577
FXUS61 KPHI 180831
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
331 AM EST Sat Jan 18 2025

.SYNOPSIS...
A warm front will continue lifting north of the area through
this morning. A cold front will then push south and east into
the area late tonight before a low pressure system develops
along it and lifts north and east up the coast Sunday into
Sunday night. Thereafter, a bitterly cold airmass builds in
through the middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As an expansive upper-level trough continues to become carved out
from about central Canada to the southern Plains through tonight, a
leading shortwave trough will lift up across the Mid-Atlantic and
Northeast later this morning and into this afternoon. This feature
will be accompanied by a south-southwest low-level jet of around 40
knots at 925 mb. This will assist in increasing the warm air
advection for a time today ahead of a strong cold front that will
arrive later tonight. A very weak wave of low pressure arrives by
midday along with a warm front also advancing northward at least
some. The main warming looks to get into the coastal plain areas
with high temperatures this afternoon climbing through the 40s with
even 50 degrees possible in southern Delaware. The overall forcing
for ascent with this feature looks to be on the weak side, however
some guidance is showing a brief period of somewhat stronger lift
northward especially across northern New Jersey back into northeast
Pennsylvania. This should result in some rain showers developing
from south to north during the mid to late morning hours.

The model forecast soundings show some initial moistening in the
lowest levels with some lingering dry air aloft for a time, and this
can sometimes lead to some drizzle (freezing drizzle where
temperatures are at or below freezing). However, the overall air
mass initially looks to dry to support much of anything at the
surface and therefore will need to wait until some deeper moistening
occurs later in the morning. By this time, temperatures for most
areas are forecast to be above freezing along with increasing dew
points. As a result, mainly some rain showers. Some snow showers are
expected though across mostly the Pocono area given colder air
remaining longer, with even some sleet possible for a time. Any
snow/sleet accumulations there are expected to be less than an inch.
The warming layer aloft looks weak enough plus with some initial
column cooling due to drier air should keep the risk of any light
freezing rain rather low. Elsewhere, rainfall amounts look to be
less than a tenth of an inch with the greatest PoPs peaking for a
time across the northern half or so of the region. Any showers
should start to end by mid to late afternoon.

As we go through tonight, a strong cold front is forecast to
gradually slide across our region however lift looks rather weak and
therefore no precipitation is currently forecast with it. More
cloudiness however will linger and a light wind will eventually
start to shift to the west and northwest overnight as the front
moves through. Some drier and colder air will then start to seep
south and eastward through the early morning hours on Sunday.
Temperatures are forecast to drop below freezing across the region
overnight, however most surfaces should be drying off by then to
limit any icy spots. We then quickly look back to the west and
southwest as a strong shortwave shifts across the Tennessee Valley
region late tonight. This will begin to activate some lift
approaching from the southwest toward daybreak. This may start to
bring in some light rain/snow (possible sleet) toward some of our
southwest counties close to daybreak Sunday. The main show though
holds off until during the day Sunday and therefore PoPs are on the
very low side until then.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
The main focus for this period is the storm system still on tap to
affect the area Sunday followed bitterly cold air in its wake
heading into the new week.

Looking at the big picture as far as the Sunday storm system, there
are still some notable differences in computer model guidance
regarding the strength and track of the low and this will impact
both the precipitation types and amounts. To start the period early
Sunday, there will be a cold front moving in from the north and west
that will tend to stall out as it nears the coast. This frontal
boundary will stretch all the way down into the southeastern states
and it is along the boundary that there will be a developing low
located over the Carolinas early Sunday morning. Meanwhile, there
will be energy aloft rounding the base of a large and deep long wave
trough setting up over the central and eastern CONUS. As this energy
aloft interacts with the front and developing low, expect low
pressure to move north and east through the day Sunday while
beginning to deepen. As alluded to above, there are still
differences in guidance regarding the track the low takes with the
NAM and the ECMWF favoring a track more offshore which would result
in a weaker but colder system for the area. Meanwhile the GEM
continues to be notably far west compared to other guidance as it
tracks the low from Virginia through Delmarva and NJ towards Long
Island. If this track were to verify, we would be looking at a
mainly rain situation from around the I-95 corridor south/east with
heavier snow farther north and west. Tend to think the truth will
lie in the middle with a track roughly off the coast of Virginia
just east of NJ towards Long Island as the system deepens. This
should favor snow, mixing at times with rain and sleet near the
urban corridor with mainly rain near the coast and mainly snow north
and west of the urban corridor. In terms of timing, expect the
precip to move in SW to NE beginning by mid to late morning Sunday
with the heaviest falling during the afternoon into the early
evening. In the areas that see all snow such as Berks County into
the Lehigh Valley north and northeast through the southern Poconos
and NW NJ, snowfall rates could exceed 1 inch per hour Sunday
afternoon as the surface low deepens to the east and an area of mid
level F-GEN forcing sets up. Meanwhile near the I-95 corridor, this
is where the forecast will be the trickiest in terms of Ptype. With
temperatures right around if not just above freezing here as the
precip begins, it may actually start as rain before turning over to
snow as dynamic and evaporational cooling effects take hold. Then,
as the low tracks up the coast through the day, a mix with or change
back to rain or sleet may occur in the afternoon for a time before
any mixing flips back to snow before ending Sunday evening. Farther
south and east towards the coast, mainly rain and mixed
precipitation is favored. Again, it`s worth stressing that there is
still uncertainty in the forecast. If the low tracks farther N/W
closer to what the GEM is showing, even the I-95 corridor could end
up with mainly rain. While a track a bit farther south and east
could mean mainly snow for these areas and potentially even closer
to the coast as well. The good news is that this will be a fast
moving system so expect precip to quickly taper off SW to NE through
Sunday evening. In terms of our forecast snow amounts, there really
hasn`t been much change with this update. We are still looking at
the potential for 5 to 10 inches over the areas in eastern PA into
northern NJ that are under a Winter Storm Watch with 3 to 5 inches
through the urban corridor and 1 to 3 inches heading farther south
and east towards the coast. In fact near the coast, our forecast
amounts are currently for less than an inch.

