The rain is expected to move in on Tuesday, but the biggest concern is what happens late Tuesday night into Wednesday.
That's when rain could fall fast enough to flood streets, overwhelm storm drains and trigger mud or debris flows, especially near hillsides and recent burn areas.
It's expected to be one of those storms where the total rain matters, but how quickly it falls matters even more.
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Even after the midweek peak, rain is expected to linger.
Showers could continue into Christmas Day and possibly beyond, making way for saturated ground, standing water and conditions that can change fast.
That's why emergency officials are urging people not to wait until the rain is falling to prep.
If you live near a hillside, a burn area or a street that floods easily, now is the time to get sandbags, clear storm drains and think about where water tends to go around your home.
Forecasters from the National Weather Service (NWS) say the worst of the storms could hit while you're sleeping overnight, so it's best to prepare while you can.
"There is a high risk of rock and mudslides through those canyon roads, but any canyon mountain, any roadway can see flooding," said Kristan Lund, a meteorologist with the NWS. "We're looking at widespread strong winds Tuesday into Wednesday, with downed trees and power outages possible."
Sandbags are already being distributed at sites in fire-impacted areas.