Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 211734
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1234 PM EST Sun Jan 21 2018

.SYNOPSIS...
High pressure moves offshore of the southeast coast through
tonight. A warm front will be gradually moving north across our
area later tonight and Monday as low pressure moves through the
Great Lakes. As the low moves into southeastern Canada, a cold
front is pulled across our area early Tuesday. High pressure
will then build across the region for the end of the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Some cooling aloft has occurred, however some low-level warm air
advection is forecast to get underway later this afternoon.
Moisture, which looks to be in a thin layer, is trapped beneath a
few inversions. These are shown nicely on the 12z Sterling, VA and
Upton, NY raobs. An area of low-level clouds (2,500-5000 feet) is
present within the inversion heights. This area of cloudiness has
been slowly expanding east and northeastward, however some breaks
are noted in satellite imagery. Overall, more clouds than sunshine
for much of the region this afternoon. Temperatures are close or
even above the forecast highs already especially near and south/east
of I-95, therefore had to raise the high temperatures some. Did
adjust the hourly temperatures some more given trends and the upward
adjustment to the highs.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
Low pressure continues to develop and strengthen over the
Central Plains as it lifts to the north. Abundant moisture in
the 1000-700mb layer overspreads the region, and thicknesses
continue to slowly rise. Latest BUFKIT soundings keep surface
temperatures above freezing throughout, even in the Poconos and
northern NJ, so no major ptype issues, as precip will remain all
rain. With limited surface moisture and not much in the way of
upper level support, PoPs will be low, and not expecting much
beyond light rain and drizzle with the increasing moisture.

Lows tonight will range from the low to mid 30s north and west
of the Fall Line, and otherwise in the mid to upper 30s.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
On Monday, low pressure will be moving through the mid-
Mississippi Valley. As this happens, the warm front across the
area will try to lift northward, but will likely not make much
progress as it remains dammed up across the area. Along and
north of where the frontal boundary stalls out, some light
rain/drizzle and patchy fog could remain through the day Monday.
It looks like temperatures will remain above freezing, so the
threat for freezing rain/drizzle has decreased. South of the
front, dry conditions will likely move back in and some clearing
may take place.

Overnight Monday into Tuesday morning, as low pressure moves
through the Great Lakes, the warm front will continue to slowly
move northward, before a cold front is expected to sweep across
the area. Again, north of the warm front, some light
rain/drizzle and patchy fog could remain into the evening. Then
as the cold front approaches and moves through the area, we
expect a period of moderate to heavy rainfall. Guidance is
showing the potential for some weak instability within the warm
sector across southern New Jersey and Delaware and Maryland.
So we have introduced isolated thunderstorms across these areas
during the morning hours Tuesday as the rainfall associated with
the frontal passage move through. Dry air will move in behind the
cold front during the afternoon. Winds will become gusty 20 to 25
mph behind the front as well.

For Tuesday night into Wednesday, the low pressure system moves
across eastern Canada, while a secondary cold front moves
across the area. We expect dry conditions across the area as any
precipitation is forecast to remain to our north. Gusty
conditions are expected on Wednesday as winds will be gusting at
least 25 to 30 mph.

High pressure returns for Thursday and Friday, but builds
offshore Saturday, leading to dry conditions through Saturday.
Winds will be less gusty than Wednesday, around 15 to 20 mph.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

This afternoon...VFR to local areas of MVFR ceilings (cloud bases
mostly in the 2,500-5,000 feet range). Low confidence on the extent
of the MVFR ceilings. Winds varying from northwest to southwest less
than 10 knots, all becoming more southwesterly.

Tonight...VFR (marginal MVFR) ceilings lower to MVFR, then to near
IFR overnight. The lowest ceilings are anticipated for RDG, ABE and
TTN where especially some light rain, drizzle and fog will also be
possible. Low confidence however regarding the ceilings. Southerly
winds less than 5 knots or light and variable.

Monday...Sub-VFR conditions improve to VFR from south to north
through early afternoon. Some light rain, drizzle or fog is possible
in the morning, especially north and west of PHL. Lower confidence
regarding the ceilings. Light and variable to southerly winds less
than 10 knots.

OUTLOOK...
Monday night-Tuesday...Conditions lower to MVFR then IFR
everywhere with periods of rain associated with a frontal
passage. Conditions improve back to VFR behind the frontal
passage. Southeast winds become westerly behind the front and
begin gusting 25-30 knots. Moderate confidence. Low-level wind
shear is possible to develop Monday night into Tuesday morning.

Tuesday night-Thursday...VFR expected. Northwest winds with
occasional gusts Tuesday night, becoming more gusty 25-30 knots
Wednesday-Wednesday night. Less gusty around 20 knots Thursday.
High confidence.

&&

.MARINE...
This afternoon...Sub-Small Craft Advisory conditions expected.
High pressure over the Southeast U.S. continues to drift
offshore. Meanwhile, a weak trough will pass through the waters
this morning. NW winds 10 KT or less will back to the SW.

Tonight...Conditions remain below SCA levels. SW winds less
than 10 KT, becoming 10-15 KT with gusts to 20 KT over DE ocean
waters by daybreak Monday.

OUTLOOK...
Monday...Conditions expected to remain below advisory levels,
but winds may periodically gust around 20 knots.

Monday night...Conditions expected to return to Small Craft
Advisory levels.

Tuesday-Wednesday...Small Craft Advisory level wind continue,
but may reach gale force.

Wednesday night-Thursday...Conditions likely to remain at Small
Craft Advisory levels.

&&

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

&&

$$
Synopsis...Gorse/Robertson
Near Term...Gorse
Short Term...MPS
Long Term...Robertson
Aviation...Gorse/Robertson
Marine...Robertson/MPS



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