Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 202026
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
326 PM EST Sat Jan 20 2018

.SYNOPSIS...
High pressure to our south will slowly shift off shore through
Sunday. A low pressure system will propagate from the southern
Plains to the Great Lakes region Sunday into Tuesday. As it does so,
expect first a warm front to arrive in our area Sunday night or
Monday, followed by a cold front arriving on Tuesday. High pressure
will then build toward our region through Friday.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
A rather mild afternoon in progress across nearly the entire region.

A zonal flow aloft continues through tonight between a trough well
to our north and a weakness aloft near the southeastern states. High
pressure at the surface centered near the southeastern states
expands northward some tonight while also shifting eastward.
Meanwhile, a surface trough in our vicinity should gradually shift
south/east and weaken. A northern stream system that tracks into the
Canadian Maritimes will try and settle a weak cold front into our
area late tonight from the north.

Some high level clouds will continue especially for the northern and
western areas given the proximity to mid and upper-level jets. There
is some weak low-level cold air advection forecast to occur
overnight as the flow turns a bit more northwest. Temperatures
should be able to drop fairly quickly this evening outside of the
urban centers despite the dew points coming up some, as clouds
should not be much or all that thick and the winds are expected to
become primarily light and variable. We used a MOS/continuity blend
overall for the low temperatures.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
As a potent closed low begins to eject out into the central and
southern Plains during Sunday, some ridging builds toward the
western Great Lakes. A more zonal flow aloft however is maintained
across much of the East with surface high pressure easing off the
southeast U.S. coast with time. A weak surface cold front trying to
work down from the north should tend to dissipate with a southern
extent. The low-level airmass is forecast to be cooler compared to
Saturday, however some low-level warm air advection is forecast to
begin developing toward late afternoon as the flow starts to back to
the west and west-southwest.

We are expecting an increase in cloud cover through the day, and
this should tend to lower with time as warming aloft takes place
owing to an inversion. It is within the inversion area where the
moisture increases and helps to develop lower clouds through the
day. Some of the guidance, especially the NAM, looks to fast with
the low-level moisture increase and therefore did not rush in the
lower clouds as fast. In addition, while blending in continuity for
the high temperatures we leaned closer to the warmer GFS/EC MOS as
the low clouds indicated by the NAM are thought to be overdone
and/or to fast.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Sunday night...We continue to watch where the best isentropic
ascent will develop through this period for the potential for
light precipitation. At this point, it appears that the favored
area will set up to the west of our region, though the GFS does
depict a narrow area of cyclogenesis over southeastern PA
through this time. For now though, it appears to be a low chance
(20 to 40 percent) across our area. If it does happen, then
there is also the potential for freezing rain across NW NJ and
the southern Poconos, as temperatures will be near freezing for
much of the night in this area. Potential remains too low to
issue an ice accumulation forecast, but if any precipitation
develops we expect it to be light.

Monday...We should see a lull in the precipitation between when the
initial isentropic lift moves north of the area and when we start to
get lift ahead of the approaching cold front. Whether this lull will
come Monday afternoon or Monday evening is uncertain as there remain
timing differences between different models.

Monday night and Tuesday...Ahead of the front, expect a very strong
low and mid level southerly jet (60+ kt at 900 mb). I mention this
because at this point we expect to have almost negligible
instability (not even elevated instability) and a robust
temperature inversion, so it is unlikely that the stronger winds
will mix down. However, if that changes, it won`t take much
mixing to see strong winds at the surface. Given the negligible
instability, do not expect any thunderstorms. Precipitable water
values could be over one inch, which is certainly above normal
for late January, but still considerably less than the 1.46
inches which was observed in the KIAD sounding on the 12th.
Additionally, very fast storm motions should limit any flooding
threat.

As for the front itself, models trended a bit slower with the front.
Current forecast is for the front to propagate through the area
through the day time hours. The low (and consequently the warm
conveyor belt) is expected to be far enough to the NW that we
shouldn`t see any precipitation behind the front.

Wednesday through Saturday...A large surface high slowly builds east
through this period, over our region and eventually off shore. We
should start the period below normal thanks to the Tuesday cold
front, but could see temperatures moderate as low level flow becomes
southwesterly late in the week.

&&

.AVIATION /21Z SATURDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Rest of this afternoon...VFR with some clouds at or above 20,000
feet. West-southwest winds 10-15 knots. Some lingering gusts to
about 20 knots are expected to subside through 22z.

Tonight...VFR with any clouds at or above 15,000 feet. Winds
becoming light and variable at most terminals.

Sunday...VFR ceilings lowering to between about 4,000-10,000 feet
through the day, however some MVFR ceilings may develop especially
north and west of PHL. Light and variable winds becoming northwest
around 5 knots, then turning southwest in the afternoon. Lower
confidence regarding any MVFR or lower ceilings developing.

Outlook...

Sunday night...MVFR or lower conditions expected across the region
mainly due to low ceilings. Some visibility restrictions also
possible with light rain. Light (less than 10 kt) southerly or
southeasterly winds. Exact timing of when lower clouds will move in
is uncertain at this time.

Monday...Starting the day with MVFR or lower ceilings. Improvement
to VFR is possible in the afternoon, primarily for KPNE, KPHL, KILG,
KMIV, and KACY. Improvement is less likely at KRDG, KABE, and KTTN.
Southerly or southeasterly winds near or less than 10 kt.

Monday night...conditions lowering to IFR or less with low ceilings
and visibility restrictions due to rain. Southerly winds of 10 to 15
kt are expected. A period of low level wind shear (LLWS) is likely
to develop late Monday night as a strong low level southerly jet
develops, though exact timing and location of the jet is uncertain
for now.

Tuesday...Starting IFR or lower Tuesday morning, but expect quick
improvement to VFR conditions behind a cold front expected during
the day Tuesday. Also expect an abrupt wind shift from southerly to
westerly winds with the cold front. Wind speeds of 10 to 20 kt with
higher gusts will be possible especially just behind the cold front.
LLWS conditions may linger into Tuesday morning along the coastal
plains. Timing of the front, and consequently improving conditions
is uncertain at this time.

Tuesday night through Thursday...VFR conditions expected. Westerly
or northwesterly winds less than 15 kt. High confidence.

&&

.MARINE...
The Small Craft Advisory for the central and northern New Jersey
Atlantic coastal waters has been cancelled.

High pressure remains centered to our south through Sunday. With a
weak or dissipating cold front settling southward late tonight and
Sunday, west-southwest winds become northwest before turning more
southwest Sunday afternoon. The conditions are expected to be below
Small Craft Advisory criteria through Sunday.

Outlook...

Sunday night and Monday...Winds and seas are expected to stay below
SCA criteria.

Monday night into Tuesday morning...SCA conditions for both winds
and seas are expected. Expect southerly winds gusting to 30 KT.
There is a chance of gale force winds especially on the coastal
waters through this period.

Late Tuesday through Wednesday...An abrupt shift to westerly winds
with gusts above 25 kt is expected behind a cold front moving
through late Tuesday.

Thursday...Westerly wind gusts near 25 kt are possible through this
period, but confidence is below average.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Johnson
Near Term...Gorse
Short Term...Gorse
Long Term...Johnson
Aviation...Gorse/Johnson
Marine...Gorse/Johnson


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