450
FOUS30 KWBC 142027
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Sat Sep 14 2024

Day 1
Valid 16Z Sat Sep 14 2024 - 12Z Sun Sep 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEASTERN U.S...

...Southeast...

Radar and satellite imagery continued to show a broad cyclonic
gyre situated across the southeastern U.S with a north- south band
of convection bisecting the a good portion Alabama into Western
Tennessee as of mid-morning. The surface reflection from what had
been Francine will continue to become more diffuse over the next 6
hrs with a degrading surface signature, leaving much of the flow
tied to the leftover mid and upper lows associated with the storm.
That said, the cyclonic spin from the tandem 850-500mb low(s) will
allow for a continued favorable moisture stream around the
circulation with small mid- level perturbations stuck over the same
areas allowing for locally enhanced ascent within the confines of
Alabama, Georgia and Tennessee. Precipitable water values generally
residing within the +1 standard deviation anomaly across the Deep
South will be sufficient enough within the pattern to yield another
round of scattered to widespread coverage of showers and
thunderstorms across similar areas to those areas that were
impacted on Friday. The difference is the magnitude of the
convective pattern will be a bit less vigorous thanks to the
weakening mid and upper low centers as denoted within all the
recent guidance height fields. Still, the combination of the
moisture anomaly and semi- favorable ascent under the remnant mid
and upper lows will be plenty to offer another round of convection
across the Deep South.

To the northeast, a stout surface ridge will nose down further into
the interior Southeast with a formidable easterly regime focused
in-of Northern GA down into the Northwest side of AL. The
combination of the cyclonic flow from the mid-level remnants and
wedging associated with the surface ridge will create a tightened
axis of convergence within the state of Alabama down into western
Georgia. The 14/12Z CAMs available through 15Z remained well represented
within the HREF mean QPF and associated neighborhood probability
field. Given the expanding footprint of diminished flash flood
guidance, the prospects of excessive rainfall will be increasing
with additional rainfall. Signals for 1-2"/hr rainfall rates would
also be sufficient to result in run off and flooding given the
antecedent conditions. A higher end SLGT risk still seemed
reasonable for this the forecast for a northwest to southeast
oriented axis from northwestern Alabama to far southwest Georgia.

...Coastal Carolinas...
Satellite and radar imagery showed showers and thunderstorms
hugging the immediate coastline of near the Northern Carolina and
South Carolina border with the heaviest rainfall off-shore. The
convection was largely being driven/focused by a weak mid-level
circulation that had little reflection at the surface. CAPE values
over land are forecast to be pretty modest...on the order of 500
to 1000 J per kg...with broad cloud cover but think there could be
enough of an on- shore component north of the circulation center to
draw some cells with 1 to 2 inch per hour rates inland.

...Arizona...

A surge of deep tropical moisture from now Tropical Storm Ileana
will make progress into the southern portion of Arizona by the
middle to end of the forecast period. With sufficient diurnal
heating anticipated this afternoon, the environment across southern
and southeast Arizona should become sufficiently destabilized to
support isolated to scattered thunderstorms in/near the terrain as
depicted by latest HRRR and NAM NEST. A Marginal risk was
maintained from previous forecast with only minor adjustments based
within the HREF blended mean QPF footprint. Still think this area
will have more favorability later on Day 2 as moisture continues
to be drawn northward.

...South Florida...

Surface trough located in proximity of South Florida will maintain
a prevailing westerly flow across the southern part of the
peninsula, placing the mean steering pattern pointed towards the
population centers along the Southeast Florida coast in time for
the sea breeze convective initiation this afternoon. Area
precipitable water were around 2.25 inch as of 14/12Z...placing
the area within the +1-2 standard deviation anomaly for the period.
This environment will be conducive for convective generation just
inland along the penetrating sea breeze in the afternoon with
locally strong convective cores producing rainfall rates between
2-3"/hr on initiation, moving towards the coast thanks to the
prevailing steering flow. The 14/12Z HREF probabilities for 3-hour
rainfall exceeding 3 hour flash flood guidance was generally in the
10 to 15 percent range during the afternoon and the probability of
rainfall rate exceeding 3 inches remained elevated. As such the
threat remained within the middle grounds for flash flood concerns
and maintenance of the Marginal Risk area with few changes along
the southeastern FL coast encompassing the population centers.

