Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
737 PM EDT Sun Mar 19 2023

Day 1
Valid 01Z Mon Mar 20 2023 - 12Z Mon Mar 20 2023

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

Roth


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

...2030Z Update...

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

Wegman



Day 3
Valid 12Z Tue Mar 21 2023 - 12Z Wed Mar 22 2023

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

...2030Z Update...

...Southern California...
No significant changes were made with this afternoon's update. The
forecast remains on track that another atmospheric river (A.R.)
will track across CA and into AZ Tuesday through Tuesday night.
There are some notable similarities with this upcoming event
Tuesday and the last one which happened last Tuesday. Both events
appear to be progressive (meaning the A.R. driving the rainfall
will keep moving and not get stuck in any one place for very
long), and are forecast to produce similar amounts of rainfall
across the L.A. and San Diego areas, the adjacent mountains, and
along the Mogollon Rim of central AZ. Much of the flooding that
occurred last Tuesday was river flooding, with flash flooding
somewhat limited. It's worth noting that the snowpack in the
Transverse Ranges is very limited to small areas of the highest
elevations, so snowmelt is not expected to be a factor. Further to
that, snow levels are likely to be even lower with this upcoming
event than the last one. In Southern California, the warmest air
associated with the A.R. is only over the area for 6-12 hours at
most before it shifts east. Prior to the A.R. passage, snow levels
are around 6,000 ft, rise to 8,000 ft with the warmest air, then
crash to 4,000 ft behind the cold front. This will greatly limit
how much runoff in the mountains will drain into the lower
elevations with this A.R. The rain (and mountain snow) will
continue lightly behind the cold front with continued onshore
flow, so the 24 hour forecasted rain is likely overestimating the
amount of rainfall that has potential to be impactful in the form
of flash flooding.

...Central Arizona...
Unlike areas further west across Southern California, expect the
higher snow levels (around 9,000 ft) associated with the A.R. to
persist for much longer, which will allow the limited snowmelt
from the higher elevations along the Mogollon Rim to contribute
more to any flash flooding in the area. Rainfall amounts are
likely to be similar, with up to 3 inches of rain expected for the
higher elevations. Far less rain is expected further south and
west, including the Phoenix metro, so the greatest chances of
flooding will be north and east of Phoenix, where topography will
exacerbate any flooding impacts due to the fast flowing water.
This area appears to have the greater likelihood of needing future
upgrades as compared with CA, though again the atypical
consistency with total expected rainfall, with amounts not
changing too much run-to-run, still makes this scenario unlikely.

Wegman

...Previous Discussion...

The latest model guidance is in good agreement with a strong
closed low diving southeast day 2 across the Northeast Pacific and
inland into central California and the Great Basin on day 3. 
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, spreading
inland into the Southwest Tuesday night into early Wednesday.  The
National Water Model streamflow anomaly map continues to show high
flows to most of the streams 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.   There is fairly
good model agreement on heavy precipitation potential across the
slight risk areas of California, with widespread 1.25 - 2.5"+
values.  Given this the previous marginal risk area across these
regions was upgraded to slight.  No changes made to the marginal
risk area 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. 

Inland across central Arizona, the previous marginal risk areas
was also upgraded to slight.  850-700 mb moisture flux values
expected to become very anomalous late Tuesday into early
Wednesday, 4 to 5 standard deviations above the mean,  over the
Southwest as the strong upper trof moves inland from Central
California into the Great Basin.  Snow level expected to be high
with much of the precip into the Mogollon Rim area falling as
rain.  Stream flow and soil moisture anomalies are not as great
across central AZ as points farther west across California, with
values at or slightly above normal.  However, given the strong
south southwesterly upslope flow into the Mogollon Rim region and
resultant heavy precip potential, model consensus 1 to 2"+ areal
average totals, an upgrade to slight risk was warranted.

Oravec


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