Scientific Forecaster Discussion
fxus61 kphi 051437
Area forecast discussion
National Weather Service Mount Holly New Jersey
937 am EST Thu Dec 5 2019
high pressure will build into the region through early Friday as a
weakening area of low pressure retreats northeast into the Canadian
Maritimes. A strong, but moisture-limited cold front will sweep
across the region Friday night into Saturday, before high pressure
builds into the region for the weekend. By Sunday afternoon, a warm
front will lift northward across the region ahead of a larger storm
system and attendant cold front that is poised to affect our region
through the beginning of next week. This cold front is forecast to
cross the mid-Atlantic Tuesday, with perhaps a secondary cold front
passing through Wednesday.
Near term /until 6 PM this evening/...
the mid-morning update only extended the pops/wx for snow
showers across the Poconos into the morning later. Clouds were
also adjusted somewhat.
A fairly potent shortwave trough is moving through the northeast
early this morning. Despite a rather moisture-starved atmosphere,
sufficient lift has been generated by the vort Max to continue to
produce some scattered/occasional snow showers in the Poconos
overnight, and these will likely linger through daybreak or so
before the ascent associated with the trough moves east of the area.
Meanwhile, cold/dry advection on the upstream side of the trough in
the low levels is slowly but surely scouring lower clouds in much of
the rest of the area. This process should continue through the early
morning, but will quickly be countered by strong mixing as daytime
heating begins. Should see a midlevel cloud deck develop during the
day before diminishing late in the day, which will impede insolation
and warming to some degree.
Regarding aforementioned mixing, expect northwest winds to increase
today as the pressure gradient generated by the passing shortwave
trough (via low pressure to the northeast and building high pressure
to the southwest) increases. Wind speeds of 15 to 25 mph will be
common, with gusts to 30+ mph or so possible.
I went slightly lower than consensus guidance for highs today given
potential for cloud cover to inhibit diabatic heating. This resulted
in highs in the 30s north of I-80 and in the 40s elsewhere.
Short term /6 PM this evening through 6 am Friday/...
expect the area to be between systems tonight, as the next northwest-
flow shortwave trough moves through the Great Lakes region during
the period. A surface ridge axis will move through the northern mid-
Atlantic overnight, which should allow for quickly diminishing winds
after sunset. This may promote a fairly rapid decrease in
temperatures during the evening (if the winds decrease quickly
enough), but this cooling will be stunted overnight by increasing
mid and high clouds in advance of the Great Lakes system approaching
the area. As a result, expect temperatures to stall after midnight
as clouds inhibit radiational cooling.
The temperature forecast is tricky, as readings will be quite
sensitive to the winds and cloud cover. I was tempted to go somewhat
above guidance, but I do think there will be a period of fairly
efficient cooling, so I was not confident to stray too much from
consensus at this point. I did, however, modify hourly temperatures
to exhibit a quick drop during the evening and very little change
after midnight. Forecast lows should be near freezing in the urban
corridor and at the beaches, and in the 20s elsewhere.
By late in the night, the large-scale lift generated by the
approaching vort Max may reach the Poconos, with generation of
flurries/snow showers possible by around daybreak. Models appear to
hold off timing until later Friday morning, but would not be
surprised to see observations of snow showers by the end of the
Long term /Friday through Wednesday/...
synoptic overview...a weak shortwave perturbation will drive a
clipper-like surface low across the Great Lakes and into the
mid- Atlantic Friday. With limited moisture, only scattered
showers and snow showers are anticipated with this system. Zonal
flow will largely dominate the eastern two thirds of the Continental U.S.
Over the weekend as a larger synoptic trough begins to dig into
the western third of the country. High pressure will begin to
build across the northeast Saturday and Saturday night, exiting
the region Sunday as the aforementioned system draws a warm
front northward across the mid-Atlantic. A significant warm up
looks to be in store after the warm front lifts northward,
though bouts of rain will work against this. The system will
take its time crossing our region, with the cold front not
forecast to move offshore until Tuesday evening. High pressure
looks to build eastward out of the Tennessee Valley Wednesday
into Thursday as the system pulls off to the north and east.
There is the possibility a weak secondary cold front may push
far enough south to bring a few scattered snow showers Wednesday
night into Thursday, though the details still remain somewhat
uncertain with this.
