DENVER (AP) _ Water buffalo meat and new-wave martinis will jazz up menus in 1988, a year that also will see booms in girdle-wearing, fly fishing and yuppie bowling, golf and softball, says ''The American Forecaster 1988.''

On the way out in 1988 will be fried chicken, Cajun food, movie sequels, parenting magazines, instant coffee and faith healing.

Kim Long, the Denver author of the 192-page fifth annual ''Forecaster,'' said reverse vacations, reverse commuting, thread bikinis, fat suctioning, smudge-proof newspaper ink, luxury cars, cuff links and blondes also will be hot in '88.

Long, 38, said the ''baby boom'' generation - the one-third of the U.S. population that was born between 1946 and 1964 - is the dominant trend-setting force and will continue to be for years to come.

''It gets boring talking about them all the time, but it's impossible to ignore them,'' he said.

Health- and status-conscious young urban professionals, the vanguard of the baby boom generation, will introduce many of the '88 trends, according to ''The American Forecaster.''

Water buffalo meat will be popular in '88 because it contains less fat and half the cholesterol of beef, and some say it even tastes better than beef. A Georgia firm already is marketing it.

Martinis will be popular again, the ''Forecaster'' predicts, but not necessarily the traditional gin-and-vermouth mix. A Cajun martini made with either red chilis or jalapeno peppers will typify the new blends.

New girdles of light, less bulky stretch material will be popular among baby boomers who have avoided the fitness craze but still want to wear the latest fashions, says the ''Forecaster.''

Fly-fishing, long considered an upper-class pursuit, has caught the fancy of yuppies, the ''Forecaster'' says. Trout Unlimited, one of the largest trout-fishing organizations, reports its typical member is 41 years old and spends more than $3,000 a year on the sport.

Similarly, snorkeling and scuba diving have become some of the hottest new recreational activities as city-dwellers head for beach vacations eager to expend money and energy on exotic pursuits.

The ''Forecaster'' says aerobics, jogging and racquetball are passe yuppie exercises, and traditional sports such as softball, bowling and golf are on the way back as baby boomers age and are less inclined toward strenuous activity. The next phase of this trend will be yuppie bowling leagues, according to the ''Forecaster.''

Another trend identified by Long is reverse vacations. ''In the last two decades, we had a record number of people migrate from one place to another,'' he said. ''Migration has slowed down, but a lot of people are taking their vacations back in the areas they moved away from.'' For example, many Florida residents are vacationing in the Catskill Mountains of New York state, says Long.

Reverse commuting already is evident in many metropolian areas where people have moved back into the city and their employers have migrated to the suburbs, says the ''Forecaster.''

Long, whose forecasts have gained such respectability that they appeared for the first time this year in the ''Old Farmer's Almanac,'' claims an 80 percent accuracy rate.

His 1987 ''Forecaster,'' for example, accurately predicted the Denver Broncos would appear in the Super Bowl and also was on the mark in targeting yuppie gardening, four-wheel-drive luxury vehicles and Caribbean cuisine as trends in '87.

His inaccuracies are just as striking. He said ''Us'' magazine would fold and that the prime-time television soaps ''Dallas'' and ''Dynasty'' would have their last seasons in 1987. All are very much alive this year.

Long has eliminated categories that caused him the most problems in the past. Gone are sports predictions, and he has hedged on political prognostications, refusing to predict who the next president will be.

''This is a prediction better suited to a psychic than a forecaster,'' Long wrote.