Washingtonians began the traditional Memorial Day exodus to resort hotels and relatives' homes yesterday, but rain and forecasts for more over the three-day weekend made it likely that 1982 would be a less-than-record year at the Atlantic Ocean beaches.

Despite the rain, vehicles were crossing eastbound lanes of the Chesapeake Bay Bridge at a rate of 2,000 to 3,000 an hour yesterday evening, police said. Major traffic back-ups were reported at the Kent Narrows bridge and many intersections on the roads across the Eastern Shore counties.

Police at the Baltimore tunnel reported traffic of 2,500 vehicles per hour each way, a normal rate for a Friday evening. "If the weather was nice and they were calling for a sunny weekend, I'm sure the traffic volume would have been more for today," said Officer Gerald Gebhart of the Maryland toll facilities police.

Meanwhile, Amtrak had extra cars standing by to handle an expected surge in travelers. At National Airport, "the terminal was fairly busy starting around noontime," said spokesman David Hess.

For coastal areas in Maryland, Delaware and southern New Jersey, the National Weather Service predicted cloudy weather for this morning, a chance of rain in the afternoon and highs in the mid-70s. Partly sunny and hazy was the forecast for Sunday and Monday, with scattered afternoon and evening showers and thunderstorms, and temperatures up to the 80s.

From Virginia Beach to the Cape Hatteras area of North Carolina, the forecast was identical for today through Monday: partly cloudy, chance of thunderstorms or showers late in the day, with high temperatures in the mid-to-upper 80s.

On the interstate highways, state troopers, some of them working overtime, were patrolling in added numbers in a coordinated effort to curb drunken driving and speeding and clear up holiday traffic snarls. Virginia registered 18 fatalities last Memorial Day weekend; Maryland had 10.

Lt. Herbert D. Northern of the Virginia State Police said that double the normal number of troopers would patrol major highways in Northern Virginia, with special attention going to the beltway and Rtes. I-395 and 66.

Municipal officials at Ocean City, meanwhile, were predicting that 150,000 to 175,000 people would show up over the weekend, though the weather could dampen that turnout. Reservations for Ocean City's 10,000 hotel rooms were running at about the same level as last year's, city spokesman Charles Lehmann said.

The four-lane Kent Narrows bridge is a traditional bottleneck on the trip to the beaches. Yesterday's traffic backups occurred even though a $2.2-million job to replace the concrete deck was completed several weeks ago and barriers that had closed off two of its lanes over the winter were removed. Backups of 10 miles were reported during the winter.

But there was good news for families headed for resorts on North Carolina's Outer Banks: It is no longer necessary to drive a 90-mile detour to avoid a closed bridge at the tiny town of Coinjock. An army landing craft struck the bridge's open span on April 23, locking it in the open position, but the U.S. Army Corps of Engineers repaired the damage and normal traffic has resumed.

Airlines expect heavier-than-usual bookings over the holidays, but not the crush loads of their traditionally busiest times, Christmas and Thanksgiving, according to National Airport spokesman Hess. Airlines serving National were not planning to provide extra flights, he said.

Amtrak spokesman Jung Ha Lee said heavy traffic was expected, but that the railroad would add extra 82-seat cars to its trains to meet the expanded load on the 100 daily departures on short-haul Northeast routes. Travelers making longer trips, such as to New Orleans or Chicago, should book in advance, he said.