Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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592
FXUS61 KPHI 210119
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
919 PM EDT Mon Mar 20 2017

.SYNOPSIS...
A weak low pressure system will slide over the Mid Atlantic tonight
into tomorrow. A cold front is expected to arrive in the region
tomorrow night. A large area of high pressure is then expected to
build south and then slide off shore through the remainder of the
work week. A warm front is forecast to lift through our region on
Friday. Low pressure is expected to approach from the west over the
weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
900 pm update: Looks like the clouds are winning the temperature
battle tonight, as the nocturnal trend downward has been impeded
considerably early this evening. Adjusted temperatures upward
for the next few hours to match trends. Dew points are running a
little high across the area as well, so lowered these overnight
by a few degrees. Overall forecast thinking has not changed
since the previous update. High-resolution models continue to
hint at light showers progressing ESE through PA overnight,
perhaps reaching our area after 2-4 am. Any showers that survive
will be very light, and with that in mind, think the probability
of measurable precipitation remains in the slight chance range
at best. No changes to PoPs/Wx grids overnight.

Previous discussion...

600 pm update: Agree with previous shift that the trends for
precip in our area overnight are downward. Most
mesoscale/storm-scale model simulations paint an awfully dry
night for the CWA, especially for the northeastern half of the
region. Weak perturbation in northwesterly flow presently in the
Midwest will progress fairly quickly ESE overnight into the
Mid-Atlantic region. The perturbation is generating a mesoscale
convective system (MCS) in portions of Indiana and Ohio at this
time. The remnants of this MCS are expected to move through the
Appalachians this evening and into the Mid-Atlantic overnight.
However, the evolution of this precipitation is the main
question, with model variability quite high in the handling of
this precipitation over the past couple of days. The general
trend has been farther south and west and with a noticeable
diminishing trend to the lee of the Appalachians. Both of these
trends make sense owing to the digging of a stronger (kicker) vort
max in southern Canada and the progression of the perturbation
into a transient ridge straddling the East Coast this evening.

As a result, reduced PoPs even more across the area overnight,
with the highest probabilities remaining in Delmarva. There is
some indication from convection-allowing models (CAMs) that
isolated/scattered light showers may move into southeast
Pennsylvania and New Jersey near daybreak, which is at least
hinted at by the 18Z GFS and 12Z ECMWF. Will look at the next
few cycles from these CAMs before biting, however.

Also modified temps/dew points rather substantially this evening
given another day of guidance too low with the former and too
high with the latter. Temperature forecast this evening is low
confidence given the small window of favorable radiational
cooling before the increasing influence of cloud cover later.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
The disturbance to the south will move offshore Tuesday and weak
high pressure will be across our area. A dry day is expected, but
there will be a good deal of cloudiness during the morning. The
clouds will tend to diminish from NW to SE during the afternoon. It
will be mild on Tue with highs in the low/mid 50s North and
mid/upper 50s South. Winds will be mostly W or NW at 5 to 10 mph.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Wednesday through Thursday...The long term period will start
off seasonably cool thanks to the passage of a cold front
Tuesday night. Temperatures both days will be 10 to 20 degrees
below normal. On Wednesday, windy conditions are possible thanks
to a tight pressure gradient behind the cold front. However,
the pressure gradient will relax quickly Wednesday night as the
high approaches the region and by Thursday winds will turn light
and variable as the high slides over the Mid Atlantic.

Friday...As the high slides further off shore, a warm front is
expected to lift through the region. Precipitation is expected
with and perhaps just ahead of the front. If we do have
precipitation ahead of the front, it could be a mix of freezing
rain and sleet from the Lehigh Valley to the north and west. A
warm layer in the mid levels is expected, and it looks like
there will be some weak lift ahead of the front. A limiting
factor however, will be a dry boundary layer already in place.
For now, will mention the possibility in the HWO.

Saturday...Should remain in the warm sector with continued
southerly flow. Have chosen to stay close to the previous
forecast which is slightly above most guidance for highs on
Saturday. There have been several recent cases of southerly
flow/warm air advection events that the MOS guidance and
especially operational models have underestimated the warm air
advection. This looks to be a similar pattern to those cases, so
will go with a forecast that is 10 to 15 degrees above normal.

Sunday and Monday...As the next low pressure system approaches
the region, there is considerable uncertainty in how far south
the cold air will get across the region. This of course, means
considerable uncertainty in what type of precipitation we might
have. For now, it looks like most of the region should see rain,
with the possible exception of the southern Poconos and NW NJ
which might again see a wintry mix especially Sunday night into
Monday morning.

&&

.AVIATION /01Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

For the 00Z TAFs...Increasing cloudiness tonight, with ceilings
lowering to around 8000 feet early this evening and to around
4000-6000 feet by 08Z to 12Z. Stray shower possible late tonight
and early tomorrow morning, with brief MVFR conditions possible
in proximity to any shower, but not confident enough to include
at any of the terminals. Ceilings are forecast to rise above 10
kft late tomorrow afternoon. Light/variable winds tonight
becoming northwesterly around 10 kts or so tomorrow afternoon.

Outlook...
Wednesday through Friday...Mostly VFR conditions are expected.
Northwesterly winds gusting near or above 25 kt are possible on
Wednesday.

Saturday...Starting VFR, but may lower to MVFR or lower late in
the day as precipitation moves in.

&&

.MARINE...
Seas continue to lower over the coastal waters, and the small
craft advisory will be allowed to expire as scheduled.

Previous discussion...

We will continue with the SCA flag until the 6 p.m. expiration time.
Winds have already diminished below criteria and seas are still a
bit over 5 ft attm. Once the flag comes down, sub-SCA conditions for
the rest of the night and Tuesday. The weather will be fair expect
for perhaps a sprinkle towards dawn across Delaware Bay or the
southern coastal waters.

Outlook...

Wednesday...Gale force winds are possible on all waters, but
most likely on the New Jersey Atlantic Coastal waters.

Wednesday night...winds will drop off quickly after midnight,
though SCA conditions may linger especially on the Atlantic
Coastal waters for much of the night. Light freezing spray will
be possible.

Thursday...winds and seas are expected to be below SCA criteria.

Friday and Saturday...winds will begin to increase Friday and
seas will subsequently increase as well. SCA conditions are
possible especially Friday night into Saturday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Wednesday morning through Wednesday evening
     for ANZ450>453.

&&

$$
Synopsis...Iovino/Johnson
Near Term...CMS
Short Term...O`Hara
Long Term...Johnson
Aviation...CMS/Johnson/O`Hara
Marine...CMS/Johnson/O`Hara



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