From NOAA's Environmental Visualization Laboratory:
NOAA's North American Model output shows that on January 1, 2012 a blast of cold, dry Canadian air will be sweeping down through the Plains and continue moving across the Eastern U.S. This image shows the relative humidity for the U.S. at 2100z on January 1st. Brown areas are dry, gray-blue areas are higher in moisture. The streamlines overlaid on the imagery show the general direction of wind flow.