Mammoth snow packs in western mountains and rain-saturated ground throughout the plains states and the Southeast threaten large sections of the United States with severe flooding this summer, the National Weather Service warned yesterday.

Devastating mud slides and flooding in Nevada and Utah this week are only the beginning of serious problems that should peak within two to three weeks, the agency said.

Flooding could affect large areas of California, Nevada, Colorado, New Mexico and Utah, the Weather Service said. Snow packs in major western mountain ranges are 200 to 300 percent above normal, and a late-spring runoff is just beginning.

Dr. Robert A. Clark, chief Weather Service hydrologist, said the flood threats have developed out of "the worst winter weather I've seen in 40 years," resulting from a phenomenon known as "El Nino," which produced an unusual shifting of the jet streams out of the equatorial Pacific Ocean.

Clark said the weather patterns have caused floods in the past six months in all but two states--Arizona and New Mexico--and warned that flooding in northern New Mexico is almost inevitable in the next few days.

He said the extent of the flooding nationwide depends largely on weather patterns in the next month, but said the long-range forecast for June calls for above-average rains in the soaked Southeast and cooler-than-normal weather in the western mountain regions.

This is the way he described the flood threat in the West:

In California, Nevada and northern New Mexico "there is no question that the worst is yet to come," probably in the next two weeks. The snow pack in the Sierra Nevada separating California and Nevada is about three times as deep as normal.

In the Rocky Mountain region near Denver, a similar situation exists, with the snow pack about 200 percent higher than usual.

In Utah, where the Great Salt Lake is at its highest level since 1924 and Salt Lake City is venting flood waters through sandbag-lined streets, the worst may be over in heavily populated areas. But Clark said that less populous areas still could be hard-hit.

About 200 people remained homeless yesterday in Bountiful, Utah, and residents were piling boulders along stream banks to prevent further flooding after a 30-foot-high wall of water and debris raged out of the Wasatch Mountains and through the small town late Tuesday. New rainstorms in the area were causing further alarm.

In Nevada, civil defense officials warned people to stay out of the High Sierra until the end of June because of the flood danger. A sudden mud slide on Memorial Day killed one man as it flowed through a popular picnicking area about 20 miles south of Reno.

Clark said that federal and California authorities have begun drawing down reservoirs in that state in an attempt to catch some of the heavy runoff. But he said serious flooding still could occur in the rich farm lands of the San Joaquin Valley.

The heavy winter rains in other parts of the country--particularly in Texas, Louisiana and Mississippi--have left farmers with severe planting problems that could impair crop production.

Clark said the ground is so saturated in large areas of the Midwest and the Southeast that relatively normal thunderstorms could cause flooding. Normally, it would take 5 inches of rain in three hours to cause flooding in those regions. But now, he said, a 1- to 2-inch rainstorm could cause rivers to overflow.

Clark said that a hurricane--the hurricane season begins this month--"would be disastrous" if it came ashore in the already inundated Gulf states or elsewhere in the Southeast.

He said the snow-pack runoff and rainfall do not appear to threaten major dams, but he said there are about 50,000 small storage dams--many of them earthen and privately owned--throughout the country and about 10,000 of them are considered "real problems."

The National Weather Service is monitoring the snow pack by satellite and has set up flood-watch stations in the most vulnerable areas, Clark said, adding that he felt reasonably certain that residents in the threatened areas would receive adequate warning.

Clark said the situation is highly unusual because it covers such broad areas of the country. Only a handful of regions--such as the Pacific Northwest and the upper Midwest north of Chicago--are in relatively normal condition and fairly free of flood threats.

"If there's more rain," Clark said, "we are in trouble."