In what could be a preview of times to come, Virginia's unemployment rate rose nearly one-half point in June, from 3.8 percent to 4.2 percent, the Virginia Employment Commission reported yesterday.

Experts said the statewide increase was principally due to an influx of students looking for summer jobs, but the rise in Northern Virginia joblessness -- from 2.0 percent to 2.2 percent -- also stemmed from the weakening economy here.

"In Northern Virginia, it has slackened more than anywhere else," said William Mezger, a research economist with the commission. "Jobs in Northern Virginia had been growing at 5 to 7 percent {per year}, while the rest of the state was growing by 3 percent. Now it has slowed down."

The Northern Virginia figures, along with information about suburban Maryland and the District, will be used in calculating the June unemployment rate for the D.C. area, which is also likely to show an increase when it is released Friday. Although the local jobless rate is one of the lowest urban rates in the nation, it masks slowing area job growth and ill health in such industries as construction, defense, retailing and finance.

Mezger said the area's construction slowdown actually appears in the jobless rates for several counties as far away as 50 miles from Washington. It is from those counties that many of this area's construction workers came, and that is where they have returned as their industry lost 7,500 jobs in Northern Virginia from June 1989 to June 1990.

In Spotsylvania County, for instance, unemployment was 5.5 percent in June, compared with 3.5 percent a year ago. In Culpeper County, the rate rose to 5 percent from 2.4 percent.

Locally, the rates were lower. Alexandria had the highest level at 2.4 percent and Falls Church the lowest at 1.6 percent. The rate for Fairfax County was 2 percent, up only slightly from 1.8 percent a year ago.

Statewide, Virginia added 27,100 jobs in June for a record employment level of 2,965,300. There were 64,200 more jobs in June 1990 than in the same month in 1989. The monthly figures are not adjusted for seasonal variations.