Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Mon Nov 17 2025
Day 1
Valid 16Z Mon Nov 17 2025 - 12Z Tue Nov 18 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF
SOUTHERN CALIFORNIA...

16z update:
Current observational trends and 12z CAM guidance continue to
solidify the current thinking and placement of ERO areas and
magnitudes moving forward through 18.12z.

Core of the warm conveyor belt/atmospheric river is starting to
reach southwest California and rounding Point Conception. Weakly
confluent 850-700mb 35-45kt southerly flow along the front and 1 to
1.25" total Pwats (around .5" in the same layer) is resulting in
400-500 kg/m/s IVT values. Though the moisture plume is narrowing
and starting to advance with upstream height-falls, the onshore
flow will intersect the Transverse Range more ideally to maintain
solid rainfall rate potential over .5"/hr (HREF probs of 70-90% on
the Transverse and 30-40% on the Peninsular Ranges). The forward
progress is likely to limit overall totals along the spine of the
terrain to 1.5-2" with an isolated 2.5" not out of the realm of
possibility. Combine this with higher soil saturation in the 55-80%
range per NASA SPoRT 0-40cm relative soil moisture values (which
are well over the 95-98th percentiles), suggestive of enhanced
runoff capability and continued flooding potential. Even urban
locations of Southern California will remain with .5-.75"
potential total in short duration maintaining solid Slight Risk
level coverage for urban flooding concerns.

Downstream, early morning (18.06z-18.12z) south to north moving
thunderstorms given modest moisture and 500+ J/kg of MUCAPE through
the Lower Colorado Valley will have some potential for enhanced
rates capable of an isolated incident of flash flooding and so
little change in placement/thinking with the downstream Marginal
Risk into S NV and W AZ as well.

Gallina


~~~~Prior Discussion~~~~
A vigorous mid-level wave (initially centered near 39.9N, 125.5W
at the start of the forecast period) will migrate south-
southeastward along the California coast today while cutting off
from westerlies farther north. As this occurs, strong ascent
(associated with areas of 0.3-0.6 inch/hr rain rates in northern
California this morning) will spread southeastward along a cold
front and also within a robust upslope regime on the western sides
of the Sierra. 1-1.2 inch PW values near the coast and aforementioned
synoptic factors will foster several areas of 1-2 inch rainfall
totals, with locally higher amounts possible across the Transverse
Ranges of southern California.

The aforementioned front will result in a focused axis of heavy
rain that will translate from north to south along the southern
California coast. Across the Slight Risk area, this heavy rainfall
will occur over both sensitive locales (from recent burn scars and
debris flows) and urban/hydrophobic surfaces. The greatest
concentration of flash flood risk will likely occur across the
Transverse Ranges and adjacent areas of Los Angeles Metropolitan
area through tonight.

Outside of this area, models indicate very isolated, but slow-
moving convective activity beneath the upper low center across
northern and central California, with heavy rainfall potentially
falling over soaked ground conditions and burn scars. Isolated
flash flood potential cannot be ruled out here.

Lastly, the cutting off upper low will spread strong mid/upper
difluence across the Lower Colorado River Valley especially in the
03-12Z timeframe. Models are consistent in developing at least one
or two bands of convective structures that drift northward across
the area fairly quickly, but also result in localized training. One
or two instances of flash flooding cannot be ruled out in this
regime.

Cook

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt