Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
911 PM EST Thursday Dec 5 2013
a cold front will cross the area tonight and move to our south on
Friday, with a series of low pressure waves moving along this front
into Saturday. High pressure is expected to build in from the
Midwest Saturday afternoon and Sunday. A warm front should then
lift north late Sunday and Monday, followed by another area of high
pressure from the Midwest for the middle of next week.
Near term /until 6 am Friday morning/...
the rain that was upstream late this afternoon no longer has the
coverage it once did. So without much rain to mix up the lower
levels and without much wind, visibilities have quickly fallen. A
dense fog advisory was issued for all of our PA zones and for
northern and central New Jersey. There's a bit more wind across our
southern zones, along the coast, and over the adjacent waters.
That said, I didn't issue an advisory for these locations.
The GFS initialization upwind looked slightly better than the
WRF-nmmb at 500mb. The GFS overall looked better at 850mb while
they were both too quick with the initial push of cold air at
925mb, but the core of the colder air in the northern plains is
colder than either model has initialized. Maybe part of the
reason for slower trending of sunday's event?
The GFS was trending too fast with the cold front passage already and its 6 hour quantitative precipitation forecast
verification areal extent was not good in the lower Ohio Valley. The
best fit is closest to the ECMWF, so we placed more of a higher pop
emphasis in the northwest part of our County Warning Area.
Given the forecast warm air advection and middle level qvec convergence, as well as the
still westward location of the 250mb jet, the best forcing aloft is
northwest. But even all of those pieces do not converge very well
(not until Friday night). We will have impulses of precipitation as the cold
front nears. This first will clip the northwestern part of our County Warning Area early,
with the next one in the Tennessee Valley slated for the overnight
hours. There was an isolated thunderstorm in the Susquehanna valley.
Temperatures are not expected to move much tonight from present
levels and the calendar day high for December 6th will likely occur
before sunrise (except maybe far se). We have opted to weigh temperatures
more heavily toward NAM MOS because of not going with the GFS faster
timing. With a broad southwest flow aloft, favor a slower cold front passage timing.
Fog, because of stronger winds aloft and most of our County Warning Area getting
into the warm sector, even if its murky, we will not issue any fog
related headlines at the present. Lowest confidence is northern
higher terrain locations in our County Warning Area where it is still foggy.
Thunder, the models are still showing some instability aloft
southeast. Overall thunder occurrence is less than modeling would
suggest with that next wave. Given the better forcing is removed
from the most unstable air aloft, we will keep it thunder free
Short term /6 am Friday morning through 6 PM Friday/...
as the front moves through our County Warning Area on early Friday morning (give or
take a couple of hours centered around 11z along the Delaware River
valley), our County Warning Area should be between impulses for a little while. We
tapered down probability of precipitation in the morning, before raising them again during
the afternoon. Once again many of the forcing mechanisms either
touch or are northwest of our County Warning Area once again favoring those areas
with the highest probability of precipitation.
Ptype issues, while southwest flow aloft "should" impede some of the
cold air, stat guidance in general is verifying too high in the Ohio
Valley behind the cold front. If one looks at the thermal fields, as they
become better aligned with the flow aloft, they slow or stop. Then a
wave of warm air advection rides along it. Because the GFS is farther east, this
stalling and wave riding (as depicted in the 925mb fields) is
farther east. The European model (ecmwf) and WRF-nmmb are farther west. Judging where
we were a couple of days ago, the modeling trend is slower. So
our mention of sleet during the daytime hours is rather late and
just in the Poconos. Bigger issues for our northwestern County Warning Area in the
Once again temperatures were weighted more toward NAM MOS because of the
cold air advection timing. A drop in temperatures will occur as the day progresses.
While there will be some gustiness in the morning just behind the
front, overall winds should not be that brisk by December standards,
although once it starts raining, it will not feel too pleasant
Long term /Friday night through Thursday/...
an active weather pattern for a good portion of the long term
forecast period with a break possible toward the end.
A frontal boundary off the coast will continue to provide a track
for waves of low pressure Friday night into Saturday.
