SLO County to see clear skies, cold nights and warm days. Here’s the forecast

San Luis Obispo County will primarily enjoy clear skies with mild afternoons and cool overnight and early morning temperatures through Thursday.

This is because the weather will be dominated by areas of high pressure off the California coastline and over the Great Basin, keeping the storm track over the Pacific Northwest.

Gentle northwesterly (onshore) winds on Friday evening and night will allow areas of low marine clouds with pockets of fog and mist to develop in the coastal regions.

Much farther inland, in the San Joaquin Valley, tule fog is forecast through early this week.

The air in the San Joaquin Valley usually contains more particulates — or pollution — than the wind coming off the Pacific Ocean. Consequently, the fog is often thicker than haze 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 collisions.

During tranquil weather, the San Joaquin Valley tule fog can last for days 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 and warmer, which makes it lighter.

The sky above the inversion layer can be as much as 50 degrees warmer than the air below. Like a cover over a bed, this further traps 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 — so only tiny amounts of sunlight penetrate the fog to the valley floor.

Along the Central Coast, moderate to fresh (13 to 24 mph with gusts to 35 mph) Santa Lucia (northeasterly) winds during the night and morning will start Sunday and continue through Thursday. Those winds will shift out of the north during the afternoon.

However, Monday morning could see fresh to strong (19 to 31 mph) Santa Lucia winds across much of the coastal region, with gusts reaching 35 to 55 mph in specific hot spots including Highway 41 from Morro Bay High School toward Atascadero (about halfway up the hills); the coastal canyons like near the Diablo Canyon nuclear power plant; and parts of eastern San Luis Obispo, including Cal Poly, San Luis Obispo High School and French Hospital Medical Center.

This condition will give mostly clear skies, with overnight temperatures dipping into the mid-30s to low 40s in the inland valleys (Paso Robles) and the mid- to upper 40s in coastal valleys (San Luis Obispo) and along the beaches.

We are at the time of the year when the coastal temperatures are warmer than the inland areas. Daytime highs will range from the high 60s to the low 70s in the coastal regions and the mid-60s in the inland locations.

A dry cold front will move over the Central Coast on Friday with gentle westerly winds and increasing clouds. In the cold front’s wake, strong to gale-force (25 to 38 mph with gusts to 45 mph) northwesterly winds and clearing skies will develop on Saturday, Dec. 14.

Along with the winds, high temperatures will only reach the low 50s in the coastal regions and mid-40s in the inland areas. The inland valleys could see lows dropping to the low 30s, while the coastal valleys the low 40s.

For the following week, the European numerical model indicates wet weather settling in the following Monday and Tuesday, Dec. 16 and 17, while the American model keeps it dry.

Surf report

A 5- to 7-foot northwesterly (300-degree deep water) swell (with an 11- to 17-second period) will arrive along our coastline on Sunday afternoon and remain at this height and period through Friday.

Gale force northwesterly winds will generate an 8- to 10-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 14-second period) next Saturday and Sunday.

Combined with this northwesterly swell will be a 1- to 3-foot southern hemisphere (210- degree deep water) swell (with a 14- to 16-second period) Tuesday into Friday.

Surface seawater temperatures will range between 55 and 57 degrees through Friday, cooling to 52 to 54 degrees next Saturday and Sunday.

On this date in weather history (Dec. 8)

1988: Santa Ana winds battered Southern California, with gusts up to 92 mph reported at Laguna Peak. The high winds removed roofs from buildings and knocked down trees and power lines, igniting five major fires and numerous smaller ones. Damage was estimated between $15 million and $20 million. (Storm Data)

2015: The day’s high temperature reached 80 degrees at the San Luis Obispo County Airport. The morning’s Santa Lucia (northeasterly) winds produced mild morning temperatures along the beaches with both Morro Bay and Diablo Canyon reporting 63 degrees at 5 a.m. At the same time, the Paso Robles Airport reported 40 degrees.

2020: Peak gusts reached 64 mph in San Luis Obispo near the high school and 54 mph at Diablo Canyon nuclear power plant. Tepusquet Canyon saw 61 mph wind gusts. This morning’s Santa Lucia (northeasterly) winds created a vast temperature differential between the inland and coastal valleys. At 6 a.m., the Paso Robles Municipal Airport reported 32 degrees, while at the same time, the San Luis Obispo County Regional Airport was at 60 degrees. Later that afternoon, the temperature hit 83 degrees, a daily record at the San Luis Obispo County Regional Airport.

This week’s temperatures

LOWS AND HIGHS, PASO ROBLES

LOWS AND HIGHS, SAN LUIS OBISPO AND COASTAL VALLEYS

John Lindsey is a retired PG&E marine meteorologist. Email him at JohnLindseyLosOsos@gmail.com or follow him on Twitter @PGE_John.