National and local real estate experts this week contended that the housing industry's more severe postwar slump has bottomed out.

Jack Carlson, chief economist at the National Association of Realtors, said at the group's annual convention in Miami that sales of existing homes will start to pick up this winter and then increase at a more rapid pace starting in the spring.

Carlson predicted that sales would rise from the current rate of about 2 million a year to 2.75 million in mid-1982 and 3.5 million by mid-1983.

This forecast ties in with an anticipated drop in mortgage interest rates, which Carlson expects to fall to 14 to 15 percent by spring.

Housing Data Reports, a local information-gathering firm, said this week that sales have gone as low as they're going to here as well.

New home and condominium sales in major subdivisions totaled 1,580 in October, virtually unchanged from September's 1,600, the firm reported. The October figure still represented a 22 percent drop from October 1980, but that was less of a decline than reported for the summer months from the year-earlier periods.

Visitor traffic to sales offices increased in the first two weeks of November from "laggard levels" in September and October, the company said.

"The upturn in activity on the part of prospective buyers is directly attributed to decreases in the prime rate and FHA and VA lending rates," Housing Data Reports said. "The upturn in traffic is indicative of the continuing pent-up demand for housing. Furthermore, increased traffic levels indicate that the public both realizes--and accepts--the wide variety of financing plans now being offered to make home buying affordable."

The NAR's Carlson, however, predicted that the long-term, fixed-rate mortgage will be back in the next three years.

"Even if the mortgage instrument is called something else, interest rates will remain constant for the life of the mortgage if the economy stabilizes," he said.