LOS ANGELES (KABC) -- This week's stormy conditions in Southern California could be merely a preview of a rain-soaked winter in the region, according to a climate scientist.
The first of two storm systems was bringing rain to parts of the Southland on Wednesday, ahead of a second, more powerful system that forecasters say will give the region a more thorough dose of precipitation.
"With this system, you can get a pretty significant flash-flood event," said Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research. "This will be a pretty El Niño-flavored storm."
In advance of the back-to-back systems, the National Weather Service issued a flood watch that will be in effect across Los Angeles County until Thursday night. In Orange County, the flood watch will be in effect through Friday afternoon for coastal and inland areas and the Santa Ana Mountains and foothills.
"The seasonal outlook still tilts toward wetter-than-average conditions in January through March," Swain said. "Again, that's just a tilt in the odds. An El Niño -- even a strong one -- it's verging on historically strong, in fact, that we may get there yet. It doesn't tell us everything we need to know."
In October, National Oceanic and Atmospheric Administration officials said a strong El Niño heavily moderates and changes the storm tracks of what America is likely to face from December to February, with an added warming boost from climate change and record hot oceans.
Most of the country is predicted to be warmer than normal with that warmth stretching north from Tennessee, Missouri, Nebraska and Nevada, along with nearly all of California. The rest of the nation is forecast to be near normal or have equal chances for warm, cold or normal. NOAA doesn't predict any part of the U.S. to be cooler than normal this winter.
A similarly large southern swath of the country is predicted to be wetter. The forecast of added moisture stretches from Massachusetts down the East Coast along most of the South below Tennessee, and extending west through Texas, Kansas, Colorado, Utah, Nevada and most of California, but excluding good chunks of New Mexico and Arizona.
All this is because of El Niño, which is a natural periodic warming of parts of the Pacific that changes weather patterns worldwide and generally heats up global temperatures, NOAA scientists said. El Niño has its strongest effects, especially in the United States, during the winter. That's when it sends the jet stream, which moves storm fronts, on an unusual path that is dominated by warmer and wetter Pacific air plunging south.
Meanwhile, forecasters estimated that much of the Los Angeles area could receive 2 to 4 inches of rain during this week's "atmospheric river" event accompanying the second storm system, with 4 to 8 inches possible on south-facing foothill areas and coastal slopes. Rainfall rates of 0.3 to 0.6 inches per hour are expected in some areas, but if thunderstorms develop, some areas could see rain falling at a rate of 1 inch per hour.
"Extensive roadway flooding, especially in low-lying areas, and flooding of creeks and streams are possible," according to the NWS. "In addition, there will be the risk for flash flooding and debris flows in recent burn scars, especially the South Fire burn scar in Ventura County. Also, there will be the strong potential for rock and mudslide activity, especially on canyon and mountain roadways. Flooding and rock/mudslide activity may lead to significant travel delays and road closures. Increased flow in rivers and streams will bring an increased threat for swiftwater rescues."
Los Angeles County lifeguards warned people to exercise caution if they visit the beach.
"Ocean conditions will also have a bit more activity with waves in the head to overhead range," lifeguards warned on social media. "Plenty of varying factors these next few days, so make sure to stay tuned, and if you decide to come to the beach make it a point to check in with a lifeguard tower prior to entering the water."
County health officials issued their standard warning for people to avoid entering the ocean water in the days following rain, noting that runoff can carry bacteria and debris into the ocean, raising the risk of illness.
NWS forecasters said Orange County will likely start seeing heavy rain late Wednesday night into Thursday.
"A stronger storm will bring several hours of steady moderate to heavy rain and gusty winds Wednesday and Thursday, with threats for flooding and thunderstorms," forecasters said.
Conditions are expected to dry out on Friday, although gusting northwest winds are expected to linger into Saturday.
Snow levels are expected to remain above 7,000 feet, with forecasters saying snow impacts from the storm will be "minimal."
Los Angeles County officials said various agencies were coordinating to ensure public safety while also hoping to capture as much stormwater as possible for future use.
"L.A. County owns a world class system of water conservation and flood protection. This system is prepared to capture and conserve valuable stormwater while protecting communities," county Public Works Director Mark Pestrella said in a statement. "Our system is primed to capture as much stormwater as possible in our efforts to increase the region's overall local water supply by 600,000-acre feet by 2045."
County officials urged residents to heed instructions from emergency responders and alert notifications; drive cautiously and slow down in wet conditions; and avoid trying to cross flooded roadways.
Daytime temperatures will remain in the 60s in the region for much of the week. Overnight lows will generally be in the upper 40s and lower 50s throughout the Southland, but will dip into the 30s in some parts of the mountains and high desert.
Thursday is the first day of winter.
The Associated Press and City News Service contributed to this report.