Weather Watch

Unseasonably sunny weather headed for rain

As much of the Northeastern seaboard continues to get hammered with record amounts of January snowfall, life along the Central Coast has been unseasonably sunny and pleasant.

The warm and dry weather over the last few weeks can be attributed to the northeasterly downslope winds and how they impact our part of the state.

Because of our proximity to the ocean, we tend to refer to them as offshore winds, but they have other names throughout the world, such as Zonda in Argentina or Bergwind in South Africa.

In California, during fall and winter, the air over the high deserts of the western U.S., cools and becomes increasingly dense.

When an area of high pressure moves over the Great Basin, the air flows in a clockwise direction around the high pressure system, forcing the cool and dry desert air down mountain slopes and canyons toward the Southern California coast. This is the start of the Santa Ana winds.

As this air mass descends toward the coast, it is compressed and its temperature rises. It’s not uncommon for the air mass to warm as much as 30 degrees. As the air heats up, its relative humidity decreases, and the air becomes very dry.

Then gravity pulls this air mass toward the ocean and funnels it through mountain passes and coastal canyons, producing strong and gusty offshore winds in one area. Amazingly, another nearby area might be completely calm.

In Santa Barbara, the Santa Ana winds are weaker than elsewhere in Southern California, held at bay by the Santa Ynez Mountains, which run west to east and parallel to the coastline.

However, a variant of the down-slope winds is known as a sundowner. These winds often begin in the evening hours as the sun sets and are typically associated with a rapid rise in temperature and decrease in relative humidity.

Sundowners often produce extreme fire danger. This is what occurred during the Montecito fire in November 2008. In October 1991, ferocious down-slope winds produced a firestorm that destroyed over 3,000 dwellings in the Oakland hills.

Moving up the California coast past San Francisco, these winds are called California northerns and can produce unbearably hot temperatures in the northern Sacramento Valley.

In San Luis Obispo, as throughout Southern California, the offshore winds tend to peak during the morning hours, when the interior is at its coolest.

On June 20, 2008, down-slope winds produced the warmest temperatures ever recorded along the Central Coast.

Cal Poly reached 113 degrees, Los Osos hit 110 degrees. On that day in Los Osos, I heard popping from pine trees, like the sound of bacon frying as pine cones exploded open to drop their seeds.

On the eastern slope of the Rocky Mountains, such winds are know as Chinook. When they get going, they can easily reach hurricane-force levels.

In 1982, the Chinook winds gusted to 140 mph at Wondervu.

These warm “snow-eater” winds live up to their regional billing and make the white stuff disappear. The greatest recorded temperature change in 24 hours caused by Chinook winds occurred on Jan. 15, 1972, in Loma, Mont.; the temperature rose 103 degrees from -56 degrees below zero to 49 degrees above zero.

This week’s forecast

Low misty coastal clouds will shroud the majority of the Central Coast early this morning giving way to rain later this morning as a somewhat cold 1,003-millibar, low-pressure system and associated cold front crosses our area from west to east.

The cold front will pass our area between 9 a.m. and noon with rain. Rain will turn to scattered showers along with a chance of thunderstorms this afternoon through tonight.

Today’s temperatures will only reach the mid- to high 50s throughout the county.

Generally 0.25 and 0.50 inches of rain is expected by tonight. Snow levels will begin in the 4,000 and 5,000 foot range and will drop to the 3,500 foot range by tonight.

Skies will clear Monday into Tuesday with chilly mornings followed by dry, cool to mild days. As a high pressure ridge builds back over California, cool and dry conditions return with dense and persistent ground fog in the San Joaquin Valley through next weekend.

A wet pattern may develop by mid-February.

Surf and sea report

The California Buoy, about 357 nautical miles west of San Francisco, reported a 23 foot west-northwesterly swell with a 15-second period Saturday.

This west-northwesterly (285-degree deep-water) swell will arrive along our coastline at 6 to 8 feet (with a 12- to 18-second period) this morning, rapidly building to 10 to 12 feet (with a 14- to 17-second period) this afternoon through tonight.

Note: wave heights will be higher at the offshore buoys.

Combined with this west-northwesterly swell will be 3- to 4-foot westerly seas today.

This swell will decrease to 8 to 10 feet (with an 11- to 15-second period) Monday.

A 5- to 7-foot northwesterly (295-degree deep-water) swell (with an 11- to 14-second period) is forecast along our coastline on Tuesday, decreasing to 3 to 5 feet Wednesday.

A 6- to 8-foot west-northwesterly (280-degree deep-water) swell (with a 16- to 18-second period) will arrive along the Central Coast on Thursday, increasing to 7 to 9 feet (with a 15- to 17-second period) Friday.

Seawater temperatures will range between 52- and 54-degrees through Tuesday.

Conservation tip

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John Lindsey is a media relations representative for PG&E. He is a weather expert and has lived along the Central Coast for more than 24 years. To subscribe to his daily weather forecast or ask a question, e-mail him at pgeweather@pge.com.

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