543
FOUS30 KWBC 121549
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1149 AM EDT Wed Mar 12 2025

Day 1
Valid 16Z Wed Mar 12 2025 - 12Z Thu Mar 13 2025

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

16Z Update: Current forecast SLGT risk across Southern CA remains
on track as there has been little run-to-run variance of locally
heavy rainfall totals in the targeted areas. The expectation is for
the cold frontal rain band to make headway down the CA coast this
period with the orientation of the low-level flow situated out of a
southwesterly alignment generating better orographic enhancement to
the precip field when acting against the coastal terrain and
windward portions of the Sierra`s. This is a pretty classic case
for SoCal flash flood prospects, heightened more than usual due to
the remnant burn scars prevalent within the San Gabriel`s and Santa
Monica Mtns. with emphasis on the Palisades and Eaton fire burn
scars. Current FFG`s within those smaller zones are ~0.5"/hr for
runoff prospects and between 0.75-1" elsewhere within the coastal
topography. 12z HREF prob fields are still generally elevated for
hourly rates >0.5"/hr (60-80%) during the time frame between 00-12z
in the D1 period which would correlate with better flash flood
concerns, especially in the areas above. The prospects for >1"/hr
rates are generally non-existent within the means with a very low
prob output via the latest HREF output (<15%). This generally means
there is a good handle on the max potential within this anticipated
period of impact, and the relatively quicker forward propagation
should limit the upper-end of any excessive rainfall thresholds.
Considering the above variables and general continuity within the
CAMs, global deterministic, and subsequent ensembles, the
previous SLGT risk was generally unchanged with the primary focus
on those more prone burn scar areas.

Further north across Northwestern CA into the foothills of the
Sierra`s, a low-end probability for urban and small stream flash
flood exists as the same frontal rain band discussed above
progresses through the rest of the state with a short window for
heavier rainfall as the frontal progression shifts south.
Orographic enhancement within any coastal ranges and the Sierra`s
will be the primary driver for those elevated rain rates capable of
producing some localized flooding concerns with complex topography.
The San Francisco/San Jose/Oakland area will be one urbanized zone
of interest through the course of the D1 given the local
susceptibility for flash flooding to occur in a hillier, more urban
zone. For more information on the flash flood threat across
Northern California, please see MPD #058. The previous MRGL risk
across the above zones was maintained with little to no change in
the risk area.

Further south into Los Angeles, a similar situation with the
urban corridor generally in a better spot for flash flooding just
due to the nature of lower FFG`s and the IVT pulse protruding a bit
further inland once south of Big Sur. Intra-hour rain rates will
once again be the driver for the flash flood threat in the area as
0.5-0.75"/hr is forecast over the metro. Local 1-2+" of total rainfall
is forecast within the LA domain, enough to warrant the area to be
within the SLGT risk with MRGL`s extended down the coast once you
get south of Long Beach.

Kleebauer

..Previous Discussion..

The forecast generally remains on track, so only minimal changes
needed to the risk areas with this update. The greatest flood risk
with this event remains over southern CA Wednesday evening into
Thursday morning. The 00z HREF continues to indicate a high
probability of hourly rainfall locally exceeding 0.5" moving into
the Transverse Range of Santa Barbara and Ventura counties as early
as ~04z, and then spreading into the San Gabriel Mountains of Los
Angeles county closer to 08z. While the probability of 0.5"+ an
hour hourly rainfall is high, the coverage of these more intense
rates will likely be localized in nature, as shown by the HREF 0.5"
per hour ensemble agreement scale probabilities (more of an
indication of coverage) which peak in the 10-15% range over the
aforementioned terrain areas.

Most of the 00z CAMs indicate a narrow line of shallow convection
moving south across these areas associated with strong cold
frontal convergence and about 100-200 j/kg of CAPE. This feature,
combined with orographic enhancement, should locally push hourly
rainfall into the 0.5" to 0.75" range over the aforementioned
terrain of Santa Barbara, Ventura and Los Angeles counties. Rates
should tend to be a bit lower outside of the terrain, although the
shallow convective line could locally push hourly rainfall over
0.5" even along lower elevation coastal areas. This axis of heavier
rain will be quick moving and only last about 2-3 hours at any one
location...which will limit total rainfall amounts and the
magnitude/coverage of the flash flood threat. However, even with
the quick movement, rainfall rates could be high enough to result
in locally significant flood impacts near recent burn scars, and
areas of minor flooding in other more flood prone urban or low
lying areas.

Elsewhere across the state of CA the deep mid level trough and
cold front will bring 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
central and northern CA 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 areas of steep terrain along with urban
and other prone low lying areas.

Chenard


Day 2
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. A shallow convective line along the
cold front will continue to support rainfall rates locally over
0.5" between 12z and 15z, with the quick frontal motion meaning
most of the higher rates should abate after 15z. 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, and this is supported by the 00z HREF.

While rainfall totals will likely be higher in the terrain where
upslope flow enhances ascent, the southward dropping line of
shallow convection should support rainfall rates locally over 0.5"
an hour even over coastal areas. These higher rates (albeit brief)
support some flood risk over this region, with sensitive urban
areas and burn scars most susceptible to flood impacts. While
rainfall on Tuesday averaged only between 0.25"-0.75", the
resulting modest soil saturation and streamflow increase could
still play a factor in locally increasing impacts when this next
round of heavier rates moves through early Thursday.

Chenard


Day 3
Valid 12Z Fri Mar 14 2025 - 12Z Sat Mar 15 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
LOWER TO MIDDLE MISSISSIPPI VALLEY INTO PORTIONS OF THE OHIO AND
TENNESSEE VALLEY...

A strong mid level low and shortwave energy ejecting east out of
the Plains and into the MS Valley will likely trigger strong to
severe convection by Friday afternoon across portions of MO, AR and
IA. This activity will then spread eastward into portions of IL,
IN, KY, TN and MS Friday evening into the overnight hours. Moisture
is initially quite limited with this system although we eventually
see a narrow corridor of higher PWs advect northward. Cell motions
are also likely going to be quite fast off to the east northeast
given the very strong wind fields in place. Thus this convection
will likely pose more of a severe risk than it will a flash flood
risk. However this is a very dynamic system with sufficient
instability for deep convection, and so expect that rainfall rates
will briefly be intense enough that we will see swaths of rainfall
that exceed current model forecasts (maybe locally over 2").
Nonetheless, this still appears to be a lower end flood threat, and
probably confined to localized impacts over any more sensitive
urban or low lying areas that pick up a quick burst of intense
rainfall rates.

The southern end of the Marginal risk over MS will see an uptick
in convection Saturday morning. The front should slow/stall over
this region with shortwave energy approaching within the base of
the longwave trough helping enhance moisture transport and large
scale ascent. Most models suggest an uptick in the flash flood risk
from possible training convection in the 12z-18z Saturday
timeframe (so just after this day 3 period), but can not rule out
some impacts beginning just before 12z, as these events can often
grow upscale a bit earlier than forecast.

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