Cooling trend brings light rain to Southern California: Timeline

Weather Forecast for Wednesday, Jan. 21

The latest forecast and air quality conditions for the greater Los Angeles area, including beaches, valleys and desert regions.

LOS ANGELES - Get ready for a weather shakeup, Southern California!

The region is trading its recent warmth for a cool, gray stretch as a storm system moves in from the Pacific, according to the latest forecast.

You can expect a significant drop in temperatures, deepening fog, and light rain through Friday before a drier and warmer pattern returns early next week.

What we know:

A high-pressure ridge that previously brought warm weather is breaking down as an upstream trough moves toward the California coast, according to the National Weather Service.

This shift is strengthening onshore flow, which will deepen the marine layer and bring patchy dense fog to coastal and valley areas. Temperatures will drop significantly through Friday.

Rainfall amounts are expected to be light, generally 0.25 inch or less, though the highest totals will likely occur across the south-facing slopes of the mountains, the NWS said.  Snow levels will remain high, keeping any accumulation limited to mountain peaks above 5,000–7,000 feet.

Local perspective:

A deepening marine layer will spread significant cooling and patchy dense fog across San Diego, Riverside, Orange, and San Bernardino counties through Friday. 

While a low-pressure system moving toward northern Baja California will keep the best chances for light rain between Thursday night and Saturday morning, rainfall totals are expected to be modest, generally ranging from 0.10 to 0.25 inch near the coast and up to 0.50 inch in the mountains.

High temperatures will bottom out in the lower to mid-60s for most coastal and valley areas on Friday.

Graphic courtesy National Weather Service Los Angeles

Timeline:

• Wednesday – Friday: Cooling trend continues with increasing cloud cover and periods of light rain or drizzle.
• Friday Night: The storm system moves south, ending most precipitation chances while keeping temperatures cool.
• Saturday – Sunday: A gradual warming trend begins as high pressure starts to rebuild over the West Coast.
• Monday – Wednesday: Drier offshore flow reestablishes, bringing warmer temperatures and the potential for advisory-level Santa Ana winds in Ventura and Los Angeles counties.

Graphic courtesy National Weather Service San Diego

Graphic courtesy National Weather Service San Diego

What's next:

A drier weather pattern is likely to last through the end of the month, according to the latest forecast. 

As for early next week, an offshore flow returns, bringing gusty Santa Ana winds and a return to above-average temperatures for eastern Ventura and western Los Angeles counties.