FOUS30 KWBC 220834
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
434 AM EDT Sun Sep 22 2024

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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID MISSISSIPPI VALLEY TO 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 northern United States. Latest model runs still support two
areas of more concentrated rainfall warranting a Slight Risk area--
namely over western Texas and from southern Missouri into southern
Illinois. The southern area should be more driven by diurnal
instability strong enough to support spotty 2 or 3 inch rainfall
amounts and rainfall rates topping an inch per hour. The area
farther east will have the added advantage of a surface front to
help focus moisture convergence but the upper support is not
expected to be as strong as it was on Saturday. For that
reason...confidence in occurrence is somewhat diminished. Made a
small southward adjustment to the Slight Risk area based on latest
guidance and a trimmed a bit of the northern periphery of the
Marginal given the placement of the cold front.

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...

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 OF WESTERN TEXAS...

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
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