Large Scale Summary
- Support for troughing continues in the eastern N Pacific 128-130W and also 130-145W. The main governing agent for this is the MJO with a maximum along about 170W-180. There is also an IVTinit feature located 126-140W. The main issues for California are the rain systems with their small band sizes but containing moderate to heavy rain rates.
- Tropical cyclone events continue to be a factor to consider in early Oct. Rain-bands with these are typically small in size but can produce heavy rains with flash flood potential, mainly for mountain areas in Oct.
- Fire Risk: There has been a decrease in fire risk for the Sierras of northcentral to N California due to increased moisture with the rains. Near average precipitation is suggested for SOCAL, below average for NORCAL Bay Area northward.
- There is a chance for a tropical cyclone coming into Central Baja California near Bahia Tortugas 9-10 Oct. This may bring some rains into SOCAL about 10-13 Oct.
- The combination of above average WSW to SW flow from 135W to the Central California coast and the current "pool" of unusually warm sea surface west (upstream)from California argues for an increase in support for rains even during the fall periods of mid Oct as well as mid to late Nov.
- The current tendency for troughing west of California will tend to decrease the number and strength of Santa Ana events that occur in Oct. These normally increase the upwelling along the California coast in Oct, and lead to colder sea surface. This upwelling may be suppressed for a while during the Oct-early Nov period.
- Rain Dates: OCT 1-3, (from GFS). After Oct 4: Other Oct rains: Oct 10-12, 21-22, 27-29, Nov 4-8.
Near Term Forecast
- NORCAL: Rain dates Oct 1-2,10-12, 21-22, 27-29, Nov 4-8. Predominantly mild rains, cool days/mild nights. Less warm throughout N California. The cool and wet spells focus on mostly Oct 1-2, Oct 10-12, 21-22, 27-29, Nov 4-8.
- Central California: Central Sierra Precipitation: Rain dates:Oct 1-2, 10-12, 21-22, 27-29, Nov 4-8. Warm Dry Periods: Oct 5-6.
- Southern California: Precipitation: Oct 10-12, 21-22, 27-29. SOCAL Warmer or Hot Periods: We return to dry and warm period 13-15 Oct.
- Deserts Outlook: SOCAL deserts continue seasonably warm or hot, changeable as tropical cyclones develop over S Baja and SW Mexico during this currently active season. Tropical cyclones induce development of alternatively TSTMS, then returns to recurrent upper high pressure with dry, warm or hot days. Shower dates near mountains or E SOCAL Deserts and mountains, 27-30 Sep. Oct shower dates are Oct 1-3, 10-12, 21-22, 27-29.
Medium Term Outlook: November 1 - November 30
- The IVTinit pattern along with the Madden Julian Oscillation (MJO) both are showing a tendency for troughing in the subtropical mid N Pacific near the Central California and NORCAL coast, and also near Santa Barbara Co in S California.
- Northern California: Rain dates estimate: Nov 1-2, 8-21, 25-30.
- South half of California: Rain dates estimate: Nov 1-2, 10-19, 25-26.
DISCUSSION OF NOAA/NCEP/CPC MAPS NOV-DEC 2025, JAN 2026
- A rainy start in Nov begins with rains about Nov 1-2, 8-10, 11-20, and 25-30. In Dec it turns wet with significant rains possible 17-23 with the usual snow in the NORCAL mountains, Sierras and SOCAL mountains.
- Eventually, the belt of westerlies in the mid to eastern N Pacific has support to meander far enough south to tap more subtropical moisture for storms coming into California, beginning in December 2025. The latest information for Jan 2026 shows a continuation of the near average rainfall regime that begins in mid Dec. NMME model has been showing above average rainfall for both Dec and Jan.
Figures and Text: Alan Fox, Fox Weather, LLC
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