Leaders of Washington's two richest suburbs said yesterday that their economic outlooks have deteriorated significantly and that they will be forced to make sharp cuts in local government spending next year.

In his first full day as Montgomery County executive, Neal Potter appeared before the County Council and projected a budget shortfall of at least $65 million for this fiscal year and a deficit of $100 million to $185 million next year. The region's "economic storm," Potter said, would force severe cuts in services and, most likely, employee layoffs.

In Fairfax, County Executive J. Hamilton Lambert said his county will not have enough money to give cost-of-living increases to its 30,000 school and government employees. The projected budget deficit for next year, he said, is about $70 million, $31 million higher than estimates released just seven weeks ago.

The snowballing of gloomy forecasts and predictions from the two counties, which rank among the nation's wealthiest jurisdictions, parallel grim budgetary assessments by governments throughout the area.

District officials ended fiscal year 1990 in September with a $93 million deficit, and they expect a shortfall as high as $200 million this year. The Rivlin Commission, a group that studied the city's fiscal problems, recommended eliminating 6,000 jobs, and Mayor-Elect Sharon Pratt Dixon has said she would like to cut 2,000 jobs from the city payroll.

Prince George's County Executive Parris N. Glendening has laid off 190 workers and frozen 649 vacancies to deal with a $50 million shortfall. Prince William and Loudoun counties, which have seen the bottom fall out of once-booming real estate markets, expect revenue shortfalls next year of $17 million and $30 million, respectively.

Over the years, Fairfax and Montgomery have pumped huge amounts of money into state coffers, and local officials now fear that Maryland and Virginia will turn to them to resolve statewide budget shortfalls. Lambert said rising federal deficits and soaring fuel costs could further hurt the fiscal outlook for local governments.

In Montgomery, Potter announced that he will continue the cost-cutting moves implemented by his predecessor, Sidney Kramer. Those measures included a hiring freeze on county employment, travel and contract purchases of equipment and services.

Under the freeze, no vacancies in the government can be filled without special permission. The county, which has a work force of 6,686 full-time and 3,908 part-time and temporary employees, currently has 600 vacancies.

Potter did not reveal specifics of what programs might be cut. He said everything is an option, including employee contracts and class sizes. He stressed that those would not be his first choices for cuts.

"There is no way to take up any of this without substantial cuts," Isiah Leggett (D-At Large), the council's new president, said yesterday. "You are presiding over a funeral," he told Potter.

Fairfax's deficit is in addition to a projected $60 million shortfall next year, which the Board of Supervisors erased four months ago by adopting strict limits on increases in school and county spending in fiscal 1992, which begins July 1, 1991.

Lambert, however, said he believes the board will not have to increase taxes next year -- when all nine supervisors face reelection -- "but in {fiscal year} '93 you're going to hit the wall . . . . '93 is going to be incredible."

Locally, a slump in real estate values has led to a projected flattening in property assessments. Those assessments had soared in recent years, funneling millions in property tax revenue into local government coffers, but sparking anti-tax rebellions in the District and jurisdictions around the Beltway.

In Fairfax, for instance, property assessments have increased about 78 percent since 1987. But Lambert said he expects no increase in assessments. "I've never seen that," he said.

Lambert said home sales in the county in the first 10 months of 1990 were 28.4 percent lower than in the first 10 months of 1989, and the number of building permits issued from July to September was down 31.6 percent from the same time last year.

Tax revenue from new development, he said, is projected to rise 2 percent, "the smallest rate of growth in Fairfax County in the postwar era."

On top of all that, he said, the county school system could lose an additional $30 million from Virginia next year because of the state's deepening budget crisis.

"These factors are precipitating the severest impact to county revenue collections ever experienced during any postwar recessionary period," Lambert said, adding that public confusion and an erosion of confidence are compounding the problem. "Anxiety feeds on itself, and right now we're having an anxiety feeding frenzy," he said.

Officials said both counties' school systems -- long considered sacred cows in the budget process -- were likely to face unusual pressure to cut costs.

Potter said Montgomery's school system, which makes up nearly 50 percent of the county budget, will not be exempt from the knife.

Montgomery school officials have noted that they will be hard-pressed to make any major cuts at a time of rapidly increasing student enrollment.

The last time the Montgomery faced budget problems was in the early 1980s, and it was able then to save money because school enrollment was declining. School officials are projecting an addition of at least 4,000 children next year to the current enrollment of 103,772.

Fairfax Board Chairman Audrey Moore (D) said county finance officials told her that they expect the school system to have a $16 million surplus at their mid-year budget review tomorrow. School officials denied that.

Moore, saying that the school officials did not appreciate the depth of the county's fiscal problems, urged other supervisors to join her in attending tomorrow's School Board meeting to ask that they use any surplus to help offset the expected budget crunch next year.

"I think it's important to send a message to the School Board members that the money is not going to be there next year," Moore said.

School Board Chairman Kohann H. Whitney said: "My board members are responsible people who keep up with what's going on. We are fully aware of the financial situation, both statewide and locally."