Subj : Flood Potential CA To : All From : Mike Powell Date : Fri Nov 21 2025 09:38:12 AWUS01 KWNH 211331 FFGMPD AZZ000-NVZ000-CAZ000-211930- Mesoscale Precipitation Discussion 1240 NWS Weather Prediction Center College Park MD 831 AM EST Fri Nov 21 2025 Areas affected...Inland Deserts of Southern California and Lower Colorado River Valley... Concerning...Heavy rainfall...Flash flooding possible Valid 211330Z - 211930Z SUMMARY...Prolonged moderate rain with embedded very weak showers capable of .3-.5"/hr rates. Scattered 1-1.5" totals pose longer duration flash flooding concerns (3-6hr exceedance). DISCUSSION...GOES-W WV suite shows core of deep upper-low has wobbled over the LA Basin, but a strong upstream shortwave along the western periphery of the upper-level closed low is digging sharply resulting an combined effect of tightening the overall flow across the deep cyclone while keeping the downstream older center stationary over the next 6-12hr. As a result, the core of the Atmospheric River will remain focused on NW Baja California before cyclonically turning (fairly sharply) through the Imperial Valley into the Mojave Deserts back toward the Tehachapi Range and southern San Joaquin Valley. CIRA LPW prior to the local outage showed a fairly strong sfc to 850 and 850-700mb signal with 95th-99th percentile signals in each layer, combine that with some increase in the surface to 850mb layer from favorable southerly flow off the warm Sea of Cortez and total PWats remain at or slightly above 1" into the Imperial Valley and as high as .75" into the San Joaquin Valley. VWP and RAP analysis suggest strong cyclonic curvature but also 20-30kts of flux. So while there is sub-100 J/kg of instability, the vertical ascent through the strengthening western branch of the TROWAL suggests highly anomalous flux to maintain moderate showery activity through the deserts. Rates of .33"/hr are more probable, but an isolated rate of .5" is not impossible. Still, persistent weakly banded features will result in tracks of 1-1.5" totals in a 3-6hr time period, so any isolated, enhanced vertical cell within the core may result in quicker stream/arroyo rises. FFG values being only .5"/hr and less than 1"/3hr and less than 1.5-2"/6hrs) suggest a few scattered incidents of 'flash' flooding are considered possible through early afternoon. While less certainty, there are CAMs suggesting some filtered insolation may be possible in the morning. Near zero inhibition through the profiles, suggest embedded widely scattered convective elements may evolve in the 16-19z time period, particularly closer to the warmer heat source of the Sea of Cortez across the Lower Colorado and southern Imperial Valley. Given the digging wave upstream off the SW CA coast, upstream winds may be favorable for some potential upstream redevelopment/back-building environment. Will continue to monitor those trends for any additional MPD later toward the afternoon. Gallina ATTN...WFO...HNX...PSR...SGX...VEF... ATTN...RFC...RSA...STR...NWC... LAT...LON 36451689 36001550 35231468 34241409 33061394 32331426 32491497 32691600 34241623 34841705 34971779 35591825 36151792 $$ --- SBBSecho 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1) .