Weather Watch

SLO is drier and hotter than the Mojave Desert right now — and that’s bad for fires

Our weather so far this November has been dominated by the Eastern Pacific High, an area of high pressure firmly anchored off the California coastline, and transitory high-pressure systems that have moved over the Great Basin — the space between the Sierra Nevada range to the west and the Rocky Mountains to the east.

This condition has created persistent Santa Lucia northeasterly (offshore) winds. In Southern California, these downslope winds are called the Santa Anas and may have gotten their name from the Santa Ana Mountains.

Like an International Harvester semi-truck rolling down the Cuesta Grade, air from the higher elevations of the Santa Lucia Mountains flows downward along the mountain slopes toward the Pacific Ocean, pulled by the never-ending force of gravity. These downslope winds are technically called katabatic wind, from the Greek word katabatikos, which means “going downhill.”

As the air mass descends, it warms at the rate of about 5.5 degrees Fahrenheit per 1,000 feet of descent. Meteorologists refer to this rate of warming as the dry adiabatic lapse rate.

If the air is warm at the top of the mountain range, it can be sizzling hot and bone dry by the time it reaches the valleys below. You see, as the air molecules descend into the higher atmospheric pressure close to Earth’s surface, they gain kinetic energy as they compress inward. If you’ve ever filled up a bicycle tire or a scuba tank, you’ve probably noticed them getting warmer as the pressure increased.

These Santa Lucia winds often create bone-dry relative humidity levels, clear skies and counter-intuitive time of peak temperatures.

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So far this month, the airports at Paso Robles, San Luis Obispo and Santa Maria have all seen high temperatures averaging nearly 15 degrees above average. Both Santa Maria and Paso Robles usually see a daily high of between 67 and 68 degrees, but so far this month they have averaged 81 degrees. San Luis Obispo typically reports an average high of 70 degrees in the month of November, but so far, it’s been 85 degrees.

Not only has it been warm during the day, but extraordinarily dry. In fact, Chris Arndt of www.sloweather.com wrote to say that the relative humidity levels and dewpoint temperatures on Friday were some of the lowest he has seen in 30 years of weather observation along the Central Coast.

“Relative humidity levels dropped to between 2 and 4 percent in the coastal regions with dew points well below zero on Friday afternoon,” he said.

Farther inland, the Santa Margarita Fire Department reported a dew point of minus 10 degrees Fahrenheit; we were warmer and drier than Mesa, Arizona, Las Vegas and even the Mojave Desert!

At times like this, I like to watch the dew point, rather than the relative humidity (RH), as it a better indicator of the current water content of the air. Being “relative,” related to the temperature at the time, the RH will vary with temperature changes for the same dew point.

Unfortunately, the warmer temperatures combined with lower dew-point temperatures have helped to produce near or record-low vegetation moisture levels. Consequently, it takes less heat and time to evaporate the moisture from the plants before they burn. Climate change will continue to produce increasing aridity due to warming.

And that means extremely high fire danger, unfortunately manifested this week in the devastating wildfires in Ventura and Los Angeles counties and Northern California.

“Wildfire remains an ever-present and increasingly dangerous threat to lives, property and natural resources throughout all of California. We must remain vigilant regardless of what the calendar says.” Cal Fire Information Officer Clint Bullard said.

Even though SLO County has yet to be hit with a major fire this season, everybody should be ready at all times for that potential. Here are some tips to prepare yourself:

“Ready, Set, Go” plan

Get ready: Prepare your family. Have a disaster plan with meeting locations and a plan for communications. Practice it!

Get set: Gather flammable materials from outside the home and place them in a pool or bring them inside. Do not leave sprinklers on to save much-needed water pressure for firefighting.

Go! (early): Have a first aid kit and three-day supply of water per person. 1 gallon per day per person. Take easily carried valuables and be certain to take prescribed medications.

More in-depth information may be found at www.readyforwildfire.org.

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.
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