Temporal variation of snow depth in the National Center for Environmental Prediction (NCEP) ETA model. a) Snow depth at time 3Z on 3/15/99; b) snow depth at time 0Z on 3/16/99; c) snow depth at 3Z on 3/16/99; and d) snow depth at -107.5 longitude and 40.0 latitude from 15Z on 3/14/99 to 12Z on 3/18/99. Note, for example, the disappearance and reappearance of snow over Wyoming/South Dakota and fluctuation of snow line through Montana/North Dakota.