Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 182330
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
730 PM EDT Wed Apr 18 2018

.SYNOPSIS...
A area of low pressure will move across the area tonight into
Thursday, along with its associated occluded, warm, and cold
fronts. This low will drift into the Canadian Maritimes through
Friday, which will keep northwest flow across the area with a
couple of dry surface troughs moving across the east coast. High
pressure will build into the Mid Atlantic and Northeast
Saturday through Sunday, before moving offshore Monday into
Tuesday. A coastal low may move north along the coast and affect
the area during the middle of next week.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
700 pm update: Main changes to the forecast were to modify PoPs
tonight based on latest hi-res guidance, which generally
indicate a period of light rain/sprinkles/showers from 03Z to
10Z generally north of the I-276/I-195 corridors. The HRRR
suggests a briefer period of rain but potentially with heavier
rainfall rates, whereas the WRF-ARW/NMM and NAM Nest suggest a
longer duration of lighter precipitation. My thinking is that
the briefer duration of the precipitation is more reasonable
(especially given upstream radar trends), though I am not quite
convinced of the wetter solutions yet (despite the rather potent
look to the vort max, along with the close proximity of the
surface low).

Increased PoPs slightly to the north of the I-276/I-195
corridors, but reduced them to the south (and remaining
unmentionable south of an Easton, MD, to Wildwood, NJ, line).

Other main change was to include some chances for (light) snow
to mix in with the rain late tonight in the Poconos (and far
northwest New Jersey), where temperatures will become cold
enough (and profiles will cool sufficiently) for the white stuff
to show up.

Will update storm total snow grids accordingly, though amounts
are expected to be very light (and generally only at the highest
elevations).

Previous discussion...

Surface low pressure will approach the region from the west
tonight, moving into our area late tonight. Cloud cover will
increase quickly tonight as the low moves closer to the area.
Some light rain will fall, in advance of the approaching low but
amounts are expected to remain light, with southern areas
likely remaining dry through daybreak.

Overnight lows will drop into the mid to upper 30s across the
northern zones to mid to upper 40s across the southern zones.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
700 pm update: As the surface low moves offshore tomorrow
morning, should see a brief break in precipitation before
instability showers/sprinkles move in during the late morning
and afternoon as the main trough/vort max move(s) into the
region. Precipitation amounts should be very light (and will
likely be a mix of rain and snow in the Poconos), but would not
be surprised to see some snowflakes mix in with the showers
farther southeast. Trended forecast in that direction a little
bit, but not too much given the expected temperatures/low-level
thermal profiles.

Tweaked PoPs to match the timing of the instability showers, as
depicted by the NAM Nest, WRF-ARW/NMM, and HREF. Also slowed the
onset of the stronger winds by a couple of hours, but did not
change the magnitudes that much given the model-derived mixing
profiles.

Previous discussion...

Low pressure will cross the region tomorrow, dragging a cold
front through during the late morning/early afternoon. Some
light rain will fall across the region as the low crosses the
area. Amounts will be fairly light, especially across southern
areas, with only a tenth of an inch or less expected. As
temperatures cool across the higher terrain, a few flurries or
light snow showers may occur as the precipitation starts to come
to an end toward Thursday evening.

Extensive cloud cover across the northern half of the forecast
area will keep much cooler and only into the upper 30s to mid to
upper 40s. Once you get down towards Philadelphia and points
south, skies should see a few breaks. Highs into the upper 50s
to lower 60s are expected.

Even with the sun peeking out, it will feel cooler through the
day as the winds will really start to ramp up by late
morning/early afternoon. Winds will be our of the north to
northwest at around 15 to 25 mph with gusts up to 35 mph
possible.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
A quiet start to the extended forecast for the end of the week,
through this weekend, into early next week. A return of
unsettled weather is possible by the middle to end of next week.

On Thursday night, the low pressure that affects the area
during the day will continue to move northward into the Canadian
Maritimes. Most of any precipitation associated with the low
will move to our north with the low pressure. However, there
could continue to be some snow showers across the higher
elevations, especially during the evening hours as the low-mid
level lapse rates may be high enough to interact with some left
over moisture and the passing short wave. These showers will
dissipate through the night into Friday morning as the moisture
dissipates across the area.

On Friday, the low will remain across the Canadian Maritimes,
while surface trough will likely cross the area. The trough will
be dry, and only present a shift in wind direction and possibly
some cloud cover, as it will likely not having much of an
airmass change. Winds during the day will be generally out of
the northwest, and gust 25- 30 mph, occasionally up to 35 mph at
times. Dry and breezy conditions will continue into Friday
night as well.

On Saturday, dry conditions will continue as another dry
surface trough will move across the area, before high pressure
builds into the area overnight. Winds will gust around 20 mph at
times, but it will not be as windy as Friday.

High pressure builds across the Mid Atlantic and northeast
Sunday, before moving offshore my Monday. Dry conditions will
continue for Sunday and Monday.

As we move into Tuesday and Wednesday of next week, a coastal
low is forecast to lift northward along the east coast. There is
timing differences between the GFS, ECMWF, and Canadian. With
the timing differences, there is the potential for rainfall
starting as early as Tuesday, with the most likely period
looking to be Wednesday Wednesday night. Temperatures across the
area are forecast to be above freezing, so the precipitation
would be all rain.

&&

.AVIATION /23Z WEDNESDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR to start with, but CIGs are expected to develop
rapidly and descend after 03Z. By 09Z, may see RDG/ABE flirt
with MVFR criteria. Light rain or showers are forecast
near/north of RDG/ABE/TTN, but may have a difficult time
reaching PNE/PHL/ILG and are not expected to affect MIV/ACY. Low
VFR to MVFR expected at most sites by daybreak. Winds will
become light/variable quickly this evening. Moderate confidence.

Thursday...Low VFR to MVFR conditions are anticipated at most
sites, especially during the morning, as drizzle/patchy fog may
occur near RDG/ABE/TTN and light showers subsequently move
through the Philly terminals northward and westward through mid
afternoon. CIGs will likely rise a little bit during the
afternoon as winds become westerly/northwesterly and gusty by
late morning and continuing during the afternoon (10-20 kts with
gusts 25-30+ kts). Moderate confidence.

OUTLOOK...

Thursday night...MVFR conditions possible early, before
improving to VFR overnight. Gusty northwest winds 20-30 knots.

Friday-Monday...VFR through the period. Gusty northwest winds
20-30 knots Friday; 20-25 knots Friday night; and 15-20 knots
Saturday.

&&

.MARINE...
A Gale Watch is in effect for all of our waters from 2PM
Thursday through 8AM Friday.

Tonight...Quiet conditions on the area waters with southerly
winds around 10 to 15 knots and seas around 2 to 4 feet.

Thursday...Winds will pick up out of the northwest on Thursday
and become particularly gusty during the afternoon and evening.
Soundings show a strong, deep layer of 35+ knots over the area
waters and mixing should be fairly efficient by the afternoon.
Sustained winds of 15 to 25 knots with gusts up to 35 knots.
Seas will also increase to around 4 to 5 feet by Thursday
afternoon.

OUTLOOK...

Thursday night...Gale Watch in effect for all the waters as
winds could gust up to 35 knots through the night.

Friday-Friday night...Small Craft Advisory conditions likely.

Saturday-Monday...Sub Small Craft Advisory conditions exected.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Thursday afternoon through Friday morning for
     ANZ430-431-450>455.

&&

$$
Synopsis...Robertson
Near Term...CMS/Meola
Short Term...CMS/Meola
Long Term...Robertson
Aviation...CMS/Robertson/Meola
Marine...Robertson/Meola



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