Weather Watch

California jet streams create the perfect storm for SoCal wildfires

Trump approves California disaster declaration as wildfires rage on

More than 500 structures have been damaged and more than 120,000 people have evacuated as fires burn in half a dozen counties in Southern California. President Donald Trump approved a California disaster declaration Friday, December 8, 2017, order
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More than 500 structures have been damaged and more than 120,000 people have evacuated as fires burn in half a dozen counties in Southern California. President Donald Trump approved a California disaster declaration Friday, December 8, 2017, order

Like the Great Wall of China, a ridge of high pressure has developed over the West Coast of the United States and has blocked the storm track from the delivery of precious rain this month. In fact, not only has California been abnormally dry and warm, but Oregon and Washington have also seen mostly clear skies — a December rarity.

The jet stream has taken the shape of Greek letter omega, (Ω) due to this strong and persistent ridge of high pressure. However, travel east about 1,500 miles from California to the other side of this Omega block, and it’s a vastly different story. The jet stream has been moving rapidly southward from Canada through the Midwest toward the Gulf of Mexico and has brought a frigid air mass from the Arctic southward to Texas. Last week, Corpus Christi on the southern Gulf Coast recorded snow for the first time since 2004.

Closer to home, the persistence and at times gusty Santa Lucia (northeasterly) winds combined with mostly clear skies have produced well above typical daytime temperatures. So far, they have averaged around 10 degrees warmer than normal for the month of December.

Paso Robles’ normal high for the month is 59.4 degrees, but so far this December, it’s at 68.8. San Luis Obispo has averaged 74.5; typically it’s 63.8 for this month. Santa Maria has averaged 73.8, exactly 10 degrees above normal.

Both San Luis Obispo and Santa Maria have seen typical overnight lows for December despite the clear skies, which allows the Earth’s heat to radiate out in space, but compensated for by the Santa Lucia winds. These winds are often referred to as downslope, or lovingly described by surfers as offshore winds. These downslope winds are technically called katabatic wind from the Greek word katabatikos, which means “going downhill.” As the air mass descends the side of the mountain range, it warms at the rate of about 5.5 degrees Fahrenheit per 1,000 feet of descent.

Clear skies 7_edited-1
A clear December day in Morro Bay, created by a ridge of high pressure over California. John Lindsey Special to The Tribune

On the other side of the Santa Lucia Mountains, Paso Robles has seen an overnight low average of 25.4 degrees, or about 8 degrees cooler than normal. In fact, the daily temperatures have fluctuated greatly from night to day. On Dec. 7, the Paso Robles Municipal Airport reported a 51-degree temperature swing between the morning low and daytime high.

This Great Wall of China high-pressure ridge has also extended the fire season. Last year’s rain season (July 1, 2016, through June 30, 2017) delivered above-average precipitation, which in turn created huge amounts of grasses and other seasonal vegetation. This vegetation is interwoven with large amounts of dead fuel from California’s multi-year drought.

This month’s heat and lack of rain has dramatically lowered the moisture content of vegetation. Along with warmer-than-normal temperatures, strong and persistent northeasterly winds not only help to dry vegetation through evaporation but also provide plenty of oxygen for combustion.

In Southern California, all of these ingredients have come together to create fierce firestorms driven by the Santa Ana (downslope) winds. Unfortunately, wildfires are likely to get worse in the future. Overall, warmer temperatures, cumulative winds and abundant fuel have fed into a dreadful feedback loop. Average yearly temperatures are forecast to rise by 6 degrees by the end of this century. Locally, temperature records keep falling like bowling pins. According to Cal Fire, a 300 percent increase in wildfire risk in non-urban areas of California is predicted by 2050 due to climate change.

On a more optimistic note, the long-range models are indicating that this ridge of high pressure will weaken and move eastward in about 10 to 12 days and allow the storm track to bring precious rain on Christmas Eve. However, these are long-range models are subject to a great deal of change.

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PG&E safety tip: Be sure smoke alarms are installed throughout your home. If the smoke alarm runs on batteries, or has battery back-up power, replace batteries at least once per year. If the low battery warning beeps, replace the battery immediately. All smoke alarms in your house should be tested every month using the alarm test button.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.

A period of “king tides” brought dramatic shifts in sea level to San Luis Obispo County, California, on Monday, December 4, 2017.

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