SAN FRANCISCO, California — The weather service warned drivers to stay off Northern California mountain roads as a major winter storm will bring heavy snowfall, powerful winds and torrential rains at lower elevations Thursday.

Meanwhile, predictions of widespread showers across the southern end of the state raised concern about flash flooding near wildfire burn scars later in the week.

The blizzard would help the snowpack, which is vital to the state’s water supply. The water supply is only about a quarter of its normal size for this time of winter.

The National Weather Service tweeted, “Totals will be measured in feet and mountain travel will become dangerous. Travel is highly discouraged.”

The northern mountains expect to receive 3 feet (0.9 meters) to 5 feet (1.5 meters) of snow, and up to 7 feet (2.1 meters) in localized areas.

The Winter Storm Will Head South to SoCal

The storm should reach Southern California in the early hours of Friday. Santa Barbara County issued a recommended evacuation warning for south coast communities including Montecito, where a Jan. 9 storm triggered flash floods that destroyed or damaged hundreds of homes.

The county is following a new system of alerts that emphasizes evacuations well in advance of storms rather than suggesting residents can use their discretion.

Officials hoped to decide Thursday whether to issue a mandatory evacuation order. They cannot force people to leave their homes under a mandatory evacuation order. However, authorities said they should not expect a rescue while the storm event is occurring.

“Our intention is to make sure that you have a plan, that you are prepared, and that you are safe,” said Santa Barbara County Sheriff Bill Brown. Brown warned people that just because the midweek weather was calm doesn’t mean they shouldn’t take it seriously. “Just because the skies are blue right now doesn’t mean there isn’t a threat.”

People with mobility issues or those with large animals should get out now, officials said. There were no exact estimates on how many people may have left the area.

Montecito and neighboring communities on the coastal foothills of the Santa Ynez Mountains lie below slopes burned bare last year by the largest wildfire in recorded California history as well as other fires in recent years.

Evacuation fatigue for some residents is a factor. Some evacuated multiple times during the wildfires, and then again when the mudslides hit.