In wake of the system on Sunday night, we`ll start to see the
beginning of the cold air mass take aim at the area. Skies will
begin to gradually clear late Sunday night into Monday morning, and
with a fresh snowpack in place, this should result in low
temperatures falling into the single digits/teens for most of the
area. With a stiff northwest wind filling in on the backside of the
departing low, we should see wind chills in the single digits for
most. Mostly clear skies will be on tap for Monday as strong
Canadian high pressure builds over the Northern Plains. 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.

Monday night is when we really get into the dangerously cold weather
with cold weather headlines near certain. Continuing westerly
breezes along with lows in the single digits to below zero, all
areas look to see sub zero wind chill values with wind chills in
the minus 10s to minus 20s across our N/W zones from Berks
County through the Lehigh Valley into the southern Poconos.
These dangerously cold temperatures could result in impacts to
people, animals, and infrastructure. Dangerously cold conditions
for people and animals are possible resulting in an increased
risk of hypothermia and frost bite. For infrastructure, impacts
could include freezing water pipes and high demand for heating
energy.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Dangerously cold weather will continue to impact the area through at
least the first part of the long term as an arctic airmass remains
in place. In fact Tuesday looks to be the coldest day with most of
the area not even getting to 20 degrees and the southern Poconos
struggling to get to 10. Dry weather persists though through the
day. It does potentially get interesting again Tuesday night as a
new low will be passing by to our south. This could brush the area
with a period of some snow but it looks like the heaviest
precipitation should stay to our south. It remains extremely cold
through Wednesday with not much improvement in temperatures compared
to Tuesday.

Late this coming week the airmass starts to modify somewhat with
with highs getting back into the 20s and 30s. Though these
temperatures will still be a good 10-15 degrees below average.
Another system moving up the coast could affect the area by next
Thursday night into Friday but confidence is low on this. Similar to
the midweek system, the heaviest precip may stay just off to our
south and east.

&&

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

Through today...VFR ceilings lower to a period of mainly MVFR
from about mid-morning onward along with some rain showers.
Some IFR ceilings cannot be ruled out. Visibilities lower to
MVFR at times with the rain showers. Light and variable winds
becoming south or southeast 5-10 knots, then becoming southwest
this afternoon. Low-level wind shear through about early this
afternoon as winds near 2,000 feet AGL increase out of the
south-southwest around 40 knots. Low confidence regarding timing
details.

Tonight...VFR ceilings overall should start to lower to MVFR late at
night and more toward daybreak Sunday. Southwest winds around 5
knots becoming light and variable, then becoming west to northwest
around 5 knots late. Low confidence regarding ceiling heights.

Outlook...

Sunday and Sunday night...Sub VFR with periods of IFR likely.
For RDG and ABE, snow likely. For ILG, PHL, PNE, TTN, snow may
mix at times with sleet and rain. For MIV and ACY, there could
mainly rain is favored though precip could both start and end as
snow. Precip winds down by later Sunday evening with a return to
VFR.

Monday through Tuesday...VFR. Northwest wind gusts up to 30
knots.

Tuesday night through Wednesday...Sub-VFR conditions possible.
Watching low pressure development, which could bring winter
precipitation to the region.

&&

.MARINE...
Small Craft Advisory remains in effect through early this afternoon
from Little Egg Inlet, NJ northward as southerly winds gust to 25
knots for a time. Otherwise, the conditions are anticipated to
remain below Small Craft Advisory criteria through tonight.

Outlook...

Sunday...No marine headlines expected. Rain and/or snow likely.

Sunday night through Monday...Gales possible Sunday night (Gale
Watch in effect) with SCA conditions likely on Monday.

Monday night...SCA conditions likely. Due to the combination of cold
air/water temperatures, gusty winds, and seas, freezing spray
accretion is likely.

Tuesday through Wednesday...SCA conditions not currently
anticipated. Watching low pressure development offshore. Some
freezing spray accretion possible at times due to combination of
cold air/water temperatures, gusty winds, seas.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Storm Watch from Sunday morning through late Sunday
     night for PAZ054-055-062.
     Winter Storm Watch from Sunday morning through Sunday evening
     for PAZ060-061-101>105.
NJ...Winter Storm Watch from Sunday morning through late Sunday
     night for NJZ001-007>009.
DE...None.
MD...None.
MARINE...Gale Watch from Sunday evening through late Sunday night for
     ANZ430-431-450>455.
     Small Craft Advisory until 1 PM EST this afternoon for ANZ450-
     451.

&&

$$

SYNOPSIS...Fitzsimmons/Wunderlin
NEAR TERM...Gorse
SHORT TERM...DeSilva/Fitzsimmons
LONG TERM...Fitzsimmons
AVIATION...Fitzsimmons/Gorse
MARINE...Fitzsimmons/Gorse/Wunderlin