FOUS30 KWBC 120759
QPFERD

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

Day 1
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. 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

$$
--- SBBSecho 3.20-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)