NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Sep 1, 12:05 PM EST, 1 hour 55 minutes earlier.
Area forecast discussion, Sep 1, 12:05 PM EST, 1 hour 55 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
105 PM EDT Wed Sep 1 2021
.SYNOPSIS...
The remnants of Ida will lift northeastward into the Mid-Atlantic
today, crossing our forecast area this evening. The remnant low will
continue to lift to the northeast tonight, moving into New England
on Thursday. High pressure will build in behind the departing low on
Friday. A cold front will approach from the west on Sunday, crossing
the area Sunday night into Monday.
&&
.NEAR TERM /THROUGH TONIGHT/...
...Significant flash flood and severe weather event likely to
occur in much of the area today and tonight...
Midday Update...
A Tornado Watch has been issued for most of the area until 10PM
tonight. The severe weather/tornado threat continues to look
concerning heading into later this afternoon and evening with an
unusually potent overlap of shear, forcing, and instability. The
highest threat is still expected across our southern zones, but
some risk may exist as far north as the Lehigh Valley and
north-central New Jersey.
Regarding rainfall, the axis of heavy rain has definitely been
on the northwest periphery of expectations so far. Areas such as
the Poconos are well on their way to significant rainfall
totals. Am a little concerned amounts may end up lower in and
around the urban corridor; they will need some very high rain rates
late this afternoon and evening to reach the forecast amounts
there. However, CAM guidance suggests hourly rates of 2 to 3
inches per hour are not out of the question, so a significant
flash flood threat still exists at least as far southeast as
I-95 based on the rain rate potential alone. At least another
couple of mostly dry hours are likely for the southeastern half
of the area, but rain will fill in by mid to late afternoon.
Valid portions of previous discussion follow...
The remnants of Ida will continue to affect the region through
tonight, with significant impacts expected for the area. Model
trends continue a slight northward nudge of the highest QPF
axis, with the GFS/NAM/ECMWF in good agreement that 3+ inches of
rainfall will occur near/northwest of the Fall Line (with
embedded totals 6+ inches somewhere in the southern Poconos
and/or Lehigh Valley and vicinity). The CMC remains farther
south (almost to outlier status), with the main axis generally
between I-76 and I-80. The convection-allowing models (CAMs) are
in between, and these are generally in line with our current
thinking this morning.
However, the risk of flash flooding is not only tied to total
rainfall but also duration, and the CAM solutions would suggest that
much of the rainfall south of the high-QPF axis will be occurring in
a relatively short period of time. Excessive rainfall rates (via PWs
well north of 2 inches) are likely with the main convective show
late this afternoon into the early overnight hours, and will likely
lead to several instances of flash flooding near the urban corridor.
Therefore, despite somewhat lowered QPF for the I-95 corridor
southeastward, the risk of flash flooding remains quite elevated
because of the rainfall rates. The HREF guidance provides some
insight here, with the 00z ensembles indicating probabilities of 1-h
rainfall accumulations exceeding flash flood guidance (FFG) above 50
percent in virtually all areas northwest of I-95 in our CWA during
the evening hours, despite a range of 3-5 inches in total QPF.
Notably, the probabilities of 6-h rainfall accumulations exceeding
FFG are above 90 percent in much of the same area, which conveys the
seriousness of the flooding threat for our area quite well.
Of course, that is not all. With the warm sector expected to shift
northward into at least the southern half of the CWA, the ambient
preconvective environment will be quite favorable for severe
weather. Model soundings indicate MLCAPE 1000-2000 J/kg (closer to
500-1000 J/kg near the warm front), 0-6 km bulk shear nearing 40 kt,
and SRH > 200 J/kg in the late afternoon and early evening hours.
This is an environment supportive of rotating storms, with CAM
simulations suggesting a mixed mode of short line segments and
cellular storms. Damaging wind gusts and tornadoes are the main
threats, and the SPC has nudged the enhanced risk slightly farther
north given model trends this morning. Given the coverage of
convection expected, the favorable environment is quite concerning,
with the severe potential continuing to look more and more
impressive. The main time window for severe weather is likely in the
20z to 06z time frame from west to east. Individual cells will
likely move quickly north-northeast, but the overall system will
move only slowly eastward, so training convection poses a flash
flood risk in the southern half of the CWA.
