The orders affect residents of the communities of Oak Glen, northeast Yucaipa and Mountain Home Village.
A division chief with the San Bernardino County Fire Department, who lives in Oak Glen, said he was concerned that a drainage channel that runs underneath a bridge at Oak Glen Road will almost certainly overflow by Friday morning, flooding the roadway.
Homes situated alongside a nearby curve in the channel could be at risk. Another concern is that some hills in the area have no recorded burn history. Thus, the possible mudslides and debris flows there could be the worst in memory.
Storm timeline: Here's when heavy rain will hit Southern California
"Don't wait, do it now. It's not like a fire -- you can't see it growing," Yucaipa Fire Chief Grant Malinowski warned at a Thursday morning news conference. "That whole hillside can come down at one time. It's not incremental.
"If we ask you to leave your home, for your safety please leave your home."
Speaking at the same news conference, San Bernardino County sheriff's Lt. Julie Landon said the morning's calm conditions belied the danger posed by the incoming storm.
SoCal storm: Ski resorts in San Bernardino Mountains expecting tons of fresh snow
"I know that you're looking out right now and you see the blue skies and you're not afraid," Landon said. "But tonight, when it hits -- between midnight and 2 o'clock in the morning -- you're not going to have time. It's going to be too late."