Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
1224 am EST Sat Dec 7 2013
a frontal boundary will tend to stall just off the East Coast as some
disturbances travel along it into Saturday. High pressure will
however briefly build across the northeast later Saturday and
Sunday. An area of low pressure is then forecast to track into the
Great Lakes Sunday night and Monday with a secondary low potentially
developing near the middle Atlantic coast. 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 am this morning/...
latest radar showed a back edge to the precipitation across our western areas.
And slowly working its way eastward. So will decrease probability of precipitation with this
update across the northwestern zones. Its possible probability of precipitation could end up lower than
what it currently in the grids but will look at more data before
changing further with the main pkg issuance.
The GFS initialization was Superior to the WRF-nmmb at nearly every
level and with critical temperatures at 850mb and 925mb deciding
ptype, we decided to lean the gfs's way. WRF-nmmb quantitative precipitation forecast verification
was also too slow in exiting precipitation, overall it looks as
though the GFS and can ggem are verifying the best with the actual
There will be no present change in headlines for tonight. GFS
forecast soundings are showing more sleet and we are still seeing
snow confined to 1000-500mb thicknesses less than 547dm (a hefty
high value) to our west. The latest European model (ecmwf) run's thermal field looks
a bit more isothermal, but we dont have the opportunity to see how
much warm air it has between 850mb and 700mb. Laps soundings are
currently showing 800 mb temperatures at 5c at mpo.
Thus while we are confident about the need for the advisory, the
details will be difficult to be exactly correct, thus more sleet and
freezing rain than snow in the advisory area with snow/sleet accums
averaging 1-3" and ice up to .2" in the higher terrain of the
Poconos. The warm ground should help ameliorate some of the ice
concerns initially, but temperatures are expected to fall below
freezing and this will add to the treacherousness on untreated
The next tier south, we plan on issuing an sps, as most of the
diciest transition is expected to occur near when precipitation is
cutting off with sleet being the main non liquid ptype as we head
deeper into the evening and overnight. The concern here is more
about temperatures dropping below freezing and icing any standing water
(bridges and overpasses especially). The orientation of thermal
fields suggest if we are too conservative with extending the advisory
its more for Berks County than the rest of northern New Jersey.
Lastly the rest of our cwa, ptype should be mainly rain. While some
sleet might occur to the i95 corridor, its extent and duration is
not expected to cause any problems. While the thermal differences
have narrowed, there are still some differences to the extent of sub
freezing temperatures into the Lehigh Valley. Stat guidance is
handling upstream sites fairly well and we used a combination of
laps/mavmet and GFS surface temperatures to incorporate temperature drops. Like the
last mixed precipitation event we had, the op WRF-nmmb is the warmest at the
surface and was not followed.
The main slug of precipitation is associated with a fgen forcing band
currently in the Ohio Valley and this coupled with the position of
the jet aloft will favor the northern half of our County Warning Area for the
heaviest precipitation. This feature flies into New England, so the southern
part of our County Warning Area just gets weaker DPVA toward morning. Overall it
looks as though the precipitation will cut off quickly from SW to NE across
our County Warning Area after 06-09z. By 06z the forecast middle level qvec convergence is
confined to New Jersey.
Short term /6 am this morning through 6 PM Sunday/...
quiet, but colder day on Saturday. Early probability of precipitation were scaled back and
just touch some of our southeastern County Warning Area. Skies should clear from
north to south by afternoon. The surface winds look to bypass Lake
Ontario for our cwa, so even north we are not expecting many lake
Maximum temperatures were kept pretty close to continuity and are supported by
stat guidance. While saturday's winds will be brisk, wont be that
unusual for a December day.
Long term /Sunday night through Friday/...
a wintry precipitation event is expected to arrive Sunday before
transitioning to rain across much of the area into Monday.
The overall synoptic setup is comprised of a persistent trough in
the western U.S. This feature will send some additional pieces of
energy northeastward along and northwest of a nearly stalled
surface frontal boundary. Meanwhile, cold high pressure will be
shifting into New England with a cold air damming scenario occurring
for a time. This setup favors wintry precipitation across much of
our area at least to start, however then warmer air advecting in
will start a precipitation type transition. These setups are always
more challenging as even subtle changes in temperatures at the
surface and aloft can have a huge affect on what precipitation type
is observed at the surface. In addition, there is the potential for
a weak coastal low to develop which could keep colder air locked in
longer at the surface especially north and west of Interstate 95.
Once this system exits later Monday there could be one last wave
along the frontal zone Tuesday, then a secondary cold front looks to
move through as we transition to a large dome of high pressure
building in for the second half of the week.
