National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXUS61 KPHI 210843
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
443 AM EDT Sun Apr 21 2024

.SYNOPSIS...
High pressure extending eastward from the Midwest to the Mid-
Atlantic will continue to build steadily eastward toward the
East Coast through Tuesday, while low pressure passes off the
Carolinas today and tonight. A cold front will swing through our
region on Wednesday, followed by another area of high pressure
pushing southeastward from the Great Lakes Thursday and Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
The secondary cold front has moved offshore and south of the region.
High pressure has since been building over the Central Plains with
ridging extending eastward into the Mid-Atlantic. An area of low
pressure over the Southeast will move east-northeast along a stalled
boundary before passing south of North Carolina later today.
Associated with the low are some high clouds fanning over the area,
which will increase through the morning resulting in a mostly cloudy
Sunday afternoon. Fortunately, the northward extent of the
precipitation shield will struggle to make much progress due to the
high to the west. Still think that a few showers could make their
way into southern Delaware this afternoon but any rainfall should
not total more than a few hundredths of an inch. High temperatures
will be in the mid to upper 50s for most; with only 40s up in the
Poconos.

Clouds quickly depart during the evening hours from northwest to
southeast giving way to clear skies overnight as the low moves away.
Winds are anticipated to be very light to near calm at times, so it
is expected that the boundary layer will decouple quite easily with
lows mostly in the low to mid 30s. With lower dew point depressions
tonight and expected RH values around 70%, there will likely be
areas of patchy frost around early Monday morning. Greatest
confidence is across northern NJ and into the Lehigh Valley where
the growing season is now active. However, with frost advisories
currently in effect this morning, have opted to leave any issuance
of frost headlines for tonight up to the next forecast shift.
&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
Surface high pressure centered over the lower Mississippi River
Valley with ridging extending northeastward toward the Mid-
Atlantic on Monday will shift eastward to the East Coast, with
the ridge axis pushing offshore on Tuesday. This will ensure dry
and quiet weather across our region, with stable atmospheric
conditions and mostly sunny skies. With a very dry airmass in
place along with the clear skies, nights will favor radiational
cooling and low temperatures dropping into the 30s outside of
the immediate coast and right around Philadelphia. Some patchy
frost will subsequently be possible once again early Tuesday
morning, from the Lehigh Valley north and northeastward, as well
as locally even around the Pine Barrens in southern NJ.
Meanwhile afternoons will be fairly warm with highs in the low
60s Monday rising to the upper 60s for Tuesday. That warming
will occur as a cool northwest breeze on Monday swings around
the southerly on Tuesday. Clouds will increase Tuesday night as
low pressure tracking across Quebec and northern New England
swings a sharp cold front toward our region. Showers may push
southeastward down to I-95 by dawn on Wednesday. That will
result in much milder nighttime and early morning temperatures
compared to the previous few nights, though.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
Models are in good general agreement with a dynamic upper-level
trough swinging quickly east-southeastward from the Great Lakes
across the Northeast and mid-Atlantic on Wednesday. A tightly-
wound, cold late-season upper-level close low is progged to drop
in from Quebec across northern New England to the Canadian
Maritimes Wednesday night into Thursday. At the surface, this
will manifest in intensifying low pressure tracking from near
northern New York state Tuesday night to the Canadian Maritimes
by Wednesday night, with an attendant sharp cold front pushing
through our region on Wednesday. This system will lack much in
the way of moisture without a tap to Gulf moisture, and
dewpoints may struggle to even reach 50F, so do not expect much
rainfall. Perhaps less than a quarter-inch in most cases.
However, the dynamic forcing along with rather strong winds
aloft and shear may take advantage of just enough instability
for some thunderstorms. The strength and coverage of any storms
will depend somewhat on the timing of the front relative to peak
heating on Wednesday; if there isn`t much sunshine preceding
frontal passage, then that will limit the strength and coverage.
For now we just have a slight chance mainly from Philly
northward. In any case, expect breezy conditions on Wednesday
with challenging high temperatures; possibly reaching near 70F
ahead of the front from Philly southeastward (again, depending
on the timing of the FROPA), while the Poconos may struggle to
reach the low 50s.

Heading into Thursday, cool Canadian high pressure will drop
across the Great Lakes toward the mid-Atlantic, ensuring dry and
sunny weather. Temperatures will start off cool with a threat
for some patchy frost in outlying areas, then struggle to reach
60F in most spots. Highs will moderate through the 60s Friday
and Saturday, while lows stay potentially frosty in the cooler
spots. A warm front may bring some cloudy skies with showers or
just a steady light rain in on Saturday.

&&

.AVIATION /07Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Through 12Z...VFR with some scattered high clouds. Northerly winds
around 5-10 kt. High confidence.

Saturday...VFR expected. Mostly BKN CIGs around 25000 feet in the
morning, lowering to around 10000-15000 feet in the afternoon.
Westerly winds around 7-12 kt. High confidence.

Tonight...VFR expected. Skies scattering out becoming SKC after 04Z.
Northwest winds around 5 kt. High confidence.


Outlook...

Monday and Tuesday...VFR.

Wednesday...MVFR to possibly IFR conditions at times, as a cold
front brings showers and a wind shift from southerly to W/NW.

Thursday...VFR.

&&

.MARINE...
No marine headlines are in effect through tonight.
Winds may occasionally gust up to 20 kt early this morning, but
will generally remain around 10-15 kt through tonight. Seas
around 2-3 feet.


Outlook...

Monday...NNW winds around 10 kt shifting around to southerly in
the afternoon. Seas around 2 ft.

Tuesday...SSE winds increasing to 10-15 kt in the afternoon with
some gusts around 20 kt possible. Seas 3 to 4 ft.

Wednesday...Small Craft Advisory conditions possible. Southerly
winds around 15 kt gusting to 25 kt, shifting to the W/NW later
in the day as a cold front swings through. Seas 4 to 5 ft.

Thursday...SCA possible, especially early. NW winds 15 kt with
gusts to 25 kt possible early, easing later on. Seas 3 to 5 ft.

&&

.FIRE WEATHER...
Even while we`ve had a wet spring, rainfall has been lighter the
past couple of weeks, and we are settling into a very dry
airmass. While fuel moisture conditions were still rather moist
early Saturday, with the very breezy conditions, sunshine and
humidity dropping well into the 20s, the fine fuels have dried
out quite a bit already. Winds from today through Tuesday will
be lighter than Saturday, generally around 8-14 mph with gusts
less than 20 mph. However, minimum afternoon RH in the low to
mid 30s today will drop well into the 20s on Monday, then
perhaps only rebound slightly into the low 30s on Tuesday.
Southerly winds on Tuesday may be a little breezier, possibly
challenging the 20 mph gust threshold. All this to say,
conditions will not reach Red Flag criteria today or Monday, but
could flirt with it on Tuesday. With drying fuels, that may at
least require a Special Weather Statement to highlight the
elevated fire weather conditions. Will coordinate with various
state forestry partners early this week.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Frost Advisory until 8 AM EDT this morning for NJZ001-007.
DE...None.
MD...None.
MARINE...None.

&&

$$

SYNOPSIS...Dodd
NEAR TERM...DeSilva
SHORT TERM...Dodd
LONG TERM...Dodd
AVIATION...Deal/DeSilva
MARINE...Deal/DeSilva