Brief Outlook:
Isolated thunderstorms are expected across parts of southern California, particularly in San Diego, Riverside, and San Bernardino County mountains drifting southwest towards the valleys from the afternoon into the evening. These storms will be capable of producing hail, gusty winds, brief heavy rain and lightning.
Counties and Cities Impacted: San Diego County Mountains, Valleys, Deserts, Riverside County (Riverside, Palm Springs, Riverside Mountains, Valleys), San Bernardino County (San Bernardino Mountains, Valleys, Deserts).
Upper-Level Dynamics:
An upper-level shortwave trough will be moving across the southwestern U.S., enhancing southerly flow aloft over southern California. This setup will lead to increased lift and a favorable environment for thunderstorm development. The interaction between this trough and a subtropical ridge centered over the Four Corners region will create conditions conducive to storms with a few possibly being strong, with mid-level winds expected to range between 20-30 knots, providing moderate shear to organize thunderstorms.
Surface Conditions:
At the surface, instability values are expected to reach 1000-1500 J/kg due to the hot and humid conditions, with dewpoints in the mid-60s. Effective shear will be moderate, supporting the development of discrete storms initially, which may then organize into clusters. The steep mid-level lapse rates will support strong updrafts, enhancing the potential for localized wind gusts and small hail. The monsoonal moisture will also contribute to heavy rainfall, increasing the risk of flash flooding in areas with poor drainage or recent burn scars especially.
Timing:
Thunderstorms are expected to initiate by early to mid-afternoon in the mountains and deserts of southern California, with activity spreading southwestward into possibly the valleys by late afternoon. Peak intensity will likely occur from late afternoon through the evening, with the most storms expected during this period across the mountains. The storms are likely to diminish in intensity by late evening as the atmosphere stabilizes once again.
Main Impact: local gusty winds, flash flooding, localized hail and lightning.
Sina⚡⚡
Managing Partner and Lead Forecaster of NWF Innovations & NWF Networks