Weather Watch

SLO County in for a late December dry spell. That means the ‘great gray’ in North County

Santa Claus and his reindeer will be battling gale-force northwesterly winds, cold temperatures and most importantly a few scattered rain showers and lower-level snow on Christmas Morning as they head back to the North Pole.

So far this rain season, the northern half of the Central Coast has seen near to slightly above average rainfall, while the southern half continues to experience below normal rainfall. Paso Robles Airport has recorded 3.43 inches of rain or 105 percent of average through Dec. 22. Further south, Cal Poly typically receives 6.50 inches but has recorded 6.77 inches. Diablo Canyon Power Plant reported 4.84 or 110 percent of characteristic rainfall.

Further south, Santa Maria has only seen 2.29 inches or 65 percent of average, a continuation of below usual precipitation over the last eight years.

After Christmas, it appears that a dry spell will develop for a few weeks. However, many of the long-range models are advertising a very wet January throughout California.

A late December/early January dry spell is quite normal and often produces an interesting switch in the weather pattern between the beaches and the inland valleys.

The interior is hot and dry in the summer months, while the beaches are socked in with fog and drizzle. It’s not uncommon to see beach temperatures more than 50 degrees cooler during the afternoon hours.

However, the opposite can occur at this time of the year. Last week, my family and I journeyed to Hidden Springs Tree Farm in Atascadero to get our Christmas tree. The weather when we left Los Osos was sunny and warm with temperatures in the 70s as we drove up toward Atascadero. As we traveled, we become engulfed by dense fog at the crest of the Santa Lucia Mountains.

You see, fog is water vapor that has condensed onto microscopic particles in the air and formed as a cloud at ground level. When the winds are light and air temperature decreases, relative-humidity levels rise, and the valleys begin to take on a grayish tint. When the air has reached its dew-point temperature, the relative humidity is at 100 percent and water droplets become visible to the naked eye. Often, this ghostly interior valley fog quietly creeps along like a cat toward the coastal mountains.

During offshore wind events, the Santa Lucia (northeasterly) winds can be strong and gusty in the coastal canyons and passes but are often substantially weaker in the North County. Light winds of less than 5 knots promote the formation of radiation fog by bringing more moist air into direct contact with the cold, wet ground.

As you may have observed on Saturday morning, the northeasterly (offshore) wind drove the fog like a gigantic wave from the interior valleys westward toward the crest of the Santa Lucia Mountains. On the leeward (downwind) side of the mountain, the air mass was forced downward by gravity and warmed by pressure. The sinking air produced clear, dry and warm weather along the beaches — just the opposite of what commonly occurs during summer.

The expected dry spell will probably promote the formation of Tule fog in the San Joaquin Valley. Anyone who lives in or drives through the San Joaquin Valley during the winter months will inevitably have harrowing experiences with Tule fog — especially after it rains.

And here’s why:

The air in the inland valleys usually contains more particulates — or pollution — than the wind coming off the Pacific Ocean. Consequently, the fog is often thicker than fog forming along our coastline. Visibility can suddenly decrease to near zero in only a few feet. Vehicles following too close to each other and traveling too fast into the great gray unknown can lead to massive chain-reaction mishaps. During periods of tranquil weather, the San Joaquin Valley Tule fog can last for days on end as it settles between the Sierra Nevada to the east and the coastal range to the west under a strong temperature inversion layer.

The air above the inversion layer is drier, warmer and, hence, lighter. The sky above the inversion layer can be as much as 50 degrees warmer than the air below it. Like a cover over a bed, this further serves to trap the cold and heavier air within the valley. At this time of the year, the sun is low in the southern sky, and much of its light is reflected out to space off the top of the fog layer, and only tiny amounts of sunlight penetrate the fog to the valley floor. Thankfully, storms coming out of the Gulf of Alaska often mix out the temperature inversion layer, producing clearer skies and warmer afternoon temperatures.

According to the California Highway Patrol, here are some tips for driving in foggy conditions:

▪ Drive with lights on low beam. High beams will reflect off the fog, creating a “white wall” effect.

▪ Reduce your speed and watch your speedometer. Fog creates a visual illusion of slow motion when you might be speeding.

▪ Avoid crossing traffic lanes.

▪ Travel with the driver’s window partially open. Listen for traffic.

▪ Watch for CHP pace cars to guide you.

▪ If your vehicle is disabled or you can’t continue, pull well onto the shoulder and turn off lights. Move away from your car. Consider postponing your trip until the fog lifts.

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 or follow him on Twitter: @PGE_John.
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