The National Weather Service has issued a lake effect snow warning for Jefferson and Lewis counties beginning at 8 p.m. Tuesday and continuing through 6 p.m. Thursday.   The NWS says the snow will be concentrated mostly to the south of Watertown.   The Tug Hill region could see 4 to 7 inches of accumulation Tuesday night and an additional 7 to 13 inches on Wednesday.   In St. Lawrence County, a winter storm warning will go into effect at 8 a.m. Wednesday.   The county could see an additional 1 to 3 inches of snow.