Forecast Discussion From Mount Holly
792
FXUS61 KPHI 171738
AFDPHI
Area Forecast Discussion
National Weather Service Mount Holly NJ
1238 PM EST Fri Jan 17 2025
.SYNOPSIS...
High pressure returns today ahead of a warm front that will
cross through our region Saturday bringing. This will bring a
brief return of milder weather and some rain to the region. A
strong arctic cold front will pass through on Sunday while a
secondary low develops for Sunday night bringing the potential
for snow to the region. Cold artic high pressure returns on
Monday and persists through the middle of next week with well
below normal temperatures expected.
&&
.NEAR TERM /THROUGH SATURDAY/...
The biggest concern in this period is a small (20%) chance for
very light freezing rain or freezing drizzle within a few hours
of sunrise Saturday morning in far northern Delaware and extreme
southeastern Pennsylvania.
For today and tonight, tranquil conditions and near normal
temperatures are expected as high pressure centered to the south
of our region slides by. Mostly clear skies are expected,
though clouds will start to build back late tonight ahead of the
warm front tomorrow.
On Saturday, our focus is on the warm front approaching from
the south. In general, this should result in very light
(generally only a few hundredths of an inch) rain during the day
time hours on Saturday. However, in the higher terrain of the
southern Poconos, any precipitation could be snow (new
accumulation less than one inch). Rain and snow amounts will be
light as the lift associated with this will be weak. Some rain
may linger by the coast Saturday evening.
As mentioned above though, one change from the previous
forecast is that there is now a very small chance that low level
moisture will increase enough ahead of the warm front arriving
for localized light freezing rain in far northern Delware and
extreme southeastern Pennsylvania. Model soundings show a cold
layer within the lowest 50 mb at the surface lingering for a few
hours after sunrise. Looking at the model soundings though,
there will also likely be a very deep dry layer above this, so
it may be more of a freezing drizzle set up. All that being
said, this is still very conditional on the air closest to the
surface being nearly saturated and staying below freezing for a
few hours after sunrise. Will be watching trends closely over
the next 24 hours.
In the wake of the warm front, high temperatures on Saturday
will be a few degrees higher than today, in the 40s south and
east of the fall line, from the mid 30s to lower 40s northwest
of the fall line. Though this system should be out of the area
by late evening Saturday evening, clouds will start building in
again Saturday night ahead of the cold front and Sunday system.
As such, temperatures may not drop off much Saturday night. Lows
are generally expected to range from the lower 20s to lower
30s.
&&
.SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/...
The focus of the short term period remains with the system
impacting the area on Sunday, with potential for widespread
snowfall accumulations.
For Sunday into Sunday night, the strong arctic cold front will
cross through the region early Sunday morning, with cold
advection of a much colder airmass beginning later in the day
and especially into the night. As it does so, an area of low
pressure will also be developing over the Deep South near the
tail-end of the arctic front. This area of low pressure will
ride along the frontal boundary and strengthen as it moves off
of the Carolina coastline by Sunday afternoon.
From here, forecast guidance still varies significantly,
considering it`s only 48-60 hours away. The track and strength
of the surface low will ultimately determine what type of
precipitation our area experiences Sunday into Sunday evening.
If the track of the low is close enough to the coast, a large
portion of the region is likely to observe accumulating snowfall
(similar to what the 00Z GFS and ECMWF depict). If the track of
the low, hugs the immediate coastline, or ends up inland over
the coastal plain, then the most significant snows will be
focused north and west of I-95 (depicted by the 00Z Canadian).
Such a solution would also mean rain for the immediate coast,
and perhaps as far inland as the I-95 corridor. The previous
suppressed solution appears to have fallen out of favor of the
deterministic and probabilistic guidance as of the 00Z suite. So
precipitation appears increasingly likely across much of our
area, but the exact detail still remain unclear based on the
track. Ultimately, the outcome probably lies somewhere between
the first two aforementioned solutions. PoPs have been increased
to 60-70% area wide. We have introduced more mention of rain
for the coast and coastal plain inland to I-95 with this
morning`s forecast update, and increased snowfall amounts a bit
farther inland from there.
So with all of this being said, the greatest potential for
significant accumulations remains for areas along the I-95
corridor and points north and west. Within this zone, forecast
amounts were increased about 1" across the board, and now stand
between 3-5", with locally higher amounts possible as usual.
