Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 110902
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
402 AM EST Mon Dec 11 2017

.SYNOPSIS...
High pressure will cross to the north of the region today. A low
pressure system will cross the Great Lakes region tonight, dragging
a cold front through our area late tonight into Tuesday. 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 Sunday or Monday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Vort max will be atop the forecast area for the next few hours as it
progresses eastward at the base of a broad longwave trough
encompassing eastern North America. Some flurries and light snow
showers have generated just downstream of the perturbation and an
attendant 250-mb jet streak in the Ohio Valley. The system is
severely moisture-starved, with a deep dry layer below the midlevel
moisture, so precipitation will be light. However, snow currently
occurring at the office suggests at least trace amounts are likely
with a band moving through the Delaware River and eastward at this
time. Expanded slight-chance PoPs for snow showers southward to
cover much of NJ for the next couple of hours.

Expect whatever reaches the surface will quickly move east early
this morning. Skies should rapidly clear as strong descent upstream
of the perturbation spreads across the Mid-Atlantic, though this
will be short-lived for the northwestern CWA as upper- and midlevel
moisture advects eastward downstream of the next system reaching the
Ohio Valley this evening.

MOS did reasonably well with temperatures yesterday (better than I
did), so followed suit with max temperatures today fairly closely.
In general, forecast continuity was below guidance, but I nudged
temperatures upward somewhat, especially given the warmer-than-
expected conditions overnight. Expect more snowmelt today. Light
westerly winds should prevail.

&&

.SHORT TERM /TUESDAY/...
Two vorticity maxima will affect the Mid-Atlantic tonight and
Tuesday, and a few model discrepancies continue given the very-
difficult-to-simulate interactive processes. At 00Z Tuesday, a
southern-stream vort max will be located in vicinity of Illinois,
with a digging northern-stream perturbation in Ontario. With time,
the southern-stream perturbation will accelerate eastward and weaken
as it elongates meridionally in the difluent flow downstream along
the East Coast. Meanwhile, the northern-stream vort max will pivot
eastward and acquire a negative tilt Tuesday afternoon as it noses
into the Northeast.

The combined effect of this evolution should be a focus of
precipitation in two regions: the warm-air advection regime
downstream of the deepening surface low in the Great Lakes region
and in the cold conveyor belt in vicinity of the Great Lakes and
Appalachians. The warm-air advection poses a couple of problems:
diagnosing surface temperatures Monday night as the isentropic lift
initiates precipitation in the northern Mid-Atlantic and determining
the nature of the thermal profiles during this phase of the event.
Models have trended much warmer during the past 24 hours, suggesting
that any precipitation in the urban corridor southeastward would
most likely be rain. This is especially true since the models have
also trended slower (somewhat unsurprising given the increased
amplification to the flow downstream of the perturbation), meaning
that most of the precipitation would occur near or after daybreak
for most of the CWA. Notably, the NAM remains much drier than the
GFS/ECMWF, with the former keeping most precipitation north of the
Philly area and the GFS/ECMWF extending southward to Delaware Bay
and vicinity.

There looks to be a transition zone with the precipitation Monday
night that may contain a wintry mix (rain, snow, sleet, and freezing
rain all being on the table). However, amounts look light, with
enough uncertainty with the presence of precipitation, the QPF
associated with it, and the exact locations to forgo any winter
headlines at this time. Best chances look to be north of I-76/I-
476/I-195 and timing is generally after midnight Monday night to
afternoon Tuesday. Farther to the northwest, model soundings are
decidedly snow, though surface temperatures will be near freezing on
Tuesday, which may limit accumulations even in the Poconos. At this
point, QPF with snow ratios around 10:1 would suggest potential for
1-3 inches in portions of Carbon, Monroe, and Sussex (NJ) Counties.
Once again, this is marginal enough, far out enough, and uncertain
enough to preclude issuance of winter headlines at this time.

Temperatures will be steady or even slowly rise after Monday evening
in the warm-air advection regime. A cold front will race through the
area Tuesday afternoon, with a quick switchover to west winds that
will likely become gusty. Much colder/drier air will infiltrate the
region thereafter, so highs may occur relatively early in the day
(especially in the western CWA). The southern Poconos look to remain
in close enough proximity to the surface low to the north to see
some wraparound snow or snow showers Tuesday afternoon, so increased
PoPs in this region during this time frame. Otherwise, the dropping
temperatures and increasing winds will be the main story by the
close of daylight.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
A series of low pressure systems will cross the region through the
extended period bringing repeated chances for some snow.

