Flash Flooding and Flooding in Pennsylvania, September 1 to 3, 2021

Area Forecast Discussion
National Weather Service State College PA
746 AM EDT Wed Sep 1 2021

.SYNOPSIS...
The post-tropical transition of Ida will bring extreme rainfall
and considerable flooding impacts to south central PA today
into early Thursday. Refreshingly cooler and drier (lower
humidity) air will follow through late week with fair weather
expected Thursday into the first part of the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
720AM/1120Z: Widespread moderate to heavy rain encompassing most
of the CPA early this morning, with only the far northern tier
and extreme southeast locations staying dry (for now). Max
rainfall in the past 6 hours has been focused over parts of the
south central Alleghenies (Blair, Bedford, and Cambria) with
KAOO coming in over 2 inches. Minimal changes to the near term
fcst grids as we start to transition into the nowcast or
preceding advisory/warning phase of the this extreme rainfall
event.

Previous Discussion Issued: 408 AM EDT Wed Sep 1 2021

*High risk for excessive rainfall over south-central PA
*Enhanced risk of severe thunderstorms over far southeast PA

The post-tropical transition of Ida (interaction with the mid-
latitude westerlies and quasi-stationary front draped near the
MD/PA line) will bring extreme rainfall and considerable
flooding impacts to south central PA today into early Thursday.
Strong signals in the latest hires ensemble guidance leads to
high confidence extreme 24-hr rainfall totals of 4 to 6+ inches
with local amounts up to 8 inches possible. The 01/00Z HREF
probability of 24hr QPF to exceed 100yr ARI is 70-100% which
increases confidence in extreme rainfall even more. To put this
into a more localized climatological perspective, we are
forecasting a top-5 maximum daily rainfall total at Harrisburg.
The only call-outs with regard to the QPF fcst include a
continued slight northward shift/trend overall, with the CAMs on
the south-side of the envelope of solutions. The greatest
uncertainty remains on the tight northern gradient.

In addition to the prolific volume of rainfall that is expected
to inundate south central PA, there is a conditional risk of
severe storms across the far southeast zones or southeast of the
I-81/78 corridor. The key will be how far north the warm front
can orient itself, as this boundary will serve as the northern
demarcation of meaningful tornado potential. It is not very
often that a portion of our CWA is included in a 10% tornado
probability. Strong vertical shear will be sufficient for fast-
moving, low-topped supercells with isolated, brief/weak
tornadoes possible; the question will be to the extent of
heating/instability needed to support robust updrafts. SPC kept
the enhanced risk (level 3 out of 5) virtually unchanged over
the far lower Susquehanna Valley mainly along and south of a
line from Gettysburg to Lancaster.

Considerable riverine flooding is forecast primarily on smaller
streams and creeks in south-central PA with several points
forecast to exceed minor/moderate flood levels. See the
hydrology section for more info and be sure to check the AHPS
page for the latest river forecasts.

MaxT will be -5 to -15F below climo today with 24hr deltas d/d
of the same magnitude. The heavy pcpn shifts to the east early
tonight and exits CPA by 8AM Thu.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
The departure of Ida's 1000 mb remnant sfc low off the northern
New England Coast will help to quickly amplify/deepen the SE
Canada/NE US upper trough. This will finally bring a welcome
push of cool and drier Canadian air across the Commonwealth for
at least Thursday and Friday. Look for refreshing, Autumn-like
dewpoints in the 40s (NW) and 50s elsewhere.

High temps in the upper 60s (higher elevations across the
north) to low and mid 70s elsewhere will be 7-10 deg F below
normal in most locations. Similar departures are expected with
overnight lows Thursday night and Friday night.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As the original Canadian sfc high settles south to the
Carolinas on Saturday, dewpoints and temps could rebound
slightly as a west to southwest LLVL flow develops and milder
air ridging aloft pushes east-southeast from the Midwestern
states and Upper Great Lakes.

Periods of mid-level clouds will occur Saturday with a large
llvl temp/dewpoint spread yielding just a few pancake cu or no
cu at all.

Fast quasi-zonal flow to the south of a broadening eastern
Canadian trough

A cold front approaching from the Great Lakes and 1 or 2
distinct shortwaves dropping SE through a slightly amplifying
northern stream trough will bring a chance of showers back into
the forecast for Sunday or Monday, followed by mainly fair
weather again for Tuesday with just a slight chance of an
afternoon shower.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
12Z TAFS sent.

More in the way of heavier and steadier rain now into central
PA since 530 AM.

More information below.

Several updates as of 520 AM. Overall, dry air still keeping
CIGS up. Also most of the rain on the light side, with a few
pockets of moderate to heavy rain, but not enough to saturate
the airmass yet.

Earlier discussion below.

Some slightly drier air working in from the north. This in
combination with not a lot of rain yet, has resulted in
conditions being better than guidance would have indicated.

Expect the rain to become more widespread and heavier today,
so CIGS and visibilities to lower with time.

Winds will pick up during the afternoon, along with a chance
of thunder at LNS, as a more humid airmass remains at LNS.

LLWS will also be a hazard late Wed afternoon through the
evening, with NE flow up to 45 kts in the lowest 2kft AGL.

Improving conditions from west to east late today and early
Thursday morning.

Outlook...

Thu...Improving conditions. Rain ending early. Breezy
conditions.

Fri...Dry with VFR conditions.

Sat...Dry with VFR conditions.

Sun...Chance of PM showers.