For Saturday night...a lull before the next precipitation event
arrives. Strong surface high pressure is forecast to build into the
northeast toward daybreak. This will help push the moisture conduit
to our south for a time. In addition, colder air especially in the
lower levels will seep southward resulting in the development of a
cold air damming signature. The airmass is forecast to be drier from
north to south as well. As the warm air advection aloft gets underway especially to
our southwest late, clouds are forecast to increase from south to
north. We are not anticipating any precipitation during this time
frame due to high pressure building in and the influx of drier air.
For Sunday and Monday...a messy event is in the works especially for
Sunday and Sunday night. This is all courtesy of low pressure
tracking up the Ohio Valley as middle level energy accompanies a piece
of the upper-level trough in the western U.S. There is no real
blocking downstream to hold the cold surface high in place in New
England, therefore it will be sliding offshore Sunday night and
Monday. The warm air advection induced lift in combination with increasing ascent
above a cold dome at the surface will result in an area of
precipitation from south to north during Sunday.
The forecast soundings indicate nearly a classic setup across our
area with much of the County Warning Area getting some snow and sleet before
transitioning to rain from southeast to northwest. The main
challenge though is the timing of this transition and also the
actual onset of the precipitation. The forecast soundings across
much our region, but especially the central and northern areas,
reveal a pronounced dry layer for awhile. This would tend to result
in the precipitation not reaching ground. Once this saturates,
soundings are cold enough to support snow in much of the area before
the warm air advection overtakes the cooling aspect and this introduces additional
precipitation types. As for the pops, we did slow the northward
increase quite a bit during the day Sunday given the forecast dry
layer in the soundings. The farther southern areas and the coastal
locals should see some snow and sleet but change to rain quicker
with the flow off of the ocean. This transition then shifts inland,
however confidence is lower regarding the timing.
It appears that many areas will get a coating of snow and sleet
before the change to rain begins. The frozen/freezing precipitation
may hang tough for awhile across portions of eastern Pennsylvania
and northwestern New Jersey as the low-level flow remains from the
northeast longer. This is especially a concern for the Lehigh
Valley, the Poconos and into northwestern New Jersey. There remains
the potential for a surface low to develop near the middle Atlantic
coast Sunday night in response to a tightening baroclinic zone. If
this occurs and it is strong enough, the low-level winds would tend
to hang on even longer from the northeast. This would result in a
slower warming of the low-level airmass with more froze/freezing
precipitation. It does appear that the 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.
Regarding hazards, there is a good chance for 1-4 inches of snow and
sleet to accumulate north and west of Philadelphia with a greater
chance as one GOES toward the Poconos. Farther to the southeast, some
snow and sleet accumulation is anticipated before a change to rain
occurs. We are generally leaning toward this being an advisory level
event. Given the ongoing system with some wintry precipitation
expected, we held off on headlines for this system. This event
however continues to be highlighted in our hazardous weather outlook
and also in our briefing package. Temperatures were a model blend
with a tip toward a non-diurnal trend set in the hourly grids Sunday
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 /06z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Conds were VFR very Erly this am of many areas, with some MVFR.
Precipitation was ending to the west. Expect MVFR/VFR with some pockets of
IFR through the nighttime hours.
Precipitation is expected to be exiting our region around daybreak, and
improvement to MVFR conditions is anticipated during this time
frame. Continued improvement to VFR conditions is forecast by middle-
morning Saturday as high pressure begins to build in from the
Winds during the daytime Saturday will be mainly out of the
north-northwest around 10 to 12 knots with gusts to 20 knots.
Saturday night...VFR. Northwesterly winds 10-15 knots.
Sunday through Monday...VFR to start Sunday, then clouds increase
and lower with MVFR to IFR developing. A period of snow and sleet
should overspread the area from south to north starting around midday
Sunday. There should be a transition to rain from southeast to
northwest Sunday night into Monday with kabe and krdg hanging onto
frozen/freezing precipitation the longest. There is higher
confidence with the precipitation, however lower confidence
regarding the arrival time and timing of the changeover. Some
improvement to VFR should occur Monday night especially north and
west of kphl.
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
we are going to extend the Small Craft Advisory on the ocean waters through 18z
Saturday. While seas are just touching criteria, a northerly cold
air advection surge tonight should bring a renewed chance for not
only seas remaining at Small Craft Advisory criteria, but also winds. We will also
issue a Small Craft Advisory for Delaware Bay for the overnight and morning periods
because of this enhanced wind field.
Saturday night...northerly winds may gust to near 25 knots as colder
air advects in with building high pressure.
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.
PA...Winter Weather Advisory until 7 am EST this morning for paz054-
New Jersey...Winter Weather Advisory until 7 am EST this morning for njz001.
Marine...Small Craft Advisory until 1 PM EST this afternoon for anz430-