Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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039
FXUS61 KPHI 250147
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
947 PM EDT Tue Apr 24 2018

.SYNOPSIS...
An area of low pressure over the Carolinas late this evening
will move slowly north overnight, across our area Wednesday into
Wednesday night, and then into New England by Thursday. Very
weak high pressure will build into the region Thursday, before
another low pressure system moves northward across the east
coast on Friday. A cold frontal passage is expected Saturday.
High pressure will build across the Mid Atlantic region into
early next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Latest radar imagery shows rain has overspread southern half of
the forecast area. Sfc obs show that initial pcpn so far may be
lighter than what is showing on radar. Based on current motion
expect rain to overspread remainder of forecast area over the
next few hours.

Model guidance has backed off a little on how much moisture
advection we may see. Latest runs show precipitable water values
by late tonight between 1 and 1.2 inches, which remains quite
high for this time of year, but less than what the models were
depicting with earlier runs. None the less, there remains a
potential for locally heavy rain. For the most part, it looks
like the rain amounts will not be enough to cause flooding in
our area. However, in the urban corridor from Philadelphia up to
Morris County, if the forecasted rain amounts fall in just a
few hours, some flooding is possible in poor drainage areas, so
will continue mention of this in the HWO.

Still see only very limited elevated instability. Thus will not
mention any thunderstorms in the forecast for now.

Overnight, temperatures will stay up higher than what we have
seen the last several nights, thanks to the moisture advection
and persistent cloud cover. Minimum temperatures are forecast to
be in the 40s and 50s.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Dry slot should develop in our region tomorrow morning,
bringing a quick end to the widespread rain. Depending on how
much standing water there is by daybreak, we could have some
patchy fog across the region, though it looks like more of a low
stratus set up than fog.

The lull in rain may not last very long as the trailing mid and
upper level trough could result in additional periods of light
rain, generally through the second half of the day tomorrow.
However, thanks to dry air advection moving in, the threat for
heavy rain is very limited tomorrow.

Cloud cover may limit heating somewhat, resulting in highs
generally in the 50s and 60s across the region.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
After a brief period of possible unsettled weather at the
beginning of the period, dry weather takes hold of the forecast
for the rest of the long term forecast.

Although the low pressure system that affects our area during
the day Wednesday lifts to our north Wednesday night, there will
remain a chance of showers on the backside of the low into
Wednesday evening as there remains a couple of vorticity
impulses that will move across the area.

By Thursday, very weak high pressure may briefly affect the
area, leading to dry conditions. However, on Thursday night, an
area of low pressure will be approaching the area from the
southwest. The low itself is not expected to make it across, or
near the area, until during the day Friday, or as late as Friday
evening. There are timing and locations differences with the
low between the guidance. Still, they all indicate an increase
in moisture while several short wave/vorticity impulses slide
across the area as well. This will lead to a chance of showers
from late Thursday night through early Friday night, although it
will likely not rain everywhere, nor the whole time.

Once this low lifts to our northeast, it will combine with
another low pressure as it moves out of the Great Lakes region
and across eastern Canada and northern New England. This will
pull a cold front across the area during the day Saturday. This
could bring another chance of showers during the day as the
front moves across the area. Behind this front, drier conditions
return to the forecast.

The low pressure across eastern Canada will near the Canadian
Maritimes through Monday, while high pressure across the Great
Lakes and Ohio Valley Sunday will build to our south Monday into
Tuesday. A couple of surface troughs will cross the area during
this time period, but are expected to remain dry. For Sunday
and Monday, there will be a pressure gradient in place between
the high to the south and low to the north, so a steady breeze
with gusts likely in the 20s is expected, with Sunday having the
strongest winds.

&&

.AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Latest radar imagery and sfc observations show that rain has
overspread southern TAF sites. Based on current motion expect
rain to overspread remainder of airfields over the next few
hours.

Tonight...Ceilings are expected to lower to IFR overnight and
through the early morning hours. Localized LIFR conditions are
possible. Additionally, visibility in rain, heavy at times,
could be reduced (mostly to MVFR, but occasional IFR
visibilities are possible). Easterly winds around 10 kt are
expected through the overnight hours. Moderate confidence on the
flight category, but low confidence on the timing of changes.

Tomorrow...though the rain may end at many TAF sites during the
morning, low clouds are expected to linger through the morning.
Improvement to MVFR or even VFR is possible in the afternoon,
but confidence in improving conditions is low. Winds are
expected to start easterly, but should shift to northwesterly
during the afternoon.

OUTLOOK...

Wednesday night...MVFR conditions possible early with a chance
of showers, before improving overnight.

Thursday...VFR conditions expected.

Thursday night...Generally VFR, although MVFR conditions may
begin late in the night across the southern areas as rain
approaches.

Friday...MVFR, possible IFR, with a chance of scattered
showers.

Friday night...MVFR conditions possible early with scattered
showers, becoming VFR during the evening and overnight.

Saturday...Mostly VFR. A chance of showers which may
temporarily lower conditions.

Saturday night-Sunday...VFR. Winds becoming gusty out of the
northwest Sunday morning and may gust 20-25 knots.

&&

.MARINE...
Southeasterly and easterly winds gusting above 25 kt are
expected to continue through tonight. By tomorrow morning, winds
may diminish, though seas above 5 feet are expected to continue
on the coastal waters. The small craft advisory remains in
effect until 6 AM on the Delaware Bay, and through the day
tomorrow (and into Thursday) on the coastal waters.

Rain and patchy fog could reduce visibility especially early
Wednesday morning.

OUTLOOK...

Wednesday night-Thursday...Small Craft Advisory remains as seas
expected to remain over 5-7 feet.

Thursday night-Saturday night...Conditions expected to remain
below advisory levels.

Sunday...Winds may approach Small Craft Advisory levels on
Sunday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 PM EDT Thursday for ANZ450>455.
     Small Craft Advisory until 5 AM EDT Wednesday for ANZ430-431.

&&

$$
Synopsis...Robertson/99
Near Term...Johnson/99
Short Term...Johnson
Long Term...Robertson
Aviation...Johnson/Robertson/99
Marine...Johnson/Robertson



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