Flash floods hit Tasmania's north

Flash flooding has forced the closure of a raft of northern Tasmanian streets and highways, with severe thunderstorms now lurching south towards Hobart.

All northbound lanes of the Southern Outlet in Launceston, the city's main arterial road, have been shut down after a deluge of heavy rain on Thursday night.

"Everybody's just got to use extreme caution, keep a lookout for floodwaters over the roads, and if you don't have to drive anywhere, don't drive anywhere," a Tasmania Police spokesman told AAP.

He said many more streets in Launceston and outlying areas had been closed, with police and SES officers responding to a heavy volume of calls for assistance.

The Bureau of Meteorology has issued a severe weather warning spanning half the island state, with heavy rain tipped to hit Hobart as well as the entire northeast and east coasts.