18 U.S. Code § 2074. False weather reports:Whoever knowingly issues or publishes any counterfeit weather forecast or warning of weather conditions falsely representing such forecast or warning to have been issued or published by the Weather Bureau, United States Signal Service, or other branch of the Government service, shall be fined under this title or imprisoned not more than ninety days, or both.
(June 25, 1948, ch. 645, 62 Stat. 795; Pub. L. 103–322, title XXXIII, § 330016(1)(G), Sept. 13, 1994, 108 Stat. 2147.)
This has some people really trying to figure who the mastermind behind the sharpie is.