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Posted at 12:51 PM ET, 11/29/2012

Atmospheric river to blast northern California with torrential rains

Water vapor image shows lengthy feed of moisture stretching from the central Pacific into California (NOAA)

A deep feed of Pacific moisture is streaming into the West Coast, and is forecast to produce incredible amounts of rain and mountain snow. Around a foot of rain is forecast in northern California and southwest Oregon over the next 5 days. At very high elevations, more than 100 inches of snow is possible.

This onslaught of heavy precipitation-producing weather systems flows along a phenomenon known as an atmospheric river (AR) where winds direct moisture along a narrow path. As this so-called river’s moisture plume often extends back to Hawaii, it has earned regional names such as the “Pineapple Express” and “Hawaiian firehose.”

The amount of moisture these ARs can carry is unbelievable.

“A strong AR transports an amount of water vapor roughly equivalent to 7.5–15 times the average flow of liquid water at the mouth of the Mississippi River,” writes NOAA.

Although ARs can cause severe flooding, they are a very important contributor to regional water supply.

“On average, about 30-50% of annual precipitation in the west coast states occurs in just a few AR events,” NOAA adds.

5-day rainfall forecast. (NWS HPC)
The National Weather Service forecast office in Sacramento is forecasting rainfall amounts ranging from 2 to 8 inches in valley areas to 10 to 20 inches in the mountains through Sunday. Flooding and mudslides are expected as a result. Here’s its summary of the effects expected in Sacramento and the surrounding area of central and northern California:

More storm systems are on their way to Northern California, bringing significant rainfall and windy conditions with it. Mud and debris flows are possible, and some tree branches are likely to break off causing possible power outages. High reservoir capacity will mitigate main stem river flooding concerns, though the Upper Sacramento River is forecast to approach flood stage as a result of the upcoming rainfall. Urban and small stream flooding is likely.

Model of moisture transport in 24 hours ( NOAA )
The National Weather Service’s Hydrometeorological Prediction Center indicates “moisture flux anomalies” are 3 to 4 standard deviations above normal. In plain English, this simply means the amount of incoming moisture is extraordinarily high. Up to 5 inches of rain in 24 hours is possible in northwest California, HPC says.

Because of the tropical air being transported by the AR, snow levels are relatively high, mainly above 7,000-8,000 feet. The National Weather Service forecast for Mt. Shasta in northern California, at an elevation of over 14,000 feet indicates around 200 inches of snow is possible by Sunday.

In addition to the extremely heavy bouts of precipitation expected through the weekend, strong winds are also forecast. Wind gusts to 55 mph are possible at lower elevations and up to 70 mph or so at high altitudes from central California into southern Oregon.

By  |  12:51 PM ET, 11/29/2012

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