Scientific Forecaster Discussion
fxus61 kphi 121833
Area forecast discussion
National Weather Service Mount Holly New Jersey
133 PM EST Tue Dec 12 2017
a low pressure system will cross to the north of the region today,
dragging a cold front through this evening. High pressure builds in
for Wednesday. A weak low pressure system will cross the region on
Thursday with another system moving up the coast on Friday. A brief
return to high pressure for the start of the weekend with yet
another system moving through our area late Sunday or Monday.
Near term /until 6 PM this evening/...
overall, fcst in good shape. Temperatures were adjusted upward a
few degrees across the Delaware valley with continued warm air
advection ahead of the approaching front.
Forecast looks to be on track given the most recent guidance.
Surface low is moving rapidly into the northeast early this morning,
with warm air surging northward in our area downstream. Most
precipitation is located to the north and west of the area this
morning, but this will change as the attendant vort Max moves
through the Appalachians this morning. However, as this occurs, the
vorticity will become elongated owing to downstream ridging, and
large-scale ascent will be weakening. The vort Max will also pivot
northeastward, taking the strongest lift mostly to the north of the
region. Thus, the best chances for precipitation this morning will
be associated with retreating isentropic ascent in the northern County Warning Area
and with and just in advance of the main cold front. Regarding the
former, convection-allowing models (cams) are in solid agreement,
generating around 0.05-0.15 inches of liquid-equivalent precipitation
thru 15z, generally north of the I-78 corridor. The precipitation
associated with the frontal passage appears to initiate late
this morning as jet dynamics improve (left- front quadrant of a
strong cyclonically-curved upper- level jet streak along the
coast of the southeast U.S.) And a kicking perturbation ejects
eastward into the mid-Atlantic (i.E., Favorable differential
cyclonic vorticity advection).
Both precipitation regimes will be brief, light, and generally low-
impact, but thermal profiles are favorable for a wintry mix to begin
with in portions of the Lehigh Valley and all snow in the southern
Poconos, even with near-surface temperatures warming above freezing.
Meanwhile, precipitation will be all liquid to the south as this
first frontal band moves through (generally 15z to 20z). However,
cold air will spill into the County Warning Area rapidly, and low-level
thermodynamic profiles become favorable for snow showers, even with
surface temperatures perhaps several degrees above freezing this
afternoon. With strong low-level winds and residual low-level
moisture, snow squalls are a distinct possibility, especially north
of the philly Metro. This threat looks to be mainly confined to the
afternoon hours and should end quickly after sunset.
The other story will be the winds, with west/northwest surface gusts
kicking up quickly after cold frontal passage. Expect gusts 25 to 35
mph, possibly a little higher, just after frontal passage.
Temperatures will fall noticeably during the late afternoon, setting
up a blustery period the next couple of days.
Regarding temperatures, stuck close to a blend of continuity, hi-res
2-M temperatures, and the lav. Statistical guidance will likely
perform poorly today owing to the rapidly evolving synoptic and
Short term /6 PM this evening through 6 am Wednesday/...
snow showers may occasionally impact the far northern County Warning Area tonight as
cold-air advection continues in a northwest-flow regime. Lake-effect
streamers look to affect at least the Poconos, where pops are
highest, but the models look pretty dry, so kept a dry forecast
after 00z virtually everywhere else. Will need to watch the late
night period, though, when another potent vort Max in a never-ending
series reaches the mid-Atlantic. This may enhance lift and readily
generate snow showers in the higher terrain of the far northwest
County Warning Area. Total snowfall for this area should generally be two inches or
less, though most of it may fall in the showery regime this
afternoon and tonight rather than early this morning.
The winds and the chill will be the main story tonight, however,
with west to northwest winds howling (though likely sub-advisory).
With temperatures falling to near 10 in the Poconos and the lower
20s in the urban corridor and the beaches, this will be a bitterly
cold night for the area. Wind chills near or below zero are forecast
for the southern Poconos, and will only be about 10 degrees higher
in the greater philly Metro area. Needless to say (but I will
anyway), bundle up.
Long term /Wednesday through Monday/...
a series of low pressure systems will cross the region through the
extended period bringing repeated chances for some snow.
Wednesday...windy day ahead as the low continues to slowly move to
the northeast. Cold air will fill in across the region and even
though we will see a decent amount of sun across the area, highs
will be cold and remain in the 20s to lower 30s. With the winds
blowing it will feel even colder as it will feel more like it is in
the single digits to teens outside.
