Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 211918
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
318 PM EDT Mon Oct 21 2019

.SYNOPSIS...
A cold front is expected to approach the region late in the day on
Tuesday. High pressure will build over the region for Wednesday and
Thursday before shifting offshore on Friday. Another cold front is
expected to approach the region Friday night. We could see another
low pressure system approach the region this weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
High pressure to the north which made for a very pleasant Monday
will shift further away to the northeast tonight as a frontal
system starts to approach. Mainly dry weather is expected tonight,
but clouds will be on the increase. Mid and upper level clouds will
fill in as the frontal system to the west approaches. In addition,
should see some low clouds develop also as low level moisture
becomes trapped beneath an inversion in light easterly flow with
low level warm advection. Could even see some patchy light showers
or drizzle towards daybreak especially over the far western zones
where frontal forcing will be slowly approaching, as well as over
eastern New Jersey due to enhanced moisture from onshore flow and
low pressure offshore. For the most part, however, the overnight
will be dry. Due to the increasing cloud cover, temperatures should
not fall off too much, with most areas getting down to the upper 40s
or low 50s. If clouds advance more slowly, however, we would radiate
better and drop down a few degrees cooler especially up north.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
A frontal system will affect the area on Tuesday and Tuesday night.
A strong area of low pressure will move from the western Great Lakes
up into Ontario. The low is moving more north than east, which is
keeping the approach of the front slow and also keeping our area
removed from the best forcing. Still, as mentioned this is a strong
low associated with a robust shortwave trough and good upper jet
dynamics, with a 130 to 150 kt upper jet streak expected to be to
our west Tuesday. So a period of rain is expected as the cold front
approaches. While spotty light showers are possible during the
morning, most of the rain should hold off until mid to late
afternoon and especially the evening. Rain may be moderate or
briefly heavy for a time with the main frontal band. PWATs make a
rather impressive bounce up to the 1.5 to 1.8 inch range which also
supports locally heavy downpours.

Rain amounts look to be highest to the north and west, where the
forcing is best and where there could also be areas of orographic
enhancement. There is a fair amount of spread in guidance on QPF,
with the hi res models generally running wetter than the global
guidance. The overall trend today seemed to be a little higher on
rain totals. Some areas should exceed an inch of rain especially up
towards the Poconos, and cannot rule out some localized amounts near
to above 1.50 inches. Closer to the urban corridor amounts should
drop closer to one half to three quarters of an inch, with a quarter
to half inch of rain expected in the southeast portion of the area.
Not expecting these totals to cause any hydro problems. However, if
the system really over-performs, some our northwest zones may start
to approach FFG, which has come down recently due to a couple of
rain events. But that is a low risk.

SPC also has far southern Delaware in a marginal risk for severe
weather on the Day 2 outlook. The extensive cloud cover will greatly
limit instability tomorrow, and most guidance, even hi res, suggests
no more than a couple hundred joules of MLCAPE by tomorrow evening.
So was still not confident enough to add thunder to the forecast.
Not surprisingly, shear is very strong, but just don`t think we`ll
have the instability for anything more than isolated rumbles.
However, most guidance suggests a southerly LLJ of 40 to 50 kt
moving overhead as the heaviest rain moves through. So even with
no thunder, the heavier band of rain may be capable of producing
gusty winds. A low level inversion will limit that threat, but some
localized 30-45 mph gusts may be possible with the heaviest areas of
rain.

The front should move through the area from west to east between
around 11PM and 3AM Tuesday night, with guidance now falling into
reasonably good agreement on this. Rapid clearing is likely behind
the front as winds shift to west-northwest and drier air comes
rushing in.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Overview: Will be starting the long term with tranquil and
seasonable conditions mid week. A cold front is expected to arrive
late Friday or Friday night. By Saturday, the forecast becomes much
more uncertain as it is unclear how close a low pressure system will
track to our region through the weekend.

Changes to the previous forecast: Delayed the timing with chances
for rain on Friday, but otherwise no major changes to the forecast.

Details:

Wednesday and Thursday...Large area of high pressure towards the
region before building off shore on Thursday night. Consequently, we
should have dry and mostly sunny conditions through the period.
Temperatures should be near normal with highs in the upper 50s and
60s both days.

Friday and Friday night...Cold front will approach from the west.
Models continue to depict this arriving Friday night, and any
associated pre-frontal showers arriving late in the day. Some
models, notably the GFS, depict little, if any, precipitation ahead
of this front. Thus, I adjusted the PoPs to depict this later start
time and more uncertainty.

Saturday through Monday...There is lots of uncertainty through the
weekend. The biggest question is if we will see a closed lee-side
low develop over the far southern Plains on Friday before lifting
into the Great Lakes region through the weekend. In this scenario
(most evident in the ECMWF), our region could see rain Sunday into
Monday. However, the upper level pattern is a split-flow pattern
which are notoriously hard to forecast more than a few days out. For
now have stayed close to the previous forecast and a blend of long
range guidance given the large uncertainty.

&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight... VFR conditions expected initially. A trend towards MVFR
is expected overnight as clouds fill in ahead of an approaching
frontal system. Winds should be very light overnight, with the flow
mainly out of the east but periods of calm or light and variable
conditions possible.

Tuesday...MVFR should have overspread most or all of the area by
12z. A continued downward trend in ceilings is expected through the
day. Areas of IFR are possible especially during the later afternoon
and evening. Rain showers will also slowly overspread from west to
east. Winds mainly from the east-southeast at around 10 kt.

Outlook...

Tuesday night... MVFR/IFR likely through midnight with rain.
Conditions should improve to VFR from west to east during the second
half of the night. Southeasterly winds will shift abruptly to
northwesterly overnight due to a frontal passage. Speeds around 10
kt.

Wednesday and Thursday...VFR conditions are expected with few, if
any clouds. Westerly winds of 10 to 15 kt on Wednesday, will become
light (5 kt or less) southwesterly winds by Thursday. High
confidence.

Friday...Likely to start the day with VFR conditions, however,
increasing clouds and chances for rain could lead to MVFR or even
localized IFR conditions by Friday evening. Southwesterly winds of 5
to 10 kt, should abruptly shift to northwesterly Friday night.
Moderate confidence on the overall pattern, but low confidence on
the timing.

Saturday...periods of MVFR or even IFR conditions will remain
possible as the next system could bring additional rain and low
clouds. However, the timing of this system is quite uncertain, so
there is low confidence in conditions on Saturday. Northwesterly
winds of 5 to 10 kt may continue through the day.

&&

.MARINE...
Outside of the Delaware Bay, a Small Craft Advisory is now in effect
through 2PM Tuesday. While wind gusts will gradually diminish to
under 20 kt tonight, seas will continue to linger near 5 ft. The
advisory may need to be extended further into Tuesday as seas will
likely remain near that threshold for much of the day. It does look
like there will be a period especially during the afternoon and
evening during which seas fall just below 5 ft. However, Tuesday
night, there may be an additional period of SCA conditions as west-
northwest winds increase during a frontal passage, with gusts to
around 25 kt. That is most likely after midnight.

Outlook...

Wednesday...SCA conditions are possible to start the day, but winds
and seas should gradually diminish through the day.

Thursday through Saturday...Winds and seas should stay below SCA
criteria.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 2 PM EDT Tuesday for ANZ450>455.

&&

$$
Synopsis...Johnson
Near Term...O`Brien
Short Term...O`Brien
Long Term...Johnson
Aviation...Johnson/O`Brien
Marine...Johnson/O`Brien


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