Mild September Temperatures and Unusual Rainfall Suppress Wildfire Risk in California
September in California was characterized by unusually mild and humid weather, with multiple rounds of precipitation that kept wildfire risk low across the state. While no widespread record heat occurred, temperatures were consistently above the long-term average, ranking among the top 10-20% warmest Septembers in the last 130 years, particularly in the Central Valley.
This warmth was linked to anomalously high ocean surface temperatures, which also contributed to higher-than-usual humidity. The moisture fueled significant thunderstorm activity and brought wetter-than-average conditions to central and southern parts of the state. Remnant moisture from Tropical Storm Mario led to torrential downpours, causing severe and even deadly flash flooding and debris flows in the southeastern deserts and the San Bernardino Mountains.
Fortunately, this combination of periodic rainfall and a lack of strong offshore winds resulted in minimal large fire activity. This stood in stark contrast to the Pacific Northwest and British Columbia, which experienced rare late-season heat, dryness, and continued forest fire activity into early October.
Remnants of Hurricane Priscilla to Bring Rain to Southwest
The pattern of active weather is set to continue as an offshore low-pressure system interacts with the remnants of Hurricane Priscilla. Over the next 48 hours, moisture and instability from the dissipating hurricane are expected to be drawn northeastward over Baja California and into Arizona and California’s southeastern deserts.
This will likely produce a substantial area of moderate to heavy rain across the lower Colorado Basin, with the potential for locally heavy downpours and flash flooding in southeastern California. Scattered showers and isolated thunderstorms are also possible for coastal Southern California, primarily south of Los Angeles.
Simultaneously, the offshore low will independently generate showers and isolated thunderstorms in Northern California, as cold air aloft moves over the unusually warm coastal waters.
Potential for Significant Northern California Rain Event Next Week
Looking ahead, the active weather pattern is expected to persist, with growing indications of a significant, potentially “fire season-ending,” rain event for Northern and Central California next week.
Forecast uncertainty remains high, largely due to a recurving West Pacific typhoon that is disrupting the jet stream. Such events can amplify the storm track, leading to high-impact weather. Current models increasingly suggest that a low-pressure system will position itself ideally to deliver substantial early-season rain to the northern half of the state. While the forecast could shift, the probability of a major precipitation event is rising.
Persistent Pacific Heatwave Shapes Autumn and Winter Outlook
The broader climate backdrop is dominated by a persistent and record-breaking marine heatwave spanning the North Pacific, from Asia to the North American coast. Sea surface temperatures along the West Coast remain far above average. This phenomenon is occurring alongside a developing La Niña event in the tropical Pacific.
This rare combination—an extremely warm extratropical Pacific and a cool tropical Pacific—is expected to continue through early winter. This may lead to a weaker and more northward-shifted jet stream, which typically favors drier conditions in Southern California. However, the warm ocean surface also provides more moisture for any storms that do develop, potentially making them wetter than average.
Overall, California is likely to experience warmer-than-average conditions through autumn and early winter. While the storm track may favor the Pacific Northwest, even one or two moisture-laden storms could significantly alter seasonal precipitation totals for California.
