Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
905 am EST Sat Dec 7 2013
a cold front still remains to our south and east today as a cold
Canadian high pressure system noses in briefly before pushing off to
the northeast by Sunday morning. A complex low pressure system will
then enter into the lower Great Lakes Sunday evening while a
secondary low pressure may form along the middle-Atlantic coast on
Monday. An associated cold front is forecast to cross our area later
Monday with a secondary front moving through Tuesday night or
Wednesday. A strong area of high pressure then builds in during
Wednesday and Thursday before gradually shifting to our east Friday.
Near term /until 6 PM this evening/...
strong cold high pressure in the Midwest is forecast to build
eastward through today and arrive into the northeast. A northerly
flow on the east side of this high will direct colder and especially
drier air into our County Warning Area during the course of the day. There is an
inversion present on the 12z Sterling, Virginia radiosonde observation however there is also
a pronounced dry layer above it. There is some lake effect snow
occurring off of the eastern Great Lakes, however this should remain
closer to the lakes given the more westerly flow component on the
12z Buffalo radiosonde observation.
Overall, lingering lower cloud bases are expected to thin out as
further drying is forecast. There will be some high cloudiness
around but even this may be pushed to our south for the afternoon.
As a result, most places should have an increase in sunshine
especially this afternoon. The cold air advection combined with a tighter pressure
gradient will result in an uptick in the winds with it being brisk
The hourly grids were adjusted based on the latest observations and
also incorporated the lamp/lav guidance into the afternoon. No major
changes made to the high temperatures at this time.
Short term /6 PM this evening through 6 am Sunday/...
the aforementioned high pressure will be over the region tonight, so
expect dry weather with increasing clouds in advance of the next system.
Some wind and the cloud cover by morning will prevent temperatures from
Long term /Sunday through Friday/...
still expecting a very complex system to eject into the lower Great
Lakes Sunday night into Monday as precipitation overspreads the
region from the south-southwest during this timeframe. A coastal
wave could develop along the old baroclinic zone, the old Arctic
front, and move towards the region Monday morning providing better
frontogenetic forcing and moderate precipitation...it also poses
ptype issues given the thermal profiles. It should be noted that the
00z GFS looked better at all thermal levels when compared to the
NAM, though both were a tad colder upstream and the ec was somewhere
Getting into it...
surface wave to our west and possibly to the east will allow decent
warm air advection to kick in as surface ridging retreats further
east-northeast during the day Sunday. Thermal profiles show that
everyone should be cold enough to see a light snowfall spread south
to north, though all the while the moisture aloft will be combating
dry surface air, so not quite sure how much will actually make it to
the ground, and ultimately stick. The better chance for accumulation
would be along portions of the Eastern Shore of Maryland, due to
some cold air damming feature allowing boundary layer temperatures
to stay right around 30f, and then north and west of Philadelphia,
but later in the evening as steadier precipitation moves in.
Still expecting a rapid changeover to occur across a majority of our
zones by Monday morning, though the surface temperatures will be the
tricky part. Depending on the placement of the coastal low, enough
of a northeast flow could stay in place to keep surface temperatures
from warming above freezing for a few more hours, more than what is
currently anticipated. This would mean more freezing rain, though
actual accretion amounts still need to be refined, providing at
least a winter weather advisorish type of event...still need to work
out a few more details.
Precipitation should start to taper off from northwest to southeast
later Monday, although another piece of energy running along the
baroclinic zone could toss some moisture back into at least our
southeast zones Monday night. This is of lower confidence.
For Tuesday...as an upper-level trough digs across the Midwest and
Great Lakes region, this may help to induce another wave along the
baroclinic zone to our southeast. This could result in a
precipitation shield tracking across our southeast zones especially
for a time. The details in this are of low confidence especially
related to the timing of the energy. Therefore, we favored mostly
wpc probability of precipitation which is in the slight chance to chance range with the highest
across the southeast.
For Wednesday through Friday...an upper-level trough will be moving
through the east. At the surface, strong high pressure will build
into the middle Atlantic and northeast Wednesday and especially
Thursday. The cyclonic northwesterly flow and cold air will result
in lake effect snow and some of this may get into our far northwest
zones. Most of the activity however should be confined closer to the
Great Lakes. The flow then starts to back Thursday night and Friday
and surface high pressure is forecast to gradually shift off the middle
Atlantic coast. We are anticipating cold temperatures during this
time frame although some moderation may start Friday, however at
this point it generally looks dry. We only carried some flurries
across the Poconos Wednesday and Thursday. The wpc guidance was
mainly used during this time frame as more focus was put on the
Sunday and Monday event.
Aviation /14z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today...overall VFR. Some local MVFR ceilings are expected to thin
out to VFR. North-northwest winds 10-15 knots, with some gusts to 20
Tonight...VFR. High cloudiness increases especially late from south
to north. North-northeast winds around 5 knots.
Sunday and Monday...increasing clouds along with chances for
snow/rain/sleet to fall by late afternoon. MVFR to IFR conditions
will prevail. Transition over to all rain by early Monday morning,
though there could be a few hours of freezing rain at ridge/Abe
before doing so. High confidence in precipitation type with
moderate confidence on transition timing.
Tuesday and Wednesday...some MVFR possible mainly south and east of
kphl Tuesday, otherwise generally VFR. Some rain snow possible
Tuesday mainly south and east of kphl, however confidence is on the
low side. Northwesterly winds 10-15 knots with possible gusts up to
based on latest observation and guidance will extend the Small Craft Advisory on the ocean
until 00z. It may need to be extended further, especially across the
S. Will keep the 1 PM expiration time for the Bay and monitor this
to see if any additional extensions are needed. Precipitation will come to
an end across the waters later this morning as high pressure builds in
from the Midwest.
Sunday and Monday...the pressure gradient may relax some for
sub-advisory conditions for a time Sunday. As the flow turns to the
northeast and increases some, gusts could near advisory criteria
later Sunday into Monday. This is as low pressure tracks into the
Great Lakes with a secondary low potentially developing near the middle
Atlantic coast. The seas should also build some with the
Tuesday and Wednesday...much colder air to arrive under a west to
northwest flow. There may be enough flow along with vertical mixing
for some gusts to near 30-35 knots. As of now, any gale potential is
of low confidence.
Marine...Small Craft Advisory until 7 PM EST this evening for
Small Craft Advisory until 1 PM EST this afternoon for anz430-