It was hot, steamy and insufferable yesterday as temperatures in the Washington area reached a high of 99 degrees, but by the time you read this, relief of sorts should have arrived courtesy of an advancing cool front.

Translation: For the next couple of days, the high will probably be a balmy 90.

This report comes from the National Weather Service, where forecasters insist that August is never as hot as July -- even though most Washingtonians think otherwise.

Forecasters caution, however, that just as this July set records for prolonged, oppressive heat, there is a significant chance that August will be a good deal hotter than usual.

Translation: It's beginning to look a lot like July.

"If the pattern continues, and our forecast is that it will, there's a 60 percent chance of August being hotter than normal," said Donald Gilman, chief of the prediction branch at the National Weather Service's Climate Analysis Center. "We think it's going to be a hot August."

Gilman, who operates out of the World Weather Building in Camp Springs, said it has been a really hot summer for the northern Plains and Midwest states as well as the East Coast. For the nation at large, this is "the third hot summer of the '80s -- 1980, 1983 and now 1987."

Normal temperatures for August in the Washington area average 77.6 degrees, compared with 78.9 degrees for July.

This July matched the all-time record for average high temperatures, with daily highs averaging 92 degrees. So far, August is piling up its own string of 90-plus temperatures.

The heat has been blamed for more than 90 deaths -- most of the victims elderly -- in 13 states, including about 50 in the Philadelphia area. In the Washington area, however, hospital officials say they have not experienced the wave of heat-related emergencies they expected. Medical examiners in the District said that two elderly city residents died of heat-related conditions in July but that there have been no such deaths in August. Officials in Maryland and Northern Virginia have not reported any deaths believed to be heat-related since the latest hot spell began more than two weeks ago.

The heavy, still air is doing cruel things to the air quality of the region and has been particularly taxing on people with allergies, asthma and other respiratory problems.

The Metropolitan Washington Council of Governments, which takes air pollution readings, officially declared the air in the region "unhealthful" yesterday.

However, a pollution advisory, which involves several factors in addition to the air quality index, has not been issued here since 1983, according to Trevis Markle, assistant director of COG's air quality program.

The heat wave's effect on crops and well-tended lawns has been apparent.

"Since June, we've had nothing but hot and dry weather," complained David Conrad, a county extension agent for agriculture in Prince George's County. "If things continue to go in this direction, it could be disastrous."

Crops can stand the heat better than humans can, and farm officials say the Washington area has largely recovered from the drought of last spring and summer. But the crops need rain, and the region received only 2.17 inches in July compared with a normal July rainfall of 3.88.

Tobacco, field corn and soy beans

have been hit the hardest by the dry spell, according to Conrad, and tomatoes, squash, cucumbers, cantaloupe and sweet corn have not fared much better. In Prince George's, he said, officials are estimating a 50 percent "yield loss" in field corn and a 33 percent loss of the tobacco crop if conditions continue.

Thunderstorms, while offering a bit of human relief, do not always help the crops and lawns as much as might be thought, because of runoff. But even a thunderstorm "would be a blessing right now," Conrad said.

Yesterday's high of 99 degrees at 3:05 p.m. was not a record for the Washington area, according to Scott Prosise, a regional forecaster for the Weather Service. That was set Aug. 4, 1930, when the mercury hit 102.

The temperature did reach record proportions yesterday at Baltimore-Washington In- ternational Airport, where it was 97 degrees at 3:25 p.m. The previous record there was 96, set in 1975.

Prosise said, "July is always hotter than August."

He rattled off a few temperature statistics from the past, recalling the time in 1930 when the maximum temperatures between July 18 and July 22 averaged 101 degrees. And the highest temperatures ever recorded in Maryland occurred in July 1936 when Frederick and Cumberland hit 109.

Forecaster Gilman, noting that wind and pressure patterns tend to govern the weather, cited a number of factors that have contributed to the heat wave in this part of the country. Among the forces of nature conspiring against us: extra wind from the south, fewer clouds to help block the sun, and a fickle jet stream, which last month traveled well to the north of its usual route along the Canadian border, allowing hot weather to penetrate another 200 miles north.

The cool wave expected to bring this region relief today helped ease temperatures in the baked Midwest and Plains states yesterday, breaking an 18-day heat wave. Forecasters said this will reduce the heat index -- a combination of heat and humidity that measures how hot it feels -- from "stifling" to "uncomfortable" in most areas.

"The feeling is kind of like walking through pea soup," said Linda Haase of the Illinois Department of Health. "You have to go real slow."