Friday, 30 November 2012

California: Extreme wether forecast

More weather chaos coming!

California Flood Threat From "Atmospheric River"

29 November, 2012

Meteorologists use the term "atmospheric river" to describe a long, narrow plume piping deep moisture from the tropics into the mid-latitudes. One type of atmospheric river you may have heard of is the "Pineapple Express", a pronounced plume tapping moisture from the Hawaiian Islands to the U.S. West Coast.

Amazingly, according to NOAA's Earth System Research Laboratory (ESRL), a strong atmospheric river can transport as water vapor up to 15 times the average flow of liquid water at the mouth of the Mississippi River!

Suffice to say, if an atmospheric river stalls over a particular area, significant flooding can be the result. In fact, a study by Ralph et al. (2006) found atmospheric rivers responsible for every flood of northern Calfornia's Russian River in a 7-year period.

That said, they're also important for western water supply considerations.

According to NOAA/ESRL, 30-50% of the average annual precipitation in the West Coast states typically occurs in just a few atmospheric river events.

With that in mind, one such atmospheric river is now soaking parts of the West Coast and will continue to do so through the weekend. Let's get to the forecast details

Flood Threat Through the Weekend

The graphic at the top of this article depicts the upper-air pattern that will stay in place through the weekend. Namely, a deep dip, or trough, in the jet stream is currently in place over the eastern Pacific Ocean.

This will continue to send a parade of frontal systems and upper-level disturbances into the West Coast.

The first storm arrived on Wednesday and brought rainfall from California to the Pacific Northwest. The combination of wind and rain led to hefty delays at San Francisco International Airport.

The upper-level pattern has now tapped into the atmospheric river of moisture extending from just north and west of Hawaii to the West Coast. This will continue to send the ongoing latest round of very heavy rain and gusty winds into northern California through Friday afternoon. Expect a very wet commute with more possible airport delays in the Bay Area Friday morning.

Though rain showers will continue Friday night into Saturday, the intensity should let up some before the next round of very heavy rain and strong winds arrives in northern California and southwest Oregon for Saturday night into Sunday.

Rain Forecast

Since the plume of moisture won't move appreciably for a couple of days, some locations, particularly in the coastal ranges of northwest California and the Sierra foothills, will likely pick up over 10 inches of total rainfall through this weekend, leading to flash flooding, river flooding and, in recent burn areas, debris flows.

Depending on exactly where the moisture plume sets up, this heavy rain could produce significant impacts (flash flooding, rock/mudslides, etc.) in at least parts of the Bay Area, as well.

Rainfall amounts in Southern California are expected to be much, much lighter.

Heavy Snow Confined to High Terrain

This pattern will also produce heavy snow over the Sierra, not to mention parts of the Bitterroots, Tetons, and the Washington Cascades.

For Northern California our Winter Weather Expert Tom Niziol says, "This is a relatively mild pattern, therefore snow levels will be above 7,000 feet for most of the time with impacts mainly above pass levels. To put this in perspective, the highest elevation of I-80 through Donner Pass is about 7,230 ft. Snow totals at highest elevations will easily exceed 1 to 2 feet, but once again well above where most people live."

Niziol adds, "Farther north through the Washington Cascades, snow levels will drop to 4,000 to 5,000 feet later in the weekend with a foot or more snow likely. Across the Sawtooth through Bitterroots in Idaho, heavy snowfall will also occur. Elevations will once again be above 7,000 to 8,000 feet, for argument's sake, peaks will see well over 3 feet of snow in the Sawtooth Recreation area but once again, well above where most people live."

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