Weatherman Scott Stevens claims that "Japanese gangsters known as the Yakuza caused Hurricane Katrina." Here are more details (thanks to John for sending me the link):

Scott Stevens says after looking at NASA satellite photos of the hurricane, he is convinced it was caused by electromagnetic generators from ground-based microwave transmitters. The generators emit a soundwave between three and 30 megahertz and Stevens claims the Russians invented the storm-creating technology back in 1976 and sold it to others in the late 1980s. Stevens says the clouds formed by the generators are different than normal clouds and are able to appear out of nowhere and says Katrina had many rotation points that are unusual for hurricanes. At least ten nations and organizations possess the technology but Stevens suspects the Japanese Yakuza created Katrina in order to make a fortune in the futures market and to get even with the U.S. for the 1945 bombing of Hiroshima.

Also check out Scott Stevens's website, Weather Wars, where he elaborates on his theory of scalar weather (as he calls it) and provides a lot of hurricane imagery to make his case. Stevens really seems to believe his theory, so I would classify it as a conspiracy theory rather than a hoax.

Update: Here's a page with more pictures of supposedly artificially created scalar weather phenomena.