Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Mon Sep 25 2023

Day 1
Valid 01Z Tue Sep 26 2023 - 12Z Tue Sep 26 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER MISSISSIPPI VALLEY...

...01Z Update...

Trimmed a bit off the northern edge of the Marginal Risk area
across east-central MN into WI as the focus remains squarely
inside the Slight Risk outline this evening, where some heavier
rainfall rates resides per current obs/RADAR. This could exceed
FFG values that have lowered recently due to rainfall this
afternoon.

Cut back much of the Marginal Risk area across the lower MS Valley
where activity is waning. Kept the area across TX where cells
continue to sink southward but also where there is a low-end
chance of new activity near the Mexican border overnight, but that
is likely decreasing as well.

Fracasso


...16Z Update...

No significant changes were made to any of the risk areas on the
Day 1 ERO. The area of highest concern remains over the upper
Mississippi Valley, as a slow-moving cutoff low will support
training convection up along the Mississippi River. Elsewhere,
concerns are lower for the lower Mississippi Valley into Texas,
where dry conditions should mean any flash flooding threat is
relegated to urban and flood-sensitive areas. For the OR/CA border
area, the antecedent soil conditions have been abnormally dry, so
this should mostly be a beneficial rainfall event. That said, for
burn-scar areas in particular, localized flash flooding is
possible.

A couple areas not highlighted: For southern New England, steady
light to moderate rain with rates generally below 1 inch per hour
will continue into the afternoon. While the area received several
inches of rain going into today, the light rainfall rates going
forward should not result in any flash flooding concerns.
For the Florida Peninsula, expect scattered showers and
thunderstorms to redevelop today. Much of the peninsula has been
dry lately, which should help absorption rates. Should training
cells happen to move over an urbanized or other flood-sensitive
area, highly localized flash flooding may be possible given very
high PWAT values in the area. The flood threat appears to be below
Marginal thresholds.

Wegman

...Previous Discussion...

...Upper Mississippi Valley...
A meandering closed low over the Upper Midwest provides a suitable
environment for divergent flow atop the atmospheric column today.
PWs as high as 1.25" and MLCAPE between 500-1,000 J/kg will
provide sufficient moisture and instability for developing
thunderstorms. Combined with a conveyor belt of southeasterly
850mb moisture flux ahead of an occluded front, thunderstorms
Monday afternoon and evening will be capable of producing rainfall
rates >1.5"/hr. This round of storms is also coming on the heels
of a 2-day stretch where as much as 2-3" or rainfall is forecast
through Sunday night, so soils will be a little more saturated
compared to earlier in the week. 00Z HREF probabilities have
continued to increase with 3-hr QPF > 3-hr FFGs rising to as much
as 40-50% between 21Z Monday - 03Z Tuesday along the MN/WI border.
Probabilities for 24-hr QPF > 3" was between 60-80% with >5"
probabilities for the same 24-hr winds up to 15-20%. These all
continue to support the Slight Risk that is in place. Tweaked the
inherited Marginal and Slight Risk areas to account for the latest
QPF guidance. Urbanized communities, poor drainage areas, and
nearby creeks and streams are the most vulnerable to flash
flooding.

...Pacific Northwest...
A powerful upper low approaching the Pacific Northwest will direct
an atmospheric river at the West Coast today, delivering a soaking
rain to residents from northern California to western Oregon and
Washington. The upper low off the coast is very impressive for
late September with heights within the 500-850mb layer a
remarkable 4-6 standard deviations below normal according to
NAEFS. This powerful upper trough will direct a robust 750 kg/ms/s
IVT (peaking around 9 standard deviations above normal) at
northern CA and western OR where PWs up to 1.5" translate to being
as high as 3-4 standard deviations above normal. This is
effectively the first bonafide atmospheric river of the season for
the Pacific Northwest. Fortunately, parts of far northern
California and southern Oregon could use the rain as parts of the
region are in D0-D2 drought. The primary concerns are residual
burn scars where soils will be more susceptible to possible debris
flows. The Marginal Risk remains in place as rates do not appear
to pose a considerable flood threat, but detrimental impacts could
be felt in burn scars.

...Southern Plains & Lower Mississippi Valley...
A frontal boundary stretching from West Texas to the Ozarks is
forecast to weaken and push south throughout the day while at the
same time, southerly 850mb moisture flux is expected to wain.
These features will still be present at a time when a weak 500mb
disturbance approaches from the north, providing some modest
vertical ascent atop the atmosphere. PWs are currently forecast to
reach as high as 1.75-2.0" in (1-2 standard deviations above
normal according to NAEFS) and MLCAPE between 1,000-2,000 J/kg
from central TX to the Lower Mississippi Valley (higher values
over south-central TX). The available PWs and instability should
support >2"/hr rainfall rates within the more intense cells, and
soils should be a little more saturated following Sunday's
rainfall. The Marginal Risk remains in place with any
consideration of a Slight Risk likely to be discussed in future
forecast cycles when more CAM guidance becomes available to key in
on the more at-risk areas.

