Environment Agency Wales said therain "could lead to the possibility of excess water onroad surfaces on Friday and flooding from blocked drains and small streams".
Alan Mason, operations manager for the Highways Agency, said whilst theroad had been salted on Sunday night, freezing rain had led to "perilous" conditions.
Thursday's heavy rain led to the cancellation of two mountain stages, before the fourth was scrapped because of mud ontheroad and stage five was shortened for the same reason.