Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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693
FXUS61 KPHI 220540
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1240 AM EST Wed Nov 22 2017

.SYNOPSIS...
A cold front will move through our region during Wednesday, as low
pressure tracks offshore and strengthens. High pressure will then
build eastward into the region for Thursday and Friday, before
giving way to an approaching cold front on Saturday. Another area of
high pressure will build into the region for the early part of next
week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Rain is beginning to develop to our southwest and is spreading
rapidly northeastward, likely reaching the Delmarva Peninsula
and adjacent coastal waters within the hour. Hi-res models are
honing in on a solution regarding the QPF distribution/evolution
during the night. In general, rain with embedded convective
showers should spread rapidly northeast along/southeast of the
I-295 corridor. Model soundings show a decent amount of
instability as the night progresses, likely a function of the
very strong lift occurring in the region (via a combination of
differential cyclonic vorticity advection and favorable jet
dynamics in association with the approaching shortwave trough
and phasing with a southern-stream perturbation). Low-level
frontogenesis will be occurring across this region for the next
few hours, which will enhance ascent in this area as well. There
is a strong suggestion of banding of precipitation on the far
northwest fringe of the precipitation shield, generally
along/near the I-95 corridor, in the 09Z to 14Z time window.

With the above in mind, continued to increase PoPs to the
northwest, encompassing all of the urban corridor and the
northwest suburbs. Although precipitation amounts should be
light here (generally under a quarter inch), I suspect this is
still be underplayed a little bit, especially northeast of
Philly where the banding may persist longer. Meanwhile, hi-res
models are clearly trending upward with the more convective
precip to the southeast, with half-inch to one-inch totals not
out of the question southeast of a Dover to Asbury Park line.
Also included a slight chance of thunder in the grids for the
higher PoPs areas, given the aforementioned influx of
instability.

Additionally, fine-tuned the timing of the precip, given the
clearly slower trend in the past 12 hours or so of model runs
(i.e., slowed onset of precip by about an hour or two north of
the Mason-Dixon Line).

Temperatures are a total mess, with the far northern CWA near
freezing with more ideal radiational cooling in advance of the
system and about 30 degrees warmer in southeast NJ and southern
Delaware. Expect frequent updates to hourly temperatures through
the rest of the night, as cloud cover/precipitation spreads
across the area.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/...
Rain/embedded convection will be moving rapidly eastward off the
coast during the morning, though some lingering may occur along
the immediate coast (until around noon). Winds will veer to
northwesterly and begin to gust by late morning, probably in the
20 to 30 mph range during the afternoon.

Main question is improvement of sky cover, with some of the hi-
res model soundings indicating a residual layer of saturation
below an inversion around 4000 feet or so off the surface. Two
days ago, my sky forecast crashed and burned in a similar
regime, as residual surface moisture was enough to keep a
stratocumulus deck for much of the day given the strong mixing.
This system will have weaker mixing, but given the increasing
trend of precipitation to the northwest -- thinking low clouds
might be underdone again. Later forecasts this morning may make
the sky cover a little more pessimistic.

Regarding temperatures, went closest to the MET guidance, with
the thinking that MAV is much too warm in the post-system cold-
air advection regime. Not much of a temperature climb is
expected during the day, and there may be a rather early
decreasing trend if the cold-air advection is as strong as some
of the guidance is suggesting. Nevertheless, with temperatures
in the 50s (urban corridor southeastward) to start the day, the
incoming chill will be mitigated to some degree.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Wednesday night through Friday night:

High pressure will be in place during this period with mostly sunny
skies. Temperatures will slowly warm as winds go from northwesterly
ahead of the high pressure to southwesterly on the backside of the
high pressure. Highs will generally be in the 40`s with some spots
in the 50`s on Friday. Overnight lows will drop down into the 20`s
and 30`s. Model guidance may be a bit warm in the Pine Barrens and
in the Delaware Water Gap given a favorable radiational cooling
setup.

