Atmospheric river begins: Torrential rain to trigger flooding and mudslides in California
A fast-moving, potent storm will deliver inches of rain in California and feet of snow to the Sierra Nevada. The storm will have dangerous, damaging and disruptive consequences.
AccuWeather California Expert Ken Clark analyzes the heavy rain and snow that’s barreling over the Golden State as of Feb. 13, which could lead to road closures, flooding and landslides.
The biggest storm of the winter, packing a firehose of rain across much of California, will trigger flooding, mudslides and road closures that may put lives and property at risk into the end of the week. Feet of snow will also pile up over the high country of the Sierra Nevada, AccuWeather meteorologists warn.
Rain and mountain snow are much needed in California. Storms were nearly absent prior to the last part of January back through the start of the rainy season in October, resulting in a drought in the region's soil and vegetation. The conditions, combined with dry winds known as Sundowner and Santa Ana events earlier this winter, resulted in some of the worst wildfires in terms of speed, lives lost and damage the state has ever experienced.
However, as is often the case in California, when rain falls, it does so with a vengeance.
A major storm packing a plume of moisture that has evolved into quick-moving atmospheric river will slide southward along the California coast. The firehose effect will direct a period of torrential rain along the shoreline, the Coast Ranges and the lower west-facing elevations of the Sierra Nevada, into Friday.
"A general 2-4 inches of rain will fall along the lower elevations of the California coast but with 4-8 inches in the west- and southwest-facing slopes of the coastal mountains with a Local StormMax™ of 11 inches," AccuWeather On-Air Meteorologist Ariella Scalese said.
Much of the storm's rain is likely to fall in 12 hours or less, which will rapidly run off and could lead to disastrous flash flooding, mudslides and hillside collapses in steep terrain areas.
"Making matters much worse will be the burn scars of the recent wildfires that swept through South California," AccuWeather Senior Director of Forecasting Operations Dan DePodwin said, "There is little to hold back the water and mudflows as the onset of storms demonstrated in late January."
"Some of the runoff can be toxic in some locations as it washes ash and other debris into streams and then reaches the beaches," DePodwin added.
"2-4 feet of snow will fall on the Sierra Nevada above 6,000 feet," AccuWeather Chief On-Air Meteorologist Bernie Rayno said, "Local amounts up to 6 feet can occur over some of the ridges and peaks in the California mountain range."
While this is just what the High Country and ski resorts need, it will lead to temporary problems. Roads will likely close, including along Interstate 80 and Donner Pass, California. The snow can pile up at 2-4 inches per hour, which crews are unlikely to keep up with while the storm is active.
Along with the likelihood of flooding, mudslides and feet of snow in the Sierra Nevada, the atmosphere may get wild enough to trigger thunderstorms on Thursday. Of the multiple thunderstorms that erupt in the central part of California, a few can become robust with gusty winds and hail. A smaller number still can become severe with damaging wind gusts and even a couple of tornadoes or waterspouts.
While there was no shortage of water, reservoir and stream levels were dropping following months of nearly bone-dry conditions since last spring. Dry conditions during the late spring to the start of autumn are normal. It's the lack of rain and mountain snow from midautumn through the first part of the winter that is abnormal.
Storms began to ramp up during the last days of January and into the first part of February, but rainfall and the Sierra snowpack were well short of the historical average, especially in the southern areas.
Since the start of the rainy season in October, Southern California has only received 5 to 35% of its historical average rainfall.
The storm during the latter part of this week will help bring rainfall and mountain snow back on track, but more non-flooding storms will be needed into early spring. The snow that falls on the High Country in the winter and early spring becomes the vital runoff into area streams, rivers and reservoirs for the water supply during warmer, drier months.
A period of storm-free conditions lasting a week or more will follow heavy rain and mountain snow into Friday. The next potential precipitation-producer may not occur until near the end of the month.
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