(Sturgis, SD)   I-90 between the Wyoming line and Piedmont is snow-packed and slippery this early morning.   Heavy snow is creating limited visibility, slick roads and a slow commute time throughout the Northern Black Hills.   Conditions on I-90 improve, west-bound, at about the Weigh Station, a geographic point long recognized by locals as a place where the weather, and road conditions, often take a turn one way or the other.  By mile marker 46 west-bound, conditions turn largely slush, but remain slippery in spots. On and off-ramps in the Rapid City area can also be perilous this early morning and drivers are encouraged to allow themselves extra time.  State snow plows and sanders are actively working and will undoubtedly help to improve conditions as the morning progresses.