Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 271027
Area Forecast Discussion
National Weather Service Mount Holly NJ
627 AM EDT Tue Sep 27 2016
A cold front will move through the region this morning before
stalling offshore this afternoon through Wednesday. Low pressure
systems over the Great Lakes and Southeast U.S. are expected to
drift toward the region for the later half of the week before
finally lifting into New England for the weekend.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
615 AM...Latest forecast update sent to NDFD prior to the next
scheduled ESTF. This update focused on hourly PoPs and weather
grids to reflect an earlier end time to the showers today. Just
about all of the recent hires model runs continued to back away
from the previous notion that showers would persist across the
coastal plain for much of the day. The back edge of the precip has
already made it to the I-95 corridor before sunrise. Once the
previously mentioned shortwave trough moves off the Delmarva coast
by mid morning, the bulk of the shower activity should then shift
offshore. However, a few showers will linger into the first part
of the afternoon in southern DE and near the southern NJ coast but
before eventually tapering off late.
07Z surface analysis shows a 994 mb surface low located just
north of Lake Superior. An occluded front extends southeastward
from the low all the way down the eastern side of the Appalachian
spine. The front is currently on our western doorstep and will
move eastward through the area this morning. Looking aloft, the
low near the Great Lakes is vertically-stacked so it is not going
to make much progress downstream today.
Warm conveyor belt around the eastern side of the closed low was
associated with a band of showers that was precedes the fropa. The
back edge of the precip has already pushed east of RDG-MPO as of 330
AM. Heavier showers were noted on the western side of the Chesapeake
Bay. This enhanced activity was connected to a weak shortwave
impulse (per water vapor satellite imagery) over the northern VA
Piedmont. As the shortwave tracks northeastward, expect the
heavier/more widespread showers to move into eastern MD and DE after
4 AM before shifting off the southern NJ/DE coast by about 8 or 9
AM. An isolated thunderstorm cannot be ruled out south and east of I-
95, where locally higher (elevated) instability situated near the
theta-e ridge axis. Rainfall amounts of one-half to locally one inch
are expected in these southern zones, while areas along and
northwest of the I-95 corridor generally see less than one-quarter
inch (and not much drought relief).
The back edge of the showers should make it to the I-95 corridor
around daybreak. The front is then expected to stall off the coast
later this morning and afternoon, which would keep showers lingering
along the coast through much of the day. The good news is the latest
model guidance has shifted toward a more progressive scenario with
the front making it farther offshore, and therefore precip may
still end sooner than previously thought even along the coast.
Tried to capture these near term trends in the PoP/weather grids
but further adjustments may be needed over the next few hours.
Similar to the precip shield, the back edge of the dense overcast
cloud cover is expected to progress through the area from northwest
to southeast over the course of the day. Anticipate some jet-induced
high clouds to stream across the area today though the cirrus should
be thin enough to not have a big impact on temps. The warmest
temperatures this afternoon (highs in the mid 70s) will likely
reside along the I-95 corridor and the warmer valleys farther to the
north and west as these areas will clear out early enough in the day
to receive strong daytime heating. Farther south and east toward the
southern NJ coast and lower Delmarva, clouds will hold on longer
into the afternoon, resulting in highs near 70F.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
The closed low will push slowly southward through the upper Midwest
tonight. Meanwhile, a wave of low pressure is expected to develop
along the stalled front to our south and east. We should be mainly
dry tonight, even along the coast as the secondary low tracks far
Good radiational cooling conditions may occur this evening across
inland areas under clear skies and calm winds. Therefore, temps
should drop quickly after sunset. Forecast lows are in the 40s for
the valleys in northeast PA and northwest NJ. However, there is some
uncertainty with the temperature forecast as high pressure builds
southward. A light northeasterly wind may develop overnight, which
could prevent temps from bottoming out.
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Strong ensemble signal for a heavy rain event somewhere in the Mid-
Atlantic region late this week with uncertainty on the location.
Current indications are our CWA will be on the eastern edge of it.
