The sky was overcast all morning, causing an anxious public relations crew to glance up nervously every few seconds.

It would not have done for the clouds to break and drench Gov. Harry Hughes, a dozen other public officials, a few high-ranking corporate executives and hundreds of employes who gathered last week for the formal opening of the Citicorp bank-card processing facility here.

But the rain held until minutes after the ceremony ended and employes had gone back to work at the 130,000-square-foot facility just north of Hagerstown.

It was perhaps a fitting metaphor for the atmosphere in Hagerstown and other Western Maryland communities these days as residents struggle to meet the challenge of an economy that often seems as unpredictable as June weather.

With the coming of the giant Citicorp's Credit Services facility, Washington County gained more than 300 full- and part-time jobs and the prospect of about 700 more in the next two years.

The facility, which will handle Visa and Choice bank-card operations in Virginia, Maryland, New Jersey and the District of Columbia, is among the first built by a group of corporations, including General Electric Co. and Tandy Corp., that have been drawn to Washington County by low property costs, tax incentives and an eager work force. But it was also a week of renewed concern about more layoffs at the Mack Truck plant, one of the county's major employers. Mack officials revealed plans to cut $37 million in costs by reducing wages 25 cents an hour and trimming benefits for the company's force of about 2,000 workers.

Company officials said that unless production costs decrease, parts of the operation will have to be moved to South Carolina. Similar moves in recent months have have already cost the Hagerstown plant 400 jobs. Although union leaders say they are willing to negotiate, many employes feel that additional layoffs are inevitable.

But if the developments at Mack Truck were the dark cloud over Hagerstown last week, the events at Citicorp were the silver lining for new employes and officials. "It's a great day. It's really symbolic of what's going on," said Hughes, who helped cut the ribbon at the facility.

Hughes, who is a candidate for the U.S. Senate, called the opening "the end of maybe some despair of previous years and the beginning of some great hope."

The Citicorp center was built near a plant formerly operated by Fairchild Industries, whose departure from Hagerstown three years ago meant the loss of 1,500 manufacturing jobs. It was a stiff blow to the economy of the already depressed region.

The new center is a byproduct of a deal struck three years ago between Maryland officials, who needed to channel development to high unemployment areas, and the New York-based Citicorp, which was seeking entree to offering full banking services in Maryland.

Among the new employes is Gwen Psillas, 35, who is divorced and has two teen-age children. After months of unemployment, Psillas has been hired by Citicorp to answer customer calls.

The job has given her renewed self-confidence, she said. One of several hundred chosen from among 12,000 applicants, she walked out of the interview feeling "wonderful," she said.

For another applicant, Pam Hardy, Citicorp has meant the chance to stay close to home. Hardy, 28, mother of a 3-year-old child and the wife of a corrections officer, said that commuting to Washington or Baltimore to work would have been rough on her and her family.

And for Kim Greene, 20, the data processing job that Citicorp offered her has meant a step up in pay, prestige and interest from the sales position she had before. "When I first started working here, people said, 'Don't get used to it, it's too good to be true,' " she recalled. "But it hasn't changed."

Greene is not the only person pinching herself. Ronald Bowers, chairman of the Washington County Board of Commissioners, noted that when he took office in January 1983, unemployment in the county hovered at 19 percent, and that it is now down to 6 1/2 percent. He contends that Citicorp's arrival has set the tone for an new era of prosperity.

"This company, right here, is destined to change the face of the way we do things in Washington County. It's the beginning of a new era," he said.

Whether the new era will bring long-term prosperity remains unclear. Some area residents noted that the work force at the Citicorp plant is predominantly female and paid less than the male-dominated work force at Mack and other manufacturing companies.

"Everybody wants to be optimistic when you're dealing with their job," said James Stewart, president of the union local that represents Mack Truck workers. "I wouldn't call Citicorp an alternative."

At Mack, the average pay is about $14 an hour, according to union officials. At Citicorp, where 81 percent of the work force is female, employes are "in a range above" the federally mandated minimum wage of $3.35 an hour, company officials said. But they refused to be specific, and cautioned employes against it as well.

While the unemployment rate in Western Maryland was 6.3 percent in April, down from 7.8 percent in March, it was still above the state's 4 percent average.

Moreover, unemployment dropped for the month in part because of seasonal employment opportunities tied to the tourism industry, state officials said.