Tuesday night...The surface low lifts up into New England and
we should see the snow taper off as it moves further away. Some
wrap around moisture may continue to bring some snow showers to
the Southern Poconos through the overnight period. Winds will be
increasing across the region as there is a pretty tight
pressure gradient and strong cold air advection in the wake of
the departing low. Temperatures will feel much colder as they
fall into the teens to lower 20s. With the winds starting to
ramp up overnight we will likely see wind chills into the single
digits across much of the area.

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...The winds will start to subside and take that
biting chill from the air. Overnight lows will drop back in to
the teens.

Thursday through Thursday night...A strong shortwave and a weak
surface low will cross the region on Thursday. This clipper
system will bring a chance for some 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.


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. Warmer air will try to move in in the west
to southwest flow. Precipitation may end up being mixed across
parts of the area but uncertainty as to how much warm air can
make it in exits.

Saturday through Sunday night...High pressure will slide across
the southeastern United States on Saturday, bringing us a day
of quiet weather. The weather should remain quiet through the
day on Sunday. 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 on Sunday.

Sunday night through Monday...Another system will approach the
region sometime around Sunday night or Monday. Precipitation
type may be tricky as the warm air may hang around and allow for
some rain to fall.

&&

.AVIATION /09Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...Generally VFR with CIGS around/above 5000 feet. Some
flurries are possible, especially at RDG/ABE/TTN, but no runway
accumulations are expected. Winds should be generally light from the
southwest. High confidence.

Monday...VFR with west winds 5 to 15 kts. High confidence.

Monday night...Primarily VFR, though conditions should deteriorate
to sub-VFR well to the north of PHL with a wintry mix possible in
the southern Poconos and vicinity. Timing of any precipitation looks
to be after 06Z at RDG/ABE (though conditions may be slow to reach
sub-VFR even after precipitation begins), with southern extent a
question mark (i.e., large uncertainty regarding any precipitation
reaching TTN/PNE/PHL/ILG). Winds should be light and mainly from the
south. Low confidence in CIGs/VSBYs and precipitation coverage;
medium confidence in winds.

Tuesday...Sub-VFR conditions possible before cold frontal passage,
especially for the Philly terminals north and west. Some rain
possible in the urban corridor, transitioning to snow in the
southern Poconos region. A cold front will race through the area
during the afternoon, with winds becoming west 10 to 20 kts with
higher gusts thereafter. Conditions should be VFR after frontal
passage at the TAF sites. Low confidence in CIGs/VSBYs/precipitation
coverage/frontal timing; high confidence in winds.

Outlook...

Tuesday night through Wednesday...Mainly VFR conditions expected.
Gusty west to northwest winds around 15 to 25 knots with gusts
around 40 knots possible. Confidence: Moderate

Thursday through Friday...Mainly VFR conditions expected. MVFR
or lower possible in snow showers. West to southwest winds
around 5 to 10 kts. Confidence: Moderate

&&

.MARINE...
Winds remain near or just above advisory criteria on the Atlantic
waters, so the small craft advisory was not changed with this
update. Gusts are near criteria on Lower Delaware Bay, but given the
overall downward trend expected for the rest of tonight, decided not
to issue anything here.

Another advisory will be needed after midnight Monday night/Tuesday
morning for all of the waters as south winds increase in advance of
the next storm system, but will let the current advisory play out
before issuing another one. Gusts may approach gale force on
Tuesday.

A strong cold front will pass through the waters late in the day
Tuesday. Winds will switch to westerly and likely become gale-force
quickly thereafter. A gale watch will likely be required as the
event nears.

Outlook...

Tuesday night through Wednesday night...Strong west winds with
gale force gusts likely. Seas will also remain elevated around 5
to 7 feet on the ocean. Seas will start to subside Wednesday
night.

Thursday...Small Craft advisory conditions early. Winds subside
below 25 knots and conditions remain fairly quiet on the waters
through Thursday.

Friday...Small Craft Advisory conditions possible. West winds
will gusts around 25 knots possible.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EST early this morning for
     ANZ450>455.

&&

$$
Synopsis...Meola
Near Term...CMS
Short Term...CMS
Long Term...Meola
Aviation...CMS/Meola
Marine...CMS/Meola



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