Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 150905
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
405 AM EST Sun Jan 15 2017

.SYNOPSIS...
High pressure over the Midwest will build eastward into the region
today. The high will settle overhead tonight and Monday before
moving offshore Monday night. An area of low pressure over the Great
Plains will track northeastward into the Great Lakes region on
Tuesday. Meanwhile, a warm front with this system will lift northward
through the northern Mid Atlantic on Tuesday followed by the passage
of a cold front late on Wednesday. High pressure generally looks to
be in control Thursday through Saturday. However, a disturbance may
pass close by to our south Friday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Drier air is arriving across the area early this morning and this
will continue today. High pressure west of the Great Lakes will
move eastward today. Skies will become mostly sunny this morning
and should remain that way thru the day. Temperatures will climb
into the upper 30s across the north and low/mid 40s elsewhere.
These readings are a few degrees above normal for this time of
year. Winds will be mostly North to Northwest at 5 to 10 mph.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
High pressure will settle across the area tonight. Mostly clear
skies will be across most of the area but some cloudiness will be
across Delmarva. Low temperatures tonight will be close to normal
for mid January with teens up across the southern Poconos and
northern NJ and low/mid 20s elsewhere.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
The period will start off quietly with a 1032 mb surface high
centered over the region Monday morning. The high will move offshore
Monday afternoon and night while an area of low pressure tracks
northeastward from the Central Plains to the Midwest. The low, which
is responsible for the major freezing rain event across the central
Plains, is then forecast to cut well to our northwest across the
Great Lakes, putting the Mid Atlantic on the warm side of the
system.

This is looking like mostly a rain event for the Tuesday-Wednesday
time frame except for northern areas at the onset. The majority of
guidance indicate precipitation along a warm front starting out on
Tuesday as freezing rain in northeastern PA and northwestern NJ with
a shallow (1-2 kft deep) layer of sub-freezing air trapped beneath a
well-defined warm nose aloft. Models favor a short duration of
freezing rain (around 6 hours in the southern Poconos and even less
farther south) as temperatures rise above freezing rather quickly
with the warm fropa. However, as discussed continually in recent
months, models often scour out the low-level cold air across the
higher terrain N/W of the Fall Line too quickly in these setups so
the duration of icing may be longer. Timing, in terms of when the
lower clouds arrives Monday night and when the precipitation moves
in, will help determine the extent of the freezing rain. The denser
cloud cover will limit help prevent temps from dropping much Monday
night across most of the forecast area. RH fields from the
NAM/GFS/ECMWF show the northern edge of the cloud cover slow to
progress north of I-78. This setup would allow the airmass to
radiate effectively (especially in the evening) and for temperature
to fall into the mid or even lower 20s across northeastern PA and
northwestern NJ. Conversely, lows in these northern zones may only
be in the upper 20s or lower 30s if the clouds arrive earlier in the
night. Additionally, the earlier the precipitation arrives on
Tuesday, the greater the threat exists for freezing rain. The faster
solutions (e.g., 00Z ECMWF ops) brings in the precipitation
overnight while the slower solutions (e.g., 00Z NAM) hold off the
onset until afternoon. Spread in the GEFS and SREF ensembles provide
additional evidence for timing uncertainty. The timing details will
hopefully become clearer over the next 24 hours. Nonetheless, there
is high confidence that areas north of I-80 (Carbon and Monroe
Counties in PA and Sussex County, NJ) see at least some freezing
rain. Our latest ice accumulation forecast graphic shows light as
far south as the I-78 corridor and as far east as I-287. This ice
forecast can be considered a bit bullish as it lies closer to the
90th percentile of the latest PWPF ice guidance from WPC, but after
taking into account model bias and local terrain, this approach
seemed warranted.

PoPs are highest Tuesday afternoon and night. Rain could become
widespread for a period as a secondary low developing along the
system`s triple point passes through the region. A cold front most
likely moves through sometime late Tuesday night or Wednesday
morning, which would give us an opportunity to dry out during the
day Wednesday. The 00Z GFS is much slower to develop the low and is
subsequently later with the cold fropa, resulting in a soggy
Wednesday. Although the GFS cannot be ruled out at this point, the
official forecast, which was weighted more toward WPC/SuperBlend
guidance, is more optimistic for Wednesday.

The cold air behind Wednesday`s cold front mainly has Pacific
origins and thus is not very cold for mid winter standards.
Therefore, temperatures should remain above normal during the second
half of the week and into next weekend. Thursday should be dry with
high pressure builds into the area. Confidence is not as high for a
dry Friday with a disturbance organizing over the Gulf of Mexico and
possibly passing close by to our south. High pressure looks to
regain control of our weather pattern heading into next weekend.

&&

.AVIATION /09Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

High pressure will build over the area today then move to the
east/northeast by early Monday. After the last of the low clouds
and light fog mix out shortly after dawn, VFR conditions are
expected the rest of the TAF period. Winds will be north to
northwest at 8-12 knots today and light tonight. There may an
increase in high clouds late Sun night and Monday ahead of the
next system.

OUTLOOK...

Monday and Monday evening...VFR.

Monday late night through Tuesday night...Lowering to MVFR and then
IFR as precip moves in from the west. There is considerable
uncertainty regarding the onset timing of precip and when CIGs/VSBYs
lower...It could be as early as late Monday night or as late as
Tuesday afternoon. Expect mainly rain. However, freezing rain may
occur at the onset late Monday night or Tuesday at KABE and perhaps
in the vicinity of RDG.

Wednesday...Sub-VFR conditions and SHRA in the morning should
improve later in the morning or afternoon.

Wednesday night and Thursday...Predominately VFR.

&&

.MARINE...
High pressure will build across the waters today before moving
away Monday. A gentle North to Northwest flow is expected today
and this evening before becoming Northeast later tonight. Seas on
the ocean should remain in the 2 to 3 ft range. A little early
morning low clouds and fog then fair weather is expected today and
tonight.

OUTLOOK...

Monday through Thursday...Winds will be less than 20 kt. Seas may
build close to 4 or 5 ft late Wednesday afternoon or night as low
pressure moves off the coast and strengthens offshore.

&&

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

&&

$$
Synopsis...Klein
Near Term...O`Hara
Short Term...O`Hara
Long Term...Klein
Aviation...Klein/O`Hara
Marine...Klein/O`Hara



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