Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 171430
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
930 AM EST Sun Dec 17 2017

.SYNOPSIS...
A warm front will lift from southern Pennsylvania through
southern New Jersey will life northeast of our area early
Monday. A cold front will then move through the mid Atlantic
states Tuesday night. High pressure will follow by Thursday
morning. 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.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The 12Z surface analysis indicated the cold front has become
stationary, now extending from far southern NJ into northern
Delmarva. The surface flow is weak, with light northeast winds
advecting in slightly drier air from the north, which is the
sharpest indication of an otherwise diffuse boundary. Given the deep
layer flow parallel to the front, expect it to remain stationary
much of the day. Considerable cloudiness remains across the northern
portion of the area, with partly cloudy skies across Delmarva.

As the next short wave disturbance approaches from the Ohio Valley
this afternoon, expect cloudiness to increase across Delmarva as
well. Composite radar imagery indicates echoes developing aloft
(virga) across the central Appalachians. Expect weak ascent to
increase in the vicinity of the front, which could lead to some
light showers late this afternoon, mainly confined to Delmarva. In
terms of high temperatures, given the front and cloud cover, cooler
north and warmer south. Overall though temperatures within a few
degrees of average for this time of year.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
With the latest model run, almost all operational models backed off
on the amount and extent of QPF, synoptic, and mesoscale forcing
across our area as the warm front continues to progress north this
evening. It appears this is in response to models depicting the
surface high to weaken and move away from the region slower than
previously expected. There is little, if any, frontogenesis depicted
with the warm front, and it appears that the most lift comes from
light onshore flow developing below the inversion, which is usually
not enough for measurable precip. As a result, decreased PoPs
across the northern half of the area to slight chance.

Precipitation type: If we do have precip this evening, it looks like
the mid level cloud deck should be persistent enough to allow ice
crystals to already be present. Thus, if there is precip, it should
be mostly snow. However, if the mid level cloud deck dissipates
early, freezing rain would be possible for some of the area. Given
how unlikely precipitation is at this point, there is not nearly
enough confidence to issue anything, but we will keep the mention in
the hazardous weather outlook.

Temperatures: with the warm front moving through generally the first
half of the night, expect lows to be 5 to 10 degrees above
normal.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Monday through Tuesday night:

Our region will be between a high pressure off the southeast
coastline and an approaching cold front from the west. This will
result in southerly winds and warming temperatures. Enough moisture
and lift could be present ahead of the front for some isolated
scattered rain/snow showers Monday night into Tuesday. However, the
signal for precipitation on both the operational runs and ensembles
is  fairly limited and confined to locations mainly north and west
of CWA. We will only maintain slight chance pops in the southern
Poconos.

It looks more like the increase in moisture may just lead to more
in the way of low stratus clouds at times. Fairly good agreement
with highs in the 40`s and used a MET/MAV/ECMWF blend. Low
temperatures Monday night will be warmer with more clouds in the
30`s. Right now the temperature forecast highs on Tuesday well into
the 50`s, confidence is increased with these numbers as the ensemble
mean has gotten warmer. Nighttime temperatures Tuesday night should
fall back into the 30`s.

Wednesday through Thursday night:

After the frontal passage, we return to northwest flow with cold
air advection into the region. Another period of gusty winds is not
out of the question. With high pressure building in, both days
should be mostly sunny. Right now the wind direction off the lakes
looks unfavorable for any increase in clouds or snow showers up in
the southern Poconos. Bufkit analysis shows the potential for some
gusts from the west at around 20 mph Wednesday afternoon. High
temperatures both days should fall back into the 40`s with lows in
the 20`s, opted to go on the cooler end of the ensemble guidance
given the cold air advection into the region.

Friday through Saturday night & longer term:

A warm frontal boundary lifts northward across the region as a low
pressure system tracks into the Great Lakes on Friday. Another surge
of southerly flow and increase in southerly winds will come into the
region leading toward a moderating trend with temperatures, likely
reaching well into the 50`s.

A cold frontal boundary will then approach the region from the west
on Saturday. Right now, this forecast is more in line with the
progressive ECMWF suite from 12Z 12/16 and 00z 12/17. This is based
on the GEFS/GFS suspect handling of the MJO looping in phase 6. This
suspect MJO forecast is likely causing the GEFS/GFS to slow the
approach of the next cold front too much. Although, the 00z GFS
run 12/17 has increased the speed of the front a bit.

Depending on the frontal timing, a non-diurnal temperature is
possible but not included given we are quite a ways away. Modeling
is keying in on the potential for quite a bit of rainfall
potentially with this system, raised pops a bit with this update but
if timing confidence increases these will need to come up even more.
Right now, temperatures look to warm enough to prevent mixing with
snow/ice on the front end of the system.

A quick look into Christmas week does still provide the chance of a
white Christmas with an active weather pattern that looks to turn
colder thanks to a EPO which is falling several standard deviations
below normal. The - EPO will discharge very cold air from AK
southeast into the United States just before Christmas. A lot of
difference in the model and ensemble solutions are present at this
point. One of the key differences is the mentioned cold front on
Saturday and how fast arctic air can spread eastward. Also, this
front may have areas of low pressure which track northeastward along
it as well. The CPC outlook for the 8-14 day does indicate
increased probabilities for below normal temperatures.

&&

.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VFR conditions are expected, though a mid level cloud deck
around 5000 ft AGL may advect south later today. Winds are
expected to be light and variable. High confidence.

Tonight...Mostly VFR conditions are expected overnight. There is a
small chance for precipitation at the TAF sites; if precipitation
develops then brief MVFR conditions are possible. Winds should
settle out of the east or southeast by this evening, but wind speeds
should remain 5 kt or less. Moderate confidence.

Outlook...

Monday through Tuesday night: Varying ceilings with intervals of low
MVFR or IFR stratus possible. Exact timing and lowering of ceilings
of uncertain with data varying at this point. Winds from the west
and southwest around 10 knots Monday then gusts from the southwest
at 15 knots Tuesday afternoon. Low confidence.

Wednesday through Thursday: VFR. West to northwest wind gusts around
20 knots Wednesday afternoon decreasing gradually into Thursday.
High confidence.

&&

.MARINE...
Winds and seas are expected to stay below small craft advisory
conditions today and tonight. Wind direction is expected to be
variable, eventually settling out of the east or southeast by
tonight.

Outlook...

Southwesterly wind gusts may approach 20 knots Tuesday afternoon.
However, the main period for concern is northwesterly winds
Wednesday which may approach or briefly exceed 25 knots. Data
indicating seas stay below five feet throughout the outlook
period. High confidence.

&&

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

&&

$$
Synopsis...Gaines
Near Term...Franck/Johnson
Short Term...Johnson
Long Term...Gaines
Aviation...Franck/Gaines/Johnson
Marine...Franck/Gaines/Johnson


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