Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 240201
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1001 PM EDT Thu Mar 23 2017

.SYNOPSIS...
High pressure across the middle Atlantic will move offshore by
Friday, while a warm front moves through the Ohio Valley. This
frontal boundary will become nearly stationary near our region
through Tuesday, as several waves of low pressure move along it. A
cold frontal passage is expected Wednesday, followed by Canadian
high pressure building into our area on Thursday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
High pressure was centered near the southern Mid-Atlantic coast
this evening. Nearly ideal radiational cooling under light
winds, clear skies and a very dry nocturnal boundary layer are
allowing temperatures to drop fairly quickly this evening. At 9
PM, temperatures were highly variable ranging from the mid 20s
in the southern Poconos, NJ Pine Barrens and rural valleys to 40
degrees in the City of Philadelphia. Forecast temperatures were
updating mainly using a blend of LAV/LAMP since this guidance
were verifying better so far this evening. However, none of the
available guidance were able to capture the large variations in
temperatures (sub- mesoscale) very well.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
The forecast was updated with a focus on hourly temperatures,
dewpoints, winds and PoPs since these grids will help
determine ptype and ice accrection. Differences from the
previous forecast were overall minor, negating the need to make
changes to the Winter Weather Advisory headline. Compared to the
forecast from the day shift, the onset of precip in the morning
has trended slightly slower (8-10 AM in the southern Poconos).
Any extra time will provide extra time for temperatures to
rebound after daybreak. Wet-bulb cooling look to allow a brief
period of sleet at the onset along and north of I-80. Sleet
accumulations will be minimal. Ice accretion of up to a few
hundredths are expected for Carbon and Monroe Counties during
the morning where the advisory is in effect. Light freezing rain
may extend beyond the county borders but should mainly be
confined to the highest ridges in the Lehigh Valley counties in
PA and Sussex County, NJ. At this point, cannot rule the need to
expand the advisory into Berks Co, Lehigh Valley and
northwestern NJ if precipitation arrives earlier than forecast.
This will be a short duration event as precipitation will last
for 3-5 hours and temperatures rise steadily to above freezing
through midday. See WSW for more information on impacts.

By early afternoon, expect all locations to be above freezing and
all precipitation to be rain.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
An unsettled period of weather is in store this weekend thru the
middle of next week. At the mid-levels across the conus, a generally
zonal northern stream flow will be in place, while several shortwave
disturbances traverse the southern stream. Meanwhile at the surface,
Canadian and sub-tropical high pressure systems will maintain a
frontal boundary in the vicinity of our region until a more
substantial frontal passage on Wednesday.

The primary uncertainty will be the location of the aforementioned
boundary, along with the timing of several areas of low pressure
that ride along it, and accompanying areas of precipitation. Of most
concern is the potential of mixed precipitation Sunday night.

On Saturday, the NAM and EC are most extensive with the overrunning
precipitation associated with the stationary boundary to our north.
The GFS looks more reasonable with the placement of the precip in
relation to the frontal boundary, confining PoPs to the northern
half of our CWA, and our forecast reflects this.

As the frontal boundary moves southward in response to Canadian high
pressure building to the north, scattered showers are possible
across our entire area Saturday night into Sunday. As low pressure
moves through the Ohio Valley into the Great Lakes from Sunday night
into Monday, more substantial overrunning precip is likely during
this time frame.

As mentioned previously, there is some potential for mixed
precipitation Sunday night into Monday morning. A review of plan
view 2-meter and 850 hPa temperatures, 1000 to 500 hPa and 1000 to
850 hPa thicknesses, model p-type, ens plume diagrams, model
soundings, and CIPS Analogs points to freezing rain as the primary
concern. There is a high amount of uncertainty, especially at this
range in the forecast, which translates into low confidence. The
probability for freezing rain is low at this time, therefore it was
not mentioned in the HWO (per our directives), but this will need to
reassessed with future forecast packages. Also, please note that ice
map on our website is for Friday, March 23.

Expect showers to continue Monday night into Wednesday, and another
round of more substantial precip likely centered on Tuesday night
into Wednesday, associated with a cold frontal passage. There is
uncertainty with the timing of this front, particularly given the
time range, but a gradual return to fair weather should occur on
Wednesday and continue into Thursday.

Temperature-wise, at or above normal through the period.

&&

.AVIATION /02Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR conditions will continue through at least 12Z. After 12Z, there
is an increasing risk for precipitation (expected to be all rain at
the TAF sites, but freezing precipitation will be possible N and W
of KABE and KRDG). With any rain, brief MVFR conditions will be
possible. The rain should move out by mid afternoon bringing a
return to VFR conditions.

Light and variable winds may continue through much of the overnight
hours before settling in out of the south by 12Z Friday.

OUTLOOK...

Saturday and Sunday...A period of MVFR is possible in low
clouds, especially northwest of an ACY-MIV line. Otherwise, VFR.
Southwest winds 10 to 15 knots on Saturday will shift to the
east Saturday night into Sunday, increasing to 10-20 knots.

Sunday night thru Tuesday...Periods of MVFR possible in low clouds
and fog. South-southwest winds Sunday night into Monday may shift to
the east-southeast Monday night and Tuesday, generally at or below
20 knots.

&&

.MARINE...
Winds and seas should stay below SCA criteria tonight into tomorrow
morning. By midday tomorrow, winds will increase. Gusts above
25 kt will be possible especially for the New Jersey Coastal
waters through the afternoon hours.

OUTLOOK...

If confidence increases for elevated winds and seas to linger
into Saturday night, the Small Craft Advisory (SCA) may need to
be extended further into this time frame.

As a front moves south across the waters on Sunday, winds and seas
may increase to SCA levels during the day, mainly across the
northern NJ waters.

Looking ahead to Monday and Tuesday, wave heights may build in
excess of five feet during this period. Therefore, a SCA may be
needed.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory from 8 AM to 1 PM EDT Friday for
     PAZ054-055.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 10 AM to 8 PM EDT Friday for
     ANZ450>453.

&&

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



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