Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 132056
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
356 PM EST Fri Jan 13 2017

.SYNOPSIS...
Large high pressure will build just to our north tonight, then
weaken Saturday into Saturday night as it moves offshore. An area
of low pressure will move to our south Saturday night, then high
pressure will build back to our north Sunday and influence the
weather through Monday. Then, low pressure is forecast to move
from the southern plains to the Great Lakes by Tuesday. This will
bring a warm front across the area Monday night into Tuesday
morning. A warm southwest flow will likely develop across the east
coast for the middle to end of the week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Cold front has progressed well south of the area today with cold
high pressure settling into the Great Lakes and the Northeast.
Temperatures have been nearly steady today as low-level cold air
advection upstream of the front has overtaken the region.  Large-
scale environment consists of quasi-zonal upper-level flow poleward
of a slowly retrogressing subtropical high in the eastern Gulf of
Mexico. Several small-scale perturbations embedded in the near-zonal
flow will progress across the Mid-Atlantic States through the
weekend. The first will move through the region tonight, bringing
increased mid and high clouds to the area.  Temperatures will
steadily cool this evening before the increased cloud cover slows
the downward trend late tonight. The damage will be done, so to
speak, as temperatures should be near or below freezing across the
CWA (from upper teens in the southern Poconos to around freezing at
the Delaware beaches).  Winds will continue to diminish this evening
as the surface pressure gradient relaxes.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
The next perturbation in the near-zonal upper-level flow moves
across the region tomorrow.  Though the perturbation is rather weak,
it will be sufficiently strong to produce cyclogenesis off the
Carolina Coast by 00Z Sunday.  Large-scale lift in association with
the vorticity maximum will generate precipitation in the Ohio Valley
tonight spreading into the Mid-Atlantic during the day Saturday.
Owing to the overall weak lift/dynamics in place, precipitation
should not be heavy with this system, and the strongest lift will be
south and west of the area.  Models have trended somewhat drier and
warmer with the system in the past 24 hours, with little if any QPF
generated north of I-78.

Model soundings suggest the profile will be cold enough for snow as
precipitation moves in by early afternoon.  However, surface
temperatures will creep above freezing south of the Mason-Dixon
Line, comfortably so south of Dover, so any precipitation here will
likely transition to rain by late afternoon.  Given the light QPF,
marginal temperatures, and antecedent warmth this week, not
expecting anywhere to receive more than an inch of snowfall, with
the highest axis somewhere between I-95 the central Eastern Shore of
Maryland and adjacent Delaware.

Meanwhile, vertical profiles will also be drying out in the
midlevels as the perturbation passes to the east of the region by
late afternoon.  This suggests snowfall will be more difficult to
generate as the event wears on, and precipitation may turn more to
freezing drizzle or light freezing rain where surface temperatures
remain cold enough.  However, two caveats lead to me not issuing any
headlines at this time.  The first is that such a change in the
vertical profile of moisture occurs when lift progresses away from
the region, so the transition to more of a wintry mix is not high
confidence (and may not amount to much more than drizzle if it does
occur).  The second is that where QPF is slightly higher (in MD/DE),
temperatures look to be near or above freezing.  Although minor
winter weather impacts cannot be ruled out tomorrow, there is simply
not enough confidence to warrant issuance of any winter weather
products at this time.  Stay tuned to the latest forecasts, as this
may change.

One final note:  Despite the lack of winter weather products issued
at this time, this is certainly not to say that no winter-weather
related impacts will occur.  With light snow and potentially
freezing drizzle/rain in portions of the area tomorrow, impacts are
most certainly possible!

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Low pres will move s of the area Sat night. Temps will be cold
enough from the I-95 corridor n and w for some wintry precip, but
overall qpf and pops are low. Pops have been lowered with this
fcst. Will have to keep an eye on this but no headlines will be
issued at this time.

The low moves out by erly Sun and high pres builds to the n which
will bring dry wx for at least the first part of Mon.

Then, low pres will develop over the srn plain and will move newd
twd the Grtlks thru Tue. The wmfnt assocd with this low will cross
the area Mon night. Precip will develop as erly as Mon aftn (GFS)
or hold off until Mon night or  Tue (ECMWF and CMC). For now,
will keep Mon dry. Pops increase Mon night into Tue. There is
genly good agreement wrt this aspect, though the mdls disagree on
when and where the precip is most likely to occur. The GFS is also
faster in moving the precip out. The cdfnt crosses the area erly
Wed in the GFS and then Wed into Thu is dry. In the ECMWF, a
secondary low forms on the front, it is, therefore, slower to
cross and Wednesday is wet, before fropa Wed night and a dry Thu.
The CMC is somewhere in the middle, slower (much) than the GFS but
not as slow as the ECMWF. So, for now, will hold some pops thru Wed
and see how this develops. Needless to say this is a lower than
average confidence fcst.

The good news is that temps will be quite warm thru most of the
week (after Mon)...well into the 50s in many areas. Therefore, any
precip that falls Tue into Wed will be plain rain everywhere.
Temps look to be 10-15 degrees above nrml for most of next week!!

&&

.AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Northwest winds are beginning to diminish across the area as the
cold front moves away from the region. However, cloud cover will
increase tonight, with ceilings lowering through tomorrow as the
next system moves through the region. Light precipitation is
expected to move into KILG and KMIV around 18Z Saturday with
KPHL/KPNE/KTTN/KACY/KRDG affected shortly thereafter. For now,
thinking is that precipitation will mostly start as light snow but
with time, a light wintry mix is possible. MVFR/IFR
ceilings/visibilities expected as precipitation occurs.  Most of the
precipitation should end by late evening, with some improvement to
ceilings/visibilities expected thereafter.  Winds should be at or
below 10 kts by the late evening hours, transitioning from north
tonight to east tomorrow morning to southerly by late in the day.


OUTLOOK...

Sat night...MVFR or IFR conditions Sat night. Wintry mixture of
precipitation possible, with mainly snow/sleet for ABE/RDG/TTN,
and snow/sleet/freezing rain possible for the remainder of the TAF
sites. Precip should be light and pops/qpf not all that high. Low
to moderate confidence.

Sun-Sun night...Conds improving to VFR. Moderate to high
confidence.

Mon-Wed... VFR early, lowering to MVFR, then to IFR late Mon night
into Tuesday. Chance of precipitation late Mon into Tue. A wintry
mix possible, mainly n and w erly Tue. Timing diffs develop after
that, with precip over in one mdl by Wed mrng and continuing thru
Wed in another. So its psbl conds cud improve on Wed or not. Mdt
to high confidence Mon-Tue, low on Wed.

&&

.MARINE...
Advisory conditions continue in the coastal waters this afternoon
but are becoming more marginal with time.  Conditions should become
sub-advisory levels this evening, and the Small Craft Advisory will
likely be allowed to expire as scheduled.  Winds will transition to
light easterly flow by tomorrow afternoon.  A chance for light mixed
precipitation is expected during the afternoon and evening tomorrow.


OUTLOOK...

Sat-Tue..Sub SCA conds are expected, although winds may gust
15-20 knots at times on Sun.

Wed...SCA conds psbl as swly flow increases in advance of cdfnt.
Wind cud gust to 25 kts and seas could reach 6 ft.

&&

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

&&

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


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