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000 
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

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