Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 172327
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
627 PM EST Sun Dec 17 2017

.SYNOPSIS...
A warm front will lift northeast of our area early Monday, then a
cold front moves through the Mid-Atlantic states Tuesday night. High
pressure will follow by Thursday morning while an area of low
pressure scoots out to sea to the east of the Carolinas. Thereafter,
strengthening low pressure will track northeast from the Great Lakes
region Friday, sending a warm front through our region, followed by
a cold front on Saturday. That cold front may stall just south of
our area on Sunday.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
For the 630 PM update, delaying the PoP increase some as incoming
precipitation still looks to be aloft. Adjusted some temperatures
and dew points based on the latest observations. No other changes at
this time.

Otherwise, the next short wave disturbance moves from the Ohio
Valley and through our region overnight. Composite radar imagery
indicates an area of spotty, light precipitation across the central
Appalachians in association with this feature. Meanwhile, a
stationary front across far southern NJ and Delmarva will begin to
lift north as a warm front overnight. Expect weak ascent to increase
across the region overnight. The models are highlighting two areas
in particular: Delmarva into southern NJ and the Lehigh Valley into
the southern Poconos. While precipitation southeast of the NJ
Turnpike and I-95 remains predominantly liquid with temperatures
expected to remain at or above freezing, a light and spotty wintry
mix is possible to the northwest.

In particular, there is the potential for a corridor of spotty light
freezing rain, especially between Interstate 78 and the Pennsylvania
and New Jersey Turnpikes. North of this area, the potential for
freezing rain is less, with a greater potential of light snow. There
is still model uncertainty with regard to the extent of snow versus
freezing rain. In general, the guidance appears too bullish on
freezing rain north of I-78, where sufficient ice aloft should favor
snow. Model soundings also indicate the absence of a warm nose, so
any sleet potential should be very limited.

There is the potential for snowfall up to a half of an inch. If the
potential for freezing rain to produce ice accumulation at or above
a trace of an inch increases, then a Winter Weather Advisory may be
needed. Given the light and spotty nature of the precip, and
uncertainty with regard to freezing rain versus snow, we opted for a
Special Weather Statement to cover the potential impacts at this
point. Overall, a low confidence forecast continues.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
Any precipitation is expected to end around sunrise on Monday, with
a return to generally fair weather. The warm front will move well to
the north of our region, with a continued westerly flow aloft. This
will maintain considerable cloudiness across portions of eastern PA
and northern NJ, with skies gradually becoming partly sunny to the
south. High temperatures will be above normal by several degrees,
with light westerly winds.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Hazards: None likely. A super week for the shopping economy in our
forecast area as basically this week doesn`t show much sign of
any significant winter weather...other than what might occur
tonight to the northwest and north of Philadelphia. And yes, we
have looked at the records for Tuesday and Saturday and we are
not yet forecasting within 2F of record.

500MB: A notable warm ridge of high pressure over the southeast
USA Monday night will weaken Wednesday in response to the passage
of a strong short wave through southeast Canada. Ridging begins
building back across the mid Atlantic states and southeast USA
by next Friday. Thereafter, it further strengthens along the
east coast and western Atlantic while a positive tilt trough
develops from the Rockies into south central Canada.

Temperatures:  Well it was a nice stretch of winter from the 8th
through 16th that drove the monthly average temps down. The
first 16 days of the month averaged 3.6F below normal at KMPO
and 2F below normal at KGED/KRDG and 1 to 1.8F below normal at
our other 5 long term climate sites. Well, thats over and am
unwilling to speculate whether the month will average above or
below normal after the culmination of the next 8 days of
generally above normal temperatures. Calendar day averages
Tuesday should be 10 to 15 degrees above normal, near 5F above
normal Wednesday (minimums occurring at 1159PM), near normal
Thursday, then warming above normal again on Friday, to
possibly 15 or even 20 degrees above normal Saturday and perhaps
Sunday as well. The best chance for below normal temps between
now and the 26th of December appears to be this coming Thursday.

Forecast basis: Monday night-Tuesday night is a 50 50 blend of
the 12z/17 GFS-NAM MOS with a tendency to favor the warmer GFS
(see our note on snowcover below), Wednesday the 12z/17 GFS
MEXMOS and next Thursday- Sunday (D4-8) is the 15z/17 WPC 12 hr
elements of the max-min temps/pops and 6 hrly td/wind/sky. These
D4-8 WPC max/min temps were modified higher via a compromise
with the 12z GFS MEXMOS temps or the 12z EC 2m temps.

