Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Tue Mar 21 2023

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

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

...California...

Model guidance continue focus a strong AR into central/southern
California and downstream into Arizona as a closed low approaches
the California Coastline. Low level, southwesterly winds of 35 to
55 knots will direct PW values of 0.50 to 1 inch (~3 to 4 sigma)
onshore and forced up and over the terrain. Local enhancement will
drive higher rainfall rates, nearing 0.75 inch/hour, along coastal
SoCal and the adjacent Transverse and Peninsular Ranges and to a
somewhat lesser degree to the southern Sierra foothills. Consensus
depicts areal averages 2 to 3 inches, however hi-res guidance
suggest very localized maximums of 7 to 10 inches could material
for Southern California. These rain amounts are forecast over
locations that are already experience high streamflows and near
soil saturation. The strong wind associated with this system will
also increase the chance for downed trees and debris that may clog
storm drains in the many urbanized portions of the Moderate Risk
area. Rainfall amounts in the densely populated valleys and
coastal areas will generally range between 1.5 and 2.5 inches.
When adding runoff from the steep nearby mountains where rainfall
totals may exceed 4 inches, numerous flash floods are likely. The
timing of the heaviest rain also favors added impacts, as they
will likely be ongoing during the Tuesday morning commute. The
inherited Risk Moderate and Slight Risks were broadened a bit to
the north and east.

Further north into central California the inherited Slight and
Marginal Risks are largely unchanged. This is largely due to the
heaviest rain impacting southern areas, and snow levels may never
rise above 5,000 feet for the entire event. Thus, despite
obviously favorable antecedent conditions, the lower forecasted
rain totals and much of the precipitation falling as snow farther
down the mountains will greatly limit the flash flooding threat
with latitude. 

Campbell

...Central Arizona...

The abundant moisture from the AR 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. Minor reshaping of the Marginal and
Slight Risks were needed to reflect the latest QPF trends and WPC
forecast amounts.

Campbell



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...

Rain associated with the 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
will keep much of southern California and central Arizona
sensitive to additional precipitation. With soils likely reaching
or exceeding saturation and with stream flows already forecast to
be high, there will be a continued threat for excessive rainfall
and localized risk for flooding. Therefore a Marginal Risk for
both locations was raised for this period.

Campbell

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...

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