450
FOUS30 KWBC 222013
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
413 PM EDT Sun Sep 22 2024

Day 1
Valid 16Z Sun Sep 22 2024 - 12Z Mon Sep 23 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID MISSISSIPPI VALLEY AND THE SOUTHERN HIGH PLAINS...

The focus for areas of heavy rainfall will continue to be within a
plume of deeper moisture ahead of a large-scale upper trough
making its way out over the plains towards the Mid-Mississippi
Valley. The airmass already in place had precipitable water values
generally at or above 1.75 inches...from the western portion of
Texas northeastward to where it encounters a cold front moving in
from the central Plains/Corn Belt. Maintained the Slight Risk area
over western Texas, driven by diurnal instability strong enough to
support spotty 2 or 3 inch rainfall amounts and rainfall rates
topping 1-2 inches per hour per the latest 12Z HREF neighborhood
probs. FFG values are around 2.5"/hr but some CAMs do show ~3"/hr
rates at times within/near the Slight Risk outline. Also extended
the risk area northeastward into the OKC Metro area per some
heavier CAM signals and coordination with WFO Norman.

Over southern MO eastward, surface front will help focus moisture
convergence in the warm sector but the upper support is not
expected to be as strong as it was on Saturday. Still, 12Z sounding
from SGF does show plenty of moisture (fully saturated profile, PW
~1.8" or around the 95th percentile). Morning activity has pushed
through central MO into IL and have trimmed/shifted the Slight Risk
outline to align better in the more unstable air/better
instability farther south where there is more clearing. Models have
trended a little less with QPF in this area, but still feel there
is a low- end Slight Risk threat for at least southern MO and just
into northern AR. 12Z CAMs show some 1-2"/hr (even 3"/hr) rates
this afternoon and 1-hr FFG values are as low as 1.50-2" in this
region (southern MO into northern AR).

Fracasso/Bann

Day 2
Valid 12Z Mon Sep 23 2024 - 12Z Tue Sep 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-MISSISSIPPI VALLEY EASTWARD TO THE CENTRAL APPALACHIANS...

2030Z Update...Expanded the Marginal risk area eastward to the
central Appalachians into areas of lower FFG values (~1.5"/hr) and
12Z HREF neighborhood probs of exceeding that around 10-25%. First
guess fields also suggest this expansion. Otherwise, no change to
the overall thinking but will have to watch southern MO for any
potential upgrade depending on D1 rainfall and the next set of CAM
guidance.

Fracasso

Previous discussion from 0830Z...

Surface low pressure begins to develop on Monday as shortwave
energy moving through the southern stream trough...leading to
renewed showers and thunderstorms in the Mid-Mississippi Valley
early in the day that shifts into the Ohio Valley by late Monday
night/early Tuesday morning. The best overlap of ingredients and
forcing looks appears to be from the afternoon into the early
evening across portions of southeast Missouri into parts of
southern Illinois. After that...low level flow should take on an
increasing component parallel to the front. Between the weakening
magnitude of moisture flux convergence at the front and the loss of
daytime heating/destabilization...rainfall rates should be waning.
There are a few ensemble members which generate over an inch of
rain over the Ohio Valley as upper flow backs in response to energy
in the northern stream...but the expectation is that the flash
flooding risk is minimal with those rainfall rates and dry
antecedent conditions.

Bann


Day 3
Valid 12Z Tue Sep 24 2024 - 12Z Wed Sep 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE EASTERN OHIO VALLEY AND ACROSS CENTRAL TEXAS...

2030Z Update... Expanded the Marginal Risk for the Ohio Valley into
the central Appalachians based on uncertainty in the latest
guidance, more or less matching near the new first guess fields as
well. For Texas, shrank the area a bit to concentrate on the lower
FFG areas but the QPF signal continues to decrease. This area could
be removed in future cycles.

Fracasso

Previous Discussion from 0830Z...
Ohio Valley... As energy in the southern stream interacts with
digging energy in the northern stream late on Day2...the flow
begins to back and become diffluent leading to increasing coverage
of rainfall by the time Day 3 begins. The flow should continue to
become increasingly diffluent aloft with precipitable water values
now about 1.75 inches in place. Spaghetti plots of QPF from the
ensembles show minimal support for amounts greater than 2
inches...and most of that support is from just a few GEFS members.
With antecedent conditions being so dry...it is presumed that only
isolated instances will occur that run off problems occur.

Texas
Light southeasterly on-shore flow across far southern Texas will
start to draw moisture northward again on Tuesday afternoon. As it
does so..a weakening cold front will help provide some focus for
showers and a few thunderstorms in central or western Texas. The
NAM and ECMWF were the only two operational runs from the 22/00Z
model suite to generate 2+ inch amounts of QPF while the GFS had
support from the GEFS favoring a weaker/drier solution. Given the
uncertainty...opted to maintain a Marginal risk for this cycle.

Bann


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