#251 CALIFORNIA 2012 SUPERSTORM

TAHOE NUGGET #251: CALIFORNIA’S 2012 SUPERSTORM

California’s version of a Pacific-bred “superstorm” is currently pounding the central and northern portions of the State with damaging wind gusts and torrential rain that’s generating urban and small stream flooding. Thousands are already without power in the San Francisco Bay Area and the strongest, most potent of three storm surges is still more than 24 hours away.

This graphic depicts jet stream position with core wind speeds exceeding 140 mph. Note wind gusts up to 80 mph in western Nevada. Driving back from Reno on Thursday, Nov. 29, I observed a brush fire ripping through dry sagebrush and scrub just south of Interstate 80. 

Relatively mild temperatures are keeping snow levels generally above 7,000 feet meaning mostly rain at Lake Tahoe beaches with heavy wet snow slowly piling up on the upper slopes. Even with its higher base elevation, Kirkwood Ski Resort has picked up less than two feet on the upper mountain so far due to the slushy, condensed nature of the snow.

With the arrival of the final moisture surge late Saturday through Sunday, freezing levels are projected to rise well above 8,000 feet limiting snowfall accumulations at Lake Tahoe resorts.

Projected storm total precipitation amounts approach 15 inches near Lake Tahoe.

As the first wave of moisture moved into Northern California two days ago I decided to check out the forecast for the upper elevations of Mt. Shasta where the precipitation will be all snow. According to the National Weather Service, anticipated total snowfall accumulation at 12,366 feet is at least 18 feet, with more possible. If measureable snow falls every 24 hours (which seems likely), it can be considered a single storm event that may set a new Sierra record.

California’s current single snowstorm record is 189 inches (15.75 feet) measured over 7 days at the Mt. Shasta Ski Bowl in February 1959. Only wrinkle in this formula is that the Mt. Shasta Ski Bowl (where they'll measure the snow) is much lower than 12,000 feet and the snow will lose a lot of its loft which means lesser totals. Despite the rain in Tahoe, with any luck we’ll set a new state snowfall record at Mt. Shasta this week!

Daily measured totals during Mt. Shasta's 1959 record single snowstorm event.

This storm series will continue to make headlines over the next 48 hours, but one thing is clear. The timing of this rough weather couldn’t be better. A wet and warm system like this one is the principle culprit that causes virtually all the wintertime floods that strike California and western Nevada.

In 2009, scientists spent time and money to see what would happen if California picked up 8 feet of rain in 3 weeks. The media dubbed the resulting catastrophic flood event "Frankenstorm." You have to wonder if this is an example of over-the-top newspaper hyperbole or climate change fear mongering?

Known as a wet mantle event, heavy precipitation coupled with high freezing levels pumps a huge amount of water into regional streams and rivers, sometimes even overwhelming reservoir capacity.

Deeper into the winter season the lower elevation snowpack is usually well-established and susceptible  to rapid melting, which adds water from previous cold storms to a drainage system already challenged by the typical 10 to 20 inches of rain that accompany these “atmospheric river” events on the West Coast. Several locations in the North State have reported more than 7 inches of rain in the past 24 hours alone.

Infrared satellite image from 4 kilometers above surface shows deep moisture plume surging into northern California on November 30.

These semi-tropical surges of heavy precipitation are California’s version of a hurricane with similar impacts over a large swath of territory. This time around, however, there is no low elevation snowpack to melt and regional reservoirs (including Tahoe) are all boasting hefty storage capacity. And with much of Western Nevada and Eastern California suffering serious drought conditions any spillover will help alleviate dry soil conditions there.

Even with the good news about available storage capacity, the Truckee River watershed is getting hammered and portions of the Truckee are under a flood warning, particularly between Squaw Creek and Truckee, as well as the lower stretches at Farad, Reno and Vista. The highest flows will be on Sunday (Dec. 2) due to saturated soil and continuing heavy rain.

Downstream of Squaw Valley, the Truckee River is projected to exceed flood stage by nearly 4 feet with a flow of 7,800 cubic feet per second. At Farad, near the California-Nevada border, the Truckee River is expected to hit 12,600 cfs, which is about one foot above flood stage. Moderate damage is possible to low-lying trailer and public parks, as well as some roads and bridges.

A wet mantle flood around New Year's Day 1997 caused major damage to bridges on the upper portions of the Truckee River. Generally most damage occurs on the lower reaches of the river near Reno. Flood warnings are currently posted for this stretch of river on Sunday.

This weekend the NWS is anticipating hydrological impacts similar to Truckee River floods that occurred in 1955, 1963 and 1964. Check out my overview of historic Nevada floods.

To read about the February 1963 flood event, check out Tahoe Nugget #227:

I’m currently developing a profile about how authorities handled the Truckee River Christmas flood of December 1964 to be published in an upcoming Nugget. Stay tuned and stay dry.

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2 comments (Add your own)

1. Joe Kruvczuk wrote:
So, will this help you annual rain fall stats?

Fri, November 30, 2012 @ 2:47 PM

2. Mark wrote:
Hey Joe,
We're off to a strong start this winter. Had about 7 feet of snow in October and early November and now all this precipitation. Wettest months (climatologically)
are still ahead!
Mark

Fri, November 30, 2012 @ 3:44 PM

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