Special Weather Statement, NWS Philadelphia/Mount Holly

Area Forecast Discussion
National Weather Service Mount Holly NJ
316 PM EDT Fri Jul 14 2023

.SYNOPSIS...
A deep closed upper low over south-central Canada will slowly
build to the south and east through the weekend before departing
early in the new week. An upper trough will then set up over
the Northeast. Several disturbances will spin off this low and
will pass through the region during this time.

&&

.NEAR TERM /THROUGH SATURDAY/...
Showers and a few thunderstorms have started to pop up this
afternoon as today's event commences. After reviewing some of
the recent CAM guidance and doing some mesoanalysis, the current
thinking is that areas along and west of the I-95 corridor have
the best chance to see showers and thunderstorms. The highest
instability and shear is currently over eastern PA and northern
NJ and CAMs have focused convection over these areas in the
coming hours. Worth noting that as of 18z, DCAPE values in these
locations were 1000+ J/kg, with damaging wind gusts as the
primary threat. Some small hail is possible as well, but it will
be difficult to get severe level hail with these storms.
Another important thing to note is that SE PA/Northern NJ have
the lowest FFG values across the area, and it will not take much
for flooding to occur. There is some CIN over the NJ coastal
plain which could limit things a bit, and observations at
KACY/KMIV show an overcast mid-level cloud deck, which is
limiting daytime heating/destabilization. Current thinking is
that the peak of the event will come between 21z-22z, beginning
to wind down around 01z-02z.

Following the departure of showers/thunderstorms, it should be
a relatively quiet overnight period. Some patchy fog and low
stratus should develop after midnight. Dewpoints will be very
close to temperatures at that point, both in the upper 60s and
low to mid 70s. It will feel quite muggy.

For Saturday, it does look pretty quiet for the most part. A
closed upper level low will begin to push southeast through
south central Canada, with a weak shortwave lifting through the
region. Convective initiation will be focused along the sea-
breeze, but any showers/thunderstorms will be on the more
scattered to isolated side. Precipitation chances are relatively
low compared to today and Sunday, around 15-30%. Temperatures
will get into the upper 80s with some spots getting into the low
90s. With dewpoints getting into the low to mid 70s, it will
feel tropical and heat indices will be getting close to triple
digits.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
Upper trough with strong shortwave energy digs through the
Great Lakes and Ohio Valley Saturday night. Surface low pressure
develops over the Ohio Valley Saturday night and tracks into
western Pennsylvania and western and central New York during the
day Sunday before departing. Meanwhile, a secondary low looks
to form southwest of the Delmarva Peninsula on Sunday, and that
low tracks to the north and east across New Jersey before
departing Sunday night.

Deep south to southwest flow ahead of this system will usher a
warm and humid airmass into the region. Surface dew points will
rise well into the low to mid 70s across much of the region, and
in the upper 70s in southern Delmarva, Saturday night through
Sunday evening.

Showers will become likely with a chance for thunderstorms
Saturday night as the first low passes north of the area.
Although it will be night, there should be 500 to 1000 J/kg of
surface-based CAPE, but there will also be 35 to 45 kt of 0-6 km
Bulk Shear. A few stronger thunderstorms are possible, but the
main threat will be heavy rain as PWATs will be over 2.25
inches. With winds from 1000 to 500 mb averaging 25 to 30 kt,
storms should be moving fairly quickly, so any flooding should
be localized unless there is training of storms. Areas of fog
will develop as well.

Going into Sunday, as the secondary low passes over the area,
showers and thunderstorms will spread over the area. PWATs will
be in excess of 2.25 inches, and may approach 2.5 inches. Highs
on Sunday will top off in the mid and upper 80s, and this should
yield surface-based CAPE values as high as 1500 to 2000 J/kg
along with 25 to 30 kt of 0-6 km Bulk Shear. Stronger to severe
thunderstorms will be possible with potentially damaging winds,
but once again, the main threat will be heavy rain. Given a
swath of heavy rain Saturday night, any additional heavy
rainfall will result in flooding and flash flooding. The Weather
Prediction Center keeps most of the area in a Marginal Risk for
Excessive Rainfall, but there is a part of far northern New
Jersey that is in a Slight Risk for Excessive Rainfall on
Sunday.

