Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 181110

Area Forecast Discussion
National Weather Service Mount Holly NJ
610 AM EST Sat Nov 18 2017

High pressure will move off the East Coast this morning while
an area of low pressure tracks across the Midwest. The low will
move northeastward and strengthen across the Great Lakes region
later today and tonight. A strong cold front is expected to move
through the Mid-Atlantic region early Sunday morning while the
low progresses through the Saint Lawrence Valley on Sunday. High
pressure will build to our south Monday into Tuesday, before a
cold front moves across the area Tuesday night or Wednesday
morning. High pressure builds into the area Wednesday night and
Thursday. The high weakens to our south by Friday while another
area of low pressure approaches.


Transient midlevel ridge moving over the region at this time will be
followed by a series of midlevel perturbations downstream of a
potent shortwave trough presently moving out of the Rockies into the
central plains. A surface low in eastern Kansas this morning will
deepen and move rapidly northeastward into the Great Lakes region by
late this afternoon, as the shortwave trough pivots into a more
neutral tilt via a strong 500-mb jet streak impinging upon the low-
level warm sector in the Ohio/Tennessee Valleys. Downstream
difluent/divergent flow will aid in deep large-scale ascent.
Widespread precipitation will occur downstream of the surface low
and poleward of a developing baroclinic zone stretching from
northern Illinois to the northern Mid-Atlantic region. Though the
strongest ascent should stay to our west during this period, in
closer proximity to the main vort max and associated surface low,
the predecessor midlevel perturbations, in conjunction with strong
low-level isentropic ascent, will permit widespread showers to
progress eastward through Pennsylvania and adjacent states today.

High-resolution models vary on the southward extent of the
precipitation, with the HRRR sometimes indicating coverage extending
through much of Delmarva and sometimes keeping the showers confined
north of the Mason-Dixon Line. The drier simulations are
corroborated by the 00Z NAM/NAM Nest and the 00Z GFS, whereas the
wetter simulations have support from the SREF and the Hi-Res WRF-
NMM. Given the discrepancies, used a blend of the guidance, which
naturally weighs the drier solutions a little higher (i.e., there
are more of them). General timing looks to begin 14Z-17Z, with
potential for several rounds through the course of the day,
especially in southeast PA. Instability is minimal this far east,
with model soundings showing a deep near-surface stable layer and
moist-adiabatic profiles above - in other words, no thunder mention
in the forecast. QPF through the day may be minimal in the far
southern CWA up to a half inch or so in the southern Poconos/Lehigh
Valley, with confidence in totals around average.

As the surface low to the west deepens and approaches, expect the
south/southwest winds to start increasing late in the day, though
they should be far from out-of-hand given the fairly stable near-
surface layer and the still-modest surface pressure gradient through
day`s end.

Forecast temps are a blend of ECS/MET/MAV MOS as well as input from
HRRR for potential modification from rain. Though agreement is
generally excellent among the guidance, expect some unexpected
deviations from the hourly/max temperatures owing to impacts from


The surface low in the Great Lakes region early this evening will
continue deepening and moving northeastward through the St. Lawrence
Valley as a strong cold front approaches the Eastern Seaboard late
tonight. A synoptically-induced low-level jet will be screaming
along and just downstream of the cold front. With substantial low-
level isentropic lift combined with differential cyclonic vorticity
advection downstream of the main vort max, deep large-scale ascent
will accompany the front, allowing for a band of widespread showers
to develop. General quarter to half inch QPF is expected in the
northern part of the area (where lift will be stronger), but most
everyone should see at least some precip as the front
approaches/moves through the area. Water loading will likely aid in
downward transport of the stronger winds aloft, so strong wind gusts
may occur in proximity to showers. There are some indications of
shallow/thin instability in close proximity to the front, so a
rumble of thunder is not impossible. However, the more likely threat
is contributions to surface wind gust potential via convectively
induced downdrafts.

The timing of the frontal band(s) of showers looks to be after
midnight, with potential for a relatively dry period earlier in the

As the front races eastward, winds will become westerly and
northwesterly and rapidly increase as cold air advection commences
and strong mixing occurs. The wind threat may be enhanced with the
showers but will continue well after the front has passed (see
forecast discussion for Sunday below).

Temperatures are likely to stay elevated tonight in advance of the
front, with little or no decrease in temperatures through the
evening/early overnight hours. After frontal passage, the
temperature plunge begins. Notably, precipitation should be all rain
for the southern Poconos through 6 am Sunday, as temperatures should
be just starting to make the drop by this point.


The period starts out with a strong cold frontal passage early
Sunday morning. A brisk westerly wind behind the front can be
expected on Sunday. The combination of CAA and daytime heating will
lead to steepening boundary-layer lapse rate and favorable mixing
profiles shortly after sunrise. BUFKIT momentum transfer from
NAM/GFS profiles favor frequent wind gusts of at least 40 mph thru
about mid afternoon. It appears that gusts across the area will be
on the cusp of a Wind Advisory (greater than 45 mph), especially
from about mid morning thru midday, if the strongest CAA arrives in
concert with the greatest pressure rises/(cold conveyor belt) low-
level jet. The models have trended 1-2 mb weaker with the surface
low (982-984 mb) as it passes to our north during the morning, yield
a Wind Advisory setup that is marginal enough to hold off at this
time. Temperatures are forecast to fall during the morning, then
struggle to rise much during the afternoon as CAA offsets surface

There is a potential for lake-effect snow streamers to reach the
higher elevations of northeastern PA (Poconos). It now appears that
the bulk of these snow showers should hold off until after sunrise
Sunday evening. Although snowfall accumulations are expected to be
minor, a brief snow squall could potentially lead to locally
hazardous travel in the Poconos if convection can be sustained this
far downstream (low confidence at this point).

