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5 things to know about the powerful storms hitting LA

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The strongest storms in years

Rain in Hollywood in January 2017.
Hayk_Shalunts / Shutterstock

Drizzle started falling Friday morning in Los Angeles, signaling the arrival of back-to-back storms that have the potential to generate the most rain Southern California has seen in six years, according to the National Weather Service.

At their peak, these slow-moving monster storms could produce 1 inch or rain per hour and unleash strong, gusty winds.

"This will likely go down in the record books as one of the wettest February days ever," said NBC4 forecaster Crystal Egger. NBC said some communities could see a month's worth of rain in one day in what has already shaped up to be an unusually wet winter.

As with another big storm that hit the region last month, these will be the result of a weather system known as an “atmospheric river,” which moves high concentrations of water vapor from the tropical Pacific and blasts it at the West Coast, like a fire hose.

Animation of an Atmospheric River event in January 2017
Courtesy of NOAA/ESRL/PSD

Here are five other things to know about these storms, which the National Weather Service advises should be taken “very seriously.”

1. When are heavy rains expected?

Forecasters say drizzle will gradually increase in intensity during the day to full-on showers, if not downpours. There will likely be a 12-hour period of rainfall, and half of that will be “intense,” meteorologist Tom Fisher said.

The main front is forecast to move out of LA by late Friday evening, but thunderstorms and scattered showers could linger through Saturday evening.

2. How much rain are we going to get?

In the low-lying areas in and around Downtown Los Angeles, expect 3 to 6 inches of rain. The foothills and mountains could see 5 to 10 inches, with the south-facing slopes of the foothills and mountains getting the biggest soaking.

3. How much rain has already fallen?

City News Service reports that Downtown has recorded a whopping 16.25 inches so far this season, which runs from October to April. That’s almost double the normal amount of 8.96 inches.

4. What about snow?

Since the atmospheric river will tap into warmer water vapor, snow levels will remain relatively high at first, at above 8,000 through Friday evening before falling to as low as 5,000 feet Saturday morning.

“Snow amounts will really start to add up after this afternoon as colder air aloft punches in,” the Weather Service said.

Above 8,000 feet, 1 to 2 feet of snow is possible, with 6 inches to 1 foot expected in elevations from 6,000 to 7,500 feet.

5. Will there be flooding?

Officials are bracing for what some media have dubbed a “monster” storm. Evacuations were ordered Thursday in Duarte in areas impacted by the Fish Fire.

A flash flood watch is in effect from Friday morning through Saturday morning.

“The potential for widespread flooding and flash flooding is a definite threat. Not just for the recent burn areas, but anywhere there is the potential for flooding,” the Weather Service said.