Dense fog blanketed the Chicago area Tuesday morning, causing ground stops at both O’Hare and Midway airports and temporarily disrupting holiday travel. However, the fog rapidly dissipated by mid-morning, and the return of filtered sunshine accompanied by southerly winds allowed temperatures to climb into the 50s for the third straight day. The unseasonable warmth will continue through Thursday with the 57-degree high expected on Christmas, likely to be the city’s second warmest on record, runner-up to the record 64-degree high reached on Christmas in 1982. The warmth should continue on Thursday, potentially breaking the 55-degree record high for December 26, set back in 1971. A strong storm system is forecast to develop over the weekend and will bring heavy snowfall to the upper Midwest, but rain to the Chicago area, though it could briefly change to snow before ending Sunday night.