Metro Washington's 100,000 grocery-buying, rent-paying federal and military retirees will miss out on a multimillion-dollar 4.9 percent cost of living (COL) raise they were scheduled to get next month. The September half of the twice-yearly COL catchup system will be officially eliminated next week when President Reagan signs the omnibus budget reconciliation bill.

Congress cleared the package before its summer recess but, because of its size and complexity, the bill itself will not be sent to President Reagan (now in California) until later this week. He is expected to sign it next week prior to a major speech on the economy.

One of the benefits chopped by the reconciliation bill is the system that gave U.S. retirees inflation catchup raises every six months. Instead of the September and March raises, retirees will get a single COL adjustment each year in March. This coming March, retirees will get the 4.9 percent originally scheduled for September plus any additional increase for the rest of 1981.

Failure to get the raise they had anticipated for September (it would have shown up in October checks) will be tough for many, especially the very oldest retirees with the smallest annuities and those getting very modest survivor benefits. Despite best efforts of retiree, federal and postal lobbyists, the September COL was chopped with bipartosan support.