Washington — Shoppers have enjoyed a relative holiday from inflation as the consumer price index inched up at a tepid 2.9 percent rate during the past six months. But more rapid increases may be just over the horizon, some economists warn. Climbing prices could add to pressure on interest rates, thus dampening the recovery in interest-rate-sensitive industries like housing and autos.
''The good news on prices is over for the year,'' argues Jason Benderly, chief economist at the Prudential Insurance Company of America. He expects consumer prices to rise at a 5 to 6 percent rate in the second half of 1983 and be climbing at a 6 to 8 percent rate by the end of '84.
As part of the normal economic cycle, prices tend to rise during an economic recovery. ''We will have recovery-related price increases; we are seeing them already,'' says Sandra Shaber, senior economist at Chase Econometrics, a forecasting firm.
The first step is for the price of crude materials - like iron and steel scrap - to start climbing as producers seek to regain ground lost during the recession. These higher costs then work their way through to the finished goods consumers buy.
Crude materials prices ''are starting to flash warning signals,'' notes David D. Hale, vice-president and chief economist at Kemper Financial Services Inc. He calculates that prices of the crude materials included in the producer price index rose at a 58 percent annual rate in the past three months, after excluding volatile food and fuel costs.
Sharply higher raw material costs won't immediately start a firestorm of inflation, he adds. That's because commodity prices started from such a low base that they are now only 6 percent above the levels of a year ago.
''All we are seeing so far is commodity prices responding to economic growth, '' Mr. Hale says. ''Now we have to watch to see what happens to finished goods and wages.''
But he cautions that unless the Federal Reserve Board's recent tightening of monetary policy slows the recovery's rapid pace, ''crude material prices will continue to rise rapidly and push inflation toward 7 to 8 percent by late 1984 or 1985.''
This concern about Fed policy and the effect it may have on inflation is shared by Alan Murray, an economist and vice-president at Citibank. ''We are concerned about the thrust of monetary policy. It looks to us as if the Fed is now leaning in a direction which would tend to accelerate inflation.'' He sees inflation running at a 6 to 7 percent rate by the second half of next year.
Still, not all forecasters expect a sharp pickup in inflation. Wharton Econometric Forecasting Associates, for example, expects consumer prices to rise 3.1 percent in 1983 and 5.3 percent in '84. Chase Econometrics puts inflation next year at 4.3 percent.
''That may have been rampant inflation 20 years ago, but it looks pretty good now,'' Chase economist Shaber adds. She points to several factors that should help control inflation.
First, ''wage rates are rising extraordinarily slowly.''
Data Resources Inc.'s hourly-earnings index rose only 0.1 percent in June and is only 1.9 percent above December's level. Somewhat larger raises are expected in the future, however.
Such bigger pay boosts may be balanced by the relatively rapid growth in productivity, that is, the amount a worker produces in an hour. In the second quarter productivity rose 4.2 percent, the largest rise in more than two years. While that pace cannot be maintained, the 1.5 to 2 percent growth expected in coming quarters ''should offset some of the expected firming in wage demands,'' Mrs. Shaber says.
Other expected contributors to inflation will be harder to offset. For example, Citibank thinks energy costs will continue a mild upswing as fuel demand recovers along with the economy.
Hot weather may also add to inflation. The heat wave has sent the spot price of soybeans up 13.6 percent in the past month, according to Data Resources. While grain costs may rise because of the hot weather, meat prices could fall initially as farmers send more animals to slaughter to avoid rising feed costs.
Finally, consumers will no longer benefit from price reductions aimed at reducing inventory, since the May inventory-to-sales ratio was at its lowest level in more than 15 years.