Jet Stream Meanderings : Weather: Take Hawaii and Then Stir in Alaska

The temperature was in the middle 80s in Orange County on Thursday, the last day of winter, but no records were broken. The forecast called for continued fair and slightly cooler weather through Monday.

For the weekend, temperatures at the beaches will range from 68 to 72, and inland from 75 to 83. The skies will be generally clear, with low morning and evening clouds.

The current heat wave may seem striking, arriving only three days after a sharp winter storm dropped snow on nearby mountains at elevations as low as 3,500 feet and carpeted parts of the Los Angeles basin with a layer of hailstones, but it is not unexpected.

Not Highly Unusual

These sharp changes in weather, according to meteorologist Arthur Thomas of the National Weather Service’s climate analysis branch in Suitland, Md., “are not significantly unusual,” even if they are “a little out of the ordinary.”

The cause of these swings in temperature and rainfall, he said in a telephone interview, are “steering patterns” in the upper levels of the atmosphere, large masses of air that cause the jet stream to undulate across the United States.

“When a trough in the waves is to the east of California in Nevada and Arizona,” he said, “we get a northwesterly flow of air aloft that brings in cooler air and winter storms.” When the trough is over California, a southwesterly flow brings in warm, dry air.

The jet stream, has been responsible for most of California’s unusually wet winter.

Stream Splits in Two

In mid-February, the jet stream split in two over the mid-Pacific. The two arms of the jet stream then collided just off the California coast, with one bringing cold air from Alaska and the other--dubbed the “Pineapple Express”--bringing warm moist storms from Hawaii.

The resulting storm system drenched California, dumping on some regions more than 50 inches of rain during a 10-day period last month.

During that period, many areas received 50% or more of their normal rainfall for an entire season, causing reservoirs to overflow and triggering widespread flooding.

But by the beginning of March, the jet stream had returned to its customary--albeit undulating--northerly path and California weather returned more or less to normal.

Dry and Hot

Meteorologists expect the rest of March and early April to bring a repeat of the dry--and sometimes hot--conditions that marked November and December.

In the longer term, some scientists predict that unusually wet Januaries and Februaries may persist through the early 1990s as a result of unusual geological conditions, including earthquakes and volcanic activities.

The recent bad weather is not attributable to “El Nino,” the currents of unusually warm ocean water that may be developing off the Peruvian Coast, according to Thomas’s colleague Ken Bergman.

He pointed out that the ocean temperatures there are only 3 to 4 degrees above normal now and that the condition is “quite localized.”

Future Effects

In 1982-83, when El Nino was blamed for an unusually wet winter in California, the temperatures were 10 to 11 degrees above normal and the area of warming extended into the mid-Pacific south of Hawaii.

“If we do get any effect from El Nino,” Bergman said, “it will probably be next winter.”

The jet stream is a high-speed, high-altitude flow of air that circles the Earth in a counter-clockwise motion. It controls much of the weather in the continental United States, but scientists still know very little about what causes the jet stream to shift from one path to another.

Around the end of October, the jet stream was swooping down through the Pacific, bringing a series of relatively mild rainstorms to the West Coast.

By mid-November, the air flow had shifted north, so that California, Oregon and Washington were spared while southern Alaska and northern British Columbia received higher-than-normal rainfalls.

Storm patterns returned to normal in late December and January, according to meteorologist Charles Pyke of the U.S. Corps of Engineers, with cold winter storms riding the North Pacific tracks and “dumping lots of snow into California and the Rockies.”

By the end of January, Pyke said, “a warm-water anomaly in the mid-Pacific” had developed. This region, with above-normal water temperatures, split the jet stream into two tracks, one following a northern route through Alaska and the second swinging far south toward Hawaii.

The Pineapple Express

The second track, the Pineapple Express, picked up warm, moist storms from the Pacific and carried them directly to Northern California. The storms became more violent when they met the cold air brought by the northerly branch of the jet stream.

The results of this collision were quickly felt in many parts of Northern California.

According to William Helms of the California Department of Water Resources, Vacaville, southwest of Sacramento, got 13.4 inches of rain in 10 days, about half the normal yearly total of 24.3 inches.

Buck’s Lake, above the Feather River, got 55.7 inches in 10 days; its normal yearly total is 80 inches. Yosemite in the central Sierra Nevada got 21.7 inches, compared to a normal yearly total of 33.8 inches.

Seasonal Deluge

Figures for the season to date are even higher. Blue Canyon on the American River in Northern California has so far had 86.06 inches, 176% of normal. Sacramento has had 27.93 inches, 195% of normal.

In Southern California, rainfall at Los Angeles International Airport for the season has totaled 17.72 inches, 174% of normal. The Civic Center received 17.41 inches, Pasadena 19.29, and Pomona 17.02.

Last Saturday and Sunday, the Civic Center received 2.55 inches of rain. The rain did not get farther inland until Sunday. On Sunday and Monday, San Bernardino received 1.21 inches and Northridge 1.85 inches.

The warm rains in February melted much of the snow in mountain areas, filling lakes, rivers, and reservoirs to overflowing and causing levees in several Northern California areas to break open, flooding the town of Linda.

Snow Is Back

The return of the jet flow to more normal patterns in early March has restored much of the snow, however.

Snowpacks in the southern Sierra Nevada are running at 150% to 175% above normal, according to Robert Brown of Southern California Edison Co.

Yosemite National Park reports 85.6 inches of snowpack at Tuloumne Meadows, compared to an average of 66.5 inches at this time of year.

Closer to Los Angeles, Snow Summit in the San Bernardino Mountains reports that it has received 66 inches so far this season, and now has 48 inches on the ground at an elevation of 8,200 feet. Last year, the resort received a total of 86 inches.

Scientists Divided

Scientists are divided as to why the weather has behaved so unusually. Pyke cited the warm-water anomaly in the mid-Pacific; but Bergman said “there are no obvious forcing causes (for the changes in the jet stream) this year, such as abnormal ocean temperatures.”

Bergman noted that scientists, in general, are not able to explain the meanderings of the jet stream because “we simply don’t know the answers yet.”

Gerald Kuhn of the Scripps Institution of Oceanography in La Jolla believes that the current wet winter is actually part of a long-term trend that began in 1978 and will continue through the early 1990s.

Kuhn and his colleagues have examined historical records of weather conditions and geologic activity dating back to 1819 and found that unusually wet winters tend to be associated with periods of unusual geologic activity, including volcanic eruptions and earthquakes.

Link to Volcano

He noted that Southern California experienced its greatest floods ever in the winter of 1883-84, shortly after the eruption of the volcano Krakatoa, near Java. Other eruptions and earthquakes continued until the early 1890s, and winters in California remained unusually wet.

In contrast, he said, the Earth was unusually quiet geologically from 1920 to 1960 and winters in California were exceptionally mild. From 1947 to 1977, he added, the winters were “extremely abnormal.”

Beginning with the eruption of the volcano Agung in Bali in 1963 and the Alaskan earthquake in 1964, Kuhn said, world weather patterns began to change. The biggest changes in California began in 1978.

The most significant event of that period, he noted, was the eruption of El Chichon in Mexico in 1983. The winter following that eruption was the wettest in recent history for California.