Low pressure creates pleasant fair weather

pgeweather@pge.comJuly 21, 2012 

A coastal stratus recently drifts over the Santa Lucia mountains between Atascadero and Morro Bay.

JOHN LINDSEY

No doubt about it, the past three years have brought below normal temperatures to the first days of the Mid-State Fair. This year, the opening day of the fair saw the maximum temperature just barely edge into the low 80s. The next morning, light scattered rain showers from the subtropical remnants of Hurricane Fabio dampened the streets. For two consecutive days before the start of the fair, the high temperatures only reached 73 degrees.

Last year, the first few days of the fair saw maximum air temperatures reach the 70s. Usually, the average maximum temperature at the Paso Robles Airport during July is about 94 degrees.

Two years ago, the Mid-State Fair was one of the coolest on record. In fact, many days didn’t even reach the 80-degree mark and only on one day did the temperature hit the century point. In contrast, the 2009 Mid-State Fair saw temperatures reach higher than 100 degrees on a daily basis. By the way, the warmest temperature ever recorded at the Paso Robles Airport during July was 115 degrees in 1960.

This has left many fair-goers scratching their heads, wondering what’s going on. The rest of the country east of the Rocky Mountains continues to suffer from one of the worst heat waves and droughts on record; temperatures along the coastal regions of California have been below normal. The answer is that over the last three years, a series of abnormally strong low-pressure troughs off the West Coast have produced below normal seasonal temperatures and at times a very deep marine layer.

On Friday, the latest of these low-pressure troughs lifted northward and in its place a ridge of high pressure moved over the state. That’s resulted in hot temperatures at the fair this weekend. For those who enjoy the cooler weather, take comfort in the fact that another long wave trough is expected to settle just off the California Coast and will produce below normal temperatures by Tuesday. Indeed, by Tuesday maximum temperatures will only reach the mid-80s and remain at this level through the following weekend.

If this area of low pressure develops as advertised, it will deepen the temperature inversion layer and allow coastal stratus to move into the North County.

Let me explain, normally — excluding the last couple of years — during fair time, the Santa Lucia mountain range helps to shelter many parts of the North County from the cool marine influence of the Pacific Ocean.

However, when the winds are right and the marine layer deep enough, coastal low clouds can make their way from Monterey Bay, surging southward through the Salinas Valley toward the Mid-State Fair.

Earlier this month, another trough of low pressure off the West Coast produced an extraordinarily deep marine layer. In fact, it was so deep it actually flowed over the top of the Santa Lucias and covered nearly the entire county. If next week’s low-pressure trough is strong enough, the same occurrence could happen again.

By the way, if the marine low clouds reach the ground, they produce fog. This type of fog is advection fog. Advection simply means transport.

As the 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 its dew point temperature.

When this occurs, fog will develop on the windward side of the mountain, almost like a hand of white mist covering the top of the ridge. As the air mass descends down the other side of the mountain range, it warms and the fog disappears. In the North County, this type of fog usually occurs during the spring, summer and fall.

During the winter months, the cool ground produces radiation or valley fog. This fog usually occurs at night. However, in areas bounded by high mountains, such as the Great Central Valley of Calfornia, tule fog can persist throughout the day.

For those living south of Paso Robles, the Templeton Gap provides an excellent pathway for the marine air. The Templeton Gap is a series of passes through the Santa Lucia Mountain Range that are more or less near Highway 46 from Cambria. It allows marine air to flow toward Templeton and Atascadero. The average maximum temperature in Atascadero in July is cooler than in Paso Robles, averaging about 86 degrees, according to the Air Pollution Control District’s monitoring station there. These cooler summer temperatures in Atascadero are caused in large part by the Templeton Gap.

Today’s weather report

This morning’s marine low clouds will clear from coastal valleys later this morning and from most of the beaches by this afternoon. Today’s maximum temperatures will reach the low 60s along the northwesterly-facing beaches (Morro Bay, Los Osos, Montaña De Oro and the Nipomo Mesa); the mid-60s along the westerly-facing beaches (Pismo and Grover Beach); and the high 60s along the southwesterly-facing beaches (Cayucos, Avila and Shell beaches).

In the North County, high pressure from the southwest along with limited influence of the marine layer will produce near 100-degree temperatures at the Mid-State Fair today.

Afternoon thunderstorms are possible in the southern Sierra as monsoon moisture filters in from the desert southwest. Any thunderstorms that develop will drift off the Sierra and toward the northeast.

The weather pattern begins to transition Monday as the northwesterly winds off the coast relax and the marine layer begins to deepen. By Tuesday, a long wave trough will settle just off the California Coast resulting in a persistent marine layer and cooler temperatures. This condition will produce mostly overcast skies along the beaches with only partial afternoon clearing Tuesday through Friday.

The coastal valleys will clear during the afternoon hours. However, afternoon maximum temperatures will gradually cool through the week. In the North County, high temperatures will only reach the mid-80s and remain at this level through the following weekend.

A 1,035-millibar Eastern Pacific high will nudge closer to the West Coast next weekend and produce strong to gale force (25- to 38-mph) northwesterly winds along the coastline. These winds will keep temperatures below seasonal norms, but will also clear the coastal stratus from the beaches during the afternoon and evening hours.

Today’s 3- to 5-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 12-second period) will continue at this height and period through Monday.

A 3- to 5-foot northwesterly (295-degree deep-water) swell (with an 8- to 11-second period) will develop along our coastline Monday, decreasing to 2 to 4 feet (with an 8- to 11-second period) Tuesday and remain at this level through Friday.

Strong to gale force (25- to 38-mph) northwesterly winds along the coastline will generate a 4-to 6-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 11-second period) next weekend.

An intense storm developed off the southeastern coast of New Zealand. A 1- to 2-foot Southern Hemisphere (200-degree deep-water) swell (with an 18- to 20-second period) from this storm will arrive along our coastline today.

This swell will gradually build to 1 to 3 feet Monday (with a 16- to 18-second period), and peak Tuesday into Wednesday at 2 to 3 feet (with a 14- to 17-second period).

Seawater temperature will range between 53 and 56 degrees through Friday.

Warning system test

On Tuesday and Wednesday, PG&E in coordination with the San Luis Obispo County Office of Emergency Services will be conducting a “growl test” on each of county’s 131 early warning system sirens.

This is only a test and no action is requested or required of the public.

The sirens are partially sounded several times each year — known as a “growl test” that lasts only a few seconds — and tested fully once a year. The full test is slated to occur Aug. 25.

John Lindsey, media relations representative for PG&E and local weather expert, has lived along the Central Coast for more than 25 years. To subscribe to his daily weather forecast or ask him a question, email him at pgeweather@pge.com.

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