The Fairfax County Board of Supervisors, leveling a blistering attack on management of the region's transit authority, threatened yesterday to withhold another county payment from the agency unless "measurable progress" is made toward improving its budgetary operations.

Discussing the draft of a letter the board agreed to send to the Rev. Jerry A. MOORE jr., chairman of the budget committee for the Washington Metropolitan Area Transit Authority, board chairman John F. Herrity decried current budget expenditures as "a colossal ripoff" of taxpayers. He particularly criticized workmen's compensation claims, a high worker absentee rate and the transit authority's large management and operating staff.

The renewed attack on WMATA came despite a mild attempt by Supervisor Joseph Alexander (D-Lee) to get the board to tone down its letter to Moore. Alexander is also chairman of the Metro board and was thus caught in the middle of its financial needs and the concerns of the supervisors.

"It's important for us to get moving and solve these problems and get the payments in," said Alexander, whose county already has held up paying $12 million of its share of transit costs.

But Alexander, who ended up voting to send the letter, said he hopes that a region-wide report on WMATA's troubles, expected soon, would address some of the county's concerns and clear the way for it to begin making its quarterly payments again.

"It's going to take an awful long time to do what we want done, maybe four or five months, and I just don't want to give the impression [in the letter] that we expect it all to be done now," Alexander said.

Told of the board's action, William A. Boleyn, WMATA's assistant general manager for finance, warned that the county's continued refusal to pay its share of regional transit costs could force the agency to have to borrow money as early as mid-March.