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

Day 1
Valid 16Z Wed Mar 22 2023 - 12Z Thu Mar 23 2023

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

Maintained the on-going Marginal Risk area for portions of
southern California and central Arizona.  Additional rainfall is
expected on and off throughout the day and into the evening across
this area as an upper low initially along the central California
coast fills and the lingering trough axis with cold temps aloft
tracks into the Desert Southwest later tonight.  The latest HRRR
and ARW both generate upwards of 500 J per kg of surface-based
CAPE during the afternoon as low/mid level lapse rates steepen
while moisture continues to be transported inland by west to
southwest flow at 15 to 30 kts.  Additional rainfall of an inch or
so (mainly focused in/near the terrain) will be falling on areas
that are already hydrologically sensitive to additional
precipitation. 

Bann




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

The Day 3 outlook will be updated by 2030Z.


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