441
FXUS61 KPHI 191506
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1006 AM EST Sun Jan 19 2025

.SYNOPSIS...
Low pressure across the Southeastern U.S. early this morning will
track northeastward today and be off the Mid-Atlantic coast later
this afternoon and early this evening. This low strengthens as it
continues northeastward to near the New England coast by later
tonight and then into the Canadian Maritimes into Monday. Arctic
high pressure builds into the Ohio Valley later Monday, then into
our area Tuesday into Wednesday. A system tracks to our south and
east Tuesday night and Wednesday with possibly another one at the
end of the week.

&&

.NEAR TERM /THROUGH MONDAY/...
945 AM update...Radar imagery shows an area of light snowfall
overspreading central PA and slowly filtering into eastern PA
this morning. As a result, have moved up the Winter Storm
Warnings for eastern PA and northwest NJ to all begin at 10 am.
Further south and east, precipitation has yet to organize as
well, so a start time of 1 pm for the winter weather advisory
remains as is. Additionally, the regional temperature snapshot
shows mid to upper 30s across the Philly metro, northern
Delmarva, and southern NJ with dew points inching above the 32
degree mark. This signals that precip will almost assuredly
start off as a rain/snow mix and struggle to stick at first.
That said, as motioned in the previous discussion below, the
low pressure system begins to strengthen as it moves offshore,
colder air is expected to filter in behind the low, helping
switch precip to all snow in the afternoon.

Previous discussion...As we head into the day today, that`s
when things start to get more interesting. Early in the day,
developing low pressure will be near South Carolina while the
cold front tends to stall out as it nears the coast. We are
still looking at wintry weather on the way for much of the
region an in fact the latest indications are that the low will
track a bit farther to the south and east than previously
thought which will result in a colder, snowier scenario for
areas near and south of the I-95 corridor. For this reason,
we`ve upgraded all of SE PA including Philadelphia to a Winter
Storm Warning as well as Mercer and Somerset Counties in NJ.
We`ve also expanded the Winter Weather Advisory a tier of zones
farther south to include Queen Anne`s County in MD and SE
Burlington, Ocean (excluding the coastal zone) and all of
Monmouth County in NJ. This all said, trend has also been for
the system to move in a bit later (beginning late morning to
early afternoon from SW to NE) so we`ve pushed back the start
time of the winter headlines 3 hours.

Expect precipitation to begin late morning into the early afternoon
as low pressure tracks northeast towards eastern North Carolina /
eastern Virginia. Temperatures will initially come up several
degrees through the morning so expect precip may start as a little
light rain or a rain/snow mix near the I-95 corridor with mainly snow
to the north and mainly rain farther south. However since the low
now looks to track of the coast of Virginia passing to our east off
the NJ coast this will help lock in colder N/NE winds. In addition,
dynamic and evaporational cooling effects will take hold as the
precipitation intensifies through the afternoon so expect that any
mixing at the onset will change over to snow, heavy at times, across
the I-95 corridor by the second half of the afternoon. Farther south
towards the coast, rain and mixing will last longer but even here it
should change over to a period of mostly snow by the early evening.
The heaviest precipitation looks to occur between roughly 4 and 9 PM
during which time snowfall rates could reach or exceed 1 inch per
hour as the snow falls heavy at times to do to strong forced ascent.

Heading into the latter part of the evening into the overnight, snow
will wind down SW to NE as deepening low pressure continues tracking
NE towards Cape Cod. By the time all is said and done, expect
snowfall amounts of generally 4 to 6 inches near the I-95 corridor
with 6 to as much as 9 inches north of here. South of the I-95
corridor in a swath heading from central Delmarva into southern NJ
extending NE towards coastal Monmouth County, generally expect 2 to
4 inches. South of here across our southernmost counties in Delmarva
extending towards Atlantic City NJ and points south, generally
expect 1 to 2 inches or less.

