Storms Help State Stay Even : Drought: Rain and snow in the north keep California from losing more ground. But the moisture does little to boost long-term water supplies.

A snowy weekend in the Sierra and a wet Sunday in Northern California kept the state from losing any more ground to the drought but did little to boost long-term water supplies, state officials said Monday.

The best that the state’s water experts would say for the weekend weather, which dropped half an inch of rainfall and a foot of snow, was that it soaked the soil thoroughly enough so that anymore storms should produce substantial runoff.

“You have to get a little additional rainfall every day or so or you will lose ground. This rainfall over the weekend kept us even, which is good,” said Bill Helms, spokesman for the Department of Water Resources’ drought center. The foot of snow will produce about an inch of water, he said.

If forecasts prove accurate, Helms said, California should be in line to get several moderate storms later in the week that will produce the hoped-for runoff.

Steve Burback, a meteorologist with WeatherData Inc., which provides forecasts for The Times, said storms from the Gulf of Alaska should work their way down to Northern and central California on Tuesday and again on Thursday and early Saturday.

With the jet stream arcing from the Aleutian Islands down to the Pacific Northwest and California, he said, storms should hit that area every few days, possibly until the end of the wet season in mid-April.

“Storms just follow that path with heavy rains,” Burback said.

Earlier in the year, he said, the positioning of the jet stream was responsible for the opposite effect on California. He said California was kept dry because of a high-pressure system that held the jet stream in the western provinces of Canada and allowed it to plunge into the Plains states.

As result, until March, California appeared headed for its driest year on record. With reservoirs depleted from four previous dry years, 1991 seemed to be making a name for itself. So far, the March rains have put an additional 1 million acre-feet of water into the major reservoirs in the Central Valley and north coast. An acre-foot is equal to an amount of water that a typical Los Angeles family of five would use in 18 months.

Snowfall, only 14% of normal until March, is now 44% of normal. While still 20 inches behind a normal year, rainfall has increased from 21% of average to 44%.

Even so, Helms said, the state is painfully short of water and there is only a 30% chance that 1991 will be as wet as 1990, a year that was recorded as “critically dry.”

“We’re still in a lot of trouble,” Helms said. “We’ve moved from a situation that was almost disastrous to one that is just bad.”

With the March rains, he said, this year can no longer be considered the driest on record, but it still has a good shot at being the fourth-driest year in history.

Although early March also produced heavy rains in Southern California, it is the rainfall in Northern California that will determine how much water Los Angeles and other cities have through the summer.

“People in Southern California see it rain and they want to know how much that has helped but it really doesn’t matter much in terms of their water supply,” said Helms. “They need to worry about what happens in Northern California.”

How state water supplies will be apportioned each year is determined by a measurement that the department refers to as the Sacramento River Index. The index measures the amount of runoff into the Feather, Yuba, Sacramento and American rivers basin.

In a normal year, the index is 18.9 million acre-feet. Before the March storms it was forecast as being 5.5 million acre-feet for 1991. Now, after the storms, the forecast has been changed to 7.3 million acre-feet--still well behind the other years of the current drought. In 1987, 1988 and 1990, all critical dry years, the index was 9.2 million acre-feet.

HOW RAIN COMES TO CALIFORNIA

Weather forecasters are optimistic that California will see more rain in March, now that an unusually stubborn high-pressure system is starting to ease up. For most of the rainy season, the high-pressure dome parked itself right off the California coast, blocking storms from entering the state and worsening already parched conditions. Most storms that hit the northern part of the dome were deflected to the Pacific Northwest; those hitting the southern portion were sent off to Baja California and other parts of Mexico. The first major storm system to hit California this season didn’t come until February 28. On that day, the high pressure system drifted northward enough that a tropical storm was able to swing under it and up into California. As a result, the state played a bit of catch-up and got an average 25% of its normal annual rainfall during that storm.

Recent storms

Major storms since November in inches

Date Los Angeles Sierra* San Francisco Dec.10, 1990 trace/.00 trace/.00 .31 Dec.11 trace/.00 .20 .27 Dec.12 trace/.00 .80 trace/.00 Dec.15 trace/.00 .40 .79 Dec.16 trace/.00 .40 .50 Jan. 3 .39 .8 trace/.00 Jan. 4 .41 .2 trace/.00 Jan. 9 .34 trace/.00 .42 Feb. 2 trace/.00 trace/.00 1.11 Feb. 5 trace/.00 trace/.00 .91 Feb. 27 .67 trace/.00 .42 Feb 28 2.44 1.6 .67 March 1 .68 1.8 .44 March 3 trace/.00 4.8 .51 March 4 trace/.00 2.9 .25 March 10-11 .01 .80 .50 March 12 forecast .25 .5 .25

Total precipitation (includes minor rainfall not included above)

Los Angeles Sierra* San Francisco This season 5.28 18.5 8.02 Norm 11.70 37.00 15.40

* Measured at Mammoth Pass in water content of snow, not snow depth

How storms get diverted

1) A dome-shaped high pressure system off the California coast in effect constructed a barrier to incoming storms. 2) Storm systems moving toward California from north and south have hit the high pressure system and been deflected away, keeping the state dry for much of the rain season.

How storms get through

Storms that have made it through to dampen the state came when the high pressure system moved north, removing the barrier.

The first in a series of Alaskan storms that came to California last weekend nudged over the northern part of the high-pressure dome.

Two more storms were expected to follow its path this week.

Source: Weather Data, Los Angeles Dept. Water and Power.