NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Jul 12, 1:30 PM EST, 15 minutes earlier.
Area forecast discussion, Jul 12, 1:30 PM EST, 15 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
230 PM EDT Mon Jul 12 2021
.SYNOPSIS...
A warm front across northern New Jersey and eastern Pennsylvania
today will lift to the north on Tuesday. Strong high pressure
remains anchored off the Southeast and Mid-Atlantic coasts for much
of the week as several disturbances eject out ahead of an upper
level low over the Midwest. Offshore high pressure breaks down late
this week, and then a cold front slowly works its way through the
region next weekend.
&&
.NEAR TERM /THROUGH TONIGHT/...
Midday Update...
In addition to earlier revisions to the Heat Advisory, we've
made a couple other changes to the forecast this morning.
Regarding heavy rain and thunderstorm potential, most of the
prior reasoning remains valid. Trends today have been for the
differential heating boundary to set up further south than
expected, which makes sense in light of how convection evolved
last night. CAM solutions are quite noisy today, but most of
them indicate pockets of heavy rainfall, and the overall trend
in the CAMs is a little further south with the higher QPF. Based
on this and the mesoanalysis situation, we've expanded the
Flash Flood Watch quite a bit further south, with a flash flood
threat possible at least as far south as the Philadelphia metro.
Multiple rounds of convection are also a possibility as
elevated instability may remain present well into the night even
after surface based instability is lost. This could lead to a
prolonged hydro threat.
Finally, based on the strong instability, relatively weak but non-
negligible shear, and the presence of lingering convective
boundaries (and potential for more of them later), the severe
weather threat has also increased a bit today. SPC has added a
Slight Risk to much of the area. Damaging downburst winds will be
the primary threat. An isolated tornado is possible mainly in the
vicinity of any mesoscale convective boundaries, but is a low and
localized risk. The severe risk should mainly diminish by mid to
late evening, but the hydro threat could continue for several
hours more.
Overall, an active next 12 to 18 hours is likely. Previous
discussion follows...
The remaining showers from the overnight convection have moved
to our northeast, so all areas are dry for the time being.
However, this will change as we go through the day. A frontal
boundary remains nearly stationary along the northern edge of
our forecast areas this morning. This frontal boundary is
expected to begin to slowly sag southward through the day today,
before stalling across the area later tonight. Meanwhile,
ridging aloft builds offshore of the east coast, while a
mid/upper low is in place across the Mid-Mississippi Valley.
This low is forecast to open up later this afternoon and
evening, rejoining the parent zonal flow pattern. As it does so,
an initial piece of energy will eject as a shortwave trough,
with a wave of low pressure tracking along the front into the
Northeast later today, which will lead to showers and
thunderstorms across the northern half of the CWA. This will
also cause the front to nudge southward closer to the Philly
Metro overnight in the wake of this afternoon's convection.
Daytime heating will help to destabilize the atmosphere into
this afternoon as SB CAPE values climb above 2000 to even 3500
J/kg across most of the area. With a lee-side trough once again
setting up across the area, this will help provide the focus for
convective initiation. It is also possible that storms may
initiate along a differential heating boundary in the wake of
low clouds from last night's convection. Eventually, convection
will push east- northeastward across the northern half of the
CWA, with the best chances for storms north of I-76 and I-195 in
New Jersey closer to the shortwave trough.
With weak shear and PWATs above 2 inches, heavy rainfall is a
possibility. If convection begins to train, we may see flash
flooding, thus a Flash Flood Watch remains in effect for
northern portions of New Jersey. Here, forecast rainfall amounts
range from one to one and one half inches. Some guidance is
hinting at training cells near the stalled frontal boundary,
which can translate into an area of 1-3" amounts, but confidence
is just too low when comparing CAM solutions to say for sure
where exactly this will happen. With multiple periods of heavy
rain across northern New Jersey this past week, our FFG values
are relatively low. The heaviest of the rain may fall during the
overnight period as the front slowly sags southward. This may
likely lead to upstream storm propagation to the southwest, and
enhance the threat for training thunderstorms.
Lastly, the high dew points in combination with surface heating
will raise heat index values near to above 100F across much of
the Philly Metro this afternoon. The Heat Advisory remains in
effect where is was in place before through tomorrow with more
heat and humidity, but we also expanded the advisory to include
the eastern shore of Maryland. It is less likely this area will
see stronger showers and thunderstorms, thus reducing the
chance for storm cold pool cooling to provide relief tonight.
Lows tonight won't recover much with these areas staying in the
mid to upper 70s. We also expanded the advisory into Ocean and
Southeastern Burlington Counties, but these areas may be north
of the boundary by Tuesday, so it is only in effect for today.As
the front sags southward tonight, the northern half of the area
will see lows in the low 70s north of the Heat Advisory.
&&
.SHORT TERM /TUESDAY THROUGH TUESDAY NIGHT/...
A hot and increasingly humid airmass remains in place over the
region on Tuesday. With a warm front lifting north through the
region, surface dew points will rise behind that front well into
the lower and middle 70s. Highs will top off in the upper 80s
north of the Fall Line, and in the lower 90s south of the Fall
Line. For the urban corridor, heat index values will be around
100 degrees once again. The Heat Advisory that was in effect for
Monday was extended to include Tuesday. Mid-level high pressure
remains anchored off the Mid-Atlantic coast. Meanwhile,
shortwave energy ejects out ahead of a trough in the Great Lakes
and Ohio Valley.
