NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Dec 15, 5:42 PM EST, 1 hour 18 minutes earlier.
Area forecast discussion, Dec 15, 5:42 PM EST, 1 hour 18 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
542 PM EST Thu Dec 15 2016
.SYNOPSIS...
High pressure in the Ohio Valley tonight moves to the mid-Atlantic
on Friday before moving offshore Friday night. Low pressure moving
from the Ohio Valley to Saint Lawrence Valley will lead to a warm
frontal passage on Saturday, followed by a cold frontal passage on
Sunday. A sprawling area of high pressure will settle over the mid-
Atlantic in the Monday thru Wednesday time frame, prior to another
warm front approaching the region on Thursday.
&&
.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
The artic air mass over the Midwest this afternoon, will shift
towards the Ohio Valley tonight then into Virginia by Friday
morning. Some light snow showers continue across the region this
afternoon in the cold air advection. Winds will remain gusty through
at least this evening and early overnight hours before they really
start to ramp down across the region.
With the winds remaining up after sundown and the temperatures
dropping into the single digits across Carbon and Monroe
counties,we expect wind chill values to drop down into the negative
teens. As a result, we have gone ahead and issued a Wind Chill
Advisory through Friday morning. This is first real cold night of
this winter season and you will need to be sure to cover any exposed
skin if you are venturing outside.
Elsewhere, temperatures will range from the upper single digits in
the the Lehigh Valley into Central New Jersey to the lower to mid-
teens across Delmarva, Southern New Jersey and the Philly metro
area.
&&
.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
Friday will start off very cold but temperatures should start to
rebound some as the sun comes up. While it will not be warm by any
stretch of the imagination, it will feel warmer due to the fact that
the winds will be much less gusty than they were today. In addition,
the flow turns more southerly as the high starts to move over the
area and then offshore.
Maximum temperatures on Friday will remain in the 20s, possibly
nearly 30 in some spots. The Poconos and NW NJ may have some areas
that remain in the upper teens.
&&
.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
..a winter storm watch (marginal combined half inch of freezing/frozen
pcpn upon very cold ground) is being considered for the region
from Berks County newd through the Poconos, Lehigh valley into
far nw NJ..
The time frame would vary in the two segments...Pocs and Sussex
Cty NJ roughly 06z-21z Saturday and RDG-ABE-Warren Cty NJ 06z-18z.
We see this as a 1-4" snow event with .05 to .25 icing on top.
There could be a period of MDT snow near and north of I80 Saturday
morning around sunrise.
What we do know: plan for hazardous roads for a time Saturday
morning, even down to I-95, then once treatments take effect,
improving conditions, especially Noon to 4 Pm as the pcpn lessens
and temps warm.
Friday night and Saturday- A warm front will approach the region
leading to a mixed bag of precipitation Friday night into
Saturday. A Winter Weather Advisory will likely be needed for
portions of the region northwest of I-95. A breakdown of P-Type,
timing, amounts, and impacts, including our confidence for each,
follows:
P-Type...The column will be cold enough for precipitation to begin
as snow across the entire region. Both model soundings and cross-
sections indicate strong warm air advection after midnight Friday
night as a southerly low-level jet moves across the area. The 1000
to 850 hPa low-level thicknesses indicate snow being the dominant
p-type through much of the night, with the exception of Delmarva
and far southern NJ, where a changeover to rain will occur late.
Both model soundings and cross sections indicate a bit of a warm
nose (above freezing layer) developing from 800 to 850 hPa to the
northwest of the I-95, which will support a brief period of sleet
over this area from south to north Saturday morning, prior to a
change over to freezing rain. Given more robust warm air advection
in the boundary layer does not occur until later Saturday morning,
a period of freezing rain is likely, especially northward of I-195
and PA Turnpike. Overall, moderate confidence.
Timing...Precipitation will overspread the region Friday night, but
with departing high pressure nearby, it will have to overcome a dry
air mass in place near the surface. By midnight Friday night, precip
should be reaching the ground northwest of I-95, but may struggle
to reach further east of here until late. The heavier precipitation
is expected to commence after midnight, northward of the I-195 and
PA Turnpike corridors, where deeper moisture is co-located with
stronger isentropic ascent. In fact, the heaviest precip is
favored in the late Friday night into Saturday morning period,
with the bulls eye between the I-80 and I-195>PA Turnpike
corridors. Overall, moderate confidence.
Amounts...In terms of snowfall, the highest amounts are expected
close to I-80. The snow is expected to be generally light, with a
heavier burst a few hours either side of sunrise Saturday, just
prior to the change-over. There is a narrow window in this period,
where upward vertical motion is maximized within the optimal Snow
Growth Zone (SGZ) region. In addition, the NAM also indicates some
frontogenesis. However, model snow-liquid ratio (SLR) values are
expected to decrease rapidly during this time frame, while heavier
QPF may also be shifting southward with the stronger isentropic
lift. Thus, we have decreased snowfall amounts from the previous
forecast south of I-80. While sleet is expected to be minimal, a
longer period of freezing rain is expected Saturday morning, with
measurable ice glaze as far south as the I-195 to PA Turnpike
corridors. The amount of ice is dependent on how quickly 2-meter
temps warm Saturday morning, as well as the placement of the
heavier precip during this period. The greatest ice glaze is
likely between the I-80 and I-195>PA Turnpike corridors, where
some locations may receive up to around 0.10" with isolated 0.2"
amounts. Overall, moderate confidence in snow amounts and low
confidence in ice amounts.
Impacts...
Hazardous travel conditions are possible Friday night into Saturday
morning northwest of I-95, and into Saturday afternoon north of I-78.
Saturday night thru Sunday night-
A brief lull in the precip Saturday evening before the cold front
delivers a second batch of precip late Saturday night into Sunday.
Could see some heavier downpours during this time frame. Improving
weather Sunday night night.
Monday thru Thursday- Above average uncertainty during this
period as the aforementioned front stalls just to the south of the
region and waves of low pressure develop along it. While high
pressure is expected to remain dominant, cannot rule out some
precip making it north, especially closer to the coast. It would
be cold enough for some of this precip to be in the form of snow,
during the overnight hours.
&&
.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Tonight...Mainly VFR conditions are expected to continue through the
TAF period at all of our terminals. Gusty northwest winds around 20
to 25 knots with gusts of 35 to 40 knots will start to diminish
later this evening. Winds will remain gusty overnight but gusts
should fall to 20 to 30 knots later this evening.
Friday...VFR conditions are expected. Winds will back to the
southwest and lose any remaining gustiness by mid to late morning.
Skies will be mostly clear but expect mid-high level cloudiness to
start moving in by the afternoon.
OUTLOOK...
Fri night and Sat...Mixed precipitation likely, especially from
the I-95 corridors and northwest. Predominantly IFR. Southwest
winds may gust up to around 20 knots. Moderate confidence.
Saturday night and Sunday...Rain and fog across the terminals.
Predominantly IFR. Southwest winds becoming northwest with gusts
up to around 20 knots.
Sunday night thru Tuesday...Predominantly VFR.
&&
.MARINE...
Gale Warning remains in effect for the area waters through early
Friday morning.
We may cancel the Low Water advisory shortly. Trends are that the
blowout tide will fall short of the -2.0 MLLW criterion.
Freezing Spray is occurring in the nearshore waters according to
our Buoy algorithms. So while there may only be a 2 ft chop where
the water temp is less than 44F, any vessel passing through this
into the chop, will throw spray on deck and it will likely freeze
in the 20s airmass temp.
The strong cold air advection that has been bringing gusty west
northwest winds to the waters will continue through this evening.
Winds will be around 20 to 30 knots with gusts mainly around 30
to 40 knots. A few gusts around 45 may occur earlier this evening.
Winds will start to subside as we head through the overnight hours
and we will lose the gale force gusts as well. With the gusty winds
occurring and seas temperatures in the low to mid 40s, we may
continue to see some freezing spray occur.
Seas remain elevated around 5 to 7 feet on the ocean and 2 to 5 feet
in the Delaware Bay. Seas on the ocean front should fall below
advisory criteria by Friday morning.
OUTLOOK...
Friday night...Sub-SCA conditions expected.
Saturday thru Sunday night...SCA conditions anticipated.
Monday thru Tuesday...Sub-SCA conditions expected.
&&
.CLIMATE...
Our first true shot of polar cold has moved into the region but
its not quite as cold as outlooked several days ago and so we've
withdrawn the record potential for today and Friday.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Wind Chill Advisory from 9 PM this evening to 9 AM EST Friday
for PAZ054-055.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Warning until 6 AM EST Friday for ANZ430-431-450>455.
Low Water Advisory until 9 PM EST this evening for ANZ430-431.
&&
Synopsis...Franck
Near Term...Meola
Short Term...Meola
Long Term...Drag/Franck/Meola 541
Aviation...Franck/Meola
Marine...Franck/Meola
Climate...541