As the snow tapers off by later tonight, the shift in focus will
turn to the blustery and very cold conditions pushing into the area.
By Monday morning expect lows mostly in the teens (single digits
over the southern Poconos) with wind chills in the single digits or
colder (sub zero over the Poconos!).

Blustery conditions with a mix of sun and clouds will be on tap for
Monday as strong Canadian high pressure builds over the Northern
Plains while the departed low will be over Atlantic Canada. This
will set up a good pressure gradient with WNW winds 10 to 20 gusting
20 to 30 mph at times. High temperatures will be limited to the
teens/20s with wind chills in the single digits and teens. Enhanced
northwest winds could pose a blowing snow in areas where heavier
snowfall occurs. The very cold temperatures may allow icy and
slippery conditions to persist even though precipitation will have
ended.

&&

.SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
Dangerous cold grips the region.

An upper-level trough from the Midwest to the southern Plains Monday
night shifts eastward and slides across the Northeast and Mid-
Atlantic region later Tuesday and Tuesday night. Arctic high
pressure settles into our area Monday night into Tuesday and will
continue to deliver a bitterly cold airmass to our region. A
northwest to west breeze will add a significant chill factor however
this should ease as the center of the surface high arrives.

As for some of the details Monday night, cold weather headlines are
probable. The wind chill values are forecast to bottom out below
zero for nearly the entire region, with most from 0F to minus 10F,
however around minus 15F in the Poconos. While it will be very cold,
the extent of the dangerous cold will depend on the magnitude of the
wind through Monday night as this will drive how low the
temperatures get and also the wind chills.

As we go through Tuesday, highs are forecast to be only near 10
degrees in the Poconos to the low 20s in Delmarva. A west to
northwest wind keeps noticeable cold air advection in place. The
lows on Tuesday night range from slightly below zero in the Poconos
to around 10 degrees in Delmarva. Winds will be lighter, however
just enough to produce wind chill values of minus 10F to minus 15F
in the Poconos to near zero in Delmarva. In addition, a ripple of
energy sliding by to our south and east may try and toss some
forcing for ascent northward into at least parts of our area. There
is uncertainty with this, however some of the guidance suggests a
brush of some snow Tuesday night. Given the uncertainty with how far
west and north the snow shield can get, especially with a very dry
airmass the farther north and west one goes, kept PoPs no higher
than the chance range.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
Summary...Bitterly cold conditions continue through Thursday then
some gradual improvement takes place Friday and Saturday.

Synoptic Overview...An upper-level trough is forecast to depart the
Northeast Wednesday with the flow backing and becoming more zonal
into Thursday. Another upper-level trough amplifying from the Great
Lakes to the southern Plains shifts eastward during Thursday and
moves across the East Coast during Friday before lifting out into
Saturday. At the surface, low pressure off the North Carolina coast
to start Wednesday tracks northeast and out to sea while high
pressure builds into our area from the upper Ohio Valley. The high
shifts offshore later Thursday, then a cold front arrives later
Friday as weak low pressure offshore of the Carolina coast tracks
northeastward into Saturday.

For Wednesday and Thursday...As the upper-level flow turns more
zonal Wednesday, surface high pressure builds across our area. This
will continue the bitterly cold conditions however the pressure
gradient collapses and therefore the winds will be lessening and
therefore not as much of a wind chill factor as we go through this
time frame. Quite a bit of instability offshore thanks to the arctic
air over the Gulf Stream with an offshore front and upper-level
energy. This should result in a precipitation shield which may brush
our southern Delmarva zones to far southeastern New Jersey with a
bit of light snow WEdnesday. As of now though, this looks to mostly
miss our area as surface high pressure builds in overhead with
associated very dry air. High temperatures Wednesday in the teens to
some low 20s (colder in the Poconos), then on Thursday temperatures
get into the 20s everywhere except the teens in the Poconos.
Wednesday night should be notably colder than Thursday night with
widespread lows in the lower single digits and even some a little
below zero. A Cold Weather Advisory may be needed for parts of the
area mainly Wednesday night. A secondary piece of energy coming in
from the Tennessee Valley later Thursday may energize another system
off the Southeast U.S. coast with it tracking northeastward. Will
have to watch if this system can amplify some more and track more
westward and bring some snow to at least parts of our area. Given
some timing differences though, this potential may continue into
Friday below.

