GENERAL OUTLOOK
May was a cool month with below average precipitation in most areas. A short period of warm temperatures early in the month got the snow melt moving. But, since then cool temperatures have slowed the melt. Volume forecasts decreased three to ten percent in most areas. One exception was the Upper Snake River area where above average precipitation increased volumes 3 - 8 percent.
Precipitation for the month was below average in most areas. The exceptions were the Upper Snake, parts of Southwest Washington and northwest Oregon where precipitation was 120 - 130 percent of average. The October - May accumulated precipitation for the Columbia River above The Dalles is at 107 percent of average.
With the cool temperatures and low freezing levels snow pack above the 6000 foot level remains above average, which will keep June