A major pattern change underway across western North America will include a strong surge of cold air—starting on Tuesday—across northern sections of the Rockies and High Plains.  During the second half of the week, chilly conditions will encompass much of the western U.S., along with the Plains and upper Midwest.  Late-season warmth will linger, however, across the South, East, and lower Midwest.  Mid- to late-week temperatures will remain below 32°F in much of Montana and portions of neighboring states, with minimum temperatures expected to fall below 10°F. 

…