Kleebauer/Bann

Day 2
Valid 12Z Sun Sep 15 2024 - 12Z Mon Sep 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST U.S...

2030Z Day 2 Excessive Rainfall Discussion Update...
Overall...the previous outlook looked to be standing up well with
time and the changes were few and minor. Models still point to deep
moisture lingering over the southeast but the overall threat of
excessive rainfall should be diminishing as the strength of the
upper forcing wanes. Increasing moisture and mid- and upper-level
height falls should combine to increase the excessive rainfall
concern in the Southwest U.S. as much due to the chance for
rainfall rates exceeding 0.3 inches per hour over the most
sensitive burn scars and isolated 1 inch rainfall amounts
elsewhere.

Bann


...Previous Discussion...

...Southeast...

Despite the remnant mid and upper circulation from Francine
degrading in its presentation, another period of enhanced
convective activity will spawn underneath the primary upper forcing
lingering over the Southeastern U.S. There`s a bit more emphasis on
an axis of convection developing across portions of the Lower
Mississippi Valley in-of AR and MS, extending southeast into parts
of the Deep South, mainly Southern MS and AL. Areal PWATs will
remain elevated within +1 standard deviations, enough of an anomaly
to spur up some potential heavier convective cores that could
result in some flash flood prospects, especially over any urban
zones where run off potential is greatest. Ensemble bias corrected
QPF and the 75th percentile of the NBM indicate a fairly stout axis
of heavier precip likely to occur under the influence of the upper
low and remnant energy pivoting around the general circulation. The
primary concerns will continue to be those locally heavy rates that
will reach between 1-2"/hr with some intra-hour potential for
heavier rates that typically cause more issues in this part of the
country. A SLGT risk was added to the previous forecast with an
adjustment southwest to the overall SLGT/MRGL risk areas to align
with the current trends and ensemble forecast consensus on the
heaviest precip location. The MRGL expands into the FL Panhandle to
cover the area that has been hit pretty hard in the past 36 hours
as isolated storms are still forecast for those areas that have
seen copious amounts of rainfall in the above time span.

...Southwest...

Remnant moisture from now Tropical Storm Ileana will continue to
advect northward into the rest of the Desert Southwest with a
trajectory towards the Four Corners, wrapping back eastward into NM
during the afternoon hours Sunday. The ensemble QPF footprint upped
the total precip forecast compared to the previous periods, a trend
that was noted when assessing the QPF differential from 12z to 00z.
This places the convective pattern towards those areas we`ve been
highlighting all summer for flash flood concerns thanks to a
plethora of burn scars located within the terrain across parts of
NM. The previous forecast was relatively unchanged over AZ into the
Four Corners, but the MRGL risk was expanded to include much of NM
with a focus into the terrain over the Sangre de Cristos and the
Sacramento Mountains. General QPF average between 0.25-0.75" is
expected, but maxima of >1" will be plausible anywhere within the
confines of the forecast risk area. A targeted SLGT risk is still
possible, but the scattered nature of the convection forecast
doesn`t instill enough confidence at this time. A reassessment with
the addition of CAMs guidance could sway that the other way, so
stay tuned for future updates.

Kleebauer

Day 3
Valid 12Z Mon Sep 16 2024 - 12Z Tue Sep 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EASTERN
NORTH CAROLINA...


