National Weather Service issues grim warning calling Harvey 'unprecedented,' with impacts 'unknown'

The National Weather Service is pulling no punches when it comes to Hurricane Harvey. 
The federal agency is now calling the storm battering Texas and bringing historic levels of flooding to Houston "unprecedented." 
SEE ALSO: Hurricane Harvey: A weather geek's live blog"... All impacts are unknown & beyond anything experienced," the NWS added in a dire tweet.This event is unprecedented & all impacts are unknown & beyond anything experienced. Follow orders from officials to