Southern California heat wave finally breaks as rain chances enter weekend forecast: Timeline

LONG BEACH, Calif. - Southern California is finally heading toward a significant break from its prolonged stretch of dangerous heat, with temperatures expected to drop sharply heading into the weekend. 

At the same time, moisture associated with Hurricane Rachel could bring the region its first meaningful rain chances in the forecast.

What we know:

Extreme heat warnings remain in effect through 8 p.m. Thursday for coastal and valley areas of Los Angeles County as well as coastal and inland Orange County. Temperatures are expected to moderate by 2 to 4 degrees Thursday, followed by a more noticeable 4- to 8-degree decline Friday.

The National Weather Service said temperatures will continue falling through the weekend as an early-season cold upper-level low moves south from British Columbia.

By Sunday, temperatures are expected to fall another 4 to 8 degrees, with coastal and valley communities generally remaining in the 70s. Highs Monday are expected to range from the upper 60s to lower 70s.

The cooler weather will coincide with increasing moisture from Hurricane Rachel. Light rain could reach Los Angeles County as early as Saturday morning, although forecasters say Saturday night into Sunday morning is the more likely window.

Rain chances could continue into Monday, when an upper-level low moving over the Central Coast combines with remnant tropical moisture. Forecasters are calling for showers across the region and a 5% to 10% chance of thunderstorms, according to the NWS. 

Los Angeles County could receive around a half-inch of rain, while the eastern portion of the county’s mountains could receive up to 2 inches.

What we don't know:

The biggest uncertainty is Rachel’s exact track. Forecasters expect the system to turn northeast and accelerate, potentially making landfall Sunday somewhere in northern Baja California or near the U.S.-Mexico border as a post-tropical system.

That track will help determine how much tropical moisture reaches Southern California and how significant the rainfall becomes.

Coastal impacts are also sensitive to Rachel’s position. The NWS said Sunday could bring the greatest coastal flooding risk, particularly around the morning high tide, when water levels could reach about 7.5 feet. 

Forecasters said that level could vary by roughly a half-foot depending on Rachel’s track.

Timeline:

• Thursday: Hot, although most areas should see a modest decline in temperatures. Extreme heat warnings are scheduled to remain in effect through 8 p.m.
• Friday: A more noticeable cooldown, with temperatures dropping another 4 to 8 degrees and the region’s heat risk coming to an end.
• Saturday: Increasing clouds and continued cooling. Light rain could arrive in Los Angeles County during the morning, although Saturday night into Sunday morning is considered the more likely period for showers.
• Sunday: Significantly cooler, with temperatures in the 70s across coastal and valley areas. Rain chances will continue, while coastal communities face their greatest potential for flooding and hazardous surf.
• Monday: Showers could persist, along with a slight chance of thunderstorms. Rain is expected to end by Tuesday, followed by decreasing clouds and warmer temperatures Tuesday and Wednesday.

What's next:

The coastal flood advisory is scheduled to remain in effect through 2 p.m. Tuesday as southerly swells continue affecting vulnerable beaches.

Surf could reach 3 to 6 feet along south-facing beaches Thursday and Friday. Morning and evening high tides are expected to reach 6.5 to 7 feet through Thursday, with somewhat lower evening tides Friday, according to the NWS.

Long Beach is among the communities facing particular coastal concerns. City officials have identified the Alamitos Peninsula and Alamitos Bay as especially vulnerable to flooding, beach erosion and localized damage.

Long Beach officials announced four additional sandbag distribution sites Thursday as residents prepare for possible coastal flooding, high surf and rain associated with Rachel. The following sites are scheduled to remain available through Tuesday: Fire Station 7, 2295 Elm St.; Fire Station 12, 1199 E. Artesia Blvd.; Fire Station 13, 2475 Adriatic Ave.; and Fire Station 14, 5200 E. Eliot St.

Residents who need sandbags can also use the Claremont Beach Parking Lot location, which opened Sunday and is scheduled to remain available through Tuesday. Sand and sandbags are available only to Long Beach residents, who must present identification and are limited to 10 bags per person.

The city is urging residents in low-lying areas to prepare ahead of the weekend and monitor changing weather conditions.

What you can do:

Residents should continue taking precautions against the remaining heat Thursday, including staying hydrated, limiting exposure to the sun and checking on relatives and neighbors.

As the weather turns cooler, residents in vulnerable coastal and low-lying areas should prepare for possible flooding before the weekend. People heading to beaches should also be aware of dangerous surf and rip currents and follow local warnings.

Long Beach residents preparing for possible flooding can obtain sand and sandbags at designated city locations and should consider storing filled sandbags for use during the winter storm season.