Fall/Winter 2025-26 Weather Forecast

Lodi

Aug 31

For Northern California and San Joaquin County

Executive Summary

Northern California and San Joaquin County are expected to experience a transitional weather pattern during Fall/Winter 2025-26, with brief La Niña conditions developing in fall followed by a return to ENSO-neutral conditions. A weakening polar vortex will create more dynamic weather patterns with increased potential for cold air outbreaks. The overall forecast suggests average temperatures with a wetter-than-normal winter, though precipitation outlook varies between forecasting agencies.

La Niña and Polar Vortex Interactions

La Niña Development and Timeline

Current forecasts indicate a 56% chance of ENSO-neutral conditions continuing through late summer 2025. However, brief La Niña conditions are favored for fall and early winter 2025-26:

September-November 2025: 39% chance of La Niña conditions
October-December 2025: 44% chance of La Niña conditions
November-January 2025-26: Gradual return to ENSO-neutral

Polar Vortex Disruption

Strong indicators suggest a significantly weaker polar vortex will develop for Winter 2025/2026. Three major factors are creating an almost "perfect storm" scenario:

Easterly QBO (Quasi-Biennial Oscillation) in the stratosphere
Low Arctic sea ice extent in Barents and Kara regions
Cold Pacific Ocean temperatures supporting La Niña-like conditions

Combined Effects

The interaction between weak La Niña and a disrupted polar vortex historically increases the likelihood of:

More frequent cold air outbreaks pushing south from Canada
Enhanced winter storm activity across the western United States
Variable temperature patterns with periods of both mild and cold conditions

Regional Weather Pattern Ranges

Temperature Outlook

Fall 2025 (September-November): Equal chances of above, near, or below normal temperatures for Northern California; 40-50% chance of above-normal temperatures in August transitioning to more variable conditions.

Winter 2025-26 (December-February): Average temperatures predicted; 33-40% chance of above-normal temperatures; cold blasts expected in mid-January and mid-February due to polar vortex disruptions.

Precipitation Patterns

Fall 2025: Dry and sunny conditions expected through fall across the Pacific Southwest; below-normal precipitation likely as typical for the dry season.

Winter 2025-26: Wet winter forecast with "good rain before windy conditions" in late January; equal chances of above or below normal precipitation according to NOAA CPC; 40-60% chance of above-normal precipitation in some models.

Weather Pattern/Factor	Fall 2025 Forecast	Winter 2025-26 Forecast	Likelihood/Confidence
La Niña Conditions	Brief La Niña conditions favored (39% chance Sept-Nov)	Brief La Niña through early winter, then ENSO-neutral	35-44% La Niña, 50-68% ENSO-neutral
Polar Vortex	Weakening expected, increased cold air outbreaks likely	Weaker than normal, more dynamic weather patterns	High confidence for weaker vortex
Temperature Outlook	Equal chances of above/below normal for Northern CA	Average temperatures predicted	Moderate confidence
Precipitation Outlook	Equal chances of above/below normal for Northern CA	Wet winter, equal chances (NOAA)	Low to moderate confidence
Extreme Temperature Risk	Moderate - polar vortex disruptions may bring cold snaps	Cold blasts possible January-February	Moderate - depends on polar vortex strength
Extreme Precipitation Risk	Low to Moderate - atmospheric rivers possible	Moderate to High - wet winter with wind events	Moderate - typical for wet pattern
Drought Risk	Elevated in central/southern CA, improving in north	Below normal risk due to recent wet winters	High confidence for continued improvement
Wildfire Risk	Above normal potential through September	Reduced risk due to expected moisture	High confidence for reduced risk
Flooding Risk	Low during typical dry season	Elevated risk from atmospheric rivers	High for winter, low for fall

Outlook Period	Chance Temp Above Normal	Chance Temp Below Normal	Chance Precip Above Normal	Chance Precip Below Normal
August-October 2025	40-50%	20-30%	Equal chances	Equal chances
September-November 2025	Equal chances	Equal chances	Equal chances	Equal chances
December 2025-February 2026	33-40%	30-35%	40-60% (Farmers) / Equal (NOAA)	20-30%
Full Winter 2025-26	Equal chances	Equal chances	40-60% (Farmers) / Equal (NOAA)	20-30%

Likelihood of Weather Extremes

Temperature Extremes

Cold Temperature Events: High likelihood of polar vortex-driven cold snaps in January and February. 64% of recent winters have shown westward-shifting cold patterns during La Niña-like conditions. Potential for brief but intense cold periods during vortex collapses.

Heat Events: Moderate risk during fall transition. Above-normal fire weather conditions expected through September.

Precipitation Extremes

Heavy Precipitation/Flooding: Elevated risk from atmospheric rivers during winter and December through February. Burn scar areas from recent wildfires face enhanced flood and debris flow risk.

Drought Conditions: Improving drought conditions expected due to recent wet winters. High confidence California will remain drought-free through 2025 and beyond. Central Valley reservoirs currently at or above historical averages.

Extreme Weather Events

Atmospheric Rivers: 30-50% of California's annual precipitation typically comes from atmospheric rivers. Moderate to high risk of significant atmospheric river events during winter.
Wildfire Risk: Above-normal fire potential through September 2025. Reduced wildfire risk expected during winter due to moisture.
Wind Events: Santa Ana/Diablo winds possible during dry periods. Strong winds often accompany atmospheric river events during winter.

Key Risk Factors and Preparations

Primary Concerns: Flooding and debris flows in burn areas from recent wildfires remain highly vulnerable. Variable winter weather from polar vortex disruptions may create rapid changes. Fire risk remains elevated through fall dry season.

Seasonal Transitions: October-November marks a transition from fire season to wet season; December-February is peak flood season with atmospheric river activity; January-February shows highest likelihood for cold polar vortex events.

The overall forecast suggests a dynamic winter season with the potential for both beneficial precipitation and challenging weather extremes, requiring preparedness for rapid weather pattern changes throughout the period.

Weather Outlook Charts

References

Weather

Don Bradford