W-NW winds will remain breezy Sunday night with low pressure
continuing to strengthen over southeastern Canada and high pressure
building to our southwest. A cold night is in store with continued
CAA over the Mid Atlantic but the winds should keep the boundary
layer well mixed, preventing temperatures from dropping too much
overnight. Lows range from the mid 20s in the Poconos to mid 30s in
the cities and along the coast.

High pressure builds across the Southeast U.S. Monday. Cool
temperatures (10 degrees below normal) and dry weather can be
expected to start the work week. High pressure moves offshore on
Tuesday. Southerly return flow around the high will yield a
moderating trend with temperatures returning to near normal by
Tuesday afternoon.

A cold front will likely move thru the area late Tuesday night or
Wednesday morning. The previous forecast was blended with WPC
Guidance and low chances for rain was introduced to the forecast as
a result. Models generally show an area of low pressure developing
along the front which would enhance rainfall. However, with large
spread in the models regarding the location and timing of secondary
low development, confidence in the forecast is lower than normal for
the Tuesday night-Wednesday morning period.

Cool and dry weather is anticipated for Thursday and Friday with
high pressure build in. Forecast high generally range between 40-45
degrees on Thursday and 45-50 degrees Friday.


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

Today...Conditions should remain VFR this morning with showers
moving in from the west around or after 15Z. CIGs will gradually
lower to MVFR by late this afternoon or early this evening; however,
sub-VFR conditions cannot be ruled out, at least temporarily, before
this time, especially at KRDG/KABE. It is possible KACY/KMIV see
little or no rainfall today. Winds should be generally southerly
around 10 kts, with potential for gusts up to 20 kts, especially at
KMIV/KACY. Confidence in CIGs/VSBYs is somewhat below average with
winds well above average.

Tonight...Conditions should gradually deteriorate at all sites to
sub-VFR (primarily CIGs), with a band of showers likely to move
through between 06Z and 12Z. Gusty/erratic winds may occur in
proximity to showers. Outside of showers, southwest winds 10 to 15
kts with gusts to 25 kts are possible, with quick switch to west or
northwest after cold frontal passage late tonight or early Sunday.
Winds will likely become stronger quickly after the front passes.
Confidence in overall evolution is above average, but in timing of
CIGs/wind switch is below average.


Sunday...CIGs quickly rise to VFR in most areas Sunday morning
although stratocu behind a cold front could keep high-end MVFR
CIGs lingering into the late morning in a few spots. Breezy W
winds 15-20 kt will gust to around 35 kt (possibly higher gusts
to near 40 kt). Moderate confidence on CIGs in the morning; high
confidence otherwise.

Sunday night through Tuesday...VFR. W-NW winds gusting 15 to 25
kt. High confidence.

Monday night...VFR and light winds. High confidence.

Tuesday...VFR. W-SW winds gust 15-20 kt during the afternoon.
Moderate confidence.

Tuesday night and Wednesday...Potential for rain to move in late
Tuesday night-Wednesday morning (greatest chance is along the
coast). Sub-VFR CIGs possible. Low confidence.


Gale warning has been issued for Delaware Bay starting at 1 am
Sunday (with small craft advisory conditions likely beginning
today), and a gale warning remains in effect for the Atlantic waters
beginning at 1 pm today. Strong south to southwest winds will begin
rapidly this afternoon, with occasional gusts to gale force likely
over the Atlantic waters. As a cold front approaches late tonight,
strong gusts may occur in proximity to showers before a switch to
west/northwest winds late tonight (or early Sunday), with frequent
gale-force gusts likely thereafter for the entire marine area. Seas
in the Atlantic should rapidly build to 5-8 feet by this evening and
potentially higher overnight.

Showers are also possible today, especially for Delaware Bay and the
New Jersey coastal waters.


Sunday night...Gale Warning was extended into Sunday night for
the waters. There may be a brief lull in the winds early Sunday
evening before restrengthening a bit late in the evening and/or
overnight as the low deepens well to our north and the pressure
gradient tightens overhead.

Monday through Tuesday night...SCA will be needed, initially for
winds and seas on Monday, just for seas Monday night, then for
winds and seas on Tuesday.

Wednesday...Winds and seas likely to drop below SCA criteria.


There is a potential for blow out tides on Sunday, especially
in the Delaware Bay and the tidal portion of the Delaware River,
where strong W-NW winds will effectively drain water out of the
bay. The low tide Sunday afternoon/evening seems to be the most
susceptible for low water concerns that could impact navigation
in the area.


MARINE...Gale Warning from 1 PM this afternoon to 5 AM EST Monday for
     Gale Warning from 1 AM Sunday to 5 AM EST Monday for ANZ430-


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