The Forecasters Shrug, Sweat It Out Too

The first hint that Southern California was in for a week of hot weather came last Sunday.

High over the rocky plateau of the Great Basin region of the western United States, the sky was clearing, a vast expanse of blue marred only by faint wisps of cirrus clouds. On the ground in Utah, Nevada, Idaho and Oregon, National Weather Service barometers began registering a buildup of high pressure.

It was the start of a familiar pattern that ends with hot, dry Santa Ana winds blowing through Southern California and temperatures popping up past the 100-degree mark.

But what has made the ongoing heat wave even more unbearable is a stubborn high-pressure system in the upper atmosphere; it has clamped like a glass dome over the Los Angeles region, dispersing clouds and smog and magnifying the effects of the sun’s heat.

Yet, despite all the knowledge at the disposal of weather experts, they were able to predict the heat wave’s coming only after the process that set it in motion had already begun.

“When you start asking about an exact cause, there’s not much we can give you,” said Arthur Lessard, meteorologist in charge of the National Weather Service’s Los Angeles office. “We still have a lot to learn about why weather systems work the way they do.”

Jerome Namias, former head of the National Weather Service’s climate analysis center--and now a researcher at Scripps Institute of Oceanography in La Jolla--described the heat wave as nothing more than a statistical aberration.

“True, this is a record-breaking heat in some places, but records are meant to be broken,” Namias said. “That’s the name of the game in meteorology. . . . The probability is that records will always be broken at some place at some time.”

Unable to offer causes for the heat wave, which Thursday produced a high of 106 in downtown Los Angeles, meteorologists could only describe and trace the path that led to this week’s weather, a stagnant spell that is not expected to budge until atmospheric changes in other parts of the world allow it to dissolve or move eastward.

Weather Service meteorologists, stationed on the 11th floor of the Federal Building in Westwood, began receiving information last Sunday that tipped them to the coming misery.

“We had readings from barometers in the Great Basin that showed increased pressure,” Lessard said. “At the same time, we had relatively lower pressures down here.”

The Great Basin is a stretch of semi-desert terrain and sparsely populated scrubland 4,000 to 5,000 feet above sea level. It extends through Utah, most of Nevada and parts of Idaho, Arizona and Colorado.

The growing strength of the high-pressure system over the Great Basin was part of a larger ridge of high pressure that had formed over the entire western United States, five to six miles up in the atmosphere.

Major high-pressure systems like the one covering the Western Basin are shunted around by the jet stream, a vast river of air that moves at speeds up to 200 m.p.h. miles above the Earth’s surface. Last Sunday, weather experts said, the jet stream began curling around the ridge of high pressure over the western states, preventing it from moving.

But at lower altitudes over the Great Basin area, the high-pressure system’s warm air moved naturally toward a region where the atmospheric pressure was lower and the air was cooler. That region was the Pacific Ocean, off the coast of Southern California.

“In a high-pressure system, the air sinks,” said Mike Smith, a meteorologist and president of WeatherData Inc., which provides forecasts for The Times. “When air sinks, it heats up at a rate of 5 1/2 degrees for every thousand feet it drops. It heats because it is compressing down into the lower atmosphere.”

Growing hotter as it sinks, the air from the Great Basin moves southwest toward cooler air, crossing into California and through the Panamint and Amargosa mountain ranges just east of Death Valley, then swooping down through the mountain passes. The winds were first dubbed “Santa Anas” by the early settlers of Orange County, who gave them the name because they came gusting down Santa Ana Canyon from the higher inland deserts.

In the past, Santa Anas have reached wind speeds of up to 70 and 80 m.p.h. Smith said that a few weeks ago “we had one night where the winds whipped in around Barstow and Edwards Air Force Base up around 70 m.p.h.”

Over the last several days, though, the Santa Anas have been fairly mild, gusting only to 30 to 40 m.p.h. and staying mostly in the 15-to-20 m.p.h. range.

The steady, hot breeze has been enough to blow the smog that normally hangs over Los Angeles and the inland region out toward the coast and into the Pacific. From their Westwood vantage point, Lessard said Thursday that the federal meteorologists could “see all that mustard-colored air rolling out over Catalina.”

Even without the Santa Anas, weather experts said that the Los Angeles region would still experience brutal temperatures this week. The reason, they said, is that the high-pressure system in the upper atmosphere over the western United States is now centered directly over Southern California.

Acting like a glass dome, the high-pressure system warms up the air and prevents moisture from gathering, clearing the sky of clouds, WeatherData’s Smith said. Without a cloud cover, he said, “more sunlight than usual reaches the ground and that heats things up even further--even without the roaring winds.”

Despite the discomfort caused by the 100-degree-plus temperatures, they do have at least one advantage, weather experts said.

The heat disrupts the “inversion layer” that normally traps pollutants over the Los Angeles Basin. Normally, a layer of cold air sits atop the layer of warm air over the basin, preventing the warm air from escaping and acting like a lid that holds in pollutants.

This week, the air has been so hot that it interfered with that process. As the superheated air rises, it pushes the inversion layer high enough so that smog-forming pollutants are able to escape--a process similar to taking the lid off a pot. This, in combination with the Santa Anas, has served to keep the skies remarkably smog-free during the heat wave.

Smith said he does not foresee any changes in the high-pressure system stalled over Los Angeles until similar high-pressure systems over China, Korea and the eastern Soviet Union begin to show signs of movement.

When they do, Smith said, the “weather pattern over the Pacific would become imbalanced and cause your high-pressure system to scoot to the west, toward Hawaii. But we don’t see any large-scale movement like that until middle of next week.”

Times science writer Thomas H. Maugh II contributed to this story.

HEAT AND SMOG

What has made the Los Angeles area so hot are unusually warm winds from a high-pressure area over Utah and Oregon. Normally, high temperatures bring increased smog, but the unusual atmospheric conditions have actually reduced smog.

1--Santa Ana winds blowing from inland high desert areas toward the coast are compressed, heated,and sped up as they make their way through mountain passes.

2--Winds blowing in from the ocean tend to trap the smog against the coastline.

3--The winds have pushed most smog toward coastal areas, rather than inland, the normal pattern. The weather is thus smoggy at the beach and clear inland.

4--Furthermore, the high temperatures are disrupting the inversion layer that normally traps pollutants in the Los Angeles Basin. Normally, a layer of cold air sits on top of a layer of warm air over the basin, preventing the warm air from escaping and acting like a lid that holds pollutants in. This week, the air has been so hot that it has interfered with that process. As the super hot air rose, it pushed the inversion layer high enough that smog-forming pollutants can escape the basin--like taking the lid off a pot.