Models continue to trend faster with precipitation moving out of the
region, with most areas likely drying out by daybreak Thursday.
&&
.SHORT TERM /THURSDAY/...
High pressure will build into the region on Thursday in the wake of
the departing system. The entire region is expected to dry out
during the morning, with partly to mostly sunny skies during the
afternoon. For this reason, have ended the flash flood watch for the
area at 8 am, as little or no precipitation is expected after that
time.
Main concern is winds, with a stiff north breeze likely for
much of the day, especially near the coast. However, the speed of
the system will likely keep the period of elevated winds rather
short, and the forecast speeds are not overly impressive anyway (10
to 20 mph sustained winds with occasional gusts 25+ mph). Forecast
highs are expected to be in the 70s for most of the area. This will
likely be a welcome reprieve from the frequent bouts of heat we have
dealt with the past few months.
&&
.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Behind the departing remnant surface low that once was Ida, we
should see cooler and drier conditions across our region. The upper
level low will remain nearby Thursday night but should start to lift
to the northeast late Thursday night. The more northerly flow will
allow for cooler air to move across the Mid-Atlantic and we should
have a relatively nice night in store for our area. Temperatures
will be around 5-8 degrees below normal with overnight lows dropping
into the 50s across much of the region. Along the immediate coast,
temps may remain in the lower 60s, while some of the colder spots in
the southern Poconos may actually dip into the upper 40s.
High pressure builds in for Friday and through the holiday weekend.
Daytime temps will generally be in the 70s with a gradual warming
trend from day to day. Overnight lows will also moderate as they
warm from the 50s/low 60s on Friday night to the upper 50s to mid
60s on Saturday and Sunday nights.
A cold front may approach the region late in the weekend, crossing
the region Sunday into Monday. Some showers may accompany the front
as it moves through the area. Conditions should then dry out for
later Monday and much of Tuesday.
&&
.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
This afternoon and evening...Conditions should deteriorate,
with prolonged MVFR/IFR at RDG/ABE with heavy rain and
occasional thunderstorms. For the Philly terminals and MIV/ACY,
a round of strong to severe thunderstorms is probable, with
timing generally in the 20z to 03z window. Torrential downpours
and strong wind gusts are likely. Winds outside of storms will
likely be easterly or southeasterly 5 to 15 kt with gusts to 25
kt. Notably, directions will become more erratic after 00z as
the surface low begins to move through the region. Moderate
overall confidence; low confidence on exact timing of storms and
category changes.
Late tonight and Thursday...Rapid improvement expected as the
rain/storms move east out of the region. Winds will become north to
northwest 10 to 20 kt with gusts 25+ kt possible. Moderate
confidence.
Outlook...
Thursday night through Friday...Mainly VFR. North-northwest winds
around 10 to 15 knots. High confidence.
Saturday through Sunday...Mainly VFR. West to southwest winds around
5 to 10 knots. High confidence.
&&
.MARINE...
Conditions will be deteriorating today, with southeast winds
increasing to 15 to 25 kt on Delaware Bay and the adjacent Atlantic
waters by early afternoon, spreading northward to the New Jersey
waters by early evening. A period of gale conditions is expected for
all but upper portions of the bay, though there is some question
regarding the duration of these winds, particularly given only
somewhat favorable boundary-layer profiles. As the low tracks
through the waters late tonight, there should be a relative lull as
winds become variable. Then as the low moves offshore, stiff
northerly winds are likely to develop. Another period of gale-force
gusts is expected for the New Jersey waters during the morning
before rapidly diminishing during the afternoon. Gale warnings have
been issued to cover these threats. Seas will be elevated through
Thursday, primarily between 5 to 8 feet.
Regarding storm chances, they should increase quickly during the
day, with a round of strong to severe storms possible from late
afternoon into the overnight hours. Waterspouts, strong wind gusts,
and torrential downpours are possible.
Outlook...
Thursday night through Friday... North to northwest winds around 15
to 20 knots will continue to drop overnight into Friday. While a few
gusts around 25 knots can't be ruled out, most gusts look to remain
below 25 knots. Seas will remain elevated into Thursday night and
will gradually drop through the night into early Friday. Some
lingering 5 foot seas are possible, especially across the southern
waters into midday Friday but expect most of the ocean waters to
fall to 3 to 4 feet on Friday.
