Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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281
FXUS61 KPHI 250423
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1223 AM EDT Wed Apr 25 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 THIS MORNING/...
Light to moderate rain has overspread the region, but an area of
heavy rain lies just southwest of the Delmarva Peninsula. This
area lifts to the north and east and impacts the region through
daybreak or so.

Model guidance has backed off a little on how much moisture
advection we may see. Latest runs show precipitable water values
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. Nonetheless, 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.

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 THIS MORNING THROUGH 6 PM THURSDAY/...
Dry slot should develop in our region Wednesday 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 /THURSDAY 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 /04Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Overnight...MVFR CIGs lower to IFR during the overnight.
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.

Wednesday...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 Wednesday 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 5 AM on the Delaware Bay, and through the day
Wednesday (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 early this morning for
     ANZ430-431.

&&

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



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