Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 060852
Area Forecast Discussion
National Weather Service Mount Holly NJ
352 AM EST Tue Dec 6 2016
High pressure over the northeast will give way to developing low
pressure off the Carolinas into tonight. Meanwhile, low pressure
nearing James Bay sends a cold front thru our area on Wednesday,
sweeping the Carolinas low well offshore. The low pressure over
James Bay will move to Atlantic Canada into Friday, while high
pressure over the central United States moves into the middle
Atlantic this weekend. A warm front may approach our region by
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
High pressure was centered over the forecast area early this morning
while low pressure was located in Mid South. The high will retreat
into New England today as the low lifts northeastward toward the
Ohio Valley. This primary low will weaken this afternoon as the
system transfers its energy to a secondary low organizes along the
warm front near the NC coast.
National radar mosaic loop early this morning shows a broad shield
of precip with this storm over the Southeast states that had
advanced northward into southern VA. Deep convection and organized
thunderstorms was ongoing across the northern Gulf Coast states. The
leading edge of the rain will reach our southern most zones
(southern DE-eastern MD) by midday. The rain is then expected to
expand northward into the Philadelphia metro by mid afternoon and
then the I-78 corridor closer to sunset. Locations near I-80 may see
wet snow or sleet mix in with the rain initially. Moderate rain will
likely be falling over the southern half of the forecast area during
A Winter Weather Advisory was issued for the Carbon and Monroe
Counties in PA and Sussex County, NJ starting at 4 PM (see short
term section for a discussion on accumulations and impacts). There
is still some uncertainty in the onset timing of precipitation in
our northern most locations (north of I-80) but it could arrive as
early as the evening rush (around 4-6 PM). It will take some time
for the column to saturate and precipitation to reach the ground
with a residual dry airmass in place, so a best case scenario would
be for precipitation to hold off until after the commute home since
these northern-most zones will likely see snow at the onset.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
The coastal low moves off the coast (somewhere between KORF and
KHSE) early this evening. The surface low will deepen to around 1000
mb tonight as it heads out to sea. Steady precipitation will
continue into the evening across the area. Ptype will be rain across
the majority of the forecast area except the far north.
Numerous hi-res models show a band of heavier precipitation arriving
during the evening across NE PA and NW NJ. Wet snow may mix in with
the rain at times farther south into the Lehigh Valley and I-78
corridor but little or no snowfall accumulation is expected.
Forecast soundings indicate sufficient dynamical cooling during this
time to change precip to all snow in the southern Poconos and in NW
NJ (roughly NW of I-287). The latest thinking is these areas may see
several hours of moderate to locally heavy snowfall this evening. We
are forecasting snowfall accumulations of 2-4 inches mainly along
the ridges above 1000 ft in elevation from roughly I-80 northward.
An elevated warm layer should surge northward in concert with the
arrival of SW low-level jet, turning precip into all liquid
overnight across the far north. Some of the colder model solutions,
including the NAM, RGEM, and NCEP WRF NMM show temperatures
remaining steady at or just below freezing at higher elevations in
the southern Poconos during the overnight. Leaned more toward these
colder solutions as some of global models tend to erode the residual
colder layer near the surface too quickly. The FRAM based on input
from our forecast database gives us a few spots with several
hundredths of an inch of ice across northern Carbon and Monroe
The rain will gradually taper off from SW to NE late tonight but
spotty drizzle may persist thru daybreak.
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
A long wave trough over the central US during the mid-week will
gradually move east, with a ridge building over the eastern US this
weekend, followed by a shortwave traversing the upper MS Valley and
Great Lakes early next week. The primary uncertainty is with regard
to shortwave energy moving through the region on Thursday, and the
amplitude/timing of the next shortwave early next week.
For Thursday, model disagreement centers on the amount of moisture
associated with the shortwave. The NAM/GFS appear to be too robust,
with a questionable amount of moisture given the synoptic pattern.
We leaned more toward the ECMWF, and concentrated PoPs across the
northwest, where the strongest lift and deepest moisture coincide.
Overall, expect a chance of rain/snow showers on Thursday, with a
dusting of snow possible across the higher terrain.
A cold front moves through the region Thursday night, with gusty
northwest winds to follow through Friday. Model soundings show the
potential for winds to gust up to around 30 mph Friday afternoon.
During this time frame, some snow showers may also work their way
into the area, especially northwest of I-95-276-76, where the BTV
Snow Squall Parameter values are the highest.
Moving into the weekend, snow showers may continue across northwest
NJ and the Poconos into Saturday. The next storm system approaches
on Sunday, with the aforementioned model uncertainty regarding the
attendant shortwave amplitude/timing. We leaned more toward a GFS/
UKMET compromise, in between the slower ECMWF and faster Canadian.
This would bring an increase in cloudiness on Sunday, with precip
overspreading the region Sunday night. The GFS thermal profiles
support wintry precip initially with a changeover to rain Monday,
while the slower ECMWF is much warmer, suggesting a primarily rain
event. Given the antecedent cold air mass in place, wintry precip
is plausible at the onset, with a changeover to rain.
.AVIATION /09Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
VFR this morning. Rain will move into the area from S to N this
afternoon. Expect CIGs and VSBYs to drop to MVFR levels shortly
after the rain starts. A band of moderate rain is expected to move
through the area between approximately 21Z this afternoon and 03Z
this evening, resulting in IFR conditions. RA may could briefly mix
with SN at RDG and ABE, which could result in temporary restrictions
Rain tapers off from SW to NE late tonight, but low clouds will
remain over the area. Consequentially, IFR conditions will linger
into the overnight.
N-NW winds around 5 kt will veer from the NE later this morning and
then increase out of the E this afternoon. Winds then shift out of
the NE tonight. Winds may become gusty from the I-95 terminals
eastward late this afternoon and tonight.
Wednesday thru Saturday...
Predominantly VFR at all TAF sites through the period.
Northwest winds may gust up to around 25 knots Thu night into
A Gale Warning was issued for tonight for the coastal waters of DE
and S NJ as well as the lower DE Bay. Easterly winds will strengthen
to the north of a coastal low. BUFKIT soundings from several models
(non-GFS) indicate the potential for 35-40 kt gusts over these
southern zones tonight. A SCA was issued for the northern coastal
waters and for the upper DE Bay with gusts 25-30 kt possible
tonight. Seas will build to 4-7 ft in response to the strengthening
Wednesday...Although northwest wind gusts are expected to remain
below 25 knots, seas are forecast to remain elevated, at or above
5 FT. Thus, a Small Craft Advisory for hazardous seas may be
needed for the ocean waters.
Wednesday night and Thursday...Sub-SCA winds and seas anticipated.
Thursday night thru Friday night...Northwest winds are forecast to
increase Thursday night, with frequent gusts meeting SCA criteria,
along with isolated gale force gusts possible late Friday.
Saturday...Sub-SCA conditions are anticipated at this time.
PA...Winter Weather Advisory from 4 PM this afternoon to 6 AM EST
Wednesday for PAZ054-055.
NJ...Winter Weather Advisory from 4 PM this afternoon to 6 AM EST
Wednesday for NJZ001.
MARINE...Gale Warning from 6 PM this evening to 6 AM EST Wednesday for
Small Craft Advisory from 6 PM this evening to 6 AM EST
Wednesday for ANZ450-451.
Small Craft Advisory from 6 PM this evening to 4 AM EST
Wednesday for ANZ430.
Gale Warning from 6 PM this evening to 4 AM EST Wednesday for