Low pressure departs Sunday night and a cold front passes
through the region late. Showers and thunderstorms will taper
off from west to east. It will take some time for drier air to
filter into the region, and given abundant low level and surface
moisture from the rainfall, patchy fog will develop Sunday
night.

A ridge of weak high pressure noses in from the southwest on
Monday. Conditions should be dry, mostly sunny, and dew points
will be a bit lower, generally in the mid to upper 60s. Highs
will top off in the upper 80s to low 90s. There should even be
some mixing in the afternoon that would lower dew points during
the peak heating of the day and may result in winds gusting 20
to 25 mph.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
Closed upper low will be over the southern portion of Hudson
Bay, Canada Monday evening and will lift north into the northern
Province of Quebec. An upper trough will extend down from this
low and will lie over the eastern Great Lakes and Northeast
Tuesday through Wednesday. Zonal flow sets up Thursday before
another mid-level trough approaches on Friday. Although the
weather will still be unsettled, especially on Tuesday,
conditions overall and throughout next week may not be quite as
active as they were this week.

Some weak shortwave energy passes through Monday evening, and
there will be some isolated showers and thunderstorms. Most of
the night should be quiet.

The base of the upper trough passes north of New York state,
and low pressure will develop west of the region and will pass
through the region on Tuesday. Deep south to southwest flow
ahead of this system will usher a hot and humid airmass into the
Northeast and Mid- Atlantic with highs in the upper 80s to low
90s and surface dew points in the upper 60s to low 70s. There is
a good chance for convection on Tuesday, especially in the
northern zones, as low pressure and a cold front work their way
through the region.

Showers and thunderstorms taper off Tuesday night, and then it
will be quiet into Wednesday. Some weak shortwave energy may
touch off isolated afternoon showers and thunderstorms. Cooler
and less humid on Wednesday with highs in the mid and upper 80s
and surface dew points in the low to mid 60s.

As another upper trough digs down into the Great Lakes, more
shortwaves will rotate around the base of the trough and will
pass through the region for the end of the week. These will once
again bring chances for showers and thunderstorms Thursday and
Friday. Highs will generally be in the upper 80s with dew points
in the upper 60s.

&&

.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Rest of today (through 00z)...Primarily VFR outside of
convection. Showers and thunderstorms expected to develop in the
vicinity of terminals as early as 19/20z, continuing through
about 00/01z. Winds will generally be out of the southwest
around 5-10 kt, with higher gusts possible in thunderstorms.
Moderate confidence overall.

Tonight (00z-12z)...Thunderstorms linger into the early
evening, which could bring some brief restrictions. Conditions
likely go VFR for part of the night before some fog/stratus
develops, bringing CIGs/VSBYs down to MVFR/IFR levels.
Introduced a temporary period of LIFR at KMIV, but confidence
remains low in exact timing for the onset of fog and stratus.
Moderate confidence in occurrence of restrictions after
midnight, continuing into the early daytime period Saturday.

Saturday...VFR once low clouds/fog break during the mid-
morning. Scattered afternoon showers and thunderstorms may
result in brief restrictions. Southerly winds around 5-10 kt.
Moderate confidence.

Outlook...

Saturday night...Sub-VFR conditions in SHRA/TSRA and fog. S
winds 5 to 10 kt. Moderate confidence.

Sunday through Sunday night...Sub-VFR conditions in
SHRA/TSRA/fog. Rain may be heavy at times. SHRA/TSRA taper off
Sunday night, but restrictions may continue in fog. S winds 10
to 15 kt with gusts up to 20 kt, becoming LGT/VRB at night.
Moderate confidence.

Monday through Monday night...VFR. Isolated SHRA/TSRA possible
at night. W winds 5 to 10 kt, becoming S at night. Moderate
confidence.

