Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Mon Mar 20 2023

Day 1
Valid 12Z Mon Mar 20 2023 - 12Z Tue Mar 21 2023

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell


Day 2
Valid 12Z Mon Mar 20 2023 - 12Z Tue Mar 21 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS COASTAL
AREAS OF SOUTHERN CALIFORNIA AND CENTRAL ARIZONA...
...Southern California...

A strong closed low diving southeast across the Northeast Pacific
and inland into central California and the Great Basin will direct
another atmospheric river (AR) into California and Arizona through
Tuesday and the overnight hours. Strong 850 to 700 mb moisture
flux expected on the south side of this closed low, impacting
areas from the central California coast range , southeast through
the Transverse and Peninsular Ranges and into the southern Sierra
during the day on Tuesday, progressively spreading inland into the
Southwest Tuesday night into early Wednesday . The National Water
Model has been showing several streams with high flows  across the
central California coast range, southeast into coastal southern
California, including the Transverse and Peninsular Ranges, which
coincides with anomalous soil moisture values as per NASA SPoRT
maps. The latest guidance continue to depict a broad area of 1 to
3+ inches over the southern to central Valley/foothills of the
Sierra with the heaviest precip here expected to fall as snow in
the Sierra, with no significant contribution to runoff from snow
melt expected. The snowpack in the Transverse Ranges is very
limited to small areas of the highest elevations which will
greatly limit or negate any snowmelt contributing to any flooding.
The Marginal and Slight Risk areas remain in good order but
required minimal adjustments to reflect the latest WPC QPF and
trends.

...Central Arizona...

Higher snow levels are expected for the terrain of central Arizona
as the AR reaches the area, which will limited snowmelt along the
higher elevations of the Mogollon Rim to contribute more to any
flash flooding in the area. With PW anomalies creeping up to +_4
and +5 standard deviations, most of the guidance are signaling up
to 3 inches of rainfall for the higher elevations, with lesser
amounts forecast for locations to the south and west. The
conditions for excessive rainfall and rapid runoff will be more
favor for points north and east of the Phoenix metropolitan area.
The inherited Slight and Marginal Risk areas remain reflective of
the level of threat for flooding concerns. Therefore, no
adjustments were made with this issuance.

Campbell


Day 3
Valid 12Z Wed Mar 22 2023 - 12Z Thu Mar 23 2023

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell


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