Wednesday night through Thursday...the winds start to decrease
through the overnight period and that bitter chill will start to
wane. Overnight lows will drop back in to the teens to lower 20s.
A strong shortwave and a weak surface low will cross the region late
Wednesday into Thursday. This clipper system brings a chance for
snow showers across the area. The cold air looks to remain in place
but enough warming may occur closer to the coast to allow for some
mixing with rain to occur. The precipitation should be clearing out
by the afternoon with any lingering showers clearing the coast
through Thursday evening.
Thursday night...high pressure will cross the region overnight and
we should have a brief break between systems. Lows will fall into
the teens to mid 20s across the region.
Friday through Friday night... some disagreement in the models with
respect to Friday and a low pressure system developing to our south
and then up the coast. Current guidance has the low staying south
and moving to the northeast but just how much moisture will make it
to our area is unknown as the proximity to the low will be key.
Precipitation may end up being mixed across parts of the area but
uncertainty as to how much warm air can make it in exits. The GFS
has a deeper low and has it closer to the coast while the European model (ecmwf)
continues to have a much weaker low (until it reaches the higher
latitudes) and further from the coast. Will continue to mention the
chance for some snow (maybe some mix along the coast) for Friday.
Both models show the low pulling away fairly quickly so any
precipitation should end from west to east Friday night.
Saturday through Sunday...high pressure will slide across the
southeastern United States on Saturday, bringing US a day of quiet
weather. The weather should remain quiet through at least part of
the day on Sunday. Precipitation from the next system may start to
arrive by Sunday afternoon but confidence is low with respect to
timing this far out. Temperatures look to be a bit more moderate
over the weekend as warmer air arrives. Temperatures look to be in
the 30s to lower 40s on Saturday and in the upper 30s to upper 40s
Sunday night through Monday...another system will approach the
region sometime around Sunday night or Monday. With the warm air
slated to hang around through Monday, we may see a broader area of
rain and/or mix of rain and snow as the precipitation overspreads
the region. Lots of uncertainty with this late period system.
Aviation /18z Tuesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today...generally VFR, though brief sub-VFR conditions may occur
with a band of showers that develops and progresses through the
area later this morning and this afternoon. Cigs should
generally stay in the 3500-7000 ft range today outside of
precipitation. Rain/snow showers may develop this afternoon,
especially north of phl, with a snow squall not out of the
question. So far, coverage and timing remain too low/uncertain
for inclusion in any of the tafs. Winds will veer from south to
west today and become strong after a cold front passes the area
this afternoon. Winds 5-15 kts this morning become 10 to 20 kts
with gusts 30+ kts possible by evening. Low confidence in
precipitation coverage/timing; medium confidence in cigs and
Tonight...generally VFR (though sub-VFR possible in the southern
Poconos occasionally with snow showers). West or northwest winds
15 to 25 kts with gusts 25 to 35 kts likely. Medium confidence
in cigs/vsbys; high confidence in winds.
Wednesday...mainly VFR conditions expected. Gusty west to northwest
winds around 15 to 25 knots with gusts around 40 knots possible. A
few snow showers possible, mainly north and west of kabe/krdg.
Thursday through Friday...mainly VFR conditions expected. MVFR or
lower possible in snow showers. West to northwest winds around 5 to
10 kts. Confidence: moderate
Saturday...mainly VFR conditions expected. West to southwest winds
around 10 knots. Confidence: moderate
Gale Warning now in effect for all waters thru Wednesday. Gale
for the ocean extended into Wednesday night. Today, winds will
be primarily southwesterly in advance of a cold front that will
reach the waters this afternoon. After the cold front passes,
winds will switch to west or northwest and become very strong
(speeds of 20 to 35 kts with gusts to 45 kts). There is a non-
zero chance of storm criteria being reached in spots tonight,
though the most likely outcome is a strong gale event.
Wednesday and Wednesday night...gale force gusts expected through
the day with gusts of 40 to 50 knots possible. Winds should fall
below gale force late Wednesday night. Seas will also remain
elevated around 5 to 7 feet on the ocean. Seas will start to subside
Wednesday night. A Small Craft Advisory will be needed once the gales
Thursday through Friday...Small Craft Advisory conditions may
continue into the early afternoon on Thursday. Winds subside below
25 knots by the afternoon and remain fairly quiet on the waters
Friday night through Saturday...Small Craft Advisory conditions
possible. West to northwest winds will gusts around 25 knots
Marine...Gale Warning until midnight EST Wednesday night for anz450>455.
Gale Warning until 6 PM EST Wednesday for anz430-431.