Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 210814

Area Forecast Discussion
National Weather Service Mount Holly NJ
414 AM EDT Sat Apr 21 2018

High pressure over the Great Lakes builds east today, and
eventually moves offshore on Monday. Low pressure over the Gulf
Coast states tracks to the east, and slowly impacts the Mid-
Atlantic during the mid-week period. Another area of low
pressure may affect the area at the end of next week.


Main forecast concern is temperatures this morning, with
current temperatures running about 2-4 degrees lower than
forecast (locally more), as clear skies and light to calm winds
have allowed for nearly ideal radiational cooling conditions.
The favorable valleys and rural locations have tanked in the
past hour or two temp-wise, with locations in the Pine Barrens
already reaching freezing. Upgraded the frost advisory to a
freeze warning given these trends (though one may question the
effectiveness of such an upgrade so late in the night) for the
Pine Barrens and zones north/west of Philadelphia (where temps
32-35 degrees are observed at this time).

After this morning`s chill, today should be a nice day, as
northwesterly midlevel flow and building surface high pressure
allow for dry conditions with temperatures just a little below
seasonal averages. Winds should be a little lower today as the
surface pressure gradient slackens with the approaching high. A
250-mb jet streak and accompanying vort max should provide some
lift in the upper levels to allow for some increasing clouds.
These may limit temperatures from climbing a degree or two, and
I generally reduced highs by this amount today.


Northwest midlevel flow will continue tonight as a surface
ridge remains over the region. Some perturbations in the
midlevel flow will approach the region by morning, but there is
little lift with these perturbations. Other than some increased
mid and upper-level clouds, not expecting much in terms of
sensible weather impacts.

This means that temperatures will once again be the primary
forecast concern. Could see temperatures flirting with freezing
again in portions of the area with the growing season underway,
but tonight`s environment will be a little less favorable. For
one thing, if increased cloud cover is present, some radiation
may be re-emitted to the surface (though the height of the
clouds will not hamper nocturnal cooling as much as one may
anticipate). This will be especially true in the southern CWA,
but how far north these clouds persist may prove critical in how
far temperatures fall in more susceptible locations such as the
Pine Barrens. At this point, way too uncertain/marginal to
issue additional frost/freeze products. My suspicion is that no
additional products will be necessary.


High pressure over the area Sunday slowly drifts offshore Sunday
night, and remains over the western Atlantic Ocean waters
through Monday night before drifting offshore on Tuesday. Plenty
of sunshine and mild, though temperatures will generally be a
few degrees below normal during the day. At night, with clear
skies and light winds, radiational cooling conditions will allow
for colder conditions in the mountains and in the Pine Barrens
of NJ.

During this time, low pressure over the Gulf Coast states will
slowly track to the east, and will be over the Southeast U.S by
Tuesday evening. A warm front just southwest of the region lifts
north through the region Tuesday night as the low tracks north
along the coast.

Though there are rain chances during the day Tuesday lifting
from south to north, the best chances will be Tuesday night as
the low lifts to the north. The low then slowly works its way
north through Wednesday, will be south of Long Island Wednesday
night, and departs on Thursday. Although there may be a period
on Wednesday where locally heavy rain is possible at times, not
expecting a prolonged period of widespread heavy rain with this

Weak high pressure builds through the region late Friday and
Friday night. Another cold front approaches on Saturday.

Temperatures during this time will generally be at or below
normal levels for most of the week, but temperatures trend
warmer towards the end of next week.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VFR with SCT-BKN high clouds. Northwest winds 5 to 15
kts. High confidence.

Tonight...VFR with SCT-BKN high clouds, especially south of
PHL. Winds should become light and variable. High confidence.


Sunday through Monday night...VFR. Generally light winds. High

Tuesday...Generally VFR. Clouds build from south to north. E-SE
winds 10-15 kt.

Tuesday night through Wednesday...MVFR/IFR in rain and fog. E-NE
winds 10-15 kt.


Fair weather and sub-advisory winds/seas are expected through
tonight. Northwest winds 10 to 15 kts (possibly gusting to 20
kts or so) this morning will likely switch to southwest this
afternoon before switching back to northwest or north tonight.
Seas should remain near or below 3 feet.


Sunday through Tuesday...Tranquil conditions on the waters with
sub-SCA conditions.

Tuesday night and Wednesday...Rain, fog, and gusty E-NE winds as
low pressure impacts the waters. SCA conditions expected.


PA...Freeze Warning until 8 AM EDT this morning for PAZ060-101>104-
NJ...Freeze Warning until 8 AM EDT this morning for NJZ012-013-015-
     Frost Advisory until 8 AM EDT this morning for NJZ016>018-021.
DE...Frost Advisory until 8 AM EDT this morning for DEZ001>003.
MD...Freeze Warning until 8 AM EDT this morning for MDZ008.
     Frost Advisory until 8 AM EDT this morning for MDZ012-015-020.


Near Term...CMS
Short Term...CMS
Long Term...MPS
Marine...CMS/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.