Mesoscale Precipitation Discussion 1231
NWS Weather Prediction Center College Park MD
700 PM EDT Wed Sep 09 2026

Areas affected...Southern CA

Concerning...Heavy rainfall...Flash flooding possible

Valid 092300Z - 100400Z

SUMMARY...Isolated flash flooding may develop along portions of
the Transverse and Peninsular Ranges of southern CA into the
southern Sierra Nevada through this evening. Peak hourly rainfall
over 1 inch will be possible with 1-2 inch totals through 04Z.

DISCUSSION...GOES West visible imagery and regional radar
reflectivity as of 2240Z showed isolated showers/thunderstorms
over eastern Tulare County of the Sierra Nevada and along the
Transverse and Peninsular Ranges, with attempts at additional
development with areas of towering cumulus. The environment across
southern CA contained highly anomalous moisture with the 12Z
soundings from NKX and VBG showing precipitable water values at or
above the SPC climatological max for early September. SPC
mesoanalysis data from 22Z showed MLCAPE of approximately 500-1000
J/kg over Orange, San Diego and western Riverside counties with
lower values of near 100-250 J/kg into Ventura County where a pair
of backbuilding cells was located.

While forcing is weak, outside of the elevated heat sources of the
mountains, southern CA's placement beneath the western edge of a
mid to upper-level ridge has resulted in very slow cell motions of
about 5 kt. With still some time for daytime heating, additional
shower/thunderstorm development will be possible over the next
couple of hours with potential for peak hourly rainfall over 1
inch and totals of 1-2 inches from the southern Sierra Nevada into
the Transverse and Peninsular Ranges. Any cells are expected to
remain isolated to terrain features.

Most of the CAM guidance suite has had a good handle on the
ongoing activity but the coverage of heavy rain due to isolated
thunderstorms/showers should remain low. However, within the lower
coverage of storms, there will be some limited potential for
hourly rainfall in excess of 1 inch and isolated flash flooding
potential. The loss of daytime heating after sunset should allow
any ongoing storms to rapidly dissipate.

Otto

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

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

LAT...LON   36831824 36611802 35811797 35441817 35191827
            35041842 34921846 34811839 34751822 34701798
            34641776 34571746 34541721 34491696 34451679
            34381655 34331635 34261609 34111584 34001565
            33861555 33761554 33691567 33691587 33771603
            33861627 33861638 33771644 33631641 33471626
            33331620 33201620 33041607 32971604 32711600
            32601607 32531635 32521653 32581661 32901677
            33311699 33551712 33851708 33951709 34061716
            34111733 34121768 34181794 34241815 34331842
            34411865 34431894 34461922 34521943 34601960
            34671966 34801965 34901953 34961936 35001913
            35021879 35291858 35651847 36261847 36491855
            36761850