The blanket of snow expected to be covering the mountaintops around the Rogue Valley this morning should begin creeping toward the south valley floor by tonight, weather forecasters said.

The blanket of snow expected to be covering the mountaintops around the Rogue Valley this morning should begin creeping toward the south valley floor by tonight, weather forecasters said.

The National Weather Service office in Medford issued a winter weather advisory last night that will run through Wednesday morning in the southern Cascades and Siskiyous. Also, a winter storm watch is set to go into effect across Southwest Oregon beginning tonight and extending through Wednesday afternoon.

Last night's winter weather advisory was calling for 12 to 17 inches of new snow starting at 4,000 feet, with the heaviest accumulation falling above 5,000 feet.

Beginning this evening, cold air will push in from the northwest, dropping snow levels across the region.

"The question is how low will it go," said National Weather Service meteorologist Mike Ottenweller.

The storm watch said that up to 5 inches of snow might fall at the 1,500 -foot mark, with a chance of an inch or less below that elevation.

Medford sits at an average elevation of about 1,350 feet while Ashland is about 1,900.

Northwest winds of 15 to 25 mph with gusts to 35 mph make blowing and drifting snow possible at all elevations as the cold front moves in Tuesday night.

Snow is likely to continue off and on through Wednesday, then taper off on Thursday. However, cold temperatures will linger, the forecast reports. On Thursday, highs are expected to remain in the 30s and overnight lows could dip to around 20 degrees in areas of the Rogue Valley, Ottenweller said.

After Tuesday night, the snowfall is expected to create hazardous driving conditions at the Siskiyou Summit on Interstate 5, and on highways 62, 66, and 140. As well as at Hayes Hill on Highway 199, and Jacksonville Hill on Highway 238.