Howard County, which during the past decade established a reputation as a veritable magnet for affluent home buyers, will lead the Baltimore area in population growth and median income into the 1990s, according to a new Regional Planning Council forecast.

The county's population is expected to increase from 156,000 to 186,000 within five years, accounting for nearly half of the new residents in the region that includes Carroll, Harford, Anne Arundel and Baltimore counties and Baltimore City, the report states.

Most of the county's new residents are expected to settle in Columbia, followed by North Laurel and Ellicott City, as they occupy the estimated 15,600 houses and apartments that are expected to go on the market by 1992.

The county is expected to assume a disproportionate share of the region's wealth in coming years, according to the report. The median annual family income in Howard, which already is $17,000 higher than the figure for the region, is expected to grow to $61,000 by 1992, a $13,000 increase over the current county figure and $21,000 higher than the $40,000 median predicted for the region.

The statistics "are a reflection of the county's newness," said Josef Nathanson, the planning council's director of economic research.

"So many people have migrated there in the last 15 years, and they've been able to meet the requirements of the housing market. That's one of the reasons why the county is looking at new ways of obtaining afforable housing," he said.

The report, titled Economic Outlook to 1992, predicts a stable, though not spectacular, growth in the number of jobs that will be created in the region. Total employment is expected to increase by 32,800 jobs during the next five years compared with the 105,000 jobs created since 1982, it states. Baltimore County is expected to lead the region with 11,800 jobs created, while Howard, with an estimated 9,300 jobs created over five years, comes in second in the region.

According to the report, shopping is one area where Howard County does not meet expectations. Based on the size and wealth of its population, the county would be expected to account for 8 to 10 percent of the region's retail trade. But because many county residents shop elsewhere, during the next five years the county's share of retail trade is expected to grow only to 6.6 percent, or $1.3 billion.

Uri P. Avin, director of Howard's Office of Planning and Zoning, said the statistics have been figured into the county government's plans for building schools and improving roads. He added that planners will have to keep an eye on projections in coming years to make sure that growth, which is expected to fall off after 1992, does not surprise officials by continuing at its rapid pace.