Anyone who lives in or drives through the San Joaquin Valley during the late fall or winter months will inevitably have harrowing experiences with tule fog.
Fog is water vapor that has condensed onto microscopic particles in the air and formed a cloud at ground level.
When much of Earth’s heat is radiated out to space, usually on clear, windless nights, it cools the moist, dense and heavier layer of air near the valley’s floor.
When air temperature decreases, humidity levels increase and the valley begins to take on a grayish tint.
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When the air has reached its dew point temperature, the relative humidity is at 100 percent. Water droplets become visible to the naked eye. At that point, tule fog can rapidly develop.
The air in the San Joaquin Valley usually contains more particulates or pollution than the air 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, like what we’re currently experiencing, 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. Like a cover over a bed, this further serves to trap the cold, dense 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 back out to space off the top of the fog layer.
Very small amounts of sunlight penetrate the fog and warm the valley floor. Afternoon high temperatures may only reach the high 30s with little temperature change through the night.
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 CHPl website www.chp.ca.gov /html/fog-tips.html, 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 actually 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 car is disabled or you can’t continue, pull well onto the shoulder and turn off lights. Move away from your vehicle. Consider postponing your trip until the fog lifts.
This week’s forecast
Two areas of high pressure, one over the Great Basin and the other about 400 miles to the west-northwest of San Luis Obispo (Eastern Pacific High) will keep the storm track far to the north for an extended period.
These two surface highs will continue to produce moderate to fresh (13 to 24 mph) northeasterly (offshore) winds and unseasonably warm and dry weather.
The high temperature reached 80 degrees at the San Luis Obispo County Regional Airport on Saturday.
The record high temperature for Jan. 15 was 83 degrees set in 1976.
High temperatures will range from the high 60s to low 70s in the North County (Paso Robles) and along the northwesterly (Morro Bay and Los Osos) facing beaches.
High temperatures in the coastal valleys (San Luis Obispo) and along the southwesterly (Avila Beach and Cayucos) facing beaches will reach the low to mid-70s.
Overnight lows will be mild, mostly in the 40s and 50s throughout San Luis Obispo County.
Dry and unseasonably warm weather will continue through Martin Luther King Jr. Day. Gusty northeasterly (offshore) winds and clear skies with cool nights and mild days are expected Tuesday through Friday.
The mid-latitude westerly winds are picking up, indicating the possible onset of unsettled weather the following week.
Surf and sea report
A 4- to 6-foot northwesterly (300-degree deep-water) sea and swell (with a 5- to 12-second period) is forecast along our coastline this afternoon, increasing to 5 to 7 feet (with a 5- to 12-second period) Monday.
A 6- to 8-foot west-northwesterly (290-degree deep-water) sea and swell (with a 7- to 18-second period) will arrive along our coastline Tuesday, increasing to 7 to 9 feet (with an 8- to 17-second period) Wednesday.
This northwesterly sea and swell will remain at this height but with a gradually shorter period through Thursday.
A very large 936-millibar storm is forecast to develop off the Kamchatka Peninsula later today into Monday.
This storm will produce hurricane-force winds and fully developed seas of nearly 60 feet by Tuesday near the International Date Line.
A 7- to 9-foot west-northwesterly (285-degree deep-water) swell (with a 20- to 22-second period) from this massive storm will arrive along the San Luis Obispo coastline Friday, increasing to 11 to 13 feet (with an 18- to 20-second period) next weekend.
Note: Swell heights will be higher at the offshore buoys.
Westerly winds from this predicted storm will continue to produce median to high-energy wave trains along the coast for most of the following week.
This storm is also expected to produce very high swell conditions along the northwesterly facing shores of the Hawaiian Islands by Thursday.
Seawater temperatures will range from 52 to 55 degrees through Friday.
A special thanks
I’d like to thank the Morro Bay chapter of the American Association of University Women for having me speak to the group last week.
If you see downed power lines, leave the area immediately and then call 911 and PG&E at 1-800-743-5000.
John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for nearly 25 years. To subscribe to his daily weather forecast or ask him a question, e-mail email@example.com.