One of the wettest and bleakest springs on record continued to hammer the country from coast to coast yesterday, leaving parts of New England flooded by four inches of rain and travelers stranded by 18 inches of new snow in California's Sierra Nevada.

The latest deluge of foul spring weather has left rivers in at least 21 eastern states at various stages of flooding, the National Weather Service said, and added to a near-record 50-foot winter snowfall in parts of the Sierra, threatening California with the possibility of major late-spring flooding.

The record rains and unusual cold have squelched eight consecutive weekends along much of the Eastern Seaboard, including the Washington area.

All this is part of a larger weather pattern that began with unusual winds and currents in the mid-Pacific Ocean that had produced the fifth warmest winter on record. That pattern resulted in a winter far warmer than normal, and now, said Don Gilman, the chief forecaster at the Weather Service, "What we are seeing is the rather spectacular breakup of a strong weather pattern that held throughout the winter."

In recent weeks, the unusual nationwide weather patterns have deluged the Gulf Coast with spring storms, sent the Mississippi River to within one foot of the top of its 51-foot levee system at Cairo, Ill., and hit other parts of the Midwest with a spring weekend storm that dumped two feet of snow on sections of Nebraska and Iowa. Even the nation's vacation spas have been danger spots. A week ago Virgin Island vacationers were drenched with 18 inches of rain in 24 hours, with the worst of the Caribbean storm striking St. Thomas.

The freakish spring followed a winter that, while mild in the East, punished the West Coast with fierce storms, eroding large parts of the coastline.

The winter weather patterns left much of the nation primed for unusual flooding, the Weather Service said, with the ground saturated and unable to absorb spring rains throughout most of the East while reservoirs are already full in California with much of the annual runoff from the mountain snow pack still to come.

One of the chief factors determining the weather is the jet stream, the seven-mile-high, 200 mph winds that steer weather movement across the United States.

Normally, the jet stream enters North America in Canada, then dips back down toward the United States across the Midwest. It continues south often as far as the Gulf Coast before turning back up the East Coast, thus moving across the country in a wavy motion.

But this past winter, because of unusual conditions in the Pacific that are called "El Nino," the jet stream moved across North America in a straight line with especially powerful winds hitting California, the Gulf states and Florida. The name "El Nino" means "the child" in Spanish and comes from a Peruvian fisherman who noted that the unusual winds and currents occur around the Christmas season.

Such a straight, powerful flow avoided for the most part bringing down cold Canadian and Arctic air into the United States. This made the 1981-82 winter the fifth warmest in the 52 years that records have been steadily kept.

But beginning in March the strong, straight flow of the jet stream began to break up as the winds slowed. The jet stream began to meander up and down across the United States slowly, spinning off storms as it moved. Heavy cloud cover kept the sun from warming the earth, and so in addition to a stormy spring it has been a cold one.

Gilman said he is not yet certain whether the pattern will settle down to a normal spring in May, or continue chaotic and wintry.

The larger cause of both the warm winter and cold spring is the unusual pattern of winds dominating the Pacific and creating disastrous weather all around the Pacific basin.

In the east part of that basin, from Peru to California, floods have cost hundreds of lives and hundreds of millions of dollars in damage, according to conservative Weather Service estimates.

In the western Pacific basin, drought has struck Australia, Indonesia, the Philippines and India. The damage reports from those areas include more than 400 dead and over $4 billion in property loss. These counts do not include the massive damage to fisheries from changing sea temperatures and currents.

The weather system that has wreaked this havoc is one that recurs with varying strength every three to eight years. In it, the eastern and central Pacific develops strong low pressure areas while the western Pacific develops high pressure areas. The trade winds, which normally flow to the west, stop or reverse themselves and flow to the east. Water depths and currents shift with the winds.

Though the connection is not well defined by meteorologists, the pattern of winds associated with the "El Nino" storms in South America also causes abrupt changes in North American weather, creating such anomalies as the unusually strong and straight jet stream flow of this past winter.

There is some danger that the current weather patterns generated in the Pacific could continue strongly through the summer and into next fall, shortening the growing season severely and creating an extremely cold winter next year, a Weather Service forecaster said.

Changes of weather over the next month should tell, Gilman said.