Subseasonal forecasts by CW3E show strong multimodel agreement on above-normal atmospheric river activity across the U.S. West Coast during mid- to late December, with the strongest signals over Northern California. Forecast regime tools indicate a transition from a West Coast Ridge to a Pacific Ridge during this period, altering temperature and precipitation patterns across the region.
Atmospheric river over the Pacific Ocean at 20:00 UTC on December 9, 2025. Credit: NOAA/GOES-West, RAMMB/CIRA, The Watchers
Subseasonal outlook issued by the Center for Western Weather and Water Extremes (CW3E) and initialized on December 8 shows strong ensemble agreement on above-normal atmospheric river activity across the U.S. West Coast during mid- to late December, with the highest confidence over Northern and Central California.
The NCEP, ECCC, and ECMWF systems all indicate enhanced atmospheric river (AR) frequencies compared to their climatologies, though ECMWF shows weaker anomalies and lower ensemble agreement.
The consistent signals place Northern California as the primary region for elevated AR activity in mid- to late December.
During Week 2, from December 15 to 21, all models indicate above-normal AR activity across California. Confidence is highest in NCEP and ECCC. Ridging forecasts show strong South-ridge and West-ridge activity, a configuration that usually enhances moisture transport into the Pacific Northwest and Northern California while reducing precipitation in Southern California.
The IRI weather-regime tool shows a moderate likelihood of transition from a West Coast Ridge to a Pacific Ridge, shifting temperature and precipitation anomalies toward cooler and wetter conditions in the northern part of the region. Hybrid-regime probabilities for December 15 to 21 support this pattern, favoring wet conditions in Northern California and continued dryness in the south.
Image credit: CW3E Week 3 forecasts, covering December 22 to 28, maintain the signal of above-normal AR activity across Northern and Central California.
NCEP and ECCC show slightly to clearly above-normal frequencies, while ECMWF retains weaker anomalies. South-ridge activity remains the most likely ridge type through Weeks 3–4, a pattern historically paired with wet anomalies in Northern California and reduced precipitation farther south.
The hybrid-regime tool suggests a continuation of wet conditions in Northern California during the first half of Week 3, followed by increasing uncertainty as regime probabilities become less distinct later in the period.
Image credit: CW3E Forecast confidence declines as we enter Week 4, from December 29 to January 4. NCEP and ECCC maintain slightly above-normal AR activity across Northern and Central California, while ECMWF spans slightly below-normal to slightly above-normal values.
Signals diverge in Southern California, ranging from near-normal to slightly above-normal AR activity, depending on the model. The hybrid-regime tool identifies no dominant pattern for the period.
Image credit: CW3E Large-scale climate drivers offer limited guidance, with the Madden–Julian Oscillation remaining weak in Phase 7, a configuration with minimal influence on mid-latitude precipitation patterns. Forecasts indicate slight eastward propagation into Phase 8 early in Week 1, followed by stationary behavior and gradual strengthening, but expected impacts on West Coast precipitation remain small.
Image credit: CW3E Hydrologic conditions as of December 8 show substantial early-season precipitation across California. Northern California stands at 70–130% of normal, Central California exceeds 150%, and Southern California exceeds 200% of normal values.
Most major reservoirs remain near or above normal levels for this time of year, and drought conditions are limited statewide. Hydrologic baselines increase sensitivity to AR-related inflows during Weeks 2–3, when confidence in above-normal AR activity is strongest.
Across all subseasonal tools, Weeks 2–3 present a coherent pattern — enhanced AR activity directed toward Northern California and the Pacific Northwest, ridge-driven drying across Southern California, and an emerging shift toward a Pacific Ridge circulation regime.
Confidence decreases sharply entering early January as ensemble spread increases and circulation-regime probabilities lose structure.
References:
1 Subseasonal Outlook – CW3E – December 9, 2025
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