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0001Q6 important implications regarding concerns of those who worry that enhanced precipitation over the mountains may cause a decrease over the plains. Water available to be converted to precipitation in the Pacific air mass which has been forced up the orographic barrier has been depleted by the time it has reached the plains. It would take another orographic barrier which is higher than the Rockies to squeeze out any more precipitation. However, it seems conceivable that storms (such as closed lows) which provide much of their own air mass lift may produce less precipitation over the plains if pre- cipitation were augmented over the mountains and if Gulf moi sture were not avai 1 ab le over the pl ains. I have seen no evidence that this has occurred. This is simply one of the hypothesis that might be tested in a study of extra-area effects. "Winter precipitation is derived mainly from stratus-type, clouds associated with large-scale disturbances covering large areas, whereas cumulus-type clouds produce most summer pre- cipitation [3J. Major storms are significant contributors to runoff, and most major storms occur in the four-month period September through December [6J. It is extremely rare for one major storm to produce heavy precipitation throughout all the sub-basin areas. Most storms affect only a limited number of the sub-basins [8J. The occurrence or absence of a few large storms generally spells the difference between a season of large or small snow accumulations. The variability of winter precipitation from year to year is quite high [7J. The greatest variation occurs in the southern part of the basin [IOJ. "In general, a relatively few number of heavy precipitation days give substantial amounts of the seasonal snow. Five percent of the heaviest snowfall days give about one-fourth of the seasonal total, and the highest 15 percent gives approxi- mately one-half of the total. Moreover, days of heavy snows sometimes occur in pairs when a large storm system is affect- ing the region [3, 6, 7, 9J. "An average of 20 to 30 storms occur over the basin during the six-month period November through April. Only three large storms producing over 0.40 inch are expected to occur during a typical year [6, 9J. Higher frequencies of precipitation events and greater precipitation yields occur on the high mountain barriers. There is some evidence that a higher frequency of winter storms occur in the northern part of the basin [8J, and that higher precipitation-yield storms occur in the south [llJ." [8J There is a great variability in aspect of the mountain barrier. environmental effects of winter snowfall- according to elevation and the This fact is important in gaging the weather activity - on human activity, on 9 ------,,--- --------------.-