When a group of government agents (i.e. FBI, DEA, ATF, etc.) are enacting a search warrant on a premises. So named because of the large amount of agents that wear trenchcoats as part of their attire. Term used more often in colder regions of the country. Can also apply to municipal law enforcement detectives as well.
"Did you see the trenchcoat convention at the meth lab down the street? Looks like they finally busted those guys."