Precipitation from these waves will affect our region through
Saturday morning, and with the infusion of cold air, a wintry mix
is expected near and north of the I-78 corridor. Rain will be the
predominate weather type through most of the Friday evening
commute, but then a changeover is expected from north to south
during the late evening hours. There is also the possibility of a
prolonged period of freezing rain...especially in the higher
elevations of Carbon/Monroe/Sussex counties. Total ice
accumulations are expected under one-quarter inch, so this would
be in the advisory range.
Precipitation should end by late Saturday morning as drier air
moves in courtesy of a high pressure system nosing in from the
Midwest. The only exception could be our far southern counties
which may see precipitation into early Saturday afternoon, but
there temperatures will be warm enough for all rain.
Then high pressure lifts into the northeast on Sunday, which will
allow a warm front to begin lifting into the area late Sunday
night. This warm front will be associated with a low pressure
system moving into the Great Lakes region at that time. As the low
continues to lift into southeast Canada on Monday, it will
eventually drag a cold front across our area. Several short
waves/vorticity impulses are expected to affect the area during
this period which will allow periods of precipitation to occur
through the time frame. With cold air in place, this precipitation
will start out as a wintry mixture on Sunday into Sunday night,
before changing to rain from south to north on Monday as warmer
air moves in. Our northern zones could hold on the the wintry
mixture longer into Monday morning as cold air could get dammed up
before the warm surge pushes through. The precipitation should
begin to dissipate Monday night into Tuesday as the frontal
boundary pushes farther offshore. There still will be a small
chance however until the front and it's moisture completely pull
Drier weather is currently expected for Wednesday and Thursday as
high pressure begins to build into the area from the west.
Aviation /02z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
The 18z tafs remain MVFR to IFR throughout with more of an emphasis
on cloudiness as being the impactor.
For the rest of this afternoon there is still low cloudiness
abounding and to our south. A pocket of clear skies is having
difficulty leaving northern Maryland and is filling. We kept
occasional ceilings as IFR with just a narrow window of exclusive MVFR
late. At the northwest terminals confidence is getting lower by the
minute about ever becoming MVFR. South winds will average 5 to 10
For this evening IFR ceilings with MVFR visibilities prevailing. We are
keeping similar wind speeds and direction.
For the overnight, continuing IFR conditions with rain moving in
as the front nears. We will carry low level wind shear for several hours
preceding the cold front passage as the low level jet is forecast to reach 40 to
50 kts at 2k. South to southwest surface winds should remain
relatively light at speeds of 5 to 10 kts.
There will be a period of improvement to MVFR ceilings and VFR visibilities
after the cold front moves through (approximately 09z-12z). Northwest
winds should average 10 kts with some gustiness for a few hours
following the cold front passage.
The next shot of overrunning rain should then bring higher
confidence MVFR conditions late Friday morning and more likely
Friday afternoon. Winds should veer to the north and become
Friday night...MVFR/IFR conditions expected with periods of rain at
the southern taf sites and a wintry mix possible at ridge and Abe.
Gusty winds in the 15-20 knots range area also possible.
Saturday-Saturday night...VFR. Gusty winds 15-20 knots possible.
Sunday-Monday night...MVFR/IFR probable with rain/wintry mix midday
Sunday into Monday. Possible improvement Monday night.
we will continue the Small Craft Advisory on the ocean through Friday. The southerly
flow is predicted to increase as are the swells. The low level jet is
predicted to reach around 50 kts which would normally translate to
about 30 kts peak gusts. This is warmer air over cooler water, so we
did lower peak gusts below this. The core of the jet is predicted to
bypass Delaware Bay, so no headlines for the Bay. We will cancel the
dense fog advisory as winds should increase mixing and the available
traffic cams are only showing isolated dense fog.
Outlook... Friday night-Monday...Small Craft Advisory, or near
advisory, conditions are possible through the period as seas may
remain near 4-6 feet and winds could gust 20-25 knots. Monday
night and Tuesday...sub-advisory conditions expected.
PA...dense fog advisory until 6 am EST Friday for paz054-055-
New Jersey...dense fog advisory until 6 am EST Friday for njz001-007>010-
Marine...Small Craft Advisory until 6 PM EST Friday for anz450>455.