Wednesday through Friday:
An area of low pressure (most noted in the upper levels) is
expected to move southeastward toward our region from the Great
Lakes and another area of low pressure will be forming in the
Carolinas ( main surface low). The exact locations of these
systems will determine where the highest risk for an excessive
rainfall event will be. The GFS and Canadian operational models
are more progressive with the upper low to our northwest which in
turn forms the surface low just to our south. In turn a deep but
narrow moisture axis ahead of these features tracks from the
Eastern Gulf and Atlantic Oceans northwestward into DE and Eastern
PA sending PW values from 1.5-2 inches, very high for this time of
year. Sufficient lift and dynamics are present coupled with
marginal instability to allow for heavy showers within this
moisture axis. A few thunderstorms would be possibile as well with
a tall skinny CAPE profile normally seen with more aggressive
heavy rainfall events. A hindrance of the heavy rainfall threat
looks to be fast moving convective elements in the axis though.
This outcome would bring several inches of rainfall to areas south
and west of Philadelphia.
However, the NAM and the ECMWF model have all of these features
further westward which would shift the moisture axis further inland
to areas like Baltimore and State College. Waffling on the locations
of the players and their respective locations is evident in the
ensembles. Many members do suggest an axis of heavy rain sets up
but are variable on location. The axis looks to be narrow, maybe
only 100 miles wide.
Overall, model trends have been to the west with the 00z cycle. So
the forecast features heavy showers from Chester county PA south
into Delmarva with just lighter showers from Philadelphia and
Showers will likely begin to develop from south to north across the
region during the afternoon Wednesday. Precipitation is then likely
to become more widespread along and near the main axis of moisture
Wednesday night. This axis then could move little through Friday but
gradually become cutoff from the deeper moisture. By Friday, the
precipitation should become much lighter regionwide even with both
areas of low pressure lingering over the Mid-Atlantic and Ohio
Rainfall amounts from the individual ensembles are as impressive as
the operational runs, so some totals over a few inches are likely
just west of our region. It should be highlighted that areas outside
of the heavy axis Wednesday night and Thursday may not see much more
than lighter showers with rainfall under an inch total eventhough
it rains off and on for a couple of days.
Diurnal temperature swings should be very limited with clouds and
rainfall with temperatures staying in the 60`s for most locations
Thursday and Friday. Ahead of the rain, temperatures will be in the
Winds will be out of the east from 10-15 mph on average Wednesday
Friday night through Monday:
Lingering low pressure both at the surface and mid-levels could
keep a few light showers around. Some breaks in the clouds are
likely as well. The clearing process will be gradual. This will
allow for high temperatures to get back into the 70`s for most of
the region, with lows falling back into the 50`s. Ensemble
guidance is likely a few degrees to cool by Sunday, not accounting
for the more abundant breaks and warmer boundary layer
.AVIATION /10Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
MVFR ceilings w/ showers continue across the area. Lower clouds
below 1 kft are expected to move in from the west behind the showers
between 08-12Z. Do not expect IFR conditions to persist more than 2
or 3 hours. Improvement to VFR as lower clouds clear out is forecast
to occur from northwest to southeast this morning. Lower clouds may
linger toward midday or early afternoon near the coast (ACY). SW
winds currently will become W-NW after sunrise. Winds should back
toward the SW during the afternoon. Wind speeds will be
light...generally under 10 kt.
VFR this evening. Patchy fog may develop overnight at the fog-prone
terminals, resulting in MVFR or locally IFR visibility early
Wednesday through Friday night: Mainly MVFR or IFR with periods
of showers. easterly winds around 10 knots.
Saturday: Improving to VFR with isolated showers. East winds 5-10
SCA continues for the waters thru early this morning. S-SW winds
have mainly been below SCA criteria thru most of the night but there
is a potential to see a brief (2 hour) surge of winds near 25 kt.
Seas in our nearby offshore buoys are around 4-5 ft. Therefore, will
let the SCA continue.
Winds will become W or NW and diminish behind a cold front late this
morning diminish. Winds and seas should remain below SCA criteria
this afternoon and tonight.
Seas building to five feet by Wednesday night with low pressure
nearby. Seas will likely be very slow to drop as the low pressure
systems linger. Winds will be easterly with SCA gusts by Wednesday
night. Some gusts may reach gale force from ACY northward
Wednesday night and Thursday. Winds should decrease by Saturday
below SCA criteria.
Rip Currents... The moderate risk for the formation of dangerous
rip currents is forecast at the NJ/DE beaches today, especially
this morning. Conditions may improve to yield a low risk by this
afternoon and winds turn offshore and decrease.
Spotty minor tidal flooding on the oceanfront is possibile with
the Thursday and Friday high tidal cycles.
MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ430-