Weather Watch

Lay of our land shapes our weather

During a visit to Hearst Castle with my family, we stood outside and marveled at the incredible beauty and diversity of the Central Coast landscape.

From the top of the mountain, we could easily see Point Sierra Nevada to the northwest. And to the southeast, we could see Morro Rock, Point Buchon and even Point Sal.

This short distance, about 60 miles as the crow flies, has a range of topography that can affect the weather.

I have seen cold fronts fall apart and seemingly vanish between Ragged Point and Point Sal, or actually intensify and stall over a particular section of this coastal region, producing copious amounts of rain over one part while leaving other areas relatively dry.

San Luis Obispo County’s topography plays an important role in our various rainfall anomalies. The coastal mountains force relatively warm, moisture-laden winds blowing horizontally from the Pacific Ocean to turn vertical or upward.

As the air mass is lifted up over our coastal mountains (upwind), it cools and eventually reaches its dew point temperature. When this occurs, either clouds, rain, sleet or snow will develop on the windward side of the mountain.

Like squeezing a wet sponge, moisture from this air mass is released in the form of precipitation. This is also called orographic enhancement or uplift.

Areas near or just past the summit will often receive the greatest amount of rain.

On the leeward (downwind) side of the mountain, the air mass is forced downward by gravity and warmed by pressure. The sinking air is drier after losing much of its moisture on the upwind side of the mountain range.

Locations on 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.

Another example of this can be seen in the Sierra Nevada. The lush and fertile Sacramento Valley is fed by the western slopes of the Sierra Nevada, while its eastern slopes descend to the vast Nevada desert. Of course, due to orographic enhancement, wind direction will also affect rainfall totals.

Thunderstorms can wreak havoc with local rainfall totals. Last month, I was asked by Diane Weatherford, the chairwoman of the Morro Heights Holiday Luminarias, what the weather would be like the night of Dec. 13.

I told her it would be partly cloudy but dry after four days of rain. That night, my family and I headed to see the luminarias, driving from Los Osos to Morro Bay.

While on South Bay Boulevard, just past Turri Road, we hit a thunderstorm with heavy rain. My daughter said, “I thought you said it is going to be dry in Morro Bay tonight.”

All I could think about were hundreds and hundreds of wet and soggy luminarias. Amazingly, by the time we got to the Morro Bay State Park, not more than a half a mile away, it was dry. It was one of the more beautiful holiday displays that I have seen.

This week’s forecast

A weak cold front passed our area early Saturday morning and produced a little less than a tenth of an inch of rain in most areas. That small amount of rain brought Los Osos to 10 inches of rain for the month of January. The average is about 4.5 inches.

Most locations along the Central Coast are now above seasonal averages for this year. The Diablo Canyon Ocean Lab has recorded more than 12 inches, or about 115 percent of normal for the season. Unofficial data from the Sierra indicate an above-average snowpack.

Patchy fog will develop north of the Cuesta Grade this morning. Otherwise, mostly clear skies and warmer temperatures will welcome us today as a 1,020-millibar high moves toward Great Basin.

Today’s high temperatures will range from the low 60s in the interior to the mid to high 60s in the coastal valleys and our beaches.

A weak weather system will pass to the north of San Luis Obispo but will produce partly to mostly cloudy weather with a few sprinkles late Monday through Tuesday.

A stronger weather system will cross the Central Coast on Wednesday morning with rain and moderate to fresh (13 to 24 mph) southerly winds.

A very wet and potentially windy weather pattern will develop when a moderately strong low-pressure system crosses our area through the day Thursday.

Periods of heavy rain and gusty southerly winds will continue through Saturday with rain or showers continuing next Sunday.

At this time, it looks like wet, but less windy, weather could continue into the following week.

Surf and sea report

This morning’s 6- to 8-foot west-northwesterly (285-degree deep water) swell (with a 14- to 16-second period) will decrease to 3 to 5 feet on Monday.

Today’s charts are indicating a number of Eastern Pacific storms developing off the California coast next week. These systems will produce a series of southerly seas and westerly swell trains.

A 6- to 8-foot westerly (270-degree deep-water) swell (with a 13- to 17-second period) will arrive along our coastline on Tuesday and will remain at this height and period through Thursday.

Combined with this westerly swell on Tuesday through Thursday will be 4- to 6-foot southerly (180-degree shallow-water) seas.

Preliminary analysis:

A 990-millibar storm is forecast to develop about 300 miles to the west of San Luis Obispo on Thursday. If this storm develops as advertised, 7- to 9-foot southerly (180-degree shallow-water) seas will develop along our coastline on Friday, followed by an 11- to 13-foot westerly (270-degree deep-water) swell on Saturday, increasing to 13 to 15 feet next Sunday.

Conservation tip

Learn about Pacific Gas and Electric Co.’s proposed WaveConnect Project in Santa Barbara County — a proposal to generate power from waves — at one of the following public meetings:

Wednesday: Santa Barbara Public Library, Faulkner Gallery, 40 E. Anapamu St., 5:30 to 8:30 p.m.

Thursday: Betteravia Government Center, Board Hearing Room, 511 E. Lakeside Parkway, Santa Maria, 6 to 9 p.m.

John Lindsey is a media relations representative for Pacific Gas and Electric Company. He is also a local weather expert and has lived along the Central Coast for more than 22 years.