Friday...a shortwave trough will slip across the Great Lakes and
into New England, deepening as it does so by Friday night. A weak
surface low will slip across the PA/New York border, eventually making its
way across the northern portions of our County Warning Area Friday night. The main
area of forcing for ascent still looks to remain north of our cwa,
however, a few light snow showers with light accumulations will be
possible across the Poconos and northwest New Jersey. The lack of moisture should
keep the rest of our area mostly dry through Saturday. Highs should
range from the mid 30s to near 50 across Delmarva Friday and from
the upper 20s to low 40s Saturday behind this system.
A fairly strong surface high will quickly build across the Ohio
Valley and translate into the northeast into Sunday, leading to
warmer weather as a warm front lifts northward across the mid-
Atlantic and return flow Ushers in warmer, southwesterly flow. Highs
look to range from the upper 30s to the lower 50s.
As the synoptic trough across the inner-mountain west begins to
shift eastward, a strong surface low will begin to develop across
the middle of the country. Several rounds of rain look to move out
of the Ohio Valley and into the mid-Atlantic as a series of vort
maxima roll eastward ahead of the evolving system. The first of
these looks to arrive Sunday night with another Monday afternoon
into Monday night. There is some disagreement in how quickly the
front will ultimately move through the region with the GFS being the
much more progressive of the two, while the ec keeps a more shy
approach and sluggishly pushes the front through the region, not
completing fropa until Wednesday afternoon. This could be
problematic in terms of excessive rainfall and will need to be
watched over the coming days to see if this trend continues. Highs
will be quite mild ahead of this system with widespread 50s and
possibly even 60s into Delmarva Monday and Tuesday. Slightly cooler
highs Wednesday (depending on the timing of the front, of course)
with upper 30s and 40s. A similar story Thursday with even cooler
highs in the upper 20s and 30s.
Aviation /15z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...mainly VFR, though some cigs around 3500-7000 feet may
develop/continue during the day. West to northwest winds 10 to
15 kt with gusts 20-25+ kt. Moderate confidence.
Tonight...VFR with northwest winds diminishing to around or
below 10 kt by late evening. High confidence.
Friday...generally VFR, though some rain/snow showers will be
possible mainly near and north of Abe. MVFR ceilings also possible for
ttn, pne, rdg, and Abe. Southwest winds around 10 knots, becoming
west and northwest by late in the afternoon. High confidence in
winds; moderate confidence in restrictions.
Saturday and Sunday...VFR prevailing. Northwest winds from 5-10
knots becoming light and variable Saturday night. Southerly winds
from 5 to 10 knots Sunday.
Monday...sub-VFR conditions probable as showers move into the region
early Monday morning. South-southwest winds from 10-15 knots.
Tuesday...sub-VFR conditions, especially in the morning with showers
ahead of a cold front that will pass through the region in PM.
Southwesterly winds from 10 to 15 knots with gusts from 20-25 knots
turning westerly behind the cold front Tuesday night. Low confidence.
a decent pressure gradient has set up across the waters behind the
departing potent midlevel system this morning. The strong northwest
winds will continue on the waters today. Gusts have already reached
advisory criteria on the open waters and on Delaware Bay. Advisory
conditions are expected to continue for Delaware Bay until early
this evening. Here, the advisory expires at 7 PM. On the Atlantic
waters, advisory conditions should persist through late evening,
so extended the advisory through midnight. Winds should diminish
thereafter as high pressure moves in.
Seas of 3 to 5 feet are forecast on the waters today and tonight.
Friday and Saturday...a period of Small Craft Advisory wind gusts is
possible Friday afternoon and Friday night, then winds will begin to
diminish into Saturday afternoon, dropping below 25 knots.
Sunday...sub-sca criteria expected with light and variable to
southwest winds below 10 knots.
Monday...southwesterly winds increase later in the day, with gusts
approaching 25 knots. This is less certain however given a warm air
advection regime. Seas building to 4 to 6 feet.
Tuesday...Small Craft Advisory conditions likely with southwest winds from 15 to 20
knots and gusts from 25 to 30 knots. Seas will remain elevated
from 8 to 10 feet.
Marine...Small Craft Advisory until midnight EST tonight for anz450>455.
Small Craft Advisory until 7 PM EST this evening for anz430-