This is the latest from the L.A. Times
- The Woolsey fire has scorched 70,000 acres, burning homes in Malibu, Westlake Village and Thousand Oaks while threatening parts of Simi Valley and West Hills. At least 250,000 residents have been evacuated.
- The Hill fire pushed through canyons to the edge of Camarillo Springs and Cal State Channel Islands. The 4,500-acre fire is now 25 percent contained.
- The Camp fire in Northern California’s Butte County has left nine dead and destroyed nearly 6,500 structures. It’s the state’s most destructive fire.
Well over half of the state has red flag warnings. This is due to the Santa Ana winds.
The National Weather Service defines Santa Ana winds as “Strong down slope winds that blow through the mountain passes in southern California. These winds, which can easily exceed 40 miles per hour (18 m/s), are warm and dry and can severely exacerbate brush or forest fires, especially under drought conditions.”
The Santa Anas are katabatic winds—Greek for “flowing downhill”, arising in higher altitudes and blowing down towards sea level. Santa Ana winds originate from high-pressure airmasses over the Great Basin and upper Mojave Desert. Any low-pressure area over the Pacific Ocean, off the coast of California, can change the stability of the Great Basin High, causing a pressure gradient that turns the synoptic scale winds southward down the eastern side of the Sierra Nevada and into the Southern California region.
Cool, dry air flows outward in a clockwise spiral from the high pressure center. This cool, dry airmass sweeps across the deserts of eastern California toward the coast, and encounters the towering Transverse Ranges, which separate coastal Southern California from the deserts. The airmass, flowing from high pressure in the Great Basin to a low pressure center off the coast, takes the path of least resistance by channeling through the mountain passes to the lower coastal elevations, as the low pressure area off the coast pulls the airmass offshore.
These passes include the Soledad Pass, the Cajon Pass, and the San Gorgonio Pass, all well known for exaggerating Santa Anas as they are funneled through. As the wind narrows and is compressed into the passes its velocity increases dramatically, often to near-gale force or above. At the same time, as the air descends from higher elevation to lower, the temperature and barometric pressure increase, warming about 5 °F for each 1,000 feet it descends (10 °C for each 1,000 m). Relative humidity decreases with the increasing temperature. The air has already been dried by orographic lift before reaching the Great Basin, as well as by subsidence from the upper atmosphere, so this additional warming often causes relative humidity to fall below 10 percent. The end result is a strong, warm, and very dry wind blowing out of the bottom of mountain passes into the valleys and coastal plain.
During Santa Ana conditions it is typically hotter along the coast than in the deserts, with the Southern California coastal region reaching some of its highest annual temperatures in autumn rather than summer.
While the Santa Anas are katabatic, they are not Föhn winds. These result from precipitation on the windward side of a mountain range which releases latent heat into the atmosphere which is then warmer on the leeward side (e.g., the Chinook or the original Föhn).
If the Santa Anas are strong, the usual day-time sea breeze may not arise, or develop weak later in the day because the strong offshore desert winds oppose the on-shore sea breeze. At night, the Santa Ana Winds merge with the land breeze blowing from land to sea and strengthen because the inland desert cools more than the ocean due to differences in the heat capacity and because there is no competing sea breeze.
The Santa Ana winds and the accompanying raging wildfires have been a part of the ecosystem of the Los Angeles Basin for over 5,000 years, dating back to the earliest habitation of the region by the Tongva and Tataviam peoples.
Consider having to move a quarter million people to safe areas as they have had to do in California… this would be like depopulating both Grand Rapids and Kalamazoo. Entire communities have been burned along with thousands of homes. California has 40 million people compared to 10 million for Michigan.
I have never considered visiting or living in California mainly due to earthquake and fire hazards. The San Andreas runs through the state and is long over due for a major quake.
The 1906 San Francisco earthquake was the last quake greater than magnitude 7 to occur on the San Andreas Fault system. The inexorable motions of plate tectonics mean that every year, strands of the fault system accumulate stresses that correspond to a seismic slip of millimeters to centimeters. Eventually, these stresses will be released suddenly in earthquakes.
But the central-southern stretch of the San Andreas Fault has not slipped since 1857, and the southernmost segment may not have ruptured since 1680. The highly urbanized Hayward Fault in the East Bay region has not generated a major earthquake since 1868.
Reflecting this deficit, the Uniform California Earthquake Rupture Forecast estimates that there is a 93 percent probability of a 7.0 or larger earthquake occurring in the Golden State region by 2045, with the highest probabilities occurring along the San Andreas Fault system.
Enough of this scary talk, though to some cold and snow are nothing to laugh about. We currently have 28° in Otsego, yesterday we made it to 34° with a coating of snow on the ground. We had maybe an inch or two of snow, hard to tell with it packing down so quickly. We also had a lot of leaves on top of the snow from the winds which blew through.
The cold air will continue until Thursday when we may break 40° but cold air will return next weekend.
Happy Veterans day to all of those who have served in our armed services – thanks to all of those who have dedicated their lives for the freedoms we enjoy. Thanks to SS for the photo from D.C. she took on a recent trip.