Tuesday through Tuesday night...Primarily VFR. Afternoon SHRA
and TSRA will result in sub-VFR conditions and gusty winds.
Conditions improve Tuesday night. SW winds 10 to 15 kt with
gusts up to 20 kt, becoming W to NW 5 to 10 kt at night.
Moderate confidence.

Wednesday...VFR. W winds 5 to 10 kt. Moderate confidence.

&&

.MARINE...
No marine headlines anticipated through Saturday. Southerly
winds around 10-15 kt with gusts up to 20 kt. Seas 3 to 4 feet.
Periods of showers and thunderstorms along the waters later
today and into tonight.

Outlook...

Saturday night...Sub-SCA conditions. S winds 10 to 15 kt with
gusts up to 20 kt. Seas 2 to 4 ft. VSBY restrictions in showers
and thunderstorms with heavy rain.

Sunday through Sunday night...Marginal SCA conditions. S winds
15 to 20 kt with occasional gusts to 25 kt, becoming W to NW 5
to 10 kt Sunday night. Seas 3 to 4 ft. Showers and thunderstorms
with heavy rain and gusty winds. Showers and thunderstorms
taper off Sunday night, then VSBY restrictions in fog Sunday
night.

Monday through Monday night...Sub-SCA conditions. S winds 10 to
15 kt. Seas 2 to 4 ft.

Tuesday through Tuesday night...Sub-SCA conditions. S winds 10
to 15 kt with gusts up to 20 kt, becoming W 10 to 15 kt at
night. Seas 2 to 4 ft.

Wednesday...Sub-SCA conditions. S winds 10 to 15 kt. Seas 2 to 4 ft.

Rip Currents...

For Friday, 10 to 15 kt southerly flow shall be accompanied by
breaking wave heights of 2 to 3 ft for all locations except
Delaware beaches which will see breaking wave heights closer to
1 to 2 ft. Given these conditions along with a medium SSE/S
period swell, there is a MODERATE risk for the development of
dangerous rip currents for the New Jersey and Delaware beaches.

For Saturday, near 10 kt southerly flow shall be accompanied by
breaking wave heights of 2 to 3 ft for all locations except
Delaware beaches which will see breaking wave heights closer to
1 to 2 ft. Given these conditions along with a continued medium
SSE/S period swell, there is a MODERATE risk for the
development of dangerous rip currents for the New Jersey and
Delaware beaches.

&&

.TIDES/COASTAL FLOODING...
Will continue to have to keep an eye on some localized coastal
flooding tonight with the next high tide, mainly limited to
localized areas around Barnegat Bay once again. Those areas saw
some minor flooding in spots last night, so with little change
in conditions tonight, the potential remains, though with
astronomical tides slightly lowering, the risk of flooding
should lower slightly.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

SYNOPSIS...MPS
NEAR TERM...Hoeflich
SHORT TERM...MPS
LONG TERM...MPS
AVIATION...Hoeflich/MPS
MARINE...Hoeflich/MPS
TIDES/COASTAL FLOODING...

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Special Weather Statement
National Weather Service Mount Holly NJ
343 PM EDT Fri Jul 14 2023

PAZ103-105-142015-
Upper Bucks PA-Western Montgomery PA-
343 PM EDT Fri Jul 14 2023

...A strong thunderstorm will impact portions of northwestern
Montgomery and west central Bucks Counties through 415 PM EDT...

At 342 PM EDT, Doppler radar was tracking a strong thunderstorm over
Green Lane, or 18 miles south of Allentown, moving east at 10 mph.

HAZARD...Wind gusts up to 40 mph and pea size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Lansdale, Quakertown, Souderton, Hatfield, Schwenksville, Green Lane,
Spring Mount, Telford, Sellersville, Pennsburg, Red Hill,
Trumbauersville, Harleysville and Kulpsville.

This includes Northeast Extension between mile markers 31 and 42.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4046 7535 4026 7527 4025 7527 4026 7528
      4025 7530 4026 7555 4034 7559
TIME...MOT...LOC 1942Z 255DEG 10KT 4033 7548

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...40 MPH

Staarmann