Operational and technical considerations regarding recent forecast seasons in California

The past three runoff seasons in California have resulted in some very unusual situations with regard to the preparation and use of operational water supply forecasts in California's Sierra Nevada. The 1976 season was near record low, followind by 1977 which was record low at about 15 percent of average runoff. This was followed by 1978 at over 200 percent of average runoff in some southern Sierra watersheds. These contrasting seasons represented the record minimum season runoff, the record minimum two year runoff, and the second or third highest runoff of record in the southern Sierra Nevada. Preparaton of forecasts during these seasons posed certain technical problems in presentation and use of water supply forecast of operational purposes. In addition, very unusual climatologic conditions further complicated the problem of analysing forecast probability and updating forecasts to meet changing conditions. This paper documents some of the technical and operational problems encountered by forecasters during this sequence of seasons in California.