This post explains an approaching low-pressure system off the Southern California coast that is expected to bring unseasonable rainfall across Southern California, Arizona, and the lower Colorado River Basin.
As a meteorologist with three decades of field experience, I break down the expected timing, the primary hazards — especially flooding-in-southern-states/”>flash flooding — and practical steps residents and water managers should take now to reduce risk.
Storm overview: what the system is and where it will go
A compact low-pressure system is forming just off the Southern California coast and will push moisture inland starting on Monday.
This type of offshore development can tap subtropical moisture and produce rainfall that is out of season for the Southwest, meaning soils, infrastructure, and communities may be less prepared than during a typical wet season.
The system is forecast to spread rain into Southern California and then affect Arizona and the lower Colorado River Basin, with the most significant threat focused on Southern California.
Forecasters expect the highest risk of flash flooding to occur on Tuesday and Wednesday as bands of heavy rain move through saturated or vulnerable terrain.
Timing and regional impacts
Rainfall should begin spreading across the region on Monday, building through Tuesday and peaking on Tuesday and Wednesday.
In Southern California, sudden heavy downpours can produce rapid runoff into urban channels, arroyos, and burn-scar areas.
In Arizona and the lower Colorado River Basin, impacts will include localized flash flooding and increased runoff into river channels and reservoir systems.
The short-term hazards to life, property, and infrastructure are the primary concerns.
Risks to communities and water systems
The immediate hazard for residents is flash flooding, particularly in low-lying and flood-prone neighborhoods, near dry washes, and downstream of recent wildfire burn scars.
Roads can become impassable quickly; small streams can rise without much warning.
For the lower Colorado River Basin, the rainfall could complicate existing water management challenges.
Sudden inflows can carry sediment, alter reservoir operations, and require rapid adjustments to releases and allocations — all while managers balance supply needs and flood safety.
Practical preparedness steps
Officials are urging residents to remain alert and prepared for rapidly changing conditions.
Simple preparatory actions can save lives and reduce damage.
What this pattern tells us and final recommendations
These unseasonable storms are consistent with a broader pattern of shifting weather behavior in the Southwest. Atmospheric variability can produce strong, off-season precipitation events.
While any single storm cannot be solely attributed to long-term climate trends, the increasing frequency of unusual timing and intensity of storms is notable.
Residents and managers should treat the next several days as a period of heightened risk. Stay informed and follow local guidance.
If you manage infrastructure or water resources in the lower Colorado River Basin, coordinate with regional partners now to anticipate rapid inflows and potential sediment impacts.
Here is the source article for this story: Rare September rain returns to Southern California, Southwest | Latest Weather Clips