NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Sep 1, 2:51 PM EST, 1 hour 54 minutes earlier.
Area forecast discussion, Sep 1, 2:51 PM EST, 1 hour 54 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
351 PM EDT Wed Sep 1 2021
.SYNOPSIS...
The remnants of Ida which are across Maryland late this afternoon
will cross 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 /UNTIL 6 AM THURSDAY MORNING/...
Significant flash flood and severe weather event arriving now
and will continue for much of the evening...
Update...
A Tornado Watch continues for most of the area until 10PM
tonight. The severe weather/tornado threat continues to look
concerning heading into the 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
21z 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 /6 AM THURSDAY MORNING THROUGH 6 PM 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 WEDNESDAY/...
No changes to the long term for the afternoon. The previous
discussion remains valid.
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 /20Z 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...
Low pressure approaching the waters this evening and steep
pressure falls have created strong and gusty winds across the
Chesapeake Bay late this afternoon. This, along with tstms,
will soon arrive across our waters, a Gale was raised bit
earlier and will go into effect at 4 PM. The SCA remains for the
upper DE Bay, but Gale gusts are possible there too, but confid
is not great for prolonged Gales. No doubt SMW's will be issued
through the evening for the waters. Inexperienced boaters are
advised to get to port now. Stay safe.
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...
A MODERATE risk of rip currents is expected for the beaches of
New Jersey and Delaware the rest of 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...
Flooding has commenced across the Lehigh Valley, Berks County, the
southern Poconos, Chester County, Delaware County and New Castle
County in northern Delaware. Eventually the rain will pivot across
the entire I95 corridor and into NJ.
Expected rainfall totals increased late last night and this morning.
As a result, crests also increased. The only meaningful changes for
this afternoon lies just north and west of the I95 corridor. The
heaviest rain and associated gradient has been shifted north and
west. This has had an influence on some of the updated afternoon
river forecasts across SE PA and northern NJ, basically dropping some
forecast points by a category (e.g. major to moderate).
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 were saturated leading up this
upcoming event.
Although there have some changes in rainfall amounts, the
Schuylkill, Passaic, and Raritan Rivers are still all forecast to
flood in the moderate to major categories. In addition, the mainstem
Delaware River is still forecast to flood. Although the rain has
shifted north across the Delaware Basin, crests haven't changed
much. In short, instead of the rain falling locally over portions of
the lower Delaware, the rain is now falling to the north, but still
has to flow through lower region forecast points.
Trenton saw 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
Please refer to the inundation maps at the following link.
Inundation maps have been developed for many forecast points along
the Delaware River and across both the Raritan and Passaic
river basins.
water.weather.gov/ahps/inundation.php
&&
.TIDES/COASTAL FLOODING...
With the passage of the remnants of Ida through 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 until 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...WFO PHI
Near Term...WFO PHI
Short Term...WFO PHI
Long Term...Meola
Aviation...WFO PHI
Marine...WFO PHI
Hydrology...Krudzlo
Tides/Coastal Flooding...WFO PHI
Climate...WFO PHI