SNOW depth and MOS temperature guidance. We have viewed NAM
model snow depth and its biased far too high here in SNJ. Be
alert for possible impact on BL temps even into Monday night.
The 06z and 12x GFS snow depth also is high but not as much as
the NAM. Nevertheless it too may impact the GFS MOS temp
guidance cooler than what one would think for this pattern.

The dailies...

Monday night...Milder. Light south wind. slight chance of a
sprinkle in ne PA but not really accounted for the in the
forecast. Confidence: above average.

Tuesday...Warming nicely ahead of the next cold front. Gusty
southwest winds possibly to 20 mph in the afternoon. Confidence:
above average.

Tuesday night...maybe a sprinkle or light shower up north. Otherwise
for now, odds favor a dry cfp.  Confidence: above average.

Wednesday...P/Sunny and cooler with a gusty northwest wind to
20 or 25 MPH. Minimum temperature for the day occurs around
1159 PM. The cirrus forecast (Partly Sunny) is in association
with a southern USA short wave and sfc low heading east, of
which the northern fringe of the high cloud shield is modeled up
to about I78. Confidence: above average.

Thursday...Probably the coldest temps of the week. Light northwest
wind. Confidence: above average.

Friday...WAA begins and it may yield rain late in the day or at
night. Confidence: average on scenario but below average on
whether rain will begin Friday.

Saturday...Probably breezy and warm. It looks like at least a
period of widespread showers ahead of a CFP. Forecast POPS are
probably too conservative by 20 to 30 pct. Confidence: average.

Sunday the 24th...Confidence rather low (well below average for a
Day7), since model solutions vary regarding the timing of the
next pcpn event. This day is primarily a blend of WPC, EC and
GFS.

A look into Christmas week indicates an eventual trend to below
normal temperatures, but it probably is delayed til after
Christmas. At this time...some uncertainty still exists regarding
whether there will be snow cover or even snow on the 25th in
our area. Recent model trends are less favorable for snow but
that does not mean it cant happen.

GEFS guidance as of 12z/17 suggests considerable warming aloft
developing next weekend into early next week while a 1050MB high
in the central Rockies is associated with a significant cold
out break from the Rockies and Plains into the Great Lakes
region. GEFS 500 MB height fields and associated anomalies, as
well as ECEFS 2m temp fields suggest above normal temps
continuing in our area through Christmas.

After Christmas, trends-odds favor colder airmasses with sub
freezing temps and potential for wintry weather events.

A concern regarding teleconnections (tc)... my interpretation
of the tc patterns shows very little evidence of blocking in
the north Atlantic through at least the 27th (-PNA and +NAO).
The EPO is modeled to drop to very very low values around the
23rd through the 27th, yet all our MOS and MOS ensemble guidance
is showing aoa normal temps here in our area except for Thursday,
even when the the EPO is negative. Thursday is currently
modeled to be our coldest day of next 8 days (inclusive of
Christmas).

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR ceilings lowering to MVFR after midnight mainly
at RDG, ABE to near TTN. While some light precipitation is
possible, the probability at any one terminal is not high enough
to include in the TAFs. Mainly light and variable winds. Lower
confidence regarding the extent of the MVFR ceilings overnight.

Monday...MVFR ceilings mainly at RDG, ABE to near TTN improves to
VFR by mid morning. Elsewhere, VFR. Light and variable winds early
in the morning becoming westerly generally around 5 knots.

Outlook...
Monday night...VFR. West wind. Confidence: Average.

Tuesday...VFR.  Southwest wind may gust 20-25 kts early afternoon.
Confidence: Average.

Tuesday night...VFR.  Northwest wind. Confidence: Average.

Wednesday...VFR. Northwest wind should gust 20-25 kt.
Confidence: Average.

Thursday...VFR. Light northwest wind. Confidence: Average.

Friday.... VFR.  MVFR or IFR conds possible late. South to southeast
wind. Confidence: Average.

&&

.MARINE...
Winds shift from east tonight to southeast Monday morning and
eventually to the west Monday afternoon, as a warm front lifts north
across the waters. Both winds and seas well below SCA criteria
tonight and Monday.

Outlook...
Monday night through Friday

Southwesterly wind gusts may approach 20 knots Tuesday afternoon.
However, the main period for concern is Wednesday when northwesterly
winds should approach or briefly exceed 25 knots. Model guidance
indicates seas stay below five feet throughout this outlook period.
Confidence: Above average.

&&

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

&&

$$
Synopsis...Drag
Near Term...Franck/Gorse
Short Term...Franck
Long Term...Drag
Aviation...Drag/Franck/Gorse
Marine...Drag/Franck



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