With bare ground on Christmas Day, Chicago was in step with three-quarters of the nation, as just 24.6 percent of the U.S. reported a Christmas Day snow cover, the lowest total since 2005. That was a far cry from Christmas 2009 ,when national snow cover reached a robust 63 percent. Chicago’s Christmas afternoon high temperatures clustered in the middle and upper 40s as the city’s official 45-degree high tied for the 14th Christmas warmest dating back to 1870. Even milder weather is on tap for the rest of this week as a strong winter storm heads for the upper Midwest. Chicago will be in the storm’s warm sector and be the recipient of rain, thunderstorms and temperatures in the 50s, while portions of the northern Plains and upper Midwest brace for heavy snow and possible blizzard conditions on the storm wintry northern flank. More seasonable weather will arrive by the weekend as temperatures return to the 30s.