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FOUS30 KWBC 111555
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Tue Mar 11 2025

Day 1
Valid 16Z Tue Mar 11 2025 - 12Z Wed Mar 12 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHERN CALIFORNIA...

...16z Update...

No change to the inherited outlook, as observational and model
trends remain supportive of a localized flash flood risk across
portions of Southern CA, as described in the previous discussion
below.

There is also a non-zero threat for localized flash flooding this
afternoon/evening across southwestern and central portions of AZ,
as the deep layered low progresses eastward today. This will
provide anomalously high tropospheric moisture (PWATs of 0.75"
near the max moving average for YUM, per SPC sounding climatology)
and sufficient instability for convective initiation with
steepening lapse rates (MU CAPE ~250 J/kg). While localized
rainfall totals should be limited to 1-2" (per 12z HREF PMM), much
of this may fall in a very short period (as is typical over the
Southwest) with sub-hourly (15-min) accumulations of 0.25"+
possible (per the HRRR) with hourly rainfall rates of 0.5"+
expected (per 12z HREF 40-km neighborhood probabilities of 30-60%).
Relatively dry soils and short residence time should largely
preclude localized flash flooding (hence no additional introduction
of a Marginal risk).

Churchill


...Previous Discussion...

A deep layered low moving into southern CA will bring showers, and
even possibly an isolated thunderstorm, to the region today into
this evening. Showers will initially be on the lighter side,
however as the center of the low approaches steeper lapse rates
will support some modest destabilization and an uptick in shower
intensity between approx 21z-03z. The 00z HREF neighborhood
probability of exceeding 0.5" in an hour peaks between 60-70% just
offshore, with hourly rainfall even exceeding 0.75" in this
offshore activity. These stronger showers and isolated
thunderstorms will likely weaken as they move onshore...however
expect that hourly rainfall will still approach or exceed 0.5" on a
localized basis over land as well. Overall expecting total
rainfall amounts between 0.5" and 1" on a pretty widespread basis
over far southern CA, with localized totals of 1-2". These totals,
combined with the locally higher rates expected, support a
generally minor and localized flood risk today...mainly across any
more sensitive urban areas and/or sensitive terrain.

Chenard


Day 2
Valid 12Z Wed Mar 12 2025 - 12Z Thu Mar 13 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHERN CALIFORNIA...

The forecast generally remains on track, so only minimal changes
needed to the risk areas with this update. A deep mid level trough
and cold front will move across CA Wednesday into Wednesday night
bringing widespread rain and mountain snow to the region. The IVT
axis is rather narrow and quick moving and only forecast to peak
around 400-600 kg/m/s. When combining this modest strength and lack
of longevity of these higher values...this event will likely only
reach weak atmospheric river levels. This limitation will be
partially offset by the strength of the cold front and deep layer
forcing associated with the synoptic scale trough and upper jet.
Even so, rainfall totals over much of the state are unlikely to be
enough to cause much of a flood concern. Mainly looking at a
beneficial rainfall, although this is a strong cold front with a
well defined convergence axis along it, and this could drive some
locally higher rainfall rates, both along the coast and within the
Sacramento Valley where southerly flow could locally enhance
convergence further. These higher rates could be enough to result
in generally minor and localized flooding within urban and other
prone low lying areas.

More flood impacts are possible over southern CA, specifically
portions of Santa Barbara, Ventura and Los Angeles counties. South
to southwesterly upslope flow into the Transverse range will likely
help enhance rainfall over this area late Wednesday night into
Thursday morning. Also will likely have a bit better large scale
forcing overrunning the front at the base of the mid level trough,
helping steepen lapse rates and increasing rainfall rate potential.
Most of the weak instability is currently forecast to be post-
frontal, but could see just enough destabilization work into the
front to produce some localized convective elements within the
southward shifting rainfall axis.

We will likely get a better idea on peak rainfall rate potential
when the 12z HREF arrives today...however the 00z 3km NAM
indicates hourly rates over 0.5" and 3 hourly rainfall of 1-2". The
GEM reg has also shown some skill in past southern CA events at
depicting max rainfall potential...and it peaks at just over 2" in
3 hours between 06z and 12z. Thus still thinking hourly rainfall
peaking between 0.5" and 0.75" is likely, particularly within areas
of terrain. Although even along coastal areas convective elements
near the front could locally push rates towards 0.5" in an hour.
This is a quick moving system, and thus these elevated rates should
only last at most a few hours, but this could be enough to locally
increase the flash flood threat over the more sensitive burn scars
and any flood prone urban areas. Thus a Slight risk remains
warranted and localized significant debris flows are possible.

Chenard


Day 3
Valid 12Z Thu Mar 13 2025 - 12Z Fri Mar 14 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHERN CALIFORNIA...

The event described in the day 2 discussion will be ongoing at 12z
Thursday over southern CA. Rainfall rates locally over 0.5" an
hour will remain possible along the southward moving cold front
between 12z and 18z. The front will remain fast moving, so the
flood risk should be over by 18z. The IVT magnitudes generally are
sustained through Thursday morning and the approach the the mid
level trough axis and shortwave energy at the base of the trough
should help enhance synoptic ascent over the front. Thus expect
the area of showers and embedded heavier convective elements to
generally maintain intensity as it drops through far southern CA.
Rainfall amounts of 1-3" are possible in the terrain, with even
0.75"-1.5" across lower elevations. This should be enough to
maintain a flood risk across this region, with sensitive urban
areas and burn scars most susceptible to flood impacts. Rainfall
on day 1 (Tuesday) may be enough to increase soil saturation and
streamflows, which could result in a bit more coverage of impacts
with this 2nd system Thursday morning. Given these factors, we went
ahead and upgraded to a Slight risk over this region.

Chenard


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