Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Remove Highlighting --
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FXUS61 KPHI 200052
Area Forecast Discussion
National Weather Service Mount Holly NJ
852 PM EDT FRI AUG 19 2016
Our region will be under the influence of high pressure centered
over the Canadian Maritimes through Saturday. Low pressure will move
through the Great Lakes and into southern Canada over the weekend.
Across the mid Atlantic, this will lead to a slow warm frontal
passage Sunday, followed by a strong cool frontal passage Sunday
night. High pressure over the Ohio Valley Monday will build into our
region on Tuesday and Wednesday, then move offshore Thursday into
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
A broad area of high pressure was located over the eastern US this
afternoon. However, there looks to be a very subtle surface trough
forming this afternoon over the coastal plain. Although the flow is
very light, surface convergence near the boundary has developed this
afternoon as the sea-breeze circulation advances inland and the flow
turns southeasterly to the east of the trough axis while remaining
northwesterly farther inland.
A few showers have developed along the sea-breeze front just inland
of the Jersey shore. An additional pop-up shower or two may form
over the ridge lines in northeast PA and northwestern NJ this
afternoon but the main focus for additional convection late this
afternoon and early this evening will be in the coastal plain of NJ,
possibly reaching the I-95 corridor early this evening. Without a
source of deeper, more organized lift, coverage should continue to
be very low. SPC/LAPS mesoanalysis depicts a modestly unstable
atmosphere over the coastal plain with MLCAPE values around 1250
J/kg. There is a mid-level subsidence inversion overhead that will
hinder the potential for deeper convective updrafts.. Therefore, we
expect the bulk of this activity to be showers though a rogue
thunderstorm or two is possible through about sunset. This diurnally-
driven convection should dissipate around or just after sunset.
With a light pressure gradient overhead, expect winds to become calm
at most locations late this evening and overnight. Good radiational
cooling conditions are expected tonight with mainly clear skies and
calm winds. A light onshore flow behind the sea-breeze fropa will
advect slightly higher dewpoints inland through this evening, which
will also help prime the boundary layer for the development of
patchy fog tonight, especially toward daybreak Saturday morning.
Low temperatures range from the low-mid 60s in northeast PA/
northwestern NJ to low 70s in the cities and along the coast.
.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
The area will be situated between an old frontal boundary stalled
to our south across VA and high pressure to our northeast across
the Canadian Maritimes on Saturday. This high will nose
southwestward down the eastern seaboard. With the surface ridge
axis located off the mid-Atlantic coast, light onshore flow will
persist across the region during the day.
Similar to today, we expect to see isolated showers and storms
develop during peak heating. The two potential sources for lift
will be with the sea-breeze front and terrain circulations.
Therefore, 20 PoPs were confined to the coastal plain and farther
inland across the higher terrain.
High temperatures will be similar to today... predominately in the
mid to upper 80s with our warmest spots in the cities and interior
Delmarva possibly reaching 90F. It will be slightly cooler (low
80s) near the coast and Poconos.
.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
The primary concern continues to be the potential for heavy
rainfall and thunderstorms Sunday and Sunday night...
A negatively tilted 500 hPa short wave, with heights around -3
Standard Deviations below normal, will traverse the Great Lakes
over the weekend. The main uncertainty with this feature is the
progress a warm front makes through our region Sunday, as there is
good model agreement on a strong cool frontal passage Sunday
night. Thereafter, a period of relatively fair weather expected
Monday thru Thursday, while another cool front may approach our
region by Friday. Temperatures will average near normal through
the period for a change. In terms of the primary hazards Sunday...
Heavy rainfall potential...A southerly low-level jet around 30
knots will transport anomalously moist air into the region, with
Precipitable Water values over 2.0 inches, 2-3 Standard Deviations
above normal. In addition, the area will be under the influence of
the left-rear quadrant of the upper jet. The warm front will also
provide a source of lift as it traverses the region. The WPC
outlook has our area in a marginal risk for excessive rainfall.
We expect the heaviest rainfall potential northwest of I-95, where
urban and small stream flooding is possible. We maintained a
mention of heavy rainfall in the forecast grids.
Thunderstorm potential...This will be dependent on instability,
as shear is not ideal. In fact, the more favorable parameters
in terms of shear and instability are disjointed. Nevertheless,
strong lift could help overcome this limitation, with forced
convection a possibility. Any thunderstorms that are able to
tap the low-level southerly jet and translate this momentum to
the surface will be capable of strong winds. In addition, there is
a healthy amount of 0-1 KM shear near the warm front, so any
storms near this boundary will have the potential for at least low
level rotation. Due to uncertainty in convective outcomes and the
SPC outlook for general thunderstorms, we held off on any enhanced
wording in the forecast grids at this time.
.AVIATION /01Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Mainly VFR through this evening. An isolated pop-up shower or storm
possible in vicinity of I-95 TAF sites as well as MIV through about
sunset. Light NW to NE winds currently reside inland while SE winds
have developed closer to the coast with a sea-breeze moving inland.
Expect winds to shift out of the SE at PHL and surrounding I-95 TAF
sites within an hour or two of 00Z.
Winds will generally have an onshore component to it tonight but
they will be so light that the direction may be variable. Patchy fog
and low clouds may develop early Saturday morning. Daytime mixing
could produce a few hours of MVFR CIGS with stratocu developing
during the mid to late morning.
Mainly VFR in the afternoon Saturday although an isolated shower or
storm is possible. This could bring very localized/brief
restrictions if it moves over a terminal.
Saturday night thru Sunday night...Ceilings are expected to lower
in advance of a warm front, and these lower ceilings could persist
into Sunday night. In particular, terminals northwest of I-95 have
the greatest chance of MVFR and IFR ceilings. More widespread MVFR
and IFR is possible as heavy showers and thunderstorms cross the
region Sunday afternoon and early Sunday night. There is the
potential for northwest wind gusts up to 20 knots at all TAF sites
as the cold front crosses the region Sunday night.
Monday thru Wednesday...Predominantly VFR conditions are
Light onshore flow (10 kt or less) that developed this afternoon
will continue into tonight and Saturday. Wave heights will be in
the 1-2 ft range thru tonight. It could increase to 2-3 ft in our
coastal Atlantic waters in response to the persistent onshore flow.
Sunday...Southeast flow across the waters in advance of a warm
front could briefly reach Small Craft Advisory (SCA) criteria.
Sunday night and Monday...As the cold front crosses the waters
Sunday night, there is a better chance of northwest winds gusting
to SCA criteria for a period of time, going into Monday morning.
Tuesday and Wednesday...Winds and seas are expected to remain
below SCA criteria.
RIP CURRENTS...There is a low risk for the development of
dangerous rip currents through the evening with a light onshore
flow and relatively calm seas.
The probable risk for the development of dangerous rip currents is
low for Saturday. However, it may approach moderate in some spots
during the afternoon with the establishment of a synoptic scale
southeasterly wind around 10 mph.
-- Changed Discussion --This lists the number of 90 degree or warmer days (as of August
19) and the mean number of 90 degree days for each climate site
(computed from when the period of record began).
Site 90 deg days beginning of climate record 2016 mean POR
ABE 29 17 1922
ACY 26 10 1874
GED 31 27 1998
PHL 34 21 1873
RDG 30 21 1999
TTN 27 20 1998
ILG 29 20 1894
Recalculating temperatures for Philadelphia...a top 3 warmest on record is
possible and its highly likely to be a top 10 warmest August on record.
The numbers forecast a 79.5 to 80 degree monthly average.
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-- Changed Discussion --DIX 88D up and running but the more permanent fix will not be accomplished
til next week.
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