&&

.HYDROLOGY...
Considerable riverine flooding is forecast across south central
PA. Several points are forecast to exceed minor/moderate flood
stages, with major levels certainly in-play given the extreme
4-8" rainfall forecast across the southern basins.

&&

.CLIMATE...
Here are some rainfall records that are in jeopardy:

Maximum 1-Day Total Precipitation at Harrisburg:
1. 9.13 1972-06-22
2. 7.71 2011-09-07
3. 5.81 1972-06-21
4. 5.72 2013-10-11
5. 4.71 2017-07-23

Rainfall records for 09/01:
3.60/1952 Harrisburg
2.06/1952, 2003 Williamsport
1.15/1952 Altoona
(From remnants of Hurricane Able in 1952)

Rainfall records for 09/02:
2.69/2006 Harrisburg
3.87/2006 Williamsport
2.35/2006 Altoona
(From remnants of TS Ernesto in 2006)

Harrisburg wettest 2-day total September 1-2:
4.00 in 1952

Harrisburg wettest 2-day total August 31-September 2:
4.36 in 1952

Prelim rankings for August and Summer 2021 (JJA):
Harrisburg: 2nd warmest August; 4th warmest summer
Williamsport: 3rd warmest August; 9th warmest summer

Harrisburg average temperature (JJA)
1. 2020 77.9F
2. 1966 77.6F
3. 2016 77.3F
4. 2021 77.1F
5. 2010 76.9F

Harrisburg average minimum temperature (JJA)
1. 2020 68.2F
2. 2021 68.1F
3. 2016 67.9F
4. 2010 67.6F
5. 2005 67.3F

Harrisburg average maximum temperature (JJA)
1.  1966      90.2F
2.  1999      87.7F
3.  2020/1991 87.6F
5.  1943      86.8F
6.  2016      86.7F
7.  1988/1944 86.5F
9.  2002      86.2F
10. 2021      86.1F

Harrisburg average temperature (Aug)
1. 2016 79.1F
2. 2021 78.3F

Williamsport average temperature (JJA)
1.  2020/2016 74.4F
3.  1949/1901 74.3F
5.  1900      74.2F
6.  1995/1955 74.0F
8.  2005      73.9F
9.  2021      73.7F
10. 1991      73.6F

Williamsport average minimum temperature (JJA)
1. 1901      66.2F
2. 1899      64.0F
3. 2005/2021 63.1F
5. 2016      62.8F

Williamsport average temperature (Aug)
1. 2016 76.1F
2. 1995 75.9F
3. 2021 75.6F

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Thursday morning for PAZ012-017>019-
024>028-033>036-041-042-045-046-049>053-056>059-063>066.

&&

SYNOPSIS...Steinbugl
NEAR TERM...Steinbugl
SHORT TERM...Fitzgerald/Dangelo/Steinbugl
LONG TERM...Lambert/Colbert
AVIATION...Martin
HYDROLOGY...Steinbugl
CLIMATE...Steinbugl

Hazardous Weather Outlook
National Weather Service State College PA
448 AM EDT Wed Sep 1 2021

PAZ012-017>019-024>028-033>035-041-042-045-046-049>053-058-020900-
Northern Clinton-Clearfield-Northern Centre-Southern Centre-Cambria-
Blair-Huntingdon-Mifflin-Juniata-Somerset-Bedford-Fulton-
Northern Lycoming-Sullivan-Southern Clinton-Southern Lycoming-Union-
Snyder-Montour-Northumberland-Columbia-Schuylkill-
448 AM EDT Wed Sep 1 2021

...FLASH FLOOD WATCH IN EFFECT THROUGH THURSDAY MORNING...

This Hazardous Weather Outlook is for central Pennsylvania.

.DAY ONE...Today and tonight.

Please listen to NOAA Weather Radio or go to
weather.gov/StateCollege on the internet for more
information about the following hazards.

   Flash Flood Watch.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Please listen to NOAA Weather Radio or go to
weather.gov/StateCollege on the internet for more
information about the following hazards.

  Flash Flood Watch.

.SPOTTER INFORMATION STATEMENT...

Spotters are encouraged to report significant hazardous weather.

PAZ004>006-010-011-037-020900-
Warren-McKean-Potter-Elk-Cameron-Tioga-
448 AM EDT Wed Sep 1 2021

This Hazardous Weather Outlook is for central Pennsylvania.

.DAY ONE...Today and tonight.

The probability for widespread hazardous weather is low.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

The probability for widespread hazardous weather is low.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

PAZ036-056-057-059-063>066-020900-
Franklin-Perry-Dauphin-Lebanon-Cumberland-Adams-York-Lancaster-
448 AM EDT Wed Sep 1 2021

...FLASH FLOOD WATCH IN EFFECT THROUGH THURSDAY MORNING...

This Hazardous Weather Outlook is for central Pennsylvania.

.DAY ONE...Today and tonight.

Please listen to NOAA Weather Radio or go to
weather.gov/StateCollege on the internet for more
information about the following hazards.

   Flash Flood Watch.

An isolated, weak, short-lived tornado is possible this afternoon
mainly along and south of the Interstate 81 and 78 corridor.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Please listen to NOAA Weather Radio or go to
weather.gov/StateCollege on the internet for more
information about the following hazards.

   Flash Flood Watch.

.SPOTTER INFORMATION STATEMENT...

Spotters are encouraged to report significant hazardous weather.

MRS