Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Wed Mar 22 2023

Day 1
Valid 01Z Thu Mar 23 2023 - 12Z Thu Mar 23 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF CENTRAL SOUTHERN CALIFORNIA...

Scattered showers and thunderstorms are expected to continue into
the evening across portions of Central and Southern California as
cold air aloft and an upper trough associated with a remnant upper
low move across the region.  While widespread heavy amounts are
not expected, brief periods of heavier rainfall rates, resulting
in isolated runoff concerns cannot be ruled out.  Therefore, the
Marginal Risk was maintained across Southern California and
extended north along the coast into Central California.

Farther to the east, showers will continue to diminish as drier
air sweeps across the Southwest.  With additional significant
accumulations not expected, the Marginal Risk was removed from
Arizona.

Pereira 



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

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL OVER A PORTION
OF THE MID/SOUTHERN MISSISSIPPI VALLEY..

Introduced a Moderate Risk of Excessive Rainfall focused primarily
from northeast Oklahoma into southwest Missouri.  Still expect a
steady stream of moisture from the Gulf of Mexico as a deep-layer
trough ejects from the Southern Rockies and results in
strengthening low level flow.  Precipitable water values at or
slightly greater than 1.5 inches reaches eastern Oklahoma by
24/00Z.  At about the same time, upper level divergence from an
upper level jet aids in surface low pressure forming along a
quasi-stationary front extending from Oklahoma northeastward into
the Ohio/Tennessee Valley.  The Moderate Risk was focused mainly
in/near the Ozarks where 850 mb flow accelerates to 40 kts or more
overnight and results in increasing moisture flux convergence
along the boundary.  The 12Z HREF started to show some potential
for 1- and 3-hour rainfall amounts to exceed the respective flash
flood guidance thresholds in southwest Missouri around 24/00Z and
persist into the overnight.  In addition...there were a few
signals in the HREF for 2 inch per hour rates in the same area.
While this area is not as saturated as it was a couple of weeks
ago...there is local concern given that ground conditions are
still fairly moist.  Latest ensemble guidance showed precipitable
water values being (more than 2 std dev getting lined up nearly
parallel to the surface front so the potential for moderate to
locally heavy/excessive rainfall along the front extending.  Opted
not to extend the Moderate any farther east which has not had
quite as much rainfall recently and is not quite as hydrologically
sensitive.  The previously issued Slight and Marginal Risk areas
covered the area and only minor adjustments were made based on the
latest QPF and model mass fields. It is expected that the risk of
excessive rainfall will continue beyond the end of the Day 2
period.

Bann



Day 3
Valid 12Z Fri Mar 24 2023 - 12Z Sat Mar 25 2023

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

Maintained the Moderate Risk area without too many changes from
the issuance at 0830Z.  Showers and thunderstorms will persist
from the day 2 period in proximity to the sprawling frontal
boundary with moisture flux convergence increasing 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. Precipitable water values will surge up to 1.5 inches into
the Mississippi/Tennessee/Ohio Valleys...which is some (+2 to +2.5
standard deviations greater than climatology for late March. 
Repeating storms will track across areas soaked from recent rains
and will likely surpass local FFGs early into the Day 3 period.
The latest guidance depicts several inches of rain to fall in a SW
to Northeast orientation with above average confidence. This part
of the nation will be sensitive to additional rainfall, let alone
a large area with several inches of new accumulations. The
Moderate Risk area for excessive rainfall remained in effect from
northeast Arkansas to western Ohio with the Slight Risk area
stretches from the Arkansas/Louisiana border to western
Pennsylvania.

Bann



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