Saturday and Saturday night:

Some limited transport of moisture is possible ahead of an
approaching cold front. Some lift along the front may result in
a few showers. Another feature to watch is an area of low
pressure expected to track from near Florida northeastward to
just offshore of the Carolinas. Some ensemble members do bring a
period of rain northward into the region. However, the general
consenus is for the cold front to continue to push the low
pressure further out to sea. With warm air advection ahead of
the front, temperatures have the potential to be warmer than the
ensemble average which has been coming up the past few runs.
Highs could reach 60 in many locations Saturday before turning
colder Sunday behind the front. Northwest winds at night should
keep lows warmer and generally in the 30`s.

Sunday through Tuesday:

A fairly strong push of cold air advection will occur behind the
front. This will lead to another period of more winter like
temperatures across the region with stronger northwest winds,
gusting perhaps to 25 mph on Sunday. Some moisture associated with a
pocket of mid-level vorticity on the backside of a departing trough
may spark a few rain or snow showers across most of the region
Sunday and Sunday night. Higher terrain could act as another lifting
agent as well for the moisture. This outcome however depends on how
progressive the trough is moving through the area. Highs Sunday and
Monday will generally be in the 40`s and warming into the 50`s on
Tuesday. Overnight lows will be in the 20`s and 30`s.

&&

.AVIATION /05Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...Conditions will begin to deteriorate rapidly as rain
moves in from the southwest. MVFR conditions are possible,
especially with the heavier rain, with MIV/ACY most likely to
see longer durations of sub-VFR. Chances of showers and
associated sub-VFR conditions rapidly diminish northwest of PHL.
Winds generally light and primarily southwesterly. Confidence in
CIGs/VSBYs is low given remaining uncertainty with precipitation
timing/placement; confidence high with winds. Frequent TAF
updates are likely through the overnight hours.

Wednesday...Rapid improvement to VFR expected, though a
lingering cloud deck 3000-5000 feet may exist through at least
the morning hours. Winds switch to northwest and become gusty by
afternoon (generally 10 to 15 kts with gusts to 25 kts). MIV/ACY
may see slower improvement, generally by late morning.
Confidence is medium.

Outlook...
Wednesday night through Friday night: VFR, northwesterly winds
becoming westerly by Thursday but at or under 10 knots. High
confidence.

Saturday and Saturday night: Sub-VFR possible with some showers.
Southerly wind gusts 10-15 knots. Medium confidence.

Sunday and Sunday night: VFR, winds shifting to northwesterly with
gusts from 20-25 knots in the afternoon. Medium confidence.

&&

.MARINE...
The winds and seas are expected to diminish some through tonight,
however then increase again during the day Wednesday (especially in
the afternoon). As cold air advection develops in the wake of a
strengthening offshore low pressure system and a cold front shifting
offshore, mixing is expected to improve within stronger flow. This
will result in gusty winds. To keep it simple, went ahead and
extended the Small Craft Advisory through Wednesday (and beyond; see
the outlook section). The advisory for Delaware Bay still starts at
17z Wednesday however. Rain moves in from the south late tonight and
will continue through Wednesday morning before moving out to
sea.

Outlook...
Wednesday night through Saturday night: Small Craft Advisory
Wednesday night, then sub-SCA seas and winds from Thursday
onward. West winds shifting to southerly Friday into Saturday
with gusts of around 15 knots at times. Any westerly wind gusts
near 25 knots will diminish quickly Wednesday evening but seas
may linger near five feet through the entire night.

Sunday and Sunday night: Winds shifting to northwest with 25-30 knot
gusts possible at times. Seas building but look to stay under five
feet at this time.

&&

.EQUIPMENT...
The KDOX radar remains out of service. Replacement parts are expected
to arrive on Wednesday, November 22nd.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EST Thursday for ANZ450>455.
     Small Craft Advisory from noon today to midnight EST tonight
     for ANZ430-431.

&&

$$
Synopsis...Gaines
Near Term...CMS
Short Term...CMS
Long Term...Gaines
Aviation...CMS/Gaines
Marine...Gaines/Gorse/MPS
Equipment...Gorse



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