Holiday storm prompts dire warnings for Southern California. Here's what to know

This story will be updated. Check back for details.

Whether you call it a "pineapple express" or atmospheric river, officials are warning that a massive storm moving across Southern California could pose serious threats to the region during the holidays.

Here's what we know as of Tuesday afternoon.

Rainfall totals

Coastal and valley areas are expected to receive 4 to 6 inches of rain over the course of the storm. As much as a foot of rain is forecast for the foothills and mountains. We'll keep track of running rain totals here as the storm progresses.

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Evacuations and closures

One of the main concerns with the holiday storm is mud and debris flows, and properties near recent burn scar areas are of particular concern. To prepare, officials in Orange and L.A. counties have issued evacuation warnings orders for recent burn scar areas.

Some areas are slated to get mandatory evacuation orders over the course of Tuesday and Wednesday.

If you're unsure of whether your home is in one of the evacuation warning zones, you can see this map on L.A. County's webpage for the current emergency. Go here for the latest orders in Orange County.

Nearly 400 properties in recent burn scar areas are under an evacuation order in L.A. County that went into effect at 11 a.m. Tuesday.

"Those recommendations are based on technical science, forecasting of the current weather event and our best estimate for who would be most vulnerable," Mark Pestrella, the director of the L.A. County Department of Public Works, said Tuesday morning.

The 383 properties initially put under an evacuation order have been contacted directly by law enforcement or other emergency personnel. If no one answered their door, officials left flyers and will revisit the property Tuesday, according to the L.A. County Office of Emergency Management.

Forecast

A flood watch is in place for most of Southwest California starting 4 p.m. Tuesday until at least Thursday night.

The heaviest rain is expected to begin Tuesday night and will peak on Christmas Eve, "potentially exceeding one inch per hour," according to the National Weather Service.

There might be a lull in rain on Wednesday afternoon.

Heavy rain will continue Wednesday night and into Christmas Day. Moderate or local, heavy rain could persist through Saturday night.

The National Weather Service said wind gusts of 35 to 55 mph are likely in the coastal and valley areas of L.A. County and there's even a "small chance for a brief, weak tornado."

Here are some of the potential effects of the storm outlined by the National Weather Service:

• Widespread and significant flooding in urban areas
• Debris flows that could affect areas outside of recent burn scar areas
• Saturated soil and strong winds could lead to downed trees and power lines

This holiday week could be among the wettest the region has seen in several years.

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National Weather Service Los Angeles / X

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Take this upcoming storm seriously

Ariel Cohen, who leads the National Weather Service in the Los Angeles and Oxnard areas, told LAist's AirTalk Tuesday morning that a storm like this occurs only once every five to 10 years in the area.

That this storm is falling on a holiday week makes it a bit more concerning, Cohen said.

" The impacts are going to be potentially much higher with folks on the roadway," Cohen added. "So what you can do now is plan to not be on the road."

Cohen also said the storm's impact won't be felt equally in different parts of the county and warned that the "life-threatening" conditions will be throughout the foothills and mountains across the region.

On Tuesday, L.A. Mayor Karen Bass recommended doing any necessary travel before the rain intensifies in the evening.

"Staying off the roads whenever possible keeps you safe and frees up the lanes for our first responders and public works crews," Bass said.

(See latest road conditions in Southern California.)

National Weather Service Meteorologist Kristan Lund said that a possible lull in rain on Wednesday afternoon doesn't necessarily mean it's safe to get back on the roads.

"We encourage people to be aware that it will take less and less rain as we go through the week for more and more impacts to happen," Lund said Tuesday. "Just because one canyon doesn't have rock slides tonight, it might end up having rock slides in the next impulse."

Understanding National Weather Service warnings

Here’s an excerpt from our guide to understanding flood warnings, if any are issued:

• Flood advisories are how the NWS begins to raise the alarm. The goal is to give people enough time to take action.
• Flood watches are your indicators to get prepared to move.
• A flood warning is issued when a hazardous weather event is imminent or already happening. When one is issued for your area, you need to get to higher ground immediately.
• A flash flood warning is issued when a flash flood is coming or in progress. Flash floods are sudden and violent floods that can start within minutes.