With all of the heat and humidity over the region, there will
be a good deal of instability. However, the question will be how
far north the warm front gets and how everything lines up. If
the warm front only gets up to or just north of the Fall Line,
then surface- based CAPE values will be from 3000-4000 J/kg, and
MU CAPE values will be upwards of 4000-5000 J/kg. MU CAPE
values could get up to 2000-3000 J/kg north of the Fall Line
late in the afternoon and into the evening. Not much shear south
of the Fall Line, but north of the Fall Line, 0-6 km Bulk Shear
will be up around 25-30 kt. PWATs throughout will be around 2
inches, so thunderstorms will be capable of heavy rain.
The concern will be where the shortwaves go, as the mid-level
ride should keep those shortwaves to the west. Without those
shortwaves, there is not much synoptic forcing to produce
convection. Will carry likely PoPs in the southern Poconos,
Lehigh Valley, and back into Berks county, which will be closest
to the approaching front and those shortwaves, and chance PoPs
most elsewhere.
Given how warm and humid it is, convection may not taper off as
quickly as it otherwise would, and convection may linger
through midnight or so.
With warm and humid conditions in place, patchy fog possible
once again Tuesday night.
&&
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
No changes made to the long term.
Another hot and humid day on tap for Wednesday, but the offshore
high begins to break down late Wednesday and Wednesday night. A cold
front with an upper trough approaches from the west Wednesday and
begins to work its way through the region.
Weak high pressure passes through the region Thursday and Thursday
night resulting in a nearly rain-free day.
On Friday, another cold front approaches from the west. Convection
will develop late Friday and Friday evening as it passes through the
region. The cold front is a slow mover so it will take a couple days
to make its way through the region with several rounds of showe. rs
and thunderstorms. The front may not clear the region until Sunday.
The front will become stationary along the Mid-Atlantic going into
the new week.
High temperatures will be in the lower 90s through Saturday, and
then behind the cold front, highs will be in the mid and upper 80s
on Sunday, and then possibly a few degrees cooler than that on
Monday.
&&
.AVIATION /18Z MONDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Rest of today...Conditions have improved to VFR for most sites
and will continue to improve for those who haven't through 19z.
However, this improvement will only be temporary as showers and
thunderstorms are expected to continue to develop across the
area during the remainder of the afternoon into this evening.
Any showers or thunderstorms will likely lead to temporary lower
conditions. Winds will remain south to southwest 5-10 knots
with gusts 15-20 knots, before becoming variable later this
afternoon as a front begins to shift southward. Locally higher
winds will occur in thunderstorms. Moderate confidence overall
but low confidence on exact placement and timing of convection.
Tonight...VFR conditions will begin the evening outside of
showers and thunderstorms which will likely lead to temporary
lower conditions. MVFR then IFR ceilings are expected to
develop and move into the area through the evening and into the
overnight. ACY and MIV may be the only sites that do now drop
below VFR. Patchy fog will also likely develop overnight for
many areas that receive rain will reduce visibilities
overnight. Variable winds begin the period, before shifting to
more east or southeast 5-10 knots later this evening and
overnight. Moderate confidence.
Tuesday...IFR conditions likely begin the period from PHL
northward, with MVFR or VFR south. Ceilings lift to MVFR by
midday, and may eventually return to VFR during the afternoon.
There is a chance of showers and thunderstorms for eastern
Pennsylvania and northern New Jersey during the afternoon, which
may lead to temporary lower conditions. Light and variable
winds for much of the morning, becoming south to southeast 5-10
knots for many areas except southern New Jersey,
central/southern Delaware, and eastern Maryland which may be
south to southwest. Moderate confidence.
Tuesday night...Once any showers or thunderstorms end, VFR
conditions are expected to prevail through the overnight for
most areas. Southeast winds 5-10 knots, may become light and
variable overnight.
Outlook...
Wednesday through Saturday...Overall, a VFR forecast most of the
time. Afternoon and evening SHRA/TSRA, mainly along and west of the
I-95 corridor terminals will result in temporary sub-VFR conditions
should a storm pass over a given terminal. Confidence is moderate.
&&
.MARINE...
Sub-advisory conditions expected today, though southwesterly
winds will approach advisory levels with gusts from 20 to 25
knots for a few hours this afternoon. Seas from 2 to 4 feet.
Outlook...
Tuesday through Saturday...Overall, sub-SCA conditions. E-SE winds
on Tuesday, W-SW winds 10-20 kt Wednesday and Thursday, S-SW winds
10-20 knots Friday and Saturday. Seas 3-4 feet. The chances for
showers and thunderstorms are better inland as opposed to the
waters, but showers and thunderstorms are possible in the late
afternoon and early evening. Limited chances on Thursday.
Rip Currents...
There is a Moderate risk for the development of dangerous rip
currents today with southerly winds increasing from 20 to 25
knots and wave heights from 2 to 4 feet. Southerly winds are
expected to continue into Tuesday with waves from 2 to 3 feet,
thus we are expecting a Moderate risk for rip currents to
continue into Tuesday for the New Jersey coast and Delaware
Beaches.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flash Flood Watch through late tonight for PAZ054-055-060>062-
070-071-101>106.
Heat Advisory until 8 PM EDT Tuesday for PAZ070-071-104-106.
NJ...Flash Flood Watch through late tonight for NJZ001-007>010-012-
013-015-017>019.
Heat Advisory until 8 PM EDT Tuesday for NJZ017>019.
Heat Advisory until 8 PM EDT this evening for NJZ020-027.
DE...Heat Advisory until 8 PM EDT Tuesday for DEZ001.
MD...Heat Advisory until 8 PM EDT Tuesday for MDZ012-015-019-020.
MARINE...None.
&&
Synopsis...MPS/Franklin
Near Term...Davis/O'Brien/Robertson
Short Term...MPS
Long Term...MPS/Franklin
Aviation...Davis/MPS/Robertson/Franklin
Marine...Davis/MPS/Franklin