Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 122047
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
347 PM EST Thu Jan 12 2017

.SYNOPSIS...

A cold frontal boundary will move into region tonight and settle
south of the region for the weekend. High pressure will also
build into the Great Lakes this weekend and move eastward through New
England on Monday. Another low pressure system will track from the
Southern Plains into the Great lakes for the early and middle part
of next week.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...

The large-scale environment will make a subtle but important change
for our area over the next 24 hours. Broad upper ridge centered
roughly over Florida will retrogress slightly into the eastern Gulf
of Mexico.  This occurs as a deep trough over Ontario progresses
into Quebec by tomorrow morning.  In between, fast quasi-zonal flow
will develop in the Midwest and Northeast.  Cold front (associated
with a vort max moving through New England this evening) will be
able to progress southeastward through our region overnight thanks
to the aforementioned retrogression of the ridge but will likely
stall in the southern Mid-Atlantic Friday night as the upper-level
flow becomes oriented parallel to the boundary.  Passage of the cold
front will occur tonight, probably near or just after midnight in
the southern Poconos and Lehigh Valley and just after sunrise Friday
near the coastal waters.

Ascent associated with the vort max will be moving away from the
region as the cold front progresses through the area.  Best chances
for associated precipitation remain in the southern Poconos and
adjacent Lehigh Valley/northwest New Jersey, but these chances
should gradually diminish through the evening as lift weakens. Gusty
winds seen today will slowly decrease through the evening hours and
switch to northwest as the cold front passes.  May see some patchy
fog just before frontal passage in portions of the area but was not
confident to include in the grids at this time.  Forecast
temperatures tonight will be in the mid 30s in the southern Poconos
to around 50 near the coastal waters.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...

As the cold front progresses south of the area tomorrow, breezy
northwest winds will develop across the area.  Temperatures will
struggle to climb from early morning readings, and daytime high
temperatures will likely be well below readings seen at midnight.
Skies will rapidly clear after cold frontal passage, but this will
be short-lived as the next vort max in the Ohio Valley begins to
approach the region, with associated upper-level lift increasing.
High clouds are expected to move into the region by late in the day.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...

Friday night through Monday:

High pressure will be located over the Great Lakes with waves of low
pressure moving west to east along a frontal boundary in the Mid-
South. Moisture will ride north of the boundary as well leading to
the development of overruning light precipitation with each wave of
low pressure.

The first area of low pressure will track to our south in the
Saturday timeframe. Right now it appears the best lift for light
precipitation development will be across northern Maryland into the
Delmarva. This light precipitation is likely to break out from
southwest to northeast on Saturday. However, a fairly substantial
dry layer around 3,500 feet up in atmosphere will likely mean virga
will occur for a while across the region for a good portion of the
day. A sharp gradient of where precipitation occurs over northern
part of the forecast area will likely be present as well.


High pressure will start to shunt the overruning moisture along the
boundary further south allowing for clearing conditions over most of
the region with continuing periods of light precipitation in Delmarva
through Sunday night. The frontal boundary will then surge
northward as a warm front Sunday night and Monday with a chance
for some additional light mixed precipitation.

Precipitation type as has been all winter gets a little more
complicated. Typical model bias at this range is for the data to be
cold in the mid-levels and to warm at the surface. Overall, these
biases lead to higher prospects of ice as we get closer to an event.
The NAM and GFS are more prominent with a warm layer surging
toward Philly and ACY in the 850-925 mb range with the ECMWF only
taking the warm layer into the Delmarva. This warm layer would
allow for sleet and freezing rain to mix in with the light snow.
Temperatures as well could rise a few degrees above freezing in
Delmarva allowing for a change to rain.

Right now total precipitation looks to be around a tenth of inch
through Saturday night from a Reading to Toms River line and points
south. A few hundredths of an inch of additional precipitation currently
looks to fall for the remainder of the weekend into Monday.

Around an inch of snow could fall on a Reading to Toms River with
lower amounts further south due to mixing. Minor ice accumulations
could occur throughout the weekend as well.

Temperatures should be in the 20`s and low 30`s through Saturday,
at or a little cooler than MET/MAV/ECMWF and then fairly close to
the model data for the remainder of the weekend. Will have to
watch to see how long it takes to scour out the low level cold in
the Southern Poconos and Northwest New Jersey on Monday.


