Weather Watch

Meteorological role reversal as winter sets in

As the Central Coast moves toward winter, an interesting switch occurs in weather patterns between the beaches and the North County. 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 Saturday 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. At the crest of the Santa Lucia Mountains we were engulfed by dense fog, light winds and much cooler temperatures — a condition that is hard to imagine in the summer.

Summer often sees northwesterly (onshore) winds blow horizontally across the Pacific toward the Central Coast. The air mass is lifted over the Santa Lucias, where it cools and eventually reaches the temperature when dew forms. When this occurs, fog will develop on the windward side of the mountains (Los Osos, Morro Bay and Cambria).

In the winter, on cool nights when a moist layer of air is near the ground, tule fog quietly creeps with the wind, especially in the interior valleys. At night, much of the earth’s infrared radiation is radiated out to space, cooling the air near the ground. When the air temperature reaches the dew point, fog develops.

During offshore wind events, the northeasterlies 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 actually promote the formation of radiation fog by bringing more moist air into direct contact with the cold ground.

As you may have observed last Saturday, the northeasterly (offshore) wind drove the fog 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 normally occurs during summer.

Tuesday will mark the first day of winter. At our latitude, about 35 degrees north of the equator, the earliest sunset this year occurred at 4:51 p.m. Dec. 7. However, the shortest day of the year will be the winter solstice Tuesday, when the sun will rise at 7:08 a.m. and set at 4:55 p.m.

In other words, the earliest sunsets happen before winter solstice and the latest sunrises occur after. Earth’s orbit around the sun is not a perfect circle but rather an ellipse. During Earth’s closest point of approach to the sun — or perihelion — the Earth’s velocity actually increases and, combined with the Earth’s tilt, produces slightly earlier sunsets.

By the way, the winter solstice also marks the longest day in the Southern Hemisphere due to the 231⁄2-degree tilt of Earth on its axis. Christmas in Australia has the same weather characteristics as some areas of the United States on the Fourth of July. Kind of like the weather switcheroo between the North County and the beaches during winter and summer.

This week’s forecast

San Luis Obispo County’s topography plays an important role in our various rainfall anomalies, and Saturday’s rain certainly proves it.

The coastal mountains force relatively warm, moisture-laden southerly winds blowing horizontally from the Pacific Ocean to turn vertical or upward.

Like squeezing a wet sponge, moisture from this air mass was released in the form of persistent rain on the ocean side of the Santa Lucia Mountains.

This is also called orographic enhancement or uplift. Areas near or just past the summit receive the greatest amount of rain.

On the leeward (downwind) side of the Santa Lucias, the air was drier after losing much of its moisture on the upwind side of the mountain range.

Locations in the North County or the leeward side of the coastal range receive noticeably less rain. On the ocean side of the Cuesta Grade, the seasonal average precipitation in San Luis Obispo is about 24 inches, while on the other side in Paso Robles, it’s about 15 inches.

The upper-level charts are indicating a strong and persistent west-southwesterly jet stream centered over the Central Coast.

This river of air in the upper atmosphere will direct impulses of subtropical moisture from a nearly stationary 984 millibar storm off the Oregon coastline, producing rain and gusty southerly winds through Monday.

Rainfall amounts today through Monday should range between 0.75 and 1.5 inches in the North County, 1 and 3 inches in the coastal valleys of San Luis Obispo county with another to 2 and 5 inches in the coastal mountains.

As cooler air filters southward across California, snow levels will gradually drop to 3,500 and 5,500 feet north to south this afternoon and 2,500 and 5,000 feet north to south Monday.

Scattered rain showers are forecast Tuesday morning, then another weather system will cross the Central Coast on Tuesday evening into Wednesday afternoon bringing rain and snow above 3,500 and 5,000 feet north to south through Wednesday night.

Southerly winds with this system should reach strong to gale force (25 and 38 mph) levels Tuesday night into Wednesday morning along with the heaviest rain.

Rainfall amounts today through Monday should range between 0.5 and 1 inch in the North County, 1 and 2 inches in the coastal valleys of San Luis Obispo county with another to 2 and 3 inches in the coastal mountains.

At this time it looks like Thursday and Friday will be mostly dry, then another weather system will drop southward across our area on the Christmas weekend.

Surf and sea report

A nearly stationary 984 millibar storm off the Oregon coastline will produce an 8- to 10-foot west-northwesterly (280-degree deep-water) swell (with an 8- to 12-second period) along our coastline today, increasing to 9- to 11-feet (with an 11- to 14-second period) on Monday.

Combined with this west-northwesterly swell will be 2- to 4-foot southerly (195-degree shallow-water) seas today through Monday.

Strong to gale force (25 and 38 mph) southeasterly winds will generate 7- to 9-foot southwesterly (220-degree deep-water) sea and swell (with a 5- to 12-second period) on Tuesday through Wednesday.

A 7- to 9-foot westerly (270-degree deep-water) swell (with a 9- to 12-second period) is forecast along our coastline on Thursday, decreasing to 4 to 6 feet on Friday.

Seawater temperatures will range between 54 and 57 degrees through Friday.

Conservation tip

During the cooler nights of winter, weather-stripping around windows and doors can save a lot of energy. For more energy saving tips, visit www.pge.com.

John Lindsey is a media relations representative for PG&E. He’s a local weather expert and has lived along the Central Coast for more than 23 years. To subscribe to his daily weather forecast or ask him a question, e-mail pgeweather@pge.com.

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