...2030Z Excessive Rainfall Discussion Update...
The 14/12Z model guidance continued to show somewhat better
agreement amongst the individual model runs in terms of track and
timing of a low pressure system that approaches the southern Mid
Atlantic coastline on Monday but the spread was still considerable.
The deterministic QPF from WPC tended to be closest to the 14/12Z
UKMET run as a compromise between the faster GFS and slower ECMWF.
Given the probabilistic nature of the EROs...expanded the Marginal
and Slight areas a bit to account for uncertainty in track and in
timing rather than an increased confidence in the extent of areal
coverage. With that caveat...it does appear that some locally heavy
rainfall is possible somewhere to the Southern Mid Atlantic
region...generally confined near the coast...beginning late Monday
night or in the early morning hours of Tuesday that persists beyond
the end of the short- range excessive rainfall outlook period.

In the Western United States...the amplification of a synoptic
scale trough and lingering moisture should result in an additional
round of showers and thunderstorms with rainfall rates high enough
to produce isolated run off problems. Saw no reason to make any
changes to the on-going Marginal risk area.

Bann


...Previous Excessive Rainfall Discussion...


...Coastal Southern Mid Atlantic...

A growing consensus for developing low pressure along the Carolina
coast has popped up amongst the 00z deterministic with much of the
global suite indicating surface cyclogenesis off the GA/SC coast,
moving northwest towards the Eastern Carolinas with emphasis on
North Carolina based on the latest runs. This is a fairly complex
evolution but rooted within the proposed synoptic evolution as a
strong block residing over the Northeast and Northwest Atlantic and
steering pattern focused towards the Eastern Carolina coast
sometime on Monday. The setup would allow for a steady easterly
flow to preside over Eastern NC with the approach of the surface
cyclone increasing a deep moist advective pattern in-of the coastal
portions of the state. The National Hurricane Center (NHC) has
marked this potential disturbance with a 50% chance of developing
into a tropical system within the next 72-96 hours which would
invoke a growing confidence in something more organized that could
promote greater tropical characteristics that would undoubtedly
allow for some locally greater impacts than forecast. As of now,
the SLGT from previous forecast was maintained, but adjusted to
the north to account for the ensemble shift in the low pressure
track and within the core of the heaviest QPF. The area from Myrtle
Beach up through Cape Hatteras will have to monitor the potential
closely as the chance for locally significant rainfall amounts have
been scaled up since the last update. For now, the SLGT risk will
suffice with the opportunity for a higher risk, pending the
expected evolution of the disturbance.

...Central Gulf Coast...

A weak surface low is forecast to develop in-of the Central Gulf
coast by Monday morning with an increasing axis of convergence
within the northern flank of the circulation. Elevated PWATs along
with prominent instability located along the Gulf coast will
amplify the threat of some locally enhanced rainfall prospects
between Southern MS through Southern AL into the Western FL
Panhandle. The main threat will continue to be urban flooding, but
will have to monitor the area around Apalachicola closely thanks to
significant rainfall the past few days with more anticipated prior
to the period. This has allowed for a sharp degradation of the FFG
indices normally in place for the area. A MRGL risk was introduced
given the sneaky threat with a lot contingent on the expected
development of a weak surface reflection along a quasi-stationary
front situated along the Central Gulf coast.

...Western U.S...

Increasing moisture nosing north ahead of an approaching deepening
long wave trough axis will allow for the opportunity for widespread
convective development across much of the Southwest U.S, extending
north through the Great Basin and the Interior Pacific Northwest.
Signals have been growing steadily for a large scale ascent pattern
that would favor locally heavy precip in-of the areas above,
especially across NV/UT/ID thanks to a premium diffluent upper
pattern approaching the West Coast in the form of a potent trough,
turned closed low by Monday morning. The setup would be conducive
for totals exceeding 1" over a large area that normally does not
see these types of totals very often. The PWAT anomaly forecast
within the ensembles are approaching 2+ standard deviations above
normal with the latest NAEFS indicating upwards of +3 deviations
located across parts of NV into ID by the end of the period. If
this setup maintains continuity, there`s an expectation that a risk
upgrade may be in order, especially since the upper pattern is
favorable for widespread convective impacts into areas with
climatologically low FFG indices.

Kleebauer

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt