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

Day 1
Valid 01Z Wed Mar 22 2023 - 12Z Wed Mar 22 2023

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

...0100 UTC Update...
Overall, made only minor updates with no significant changes to
the previous outlook.


Previous Discussion...
...Central and Southern California...
The plume of deeper moisture that brought locally heavy
precipitation to portions of Southern California has now shifted
farther south into northern Baja California.  While drier air to
the north offered a short reprieve, shower and thunderstorm
activity supported by colder air and energy aloft is beginning to
move into Central and Southern California.  This activity is
expected to continue through the evening and into the overnight,
with additional locally heavy amounts expected along the favored
terrain.  Localized rainfall rates of 0.5 in/hr are expected,
fueling scattered additional totals of 1-2 inches along the
Central to Southern California coast.  Given the showery,
progressive nature of these storms, guidance is suggesting a more
isolated excessive rainfall threat.  This along with rainfall
totals from the initial round that were generally less than
expected, opted to remove the Moderate Risk with this update. 
However, the Slight Risk was left largely unchanged, noting that
instances of flash flooding are considered possible through the
evening and into the overnight, especially near burn scars and
other sensitive areas.

...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.

Pereira/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
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...

...2030Z Update...
Models continue to agree on the idea of a stripe of moderate to
potentially excessive rainfall extending from the Southern Plains
into the Ohio Valley...but the spread in the model guidance
remains large in terms of placement. Made a couple of minor
adjustments on the southwest end and the northeast end of the
Slight Risk areas but left the core of the Slight Risk area
largely in tact.  Much of the forecast reasoning below remains
valid.

Bann

...0830Z Excessive Rainfall Discussion...

The cold front associated with the AR during the D1 and D2 periods
will be tracking across the central and southern states while
shortwave energy ejects further north across the Plains. This will
result in a long fetch of moisture stream in along and ahead of an
elongated front, spanning from the Great Lakes region and trailing
southwest to Mexican border/southern Desert Southwest. This
boundary will provide a focus for showers and thunderstorms to
develop and track eastward. The best forcing and lift will likely
align near the Ozarks and central Mississippi Valley as this is
where the probability of exceeding 2-3 inches is the greatest.
Model consensus show areal averages of 2 to 4 inches. A broad
Slight Risk was inherited from the experimental Day 4 and still
encompassed where there will be an elevated threat for excessive
rainfall and isolated to scattered flooding. The Slight spans from
the Oklahoma/Texas border to western Ohio. A Marginal Risk
encloses the Slight, stretching from northeast Texas to western
Pennsylvania.

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