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Get Out The Ark? 5 Day Storm A Real Soaker

Writing an article for the California Citrograph magazine back in August about the prospects for a wet winter in California meteorologist Alan Fox wrote:

“We expect that the combined contributions of El Nino and expected PNA pattern (cold SSTA area off northern California) will encourage recurrent periods of storms moving into central and southern California during mid-December 2012 and January 2013. Because of the proximity of cold airmasses and the supply of cold air from the west and northwest into California, we expect that snow amounts will be abundant in the Sierra Nevada this season for the region that includes
the following watersheds: Upper San Joaquin, upper Kings, Merced, Stanislaus, Tuolumne, American, Yuba, and south portion of the Feather.”

Looks like the pattern may be emerging a few weeks early and with an attitude.

This week Alan’s privately circulated Fox Weather report is predicting a real soaker reminiscent of the weak El Nino year of 1969 he says – what he calls an ”humongous storm” that is now expected to hit Northern and Central California over a five day period – November  28,29,30 and December 1& 2.

Particularly strong will be rainfall totals on the 30th (Friday) he says with Sacramento getting 1.6 inches in one hour and 14 inches of rain in 24 hours hitting the Napa region. He predicts up to 2.5 inch an hour in the Feather River area.

Rainfall will be impressive in the Santa Lucia mountains on SLO’s northern boundary with a five day total of 21.6 inches on Anderson Peak and 17 inches in Big Sur, he predicts.The Santa Cruz foothills could receive 12.51 inches over this period , all sand bag numbers.

In the mid San Joaquin Valley, Fox is predicting the five day total should be 3 inches in Fresno and 2.86 in Visalia but falling well off further south with Bakersfield receiving just 0.39 inches for the five days.Southern California may not get much either. Mariposa, above Fresno, gets over 10 inches as the Sierra foothills get plenty of rain and runoff.

The upper San Joaquin watershed- key for water users in the Central Valley would get 13.4 inches of water at Tamarack at 7600 ft and 33 inches of snow,nearly 3 ft.That will make them happy at nearby China Peak. The nearby Huntington weather station has only 5 inches in the bucket so far this year with an annual average of 42 inches .So this five day total would take it to approaching  half a years supply well before Christmas,good news for farmers.

Bear Valley skiers will be happy getting near 46 inches at Gianeli at 8400 ft.

If there is good news for the snowpack there may be flooding problems in the Valley and especially the Delta region where the storm will target.

Much of Central California’s water supply and agricultural areas are protected by a set of levees along the Sacramento and San Joaquin Rivers that are in serious danger of failure during an extreme flood or major earthquake. The 1,600 miles of levees protect 500,000 people, 2 million acres of farmland, and structures worth $47 billion. Of particular concern is the delta at the confluence of California’s Sacramento and San Joaquin rivers, about 80 miles inland from San Francisco Bay. The Delta Region receives runoff from more than 40% of California, and is the hub of California’s water supply system, supplying water to 25 million people and 3 million acres of farmland.

Fox says this storm pattern is not just the largest one this year but  perhaps biblical in scale – an ARkStorm (for Atmospheric River 1000 Storm), It is a hypothetical but scientifically realistic “superstorm” scenario developed and published by the United States Geological Survey, Multi Hazards Demonstration Project (MHDP). It describes an extreme storm that may impact much of California causing up to $725 billion in damages and repair (most caused by flooding), and affect a quarter of California’s homes.

Such an event would be similar to intense California storms which occurred in 1861 and 1862.[1] The name “ARkStorm” means “Atmospheric River (AR) 1000 (k).” The name was created as a way of quantifying the magnitude of west coast storms. It also meant to be drawn as a parallel to the biblical Noah’s Ark story.

So far the NWS SF says there are some chances for flooding over the coming weekend but does not see the potential for a massive event .

Another Take

Here is another take of the effect of these upcoming storms on the SLO region from PG&E forecaster John Lindsey.

A cold front from this low pressure system will pass the Central Coast on Wednesday between 10 a.m. and 2 p.m. with
moderate gale to fresh gale force (32- to 46-mph) southeasterly winds along the coastline and moderate rain throughout San Luis Obispo County.

Rainfall amounts on Wednesday will range between 0.75 and 1.25  inches with the coastal mountains receiving over 2.00 inches. Snow will develop in the Sierra above 5,500 and 7,000 feet on
Wednesday with a foot of new snow by Thursday morning.

Moderate to fresh (13- to 24-mph) southerly winds and a few scattered rain showers are expected on Thursday.

A stronger and even wetter Gulf of Alaska storm system is forecast to move southeastward towards the Central Coast on Friday. The associated cold front will reach northern
San Luis Obispo County by Friday evening producing rain as far south as Cambria. This system will move slowly southward  with rain reaching Morro Bay by Saturday afternoon.

Widespread moderate to heavy rain will spread to the rest of the County by Saturday night and will continue through Sunday morning.

Along with the rain, fresh gale to strong gale force (39- to 54-mph) southwesterly winds are forecast along the coastline  on Sunday morning.

The storm has a subtropical tap of moisture and rainfall amounts will be impressive with 1 and 3 inches across the low elevation locations and 4 to 6 inches of precipitation in the mountains.
Snow levels will be high and generally above 7,500 feet.

Overall, it will be a very wet and windy week and weekend starting Wednesday.

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