Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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217
FXUS61 KPHI 220725
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
325 AM EDT Mon May 22 2017

.SYNOPSIS...
Low pressure tracks across southeastern Canada tonight through
Monday. An associated cold front will move through our area this
evening, then stall just offshore through Tuesday. An area of
low pressure moves across our area Wednesday morning, followed
by a cold front late Wednesday night and Thursday morning. High
pressure will build in from the southwest late Friday into
Saturday, then it shifts offshore into Sunday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
We are beginning to see the first wave of showers as of 3 AM. So
far rain totals have been relatively light. Expect that this
will continue until the second wave, associated with a second
mid level vorticity max is expected after sunrise through mid
day.

As far as the hazards, instability still appears meager, but
perhaps enough for isolated thunderstorms along and east of the
I95 corridor. The main story will be the potential for heavy
rain. At this point the axis of heaviest rain looks to be east
of the I 95 corridor through mid day today. Even so, the latest
models have backed off a bit on QPF values. Therefore, do not
expect a widespread threat, but still potential for localized
flooding especially in poor drainage areas and especially if any
thunderstorms develop. It still looks like there will be a sharp
drop off in rain totals north and west of I95.

Expect rain to taper off rather quickly late this afternoon as
the cold front approaches the region.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...

Low clouds are expected to persist through at least the first
half of the night until dry air advection dominates in the wake
of the cold front. Depending on how much rain areas see this
morning, some fog may develop as well, but the low clouds should
inhibit widespread fog development.

Models have trended slightly later with the arrival of the cold
front, as a result expect low clouds to linger a bit longer, and
thus there will be a shorter opportunity for efficient
radiational cooling, especially for the Coastal Plains.
Therefore, temperatures from the I95 corridor to the east may
not drop off that much overnight.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Long term discussion will be updated shortly.

Summary...Times of wet weather, however warmer temperatures look to
occur especially later this week and next weekend.

Synoptic Setup...An amplifying upper-level trough is forecast to
gradually shift eastward from the Midwest and Plains Tuesday night
and Wednesday. This should move out of the East Friday as some
ridging arrives and establishes high pressure off the East Coast
during next weekend. At the surface, an area of potentially
disorganized low pressure looks to move through Tuesday night and
Wednesday. If short wave energy can consolidate more ahead of the
aforementioned trough, then surface low pressure would be able to
strengthen and organize. While the pattern for awhile looks
unsettled, there is less certainty with the details especially for
the Tuesday through Wednesday time frame. We used a model/continuity
blend for Monday night through Tuesday night, then blended in the
12z WPC Guidance thereafter. Some adjustments were then made
following additional collaboration with our neighboring offices.


For Tuesday and Wednesday...An upper-level trough is forecast to
migrate eastward from the Midwest and Mississippi Valley. This will
allow for some downstream riding, however the model guidance shows
several embedded short waves within the southwesterly flow aloft.
These should result in surface low development into Mid Atlantic
region, although the parent surface low is currently forecast to
track into the Great Lakes Wednesday night. The initial surface low
should develop on the stalled frontal boundary, however timing is
less certain as some guidance is faster with this feature. It
appears the main forcing may arrive Tuesday afternoon and night,
then some drying tries to work in during Wednesday. While there is
less certainty with the details, went ahead and made some
adjustments to the PoPs with the highest values mainly from about
I-95 on south and east later Tuesday into early Wednesday.

For Thursday and Friday...As an upper-level trough shifts eastward
from the Great Lakes and Ohio Valley Thursday, surface low pressure
is forecast to move from the eastern Great Lakes toward northern New
England. Its trailing cold front should cross our area mostly
Thursday morning, however there may be a weak surface low near the
triple point as it crosses our region. Some showers are expected
with this system Thursday and enough instability should be realized
to generate some thunderstorms and therefore a chance of thunder is
included for much of the area mainly Thursday afternoon/early
evening. As the trough begins to lift out Friday, a weak cold front
or surface trough moves through. This in combination with cyclonic
flow and some instability may generate some showers especially
across the western/northern zones. The PoPs were kept on the lower
side at this time. A westerly wind will increase Friday as low
pressure strengthens across New England. There does not appear to be
much cooling though in the wake of this system.

For Saturday and Sunday...Some guidance is slower in removing the
upper-level trough from the Northeast Saturday, however overall a
narrow ridge should arrive as the next trough moves across the
Plains and Midwest. The arrival of the ridge allows for surface high
pressure to build in from the southwest before shifting offshore
into Sunday. There may be a warm front lifting to our north Saturday
night into early Sunday, however the opportunity for convection with
it is less certain this far out especially given the main energy is
currently forecast to be over the Great Lakes to the Central Plains.
As of now, included a slight chance PoP for most areas Sunday and
Sunday night.

&&

.AVIATION /07Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Ceilings will continue to lower, with widespread IFR conditions
expected after 12Z. Heavier showers look to move
through the terminals from 12Z to 18Z, but chances for rain
exist before and after this time frame. There is a slight chance
for thunderstorms but it is too uncertain at this time to
include in the TAFs.

Even as the precipitation begins to taper off after 18Z, expect
the low clouds to linger through the rest of the day time hours.
If there are breaks in the clouds late this afternoon into the
evening hours, patchy fog may develop.

Conditions should begin to improve after 06Z behind a cold front
which will bring in drier air.

OUTLOOK...
Tuesday...VFR ceilings lower to MVFR or IFR as showers develop,
especially from about PHL south and east. Lower confidence on
the timing. Northeast or east winds mostly 10 knots or less.

Wednesday...Times of MVFR/IFR conditions along with some showers. A
few thunderstorms are possible late in the day or evening. East-
southeast winds 5-15 knots.

Thursday and Friday...Some showers and thunderstorms around Thursday
resulting in times of MVFR/IFR conditions, improving Thursday night
or Friday. South to southwest winds Thursday, becoming westerly and
potentially gusty on Friday.

&&

.MARINE...
Although some gusts above 20 kt are possible this evening, we
are expecting conditions to remain below SCA criteria today and
tonight. By this evening, fog may develop on both the Delaware
Bay and the coastal waters. However, it is uncertain at this
time how widespread or dense any fog will be. Any fog that does
develop is expected to dissipate in the pre-dawn hours as
northerly winds increase.

OUTLOOK...
Tuesday...Winds may gust to near 25 knots late Tuesday across
the southern zones with seas building to around 5 feet. There is
lower confidence with the details given timing differences with
low pressure along a stalled front.

Wednesday through Friday...The winds should be below Small Craft
Advisory criteria, however seas may reach or just exceed 5 feet at
times on the ocean zones.


&&

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

&&

$$
Synopsis...Gorse
Near Term...Johnson
Short Term...Johnson
Long Term...Gorse
Aviation...Gorse/Johnson
Marine...Gorse/Johnson



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