Read more: Flash Flood Warnings? Watches? Here’s What You Need To Know

Tips for driving in the rain

Advice on driving in the rain:

• Check weather and road conditions all along your planned route.
• Slow down.
• Keep a wider-than-usual distance between your vehicle and the one in front.
• Don't drive through standing water — as little as 12 inches of rushing water can carry away most cars, and two feet can carry away SUVs and trucks.
• Make sure tires are fully inflated.
• Check windshield wiper blades and replace if necessary.

Read more: What you should do if you end up driving in a flooded area

Downed tree, power line or flooded road?

Dial 911 in an emergency.

However, if you need to report a flooded road or a downed tree, you can call the following non-emergency numbers:

• L.A. city: Dial 311 for a flooded road or downed tree. Call (800) DIAL-DWP if you see a downed power line.
• L.A. County: (800) 675-HELP
• Ventura County: (805) 384-1500
• Orange County: (714) 955-0200 or visit here.

If you're in L.A. County and need sand bags, you can find some at local fire houses.

Staying safe when the winds are high

• Watch for traffic signals that may be out. Approach those intersections as four-way stops.
• Make sure you have a battery-operated radio and flashlights. Check the batteries to make sure they are fresh. Use flashlights for lighting during a power outage; do not use candles because they may pose a fire hazard.
• If you’re in a vehicle with a fallen power line on it, stay in the vehicle and remain calm until help arrives. It is OK to use your cellphone to call 911. If you must leave the vehicle, exit away from downed power lines and jump from the vehicle, landing with both feet together. You must not touch the vehicle and the ground at the same time. Then proceed away from the vehicle by shuffling and not picking up your feet until you are several yards away. 
• Water and electricity don’t mix. Water is an excellent conductor of electricity. Do not step in or enter any water that a downed power line may be touching.
• Do not use any equipment indoors that is designed for outdoor heating or cooking. Such equipment can emit carbon monoxide and other toxic gases.
• If you use a generator, place it outdoors and plug individual appliances directly into it, using a heavy-duty extension cord. Connecting generators directly to household circuits creates “backfeed,” which is dangerous to repair crews.
• Leave the doors of your refrigerator and freezer closed to keep food as fresh as possible. Place blocks of ice inside to help keep food cold. Check food carefully for signs of spoilage. 
• Check on your neighbors to make sure everyone is safe.

Tips on staying warm

• State law requires residential units to have heating systems that can keep indoor temperatures at a minimum of 70 degrees. That means every dwelling unit and guest room offered for rent or lease should offer heating equipment.
• Use heat smartly to save money: Cranking heaters can be expensive. If money is tight, be judicious about how and when you use your utilities. For example, only use heaters at night or only set the thermostat to around 70 degrees.
• Open and close those vents: If you have central A/C, look at where the vents are around your home. Are any open in places where you don’t stay long? Practice opening and closing those so warm air only goes where you need it (most vents should have a small toggle lever). Humidifiers can also help you warm things up — and it’s useful to add moisture into our dry air.
• Adjust your wall heaters: If you have a wall heater, you can change the output by adjusting the knob (usually at the bottom). Since wall heaters can only warm the areas where they’re placed, it’s essential to close doors to rooms you won’t be in so hot air doesn’t get wasted.
• Turn on your ceiling fan (really): If you have a ceiling fan, try turning it on. This sounds counterintuitive, but there’s science behind it. TSince hot air floats up, your fan can help move it around. For warming, your fan should spin clockwise to create an updraft. Not all fans will have this option.
  

Sign up for emergency alerts

• L.A. City: Notify L.A.
• L.A. County: Ready L.A. County
• Ventura County: Ready Ventura County
• Orange County: AlertOC
• Riverside County: AlertRivCo
• San Bernardino County

How we're reporting on this

This is a developing story. We fact check everything and rely only on information from credible sources (think fire, police, government officials and reporters on the ground). Sometimes, however, we make mistakes or initial reports turn out to be wrong. In all cases, we strive to bring you the most accurate information in real time and will update this story as new information becomes available.