Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Tue Mar 21 2023

Day 1
Valid 16Z Tue Mar 21 2023 - 12Z Wed Mar 22 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS COASTAL
AREAS OF SOUTHERN CALIFORNIA...

...1600 UTC Update...
Morning radar showed a compact area of low pressure off the
California coast moving northeastward. With the amount of on-shore
flow accompanying the system and the depiction of rainfall
rates/coverage on radar...opted to extend the Marginal Risk area
northward a bit along the coastline.  No changes were made
elsewhere in California.  The latest HREF has been advertising the
potential for 1 inch per hour rainfall rates persisting into the
19Z or 20Z time frame before decreasing as the moisture plume
shifts south and eastward. Models still show another wave of
rainfall to develop and move inland across Southern California
later.  As a result...felt staying the course with a Moderate was
still warranted (especially in urbanized areas)...as southwesterly
low-level winds of 35 to 55 knots will continue to transport an
airmass with precipitable water values of 0.50 to 1 inch (~3 to 4
sigma) onshore and into a region of complex terrain.  Consensus
depicts areal averages of 2 to 3 inches for the entire
event...however hi-res guidance suggest isolated maximum amounts
in excess of 6 inches could materialize with the potential for
flooding being worsened by run-off off from nearby areas of steep
terrain. 

...Central Arizona...

The abundant moisture from this atmospheric river will usher in
heavy rains and mountain snow to the state. With the snow levels
hovering near 8,000-9,000 ft and the progressive nature of the
heavy rain will limit total rainfall, and therefore reduce
resultant impacts. Areal averages of 1 to 3+ inches are expected
before the cold front quickly pushes through. No modifications to
the Marginal and Slight Risk areas were needed after the minor
reshaping done to the areas overnight and given consistent QPF
from the models and from WPC.

Bann



Day 2
Valid 12Z Wed Mar 22 2023 - 12Z Thu Mar 23 2023

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

The models still suggest that rain associated with this AR will
persist during this period as the upper trough tracks from
southern California into the Desert Southwest. Although rain rates
will not be as impressive as those expected for the day 1 period,
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 on Wednesday.

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