A substantial majority of the nation's top economic forecasters say the recession will be briefer and less severe than the post-World War II average for downturns, a new survey shows.

Sixty-one percent of forecasters polled look for the recession to be shorter than the post-1945 average of 11 months, while 36 percent think it will be about average in length, the National Association of Business Economists said."As for the recession depth, 78 percent expect to see less than the postwar average 2.5 percent decline in GNP (gross national product), while 15 percent expect about an average decline," the association said.

The survey, conducted during the first two weeks of February, also shows that just over half of the 54 economists believe the recession will end in the second quarter. Two-thirds had predicted last November that it would end in the first three months of this year.

In the new survey, 22 percent put the low point in the current quarter, while 18 percent said the downturn will bottom out in the third quarter of 1991.

The GNP fell at an annual rate of 2.1 percent in the fourth quarter of 1990, and the median expectations of the NABE economists are for a 1.6 percent decline in the current quarter and a 0.3 percent decline in the next.

A recession generally is defined as at least two consecutive declines in the GNP, the nation's total output of goods and services.

But while the forecasters extended the length of the recession, they did not significantly alter their projection of the economy's initial recovery.

They foresee a growth rate of 1.9 percent in the third quarter and 2.5 percent in the fourth. The November poll forecast a 2.0 percent growth rate in the third quarter and 2.5 percent in the fourth.