Mesoscale Precipitation Discussion 1226
NWS Weather Prediction Center College Park MD
335 PM EST Sat Nov 15 2025

Areas affected...portions of southern CA

Concerning...Heavy rainfall...Flash flooding likely

Valid 152033Z - 160230Z

SUMMARY...The flash flood threat will continue for coastal CA into
the Transverse and Peninsular Ranges through 02Z. While the
main/widespread threat of flash flooding is expected to wane over
the next couple of hours, a lingering threat for more localized
but potentially significant flash flooding will remain.

DISCUSSION...Radar mosaic imagery at 20Z showed a band of moderate
to heavy rain that extended from the southern San Joaquin Valley
into some of the Valley/Desert regions east of the Peninsular
Ranges. This axis of heavier rain has been following a plume of
highly anomalous moisture translating eastward as seen on OSPO
ALPW imagery, out ahead of a negatively tilted mid to upper level
shortwave trough axis and vorticity max just west of the CA/MX
border. Additional heavy rainfall appeared in the waters just east
of Santa Catalina and San Clemente Islands, tied to the eastern
lobe of the mid/upper level shortwave. Rainfall over the past 6
hours has peaked near 1 inch for coastal locations into the
Peninsular Ranges while 1 to 2+ inch values have been observed
within portions of the Transverse Ranges, with hourly rainfall
between 0.5 and ~1.0 inches.

While relatively drier air was moving into southern CA as of 20Z,
the moisture remains anomalous with PW values of ~1.0 to 1.3
inches and despite continued drying of the layer, sufficient
moisture will remain through the evening to support localized high
rainfall rate potential. As the closed upper low center continues
to slowly edge closer to the coast, 700-500 mb lapse rates will
increase into the 6.5 to 7.0 C/km range and support localized
MLCAPE values in excess of 500 J/kg near the coast. Lift will also
be aided by left-exit region divergence/diffluence corresponding
to a 100 kt upper level jet max positioned south of the upper low
center. Hourly rainfall in the 0.5 to 1.0 inch range is expected,
though localized spot training could exceed these values and/or
support 0.5 to 1 inch of rain in about 30 minutes. The combination
of continued forcing and increased ground sensitivity due to
today's rainfall will likely support continued areas of flash
flooding over the next few hours. This will especially be true
where additional areas of heavy rain overlap with urban areas and
burn scars where localized higher impacts could occur. Within
sensitive burn scar regions, high rain rates are likely to result
in mudslides and debris flows.

Otto

ATTN...WFO...HNX...LOX...SGX...VEF...

ATTN...RFC...RSA...NWC...

LAT...LON   35541966 35531858 35111811 34381686 33961642
            33361612 32881611 32461613 32451668 32421710
            32521739 32741783 33201827 33431858 33891947
            34462098 35302118