Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Wed Mar 22 2023

Day 1
Valid 12Z Wed Mar 22 2023 - 12Z Thu Mar 23 2023

...THERE IS A MARGINAL FOR EXCESSIVE RAINFALL ACROSS SOUTHERN
CALIFORNIA AND CENTRAL ARIZONA...

Lingering rains from a departing AR will continue as the upper
trough tracks from southern California into the Desert Southwest.
New accumulations of 1 to 2 inches (mainly focused in/near the
terrain) that fall on areas that are already hydrologically
sensitive to additional precipitation will keep the potential for
excessive rainfall going over portions of southern California and
central Arizona.

Campbell



Day 2
Valid 12Z Thu Mar 23 2023 - 12Z Fri Mar 24 2023

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE SOUTHERN
PLAINS TO THE OHIO VALLEY...


A steady stream of Gulf of Mexico moisture will surge northward
along a frontal boundary spanning from the Great Lakes region
southwest to New Mexico and produce widespread rain from the
Southern Plains to the Ohio Valley. Low level flow transporting PW
values of 1.25 (+2 std dev) into the heart of the nation will be
nearly parallel to the frontal boundary. A southwest to northeast
orientated swath of QPF is expected to setup from Texas to
Pennsylvania with some of the highest amounts aligning in a narrow
band from eastern Oklahoma to Indiana. The 3-hr FFGs across the
general area are 1.5/2 inches with areal averages of 1 to 3+
inches forecast. There were already Slight and Marginal Risks for
excessive rainfall in effect. Given the latest trends and WPC QPF,
the Marginal Risk and Slight Risk areas were expanded south and
west across parts of Oklahoma, Texas and Louisiana. The Marginal
Risk was expanded a few counties southward from eastern Kentucky
to West Virginia. The eastern bounds of the Slight Risk was
narrowed a bit to be near/south of I-70 in Indiana and Ohio.

Campbell

Day 3
Valid 12Z Thu Mar 23 2023 - 12Z Fri Mar 24 2023

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FROM THE OZARKS
TO THE OHIO VALLEY...

Showers and thunderstorms will persist from the day 2 period in
proximity to the sprawling frontal boundary. Moisture convergence
will increase across parts of the Mississippi Valley and into the
Ohio Valley as a surface low pressure with trailing cold front
moves east across the Southern Plains. PW values will surge up to
1.5 inches (+2 to +2.5 Std dev) into the
Mississippi/Tennessee/Ohio Valleys. Repeating storms will track
across areas soaked from recent rains and will likely surpass
local FFGs early into the D3 period. The latest guidance depicts
several inches of rain to fall in a SW to Northeast orientation
with above average confidence. The EC ensemble means are showing a
sizable area from Arkansas to Ohio with a 20% probability of
exceeding 3 inches with a few speckles that have a 5% probability
of 5 or more inches of rain in a 24 hour period. This part of the
nation will be sensitive to additional rainfall, let alone a large
area with several inches of new accumulations. A Moderate Risk
area for excessive rainfall and flooding is now in effect from
northeast Arkansas to western Ohio. A Slight Risk stretches from
the Arkansas/Louisiana border to western Pennsylvania. A Marginal
Risk area covers eastern Texas to western Maryland.

Campbell





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