Lesser amounts upwards of 1-2" remain in the forecast for the
coastal plain, and amounts for the immediate coast were lowered
slightly in light of the increasing potential for some rain to
mix in during the event. The far southern coastal areas such as
Cape May and southern Delaware likely end up with less than 1",
though potential exists for higher accumulations there.
Since warning level snowfall does not currently appear likely to
occur (though this certainly remains possible for some inland
areas), we have not issued any watches at this point. Our
current forecast support mainly a widespread advisory type of
event, but given the relatively low confidence and quickly
changing forecast, we`ll hold of on any advisories for now as
well. Obviously, a lot can change over the next 48 hours as
more hi- res/short range forecast guidance comes into the
picture. Note that the forecast is likely to change, so stay
tuned for the latest details to the forecast over the next
couple days!
Following the snow potential Sunday and Sunday evening, the
heart of the arctic airmass will begin to advect into the
region overnight. Low temperatures are mainly in the teens with
minimum wind chills in the single digits (below 0 degrees for
the Poconos). Increasing northwest winds could pose a blowing
snow threat where ever the heavier snowfall occurs.
&&
.LONG TERM /MONDAY THROUGH THURSDAY/...
In the wake of the departing system and arctic cold front, the
early to middle portion of the week will feature by far the
coldest airmass so far this season. Strong (1045+ mb),
expansive, and cold, Canadian high pressure will sink southward
out of the High Plains on Monday before gradually shifting
southeast toward the Mid- Atlantic region by Wednesday and
Thursday. With such a cold/dry airmass in place, there does not
appear to be any real chance for some precipitation during this
period, however there is a slight chance for some light snow on
Tuesday night as a system offshore skirts on by to our south.
For now, the peak of the cold looks to occur during the Monday
night through Wednesday night timeframe, as 850 mb temperatures
are forecast to be between -20C to -25C during this time! So in
terms high temps, expecting mostly teens/low 20s with low temps
in the single digits (near to below zero degrees north of I-78).
The coldest daytime periods will be Tuesday and Wednesday, with
highs in the teens area wide (single digit highs for the
Poconos). Feel- like temperatures will mostly be in the single
digits during the day and below zero at night (as low as -20
degrees in the Poconos), so cold weather advisories/warnings
will likely be needed in the coming days. As the high pressure
passes overhead Wednesday night, we`ll likely have good
radiational cooling. Assuming some areas still have a solid
snowpack on the ground by then, areas farther south into the
Philly metro, southern NJ, and Delmarva could have a shot at
getting some sub-zero actual temperatures, which has not
occurred in many years for some areas. Temperatures should begin
to rebound Thursday, though likely remaining below freezing.
Following another night in the teens Thursday night, many areas
along and southeast of I-95 should finally warm above freezing
by Friday.
There is quite high confidence that this multi-day period of
dangerously cold temperatures will occur, resulting 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. Stay tuned to the latest
forecast for additional information as we get closer.
&&
.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Rest of Today...Prevailing VFR. W winds 5-10 kts becoming more
SW/SSW with time. High confidence.
Tonight...VFR. Light S winds 5-10 kts. Some sites may see winds
become light and variable during the first half of the period.
Cirrus building in with time. High confidence.
Outlook...
Saturday through Saturday night...Brief periods of MVFR or even
IFR possible with mostly rain. Winds mainly 10 kts or less. LLWS
possible.
Sunday through Sunday night...Sub-VFR conditions probable,
especially Sunday afternoon and evening. Light to moderate snow
expected. Winds 10-15 kts.
Monday through Tuesday...VFR and breezy. No significant weather
expected.
&&
.MARINE...
SCA was allowed to expire at 9 AM. Winds and seas are expected
to stay below SCA conditions through Saturday night. On the
waters, wind gusts above 20 kt are possible Saturday, but at
this point looks like they should stay just below 25 kt.
Outlook...
Sunday...No marine headlines expected.
Sunday night through Tuesday...SCA conditions likely as wind
gusts will range between 25-30 kt with seas around 5 feet. Due
to the combination of cold air/water temperatures, gusty winds,
and enhanced seas, freezing spray accretion is likely.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.
&&
$$
SYNOPSIS...Johnson/Staarmann
NEAR TERM...Johnson/MJL
SHORT TERM...DeSilva/Staarmann
LONG TERM...DeSilva/Johnson/Staarmann
AVIATION...DeSilva/Johnson/Staarmann/Wunderlin
MARINE...DeSilva/Johnson/Staarmann
NWS PHI Office Area Forecast Discussion
