(CNN) -- Significant widespread flooding is possible across much of California on Monday as more heavy rain hits the state, forecasters say.

"The longevity and intensity of rain, combined with the cumulative effect of successive heavy rain events dating back to the end of December, will lead to widespread and potentially significant flood impacts," the Weather Prediction Center said Sunday morning.

A "significant" atmospheric river event is expected to impact much of the state early this week, according to the prediction center.

Two major bouts of rain will impact the West Coast over the next few days. The concern is not just the rain, snow and wind, but there will be not much of a break in between events for the water to recede or cleanup to be completed.

"Numerous flash flooding events likely, some possibly significant, especially over burn scars," the prediction center said.

The storms come on the heels of a powerful cyclone which flooded roads, toppled trees and knocked out power to more than 500,000 customers on Saturday evening and Sunday morning, according to PowerOutage.US. And a New Year's weekend storm also produced flooding rains across the state, which is already off to a very wet start to the year.

Flooding impacts have already been reported in the city, according to San Francisco Department of Emergency Management Executive Director Mary Ellen Carroll.

"We're seeing sinkholes on our streets -- a few of them. We're seeing mudslides -- nothing significant at this point. But the more rain we get and the less time in between, we know we're going to see more of those conditions," Carroll told CNN.

The city's communications infrastructure, cellular and internet, is underground so "as we get more inundation from the rain, we're seeing more failure around those, what we call lifeline systems" for power and communication, said Carroll.

Record rainfall sets the stage

 

Over 15 million people are under flood watches across much of California ahead of this atmospheric river event which could bring several more inches of rain to the state through Tuesday.

"While some of the forecast rain totals are impressive alone, it is important to note that what really sets this event apart are the antecedent conditions," the National Weather Service office in San Francisco said. "Multiple systems over the past week have saturated soil, increased flow in rivers and streams, and truly set the stage for this to become a high impact event."

Last week, San Francisco experienced its wettest 10-day period on record for downtown since 1871. So far they have had more than a foot of rain just since December 1, and the forecast calls for an additional 3 to 5 inches of rain in the next five days.

Much of the state has already seen 5 to 8 inches of rain over the last week which has greatly saturated the soils. And additional rainfall totals of 2 to 4 inches are expected across the coasts and valleys, however the mountains and foothills are forecast to see up to 4 to 8 inches through Tuesday.

"With the heavy rain expected, and already very wet soils in place, there will be an increased risk for rock/mudslide activity in the local mountains and canyon roadways," the weather service in Los Angeles said.

This has led the prediction center to issue a Level 3 out of 4 risk for excessive rainfall for over 15 million people in the state on Monday including those in San Francisco, Sacramento, Monterey, Fresno, and Oxnard. A Level 4 out of 4 "high risk" may become necessary for Monday if the forecast guidance continues to increase rainfall totals, the prediction center wrote in their discussion Sunday morning.

The rainfall over the weekend brought renewed flood concerns for local streams, creeks, and rivers. The Colgan Creek, Berryessa Creek, Mark West Creek, Green Valley Creek, and the Cosumnes River all have gauges either currently above flood stage or expected to be in the next few days.

"Tuesday is probably the day where you'll likely need to keep a really close eye on the weather as the potential for widespread flooding of rivers, creeks, streams and roadway and urban flooding will be at its highest during the next week as all the runoff and heavy precipitation comes together resulting in a mess," the weather service office in Sacramento said.

The main concerns for the coastal communities will be widespread flooding, gusty winds, and dangerous beach and marine conditions. In the higher elevations it will be heavy snow and strong winds leading to near whiteout conditions for anyone traveling on the roads.

In addition to the heavy rainfall, another round of strong winds is expected to accompany the storm as it pushes inland, which could lead to even more downed trees and power lines causing additional power outages. As the storm pushes farther inland, heavy new snow is forecast across the Sierras which could exceed 5 feet through Tuesday.

"Plan on significant travel delays in the Sierra," the weather service office in Reno said.

Even after this major system Monday and Tuesday moves out, the possibility of yet another impactful system is already on the horizon. Guidance is slowly increasing for a new system at the end of the week to impact Northern California, though it is still too far out for more details at this point.

"Overall, there is high confidence (60-80%) that this wetter-than-normal pattern will continue through the next couple of weeks," the weather service in San Francisco said. "While we don't have details on how much rain above normal will fall, suffice it to say that the continuation of saturated soils could continue to pose hazards into the third week of January."

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