For Friday and Saturday...An upper-level trough amplifying some
eastward from the Great Lakes to the Tennessee Valley should support
one or more surface lows along an offshore front. These should be
tracking northeastward and more out to sea, however if the incoming
trough sharpens some more and can back the flow aloft then perhaps
the surface low tracks closer to the coast. A system tracking across
the Great Lakes though in the form of a weak surface low and cold
front may be a kicker and keep the offshore system there with little
or no impacts to our area. Given the uncertainty, kept any PoPs for
possible measurable precipitation on the low side. Temperatures are
forecast to not be as harsh as the arctic airmass moderates some
right into the weekend. It will still be on the cold side though,
just not as bitterly cold as previous days.

&&

.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VFR ceilings lower back to widespread MVFR around early
afternoon, then to IFR by mid/late afternoon. Precipitation
continues to develop late this morning into the early
afternoon. All snow is expected at KRDG and KABE. For KILG,
KPHL, KPNE and KTTN, mainly all snow is expected however there
may be a brief period of rain/snow mix at the onset (a bit of
sleet cannot be ruled out). For KMIV and KACY, some light rain
to start then this should mix with snow before changing to snow
prior to ending. A period of moderate (1/2SM) to heavy snow
(1/4SM) snow is probable at nearly all terminals before it ends.
West to northwest winds near 5 knots becoming north to
northeast 5-10 knots, then back to north/northwest later this
afternoon and increase to 10- 15 knots. Low confidence with
lingering fog early this morning then especially with the onset
timing of the precipitation and lower conditions.

Tonight...MVFR/IFR conditions with snow to start the evening then
conditions improve to VFR as the snow ends. Northwest winds near 15
knots with gusts 20-25 knots. Low confidence on the timing details.

Outlook...

Monday and Tuesday...VFR. West-northwest wind gusts to 25 knots
Monday, diminish some Monday night and especially Tuesday.

Wednesday and Thursday...Some snow may develop from low pressure
offshore, however this along with any sub-VFR conditions are
uncertain.

&&

.MARINE...
Winds and seas will ramp up through the day today as a low pressure
system moves up the coast. Expect Small craft Advisory conditions by
late this afternoon with a period of Gale force wind gusts expected
for tonight. Also expect rain moving in by later this afternoon
changing to snow over the waters tonight before ending. Then
freezing spray becomes a concern overnight as temperatures drop.

For Monday, winds at Small Craft Advisory levels continue along with
freezing spray.

Outlook...

Monday night...Small Craft Advisory conditions. Freezing spray
expected.

Tuesday and Wednesday...A period of Small Craft Advisory conditions
are possible later Tuesday night and Wednesday morning. Freezing
spray at times.

Thursday...The conditions are anticipated to be below Small Craft
Advisory criteria.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Storm Warning until 1 AM EST Monday for PAZ054-055-
     060>062-070-071-101>106.
NJ...Winter Weather Advisory until 1 AM EST Monday for NJZ013-014-
     016>020-027.
     Winter Storm Warning until 1 AM EST Monday for NJZ001-007>010-
     015.
     Winter Weather Advisory until 1 AM EST Monday for NJZ012.
DE...Winter Weather Advisory until 1 AM EST Monday for DEZ001.
MD...Winter Weather Advisory until 1 AM EST Monday for MDZ012-015.
MARINE...Gale Warning from 6 PM this evening to 6 AM EST Monday for
     ANZ430-431-450>455.

&&

$$

SYNOPSIS...Gorse
NEAR TERM...Fitzsimmons/MJL
SHORT TERM...Gorse/MJL
LONG TERM...Gorse
AVIATION...AKL/Fitzsimmons/Gorse
MARINE...Fitzsimmons/Gorse