...Northeast...
While it still looks to be a wet day along the southern New
England coast, there is even less instability to work with
compared to what was observed on Sunday. Rainfall rates should
remain within manageable levels today given the depleted
instability aloft and diminishing 850mb moisture transport from
northern NJ to southern New England, but cannot rule out some
nuisance ponding in poor drainage areas.

Mullinax


Day 2
Valid 12Z Tue Sep 26 2023 - 12Z Wed Sep 27 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
FLORIDA AND THE MIDWEST...

...2030Z Update...

No significant changes were made to either Marginal Risk area with
this update, as the forecast remains on track for isolated
instances of flash flooding in both areas. See the below
discussion for more details.

Wegman

...Previous Discussion...

...Florida...
There is a large reservoir of 2-2.25" PWs across the Sunshine
State with a stationary front draped over the region. At upper
levels, a deep trough over the Gulf of Mexico is providing some
sufficient divergence aloft while a large dome of high pressure in
the Caribbean will direct a upper level disturbance to approach
Florida from the South. NAEFS shows these PWs are at or slightly
above the 90th climatological percentile and MLCAPE values
reaching up to 2,000 J/kg. These parameters should be able to
support hourly rainfall rates up to 3"/hr in the most intense
storms. The other factor worth noting is vertical wind shear,
winds within the 1000-850mb level will be southeasterly but veer
and strengthen with height. This allows for surface-6km shear
values to climb as high as 20 knots. This could help keep some
storms around a little longer than the usual summer-time pulse
storms. Given these factors, chose to introduce a Marginal Risk
for Florida. This area will likely be tweaked depending upon QPF
trends in guidance over the next 24-36 hours.

...Midwest...
The closed low responsible for the Excessive Rainfall threat in
the Upper Mississippi Valley on Monday works its way southeast
towards the Great Lakes on Tuesday. A low pressure system will
funnel 850mb moisture flux on the eastern flank of its circulation
with PWS rising to around 1.25". MLCAPE is expected to range
between 500-1,000 J/kg and mean 1000-500mb RH values are likely to
average >80%. The slow progression of the closed upper low could
support training convection given slower storm motions, especially
as southeasterly low level winds on the eastern flank of the low
are oriented quasi-parallel north of the warm front. The 00Z HREF
did shows probabilities for 3-hr QPF > 3-hr FFGs as high as 40-45%
in parts of northern IL Tuesday afternoon. Some soils are a little
more sensitive than others, particularly north-central IL where
AHPS 7-day rainfall totals have been as high as 400-600% of
normal. Given these reasons, have chosen to introduce a Marginal
Risk for parts of the western Great Lakes region this forecast
cycle.

Mullinax

Day 3
Valid 12Z Wed Sep 27 2023 - 12Z Thu Sep 28 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AND OHIO VALLEY...

...2030Z Update...

...Ohio Valley...

A Marginal Risk area was introduced across portions of Ohio,
Kentucky, and small pieces of neighboring Indiana and West
Virginia with this update. A slow moving cutoff low will drift
eastward during the day Tuesday. Afternoon and evening convection
embedded within an airmass characterized with PWATs around 1.25
inches will develop ahead of/east of the cutoff low. The slow
movement of the forcing will support slow-moving/training storms
across portions of OH/KY. While the area has been much drier than
normal, the storms will have the potential to cause heavy rain,
which may persist for an extended period where storms are slow
moving or train over the same areas. Should that occur over an
urban center then flash flooding will be possible, albeit in
isolated instances.

The Marginal Risk area for portions of the Southeast remains
unchanged, with the previous discussion provided below.

Wegman

...Previous Discussion...

...Southeast...

A near carbon-copy forecast from Tuesday as there will still be a
pool of 2-2.5" PWs to go along with a stationary front draped over
Florida and an upper trough over the Gulf of Mexico. Wednesday
features a little more instability across the Gulf side of the
Florida rather than the Atlantic for now, so the focus for
Excessive Rainfall will stretch from southwest Florida on north to
southern Georgia and southern Alabama. As much as 500-1,000 J/kg
of MLCAPE will be present across central and northern Florida, and
with such a tropical air-mass in place, rainfall rates could
approach 3"/hr in the strongest storms. A Marginal Risk for
Excessive Rainfall remains in place as urbanized communities and
poor drainage areas are most vulnerable to possible flash flooding.

Mullinax


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