming days. This changing of the pattern looks to result
in temperatures across our area getting back to right around average.
Regarding the chance for precipitation, given the uncertainty opted
to keep the PoPs on the lower side which is pretty much with the
NBM output suggests.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Through 12z...VFR. West/northwest winds gradually diminishing to
around 5 kt. High confidence.

Today...VFR with increasing clouds. West-northwest winds around 5-10
kt. High confidence.

Tonight...VFR. Northwest winds around 5 kt. A few snow showers
possible at KACY-KMIV, but only 20% chance. High confidence.

Outlook...

Wednesday through Saturday...VFR with no significant weather
anticipated.

&&

.MARINE...
A Small Craft Advisory is in effect for all ocean zones through 6 AM
with a Freezing Spray Advisory in through today. Buoys have reported
ice accretion, so caution is advised when navigating the waters.

For today and tonight once the SCA expires, sub-Small Craft Advisory
conditions are expected with west/northwest winds around 10-20 kt
and seas 2 to 3 feet.

Outlook...

Wednesday...Some wind gusts up to 25 knots possible Wednesday
morning, otherwise the conditions are anticipated to be below Small
Craft Advisory criteria. Areas of freezing spray may linger at times.

Thursday through Saturday...The conditions should be below Small
Craft Advisory criteria.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Cold Weather Advisory until noon EST Wednesday for PAZ060>062-
     070-071-101>106.
     Extreme Cold Warning until noon EST Wednesday for PAZ054-055.
NJ...Cold Weather Advisory until noon EST Wednesday for NJZ001-
     007>010-012>027.
DE...Cold Weather Advisory until noon EST Wednesday for DEZ001>004.
MD...Cold Weather Advisory until noon EST Wednesday for MDZ012-015-
     019-020.
MARINE...Freezing Spray Advisory until 6 PM EST this evening for ANZ430-
     431-450>455.
     Small Craft Advisory until 6 AM EST early this morning for
     ANZ450>455.

&&

$$

SYNOPSIS...Gorse
NEAR TERM...Hoeflich
SHORT TERM...Gorse
LONG TERM...Gorse
AVIATION...Gorse/Hoeflich
MARINE...Gorse/Hoeflich