Monday, November 16, 2015

Intense November Weather in Central California...and Tornadoes?

Credit: Jacquetta Wehking

I tell my students on occasion that they live in a place that is relatively free of the most serious geological hazards. We are a fair distance from the earthquake faults that wreak havoc on the coastal cities (the San Andreas and Hayward faults, for instance). We mostly live on a river terrace so serious floods do not impact most of the city (the floodplain developments are an exception of course). We live far from volcanoes or steep slopes that can fail. But I also warn them that there is no place in the world that is entirely free of geologic hazards. I got reminded of that today. A tornado touched down just a few miles from my house, causing some serious damage, but thankfully no one was hurt. Friends were witness to the event. It was uncomfortably close.

Tornadoes are not all that rare in California. No one could mistake our funnel clouds for the monsters that regularly roar down through Tornado Alley in the central United States, but there have been 403 confirmed events between 1950 and 2013. The majority have been weak, as only 25 of them reached EF2 or more. But they do happen, and there was enough energy in the latest storm to produce one this afternoon.
It's been a wild weather month here in central California. I went walking to see the retreating edge of the tornado-forming storm from the new parkway they're building along the Tuolumne River. It's looking very different than it has for most of the last four years. It's wet. Ponds of water covered parts of the recently aligned trail.

We've had three major storms in as many weeks, and they followed two weaker storms last month. The month is half over, but it is already the second wettest November in the twenty-five years I've been keeping records at my home east of Modesto in the Great Valley. We've received 2.48 inches, which is equivalent to nearly a fifth of year's normal precipitation. It would be nice if this were a harbinger of more storms to come. We've been in the throes of the worst drought in the historical record. Last year's snowpack in the Sierra Nevada amounted to less than five percent of normal. Our reservoirs are just about empty, and the shortfall would not be made up even if we had a record year of precipitation.
As much as we hope for an end to the drought, it's a clear case of being careful what you wish for. El Nino weather years are known for producing intense weather events, causing flooding and mudslides in central and southern California. And the El Nino that has been building in the Pacific Ocean this year is a record-breaker, a storm-maker on steroids. Our storms so far haven't even been warm storms out of the south. The main effects aren't really expected for another month or so.
As I walked this afternoon, I noted that there was already a small slope failure along the trail. It did no damage, but it's a warning of what may be coming.  I was reminded of the floods of 1997 when the Tuolumne River was flowing at 70,000 cubic feet per second, the highest ever recorded. The river threatened numerous bridges, and flooded more than 1,000 homes downstream. And the flood could have been worse had Don Pedro Reservoir upstream hadn't held back the highest flows (at one point, the inflows exceeded 130,000 cubic feet per second).
One thing I am anxious to see happen is a high enough runoff to scoop out the hyacinth plants that are threatening to cover the river. The invasive weed crowds out the native plants, blocks light from the river bottom, and lowers the oxygen content of the water. It's been around for years, but the infestation has been particular bad in this year of constantly low flows.
I turned around and reveled in the sunset and fall colors that have finally arrived. I know that a lot of you live in places where the fall colors are astounding and all, but California is simply a darn nice place to live, despite all her problems. Here I was walking in a light shirt and short pants, in the middle of November still enjoying the changing leaves of the trees. I can visit snow if I want to, but I won't be shoveling it. It's been a tough four years of drought, but maybe the tide is turning. A record wet November is certainly a good start. I can do without the tornadoes, though.

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