An autopsy has concluded that a Petaluma, Calif., teenager who died after leaving South Lake Tahoe's SnowGlobe Music Festival New Year's Eve died of "probable hypothermia."Alyssa Byrne, 19, died of "probable hypothermia due to prolonged environmental exposure," according to a Monday statement from the El Dorado County Sheriff's Office."Methampethamine toxicity" and "multiple psychoactive drug ingestion" were also "significant conditions" in the death of Byrne, whose body was found in a snowbank off Pioneer Trail in the area of the festival Jan. 4.Sheriff's spokesman Lt. Pete Van Arnum declined to release exactly what drugs were found in Byrne's system. He said …