Monday night through Thursday:

A low pressure system will track into the Great Lakes with a
surge of warm air advection.  A CAD situation is showing up on
the models on Tuesday which will likely delay the warm-up till
Wednesday.

A chance of rain showers is present as the warm front
on tuesday, with additional chances of rain Wednesday and Thursday
with a front nearby and warm moist air being advected northward.
Temperatures have the potential to be 20 degrees above normal
Wednesday and Thursday with a frontal boundary to our northwest.
Long term ensemble guidance is likely underdoing the warm-up at
this point given biases to stick closer to climatology that far
out.

Longer term: Temperatures should continue well above normal in
the 8-14 day as a trough moves into the west coast and Alaska (+
EPO, - PNA) that will result in height rises along the east coast
bringing warm air northward.

&&

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

Southwest winds will slowly decrease this evening as ceilings
gradually lower generally to the 5-10 kft range.  May see a brief
period of ceilings/visibilities approaching MVFR before cold frontal
passage in the 06Z-12Z time frame.  Winds will switch to northwest
after frontal passage, with rapid improvement to ceilings and any
lowered visibilities.  Chance for showers at KABE/KRDG through this
evening but expect these to diminish before reaching the other TAF
sites.

Friday should be VFR with breezy northwest winds 10-20 kts with
gusts up to 25 kts.

OUTLOOK...

Friday night through Saturday night: Visibilities and ceilings
lowering to MVFR/IFR Saturday into Saturday night due to light
snow developing. Possibly a wintry mix for KILG, KACY and KMIV.
Winds 10 knots or less.

Sunday and Sunday night: Lingering light mixed precipitation for
KILG, KMIV and KACY with MVFR/IFR, improvement elsewhere. Winds
under 10 knots.

Monday through Tuesday night: MVFR/IFR throughout most of this
period. A light wintry mix Monday morning changing to light rain.
Winds under 10 knots.

&&

.MARINE...

Advisory conditions occurring on Delaware Bay and the coastal waters
of NJ/DE this afternoon.  Should see a sufficient decrease in winds
on Delaware Bay this evening to let the advisory expire as
scheduled.  However, winds/seas on the coastal waters should remain
elevated through this evening.  After a brief lull late tonight,
northwest winds will kick in tomorrow with more gusts exceeding
advisory criteria.  Small Craft Advisory for the coastal waters
through Friday afternoon looks good and did not change with this
afternoon`s package.

OUTLOOK...

Sub-SCA conditions expected. Highest winds will be Saturday,
around 15 knots on the ocean. Seas will fairly low of only a foot
or two throughout the period.

&&

.TIDES/COASTAL FLOODING...

Model guidance continues to show low tides tonight approaching blow
out tide thresholds. With the full moon today, astronomical low
tides will be between -0.8 and -1.0 ft MLLW. However, local
regression equation suggests that the tidal departures should be
between 0 and -0.5. Additionally, the southwest winds expected
before low tide tonight are not optimal for blow out tides.
Therefore, we still expect the total water level to stay above blow
out tide thresholds.

&&

.CLIMATE...

Record high temperatures occurred today at Atlantic City of 67
and 67 at Wilmington.

Today should be quite mild with current forecast within a couple
of degrees of record highs for many of our climate sites. Here are
the records for January 12:

Location/     Record/Year
Atlantic City   63/1913
Wilmington      63/1913
Allentown       60/1932
Reading         61/1913
Georgetown      70/1975
Mount Pocono    56/1975
Trenton         68/1890

At this time, we are forecasting that Philadelphia will stay well
below its record of 72 set in 1890.

Another round of very warm temperatures is likely to
occur Wednesday and Thursday but potential cloud cover and CAD
may limit the degree of the anomalous warmth. Will post records if
the full potential of the warm up looks to transpire.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 PM EST Friday for ANZ450>455.
     Small Craft Advisory until 7 PM EST this evening for ANZ430-
     431.

&&

$$
Synopsis...Gaines
Near Term...CMS
Short Term...CMS
Long Term...Gaines
Aviation...CMS/Gaines
Marine...CMS/Gaines
Tides/Coastal Flooding...
Climate...



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