Displayed below are 6-h forecasts of 500-mb geopotential height from the most recent 00Z or 12Z GFS model run (thick dark green contours) from F00-F84. To assist in visualizing model trends, forecasts from the GFS runs initialized 12 h and 24 h earlier are overlain at the same valid time. The thickness and darkness of the contours decreases with the age of each model run. The user can select a difference field between the current GFS run and that from the GFS run 12 h or 24 h ago. The difference is shaded according to the color scale at the bottom left.