I’ve seen this 2 or 3 times, don’t know how it got started.
Their proper name was WEATHER Underground, not Weatherman.
You don’t need a weatherman to know which way the wind blows.
This phrase is famously used in Subterranean Homesick Blues by Bob Dylan. Most famously, its lyric “you don’t need a weatherman to know which way the wind blows” was the inspiration for the name of the American radical left group the Weathermen https://english.stackexchange.com/questions/36195/origins-of-the-phrase-you-dont-need-a-weatherman-to-know-which-way-the-wind-bl
The Weather Underground Organization (WUO), commonly known as the Weather Underground, was a radical left militant organization active in the late 1960s and 1970s, founded on the Ann Arbor campus of the University of Michigan. It was originally called the Weathermen. https://en.wikipedia.org/wiki/Weather_Underground