‘Atmospheric River’ of Moisture Moving Slowly Toward Southland

After dropping “epic” amounts of rain in San Luis Obispo County, a storm front slowly headed toward the Southland today, prompting forecasters to warn of potentially damaging mud and debris flows from what could be a night of intense downpours.

The atmospheric river will roll into the area between 5 and 7 p.m. Heavy rainfall is expected between 8 and 9 p.m.

“We have sand and sandbags at fire stations 37 & 38. In the unlikely event of a power outage due to the rain, please make sure you have a working flashlight next to your bed,” said Pasadena Public Information Lisa Derderian. 

“Fortunately, we don’t have any burn scar areas from the recent Bobcat fire, but excessive water can create mud and debris flow.  Be cautious driving and allow extra time to commute tonight and tomorrow. Stay inside as much as possible because a significant amount of rain is expected,” Derderian said.

According to the National Weather Service (NWS), rain will be heavy at times in the evening. Temperatures in Pasadena are expected to drop into the mid-40s. Localized flooding is possible.

Forecasters said the “atmospheric river event” dropped nearly 14 inches of rain in some parts of San Luis Obispo County over the past 36 hours. The storm was driving slowly south, with rain expected to fall in Ventura County by early evening.

“As the storm shifts south later today and tonight, heavy rain is expected to spread into the recent burn scars in Los Angeles County, including the Lake, Bobcat and Ranch 2 burn scars,” according to the NWS.

A winter storm warning will be in effect from 3 p.m. Thursday to 3 p.m. Friday, with one to two feet of snow potentially accumulating at elevations above 6,000 feet.

A flash flood watch will be in effect Thursday afternoon through late Thursday night for the mountains and the Antelope, Santa Clarita and San Gabriel valleys, mainly affecting burn scar areas near the locations of the Lake, Bobcat and Ranch 2 fires.

Forecasters said the areas could see rain rates of 0.75 inches per hour, potentially leading to “significant mud and debris flows.”

“Residents in or below the recently burned areas are urged to take the steps necessary to protect their property,” according to the NWS. “Persons in the watch area should remain alert and follow directions of emergency-preparedness officials. Avoid hiking and camping in canyons and near creeks.”

A flood watch will be in effect for most of Orange County from Thursday afternoon through Friday afternoon. Forecasters said the heaviest rain there is anticipated to begin before midnight Thursday night, continuing through Friday morning, including a chance of thunderstorms and rain rates topping a half-inch per hour.

Winds in mountain areas will blow at 20 to 30 mph, with gusts ranging up to 50 mph. A wind advisory will be in effect for Los Angeles County mountains until 3 p.m. Thursday. Forecasters said the strongest winds are likely in the San Gabriel Peaks and the Grapevine area, making driving difficult, especially for high-profile vehicles.

Forecasters said Los Angeles County could see 1.5 to 3.5 inches of rain, with 2 to 5 inches possible in the foothills and mountains, although the largest rainfall totals are likely to occur in Ventura County.

Whenever persistent rain is in the forecast, fears are raised about possible mudslides.

“With the potential of this event, the area may approach critical thresholds for mud and debris flows in and around the recent burn areas,” according to the NWS.

Los Angeles County health officials have issued a cold weather alert through Thursday for the mountains and Antelope Valley, and through Tuesday for the Santa Monica Mountains and the Santa Clarita Valley.