Friday night through Sunday... Mainly sub-SCA conditions expected.
North to northwest winds around 10 to 15 knots becoming southerly
Saturday night and more southwesterly on Sunday.
Rip Currents...
The rip current risk will be increasing today as southeasterly
winds increase rapidly during the day ahead of a surface low
moving into the region. A MODERATE risk of rip currents is
expected for the beaches of New Jersey and Delaware today. By
late this evening, the rip current risk will be HIGH on the New
Jersey coast, likely continuing through much of the day
Thursday, as swell only slowly diminishes as the low moves
offshore. The risk will likely remain moderate for the Delaware
beaches through Thursday.
&&
.HYDROLOGY...
If rainfall totals come to fruition, a significant flooding event
will commence late this afternoon into Thursday morning and persist
across some spots into early next week.
With a widespread of 3-6 inches and locally higher totals of 6+,
especially north and west, widespread flooding, albeit in different
magnitudes, would be likely across portions of our area.
Climatologically, August is supposed to be a quiet month in regards
to flooding and severe weather. It was quite the opposite. Parts of
the region got hit with Fred. A larger part of the area got hit with
Henri. In between, rounds of thunderstorms produced flash flooding.
1/3rd of days in August had at least 1 Flash Flood Warning issued.
This has left much of the HSA very wet. Except for SW NJ, all the
other counties we serve are either running above or much above
normal in terms of rainfall. Grounds are saturated leading up this
upcoming event.
Again, if the QPF is on target, nuisance or flash flooding will
commence today with larger streams and rivers reaching flood stage
on Thursday.
The Schuylkill, Passaic, and Raritan Rivers are all forecast to
flood. In addition, importantly, the main stem Delaware River is now
forecast to experience minor to moderate flooding from Trenton
northward. If memory serves me correct, Trenton has seen flooding in
both 2014 and 2018, but during the winter due to ice jams. The last
time Trenton has flooded strictly from a rain event was in 2011.
Please refer to our AHPS webpage for river observations (courtesy of
the USGS) and our forecasts
https:/water.weather.gov/ahps2/index.php?WFO=phi
&&
.TIDES/COASTAL FLOODING...
With the passage of the remnants of Ida Wednesday and Thursday,
tidal levels will likely be elevated. However, with the faster
timing of the low's passage through the area, the highest surges
on Chesapeake Bay and on the Atlantic coast will likely be
occurring during low tides rather than high tides. As a result,
only spotty minor flooding at most is expected at this time.
&&
.CLIMATE...
Record Maximum Daily Rainfall Records for:
9/1 9/2
Site Record/Year Record/Year
ABE 4.06/1952 1.63/2006
ACY 2.18/2006 2.40/1951
55N 2.35/2006 1.43/2017
PHL 1.80/1952 1.89/2013
ILG 1.73/1952 2.95/1910
RDG 3.50/1952 2.24/2006
TTN 3.84/1940 2.05/2006
MPO 4.54/1952 2.30/1926
GED 6.22/2002 2.47/1998
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flash Flood Watch through Thursday afternoon for PAZ054-055-
060>062-070-071-101>106.
NJ...Flash Flood Watch through Thursday afternoon for NJZ001-
007>010-012>027.
High Rip Current Risk from 8 PM EDT this evening through
Thursday evening for NJZ014-024>026.
DE...Flash Flood Watch through Thursday afternoon for DEZ001>004.
MD...Flash Flood Watch through Thursday afternoon for MDZ012-015-
019-020.
MARINE...Gale Warning from 6 PM this evening to noon EDT Thursday for
ANZ450>453.
Gale Warning until 8 AM EDT Thursday for ANZ431-454-455.
Small Craft Advisory until 6 PM EDT Thursday for ANZ430.
&&
Synopsis...Meola
Near Term...CMS/O'Brien
Short Term...CMS
Long Term...Meola
Aviation...CMS/Meola/O'Brien
Marine...CMS/Meola
Hydrology...WFO PHI
Tides/Coastal Flooding...WFO PHI
Climate...WFO PHI