An unseasonably warm and dry December in Northern California has ski resort operators worried about a lack of snow, but they say they\'re staying optimistic. While last year\'s snowfall brought huge totals -- 65 feet fell on the mountains around Lake Tahoe resorts, well above the typical 50 feet -- the snow so far this year is being measured in inches, the Los Angeles Times reported Thursday. It\'s had an impact on the holiday season business, ski resorts say. \"It\'s certainly having an effect on us now,\" Joani Lynch, a spokeswoman for Mammoth Mountain, said. The resort, which had so much snow last year it stayed open until July 5, has had just 2 inches of snow in December. \"Visitor numbers are down 18 percent behind last year,\" Lynch said. Many of the mountainsides in Mammoth are brown, and only 46 of 150 available ski trails are open. The problem is not just in California, as ski resorts across the country are struggling with a lack of snow and falling business, in contrast to last year when resorts had record snowfalls and attendance. The National Oceanic and Atmospheric Administration said 47.5 percent of the continental United States was covered in snow this time last year, about double the coverage this year, USA Today reported. Still, ski operators say they remain optimistic the rest of the winter will bring plenty of snow -- and plenty of ski buffs.