Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Mount Holly New Jersey
1111 PM EST Sat Feb 13 2016

Arctic high pressure centered over the Midwest late this evening
will build over our region through the remainder of the weekend.
The high then retreats off the coast Sunday night and Monday. Low
pressure is forecast to develop over the northern Gulf Coast
states Monday and then track northeastward up the eastern Seaboard
Monday night and Tuesday. A cold front should pass through the
middle- Atlantic region Wednesday. High pressure returns for Thursday
and Friday. Another cold front may approach from the west early
next weekend.


Near term /until 6 am Sunday morning/...
with incoming Arctic high pressure spreading east into the middle-
Atlantic, the drier air mass deepens across our region. Not many
adjustments needed to overnight low temperatures as the forecast looks on
track, but we did use the lav/lamp guidance to capture the latest
hourly dewpoint and temperature trends into tonight. Gradual clearing is
expected through the night, and there could be some west-
northwest wind gusts to around 25 to 30mph through the remainder
of the late evening. Otherwise, the wind chill headlines continue
as much of the forecast area will feel wind chills to between 10
to 20 below zero, with wind chills ranging 20 to 30 below zero
across the Poconos and northwest New Jersey.


Short term /6 am Sunday morning through 6 PM Sunday/...
high pressure will crest over the region Sunday. Skies are expected
to be clear and winds will be rather light. Very cold morning
temperatures will moderate only into the teens in most areas. There
may be some low 20s across the Delaware-Maryland-Virginia however. Winds will be mostly
west or northwest at 5 to 10 miles per hour.


Long term /Sunday night through Saturday/...
Arctic high pressure settles over the area Sunday evening. High
clouds will start to stream in from the west but should be thin and
not have much of an impact on radiational cooling. Therefore, temperatures
should drop rather quickly after sunset under light winds. The high
will begin to move off the coast overnight while clouds will lower
and thicken ahead of the approaching storm system. Therefore, temperatures
may not drop as much and even rise overnight. Regardless, it will be
a cold night with lows in the single digits in the rural areas of
eastern PA and New Jersey...and teens in the cities, along the coast and in
the Delaware-Maryland-Virginia.

Our attention then turns to a potentially significant winter storm
that is expected to impact the middle-Atlantic region early in week.
The trend has been for a slightly quicker arrival of precipitation on
Monday as a strengthening southerly winds aloft produces isentropic lift.
The overrunning precipitation will be cold enough to start as snow
everywhere. It may come in quick enough to bring light snow
accumulations that could impact the morning commute on president's
day, especially toward the Delaware-Maryland-Virginia.

The synoptic setup to lock the cold air in place will not be there
with this event as the high is retreating farther offshore on
Monday. The big question is how quickly will it take for the dome of
residual Arctic air in the low-levels to retreat? The models from
yesterday's runs were showing a more amplified upstream trough with
a negative tilt that allowed for a stronger low to develop and track
farther inland (over central PA on tuesday). The European model (ecmwf) has held onto
this idea as well even with the latest run. Aside from the ECMWF,
today's 12z runs have trended a bit more progressive with the
trough, which would result in a weaker low that tracks closer to the
I-95 corridor. A track like this would imply that the warm front
will not have much resistance reaching areas to the east of the fall
line and thus the changeover from snow to rain would occur quickly
along and east of I-95 Monday afternoon and evening from southeast to northwest.
Still cannot rule out a brief period of sleet/freezing rain though
during the transition, especially with the near record cold leading
up to it. An inch or two of snow is possible before the changeover.

Farther north and west toward Berks County, the Lehigh Valley,
Poconos and northwest NJ, there will likely be more resistance from
the shallow cold air if the low tracks along or just to the east as
consensus indicates. Thus, the threat for an extended period of
mixed wintry precipitation is highest in northeast PA and northwest New Jersey with
this event. The changeover to snow may be delayed until Monday nigh
for these inland areas and would be followed by a period of sleet
and freezing rain. Some of the guidance has the warmer air
infiltrating these far northwestern areas, allowing temperatures to rise
above freezing and for precipitation to change to rain much earlier than we
forecast. Am a bit skeptical of these warm solutions as the cold air
tends to hang on longer than modeled in northeast PA and
northwestern NJ, where the orography plays a role in trapping the
shallow cold air near the surface especially in sheltered valleys.
Would not be surprised to see temperatures struggling to go above
freezing throughout the duration of the event toward the Poconos/
Delaware Water Gap/High Point. The latest snowfall forecast that
went out with the afternoon update is for 3-5 inches of snow in our
far northern and western zones as well as one- to two-tenths inch of
ice. Locally higher amounts above one-quarter inch seems possible in
the ice-storm prone Poconos. Since there is still a fair amount of
uncertainty this far out regarding snow/ice amounts and impacts, we
will hold off on a Winter Storm Watch for now.

There is a potential for heavy rain, particularly Tuesday morning,
and thus the other concern is flooding. Please see our Hydro section
below for more details.

Northwest flow will develop after the wave of low pressure tracking
along the front moves north of US into the northeast states. This
will cause the cold front to move offshore Tuesday afternoon. The
rain (except possibly a wintry mix in the far northwestern zones)
will end from southwest to northeast during this time.

After a lull in precipitation late Tuesday, another round of lighter precipitation
in association with the trailing upper shortwave trough will pivot
through the area on Wednesday. Colder air will also work into the
area so the precipitation may change over to snow in the higher elevations
and either rain/snow mix or just rain elsewhere.

High pressure builds in Thursday and Friday resulting in dry
conditions. Below normal temperatures on Thursday return to near normal

The high moves offshore by the start of the weekend as a storm
system approaches from the Great Lakes. The track of the low favors
mainly rain for US except maybe snow at the onset in interior
sections. The system looks to be embedded in a rather fast zone flow
pattern, so we are currently not anticipating a significant rainfall


Aviation /04z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.

VFR conditions will continue at the taf sites tonight and Sunday.
The drying airmass should yield less in the way of snow shower
activity tonight, as compared to earlier during Saturday. The
winds will remain gusty through the evening to around 25 or even
30 knots and then gradually diminish some late tonight. On Sunday, VFR
conditions with mostly clear skies are expected. Winds west to northwest at 8 to
12 knots, then decreasing late-day.

Sunday night...VFR and light winds.

Monday and Monday night...snow overspreading the area during the day
(perhaps morning across the southern terminals in the morning and
afternoon toward Abe. The snow may start to mix with or change to
rain late in the day, especially at ilg-miv-Acy. The transition to
rain will work into the rest of the I-95 terminals by evening. IFR
conditions likely. There is a potential for a brief period of
freezing rain during that transition. The bigger concern is for
several hours of freezing rain toward ridge and Abe. Potential for
low level wind shear Monday night with a strong thermal inversion aloft and a southerly
jet near 60 knots at 2 kft above ground level.

Tuesday...rain may be heavy at times in the morning. IFR conditions
likely. The rain will exit the area during the afternoon from SW to
NE and improve to VFR in its wake.

Tuesday night and Wednesday...mainly VFR. However, light rain may
move in and could be accompanied by local restrictions to MVFR.

Wednesday through Friday...VFR.


we've upgraded the freezing spray advisory to a warning as several
sites are showing upper end of moderate rates, close to warning
rates, and it's only going to get colder. The rest of the marine
headlines will continue with gales and freezing spray The Hazards
which will continue through the night and into Sunday morning.
There may be a few scattered snow showers, but other than that
just cold gusty winds and fair weather.


Sunday night and marine headlines expected.

Monday night and Tuesday...southerly winds increase but mixing potential
will be limited with warm air atop the colder waters. Sustained
winds 25-30 knots in our coastal waters may struggle to bring down gale
force gusts outside of convective transport. Seas will build to 6-10
feet in the Atlantic coastal waters Monday night and Tuesday.

Tuesday night and Wednesday...winds will weaken but seas may remain
elevated to at or above 5 ft, which would warrant a Small Craft Advisory.

Wednesday and Thursday....NW winds late Wednesday and Thursday
behind a cold front may gust to near Small Craft Advisory threshold.


there is potential for flooding across the hsa into the early part
of this week as heavier precipitation is expected to overspread the
region late Monday night through Tuesday. There's still a lot that
needs to unfold, but the 5-day wpc quantitative precipitation forecast of 1.25 to 2.50 inches of
liquid combined with another 0.25 to 0.50 inch of swe on the ground
would very likely cause nuisance low-lying flooding/poor drainage
flooding and possibly some minor to perhaps moderate river flooding
(rivers associated with our forecast points). This assumes most, if
not all, the precipitation falls as rain. However, if the
precipitation starts off as snow or ice, the extent of the flooding
threat decreases. At this time, the Raritan and Passaic basins have
a somewhat higher potential for possible river flooding as these
areas may be on the higher end of expected quantitative precipitation forecast amounts, but there
could also be some possible flooding concerns into eastern PA.

The uncertainty with this set-up is large. The precipitation is
heaviest late Monday night through Tuesday. The track of the low
pressure will play a role in determining temperatures and not only
affect what type of precipitation falls, but also affect what
remaining swe melts. There's about 0.25 to 0.50 inch of swe
scattered across the area. We are in a better spot as compared to
the 3rd and 4th when there was in excess of 2.00 of swe along the I-
78 corridor. As an fyi, one of the main reasons we didn't see the
flooding, that models suggested, on the 3rd and 4th was because all
the swe did not rapidly come out of the snowpack. Snowmelt can be a
tough nut to crack.

While the lack of swe is looking better across the hsa, not all the
rivers have receded back to where they were prior to the 3rd.
So rivers will start higher and the cold this weekend will surely
get the ground hard for a very efficient runoff.


near record cold is still expected for portions of our area
tomorrow morning...and the coldest air of the season so far.

Best chance for record equaling or exceeding for Valentines day:
kabe, kttn, kphl and kmpo.

Record low Max's can only occur for this event on Sunday the
14th, if at all. Those have been added recently below.

Site 2015-16 coldest so far record low Feb. 14th rer low maximum
--------------------------- -------------------- -----------

Abe 8 -1 in 1979 14-1979

Acy 10 -6 in 1979 13-1979

Phl 12 +2 in 1979 14-1979

Ilg 12 -4 in 1979 13-1979

Ridge 10 -4 in 1983

Ttn 9 0 in 1916

Ged 13 -7 in 1979

Mpo -1 -12 in 1970


we are experiencing difficulty uploading our latest storm total
snow and ice maps to our website this evening. We are working to
identify the source of this problem and to correct this issue so
that these images will Post to our website soon.


Phi watches/warnings/advisories...
PA...Wind Chill Advisory until 9 am EST Sunday for paz060>062-070-
Wind chill warning until 1 PM EST Sunday for paz054-055.
New Jersey...Wind Chill Advisory until 9 am EST Sunday for njz007>010-
Wind chill warning until 1 PM EST Sunday for njz001.
Delaware...Wind Chill Advisory until 9 am EST Sunday for dez001>004.
Maryland...Wind Chill Advisory until 9 am EST Sunday for mdz008-012-015-
Marine...heavy freezing spray warning until 10 am EST Sunday for anz430-
Gale Warning until 10 am EST Sunday for anz430-431-450>455.


near term...Kline
short term...O'Hara
long term...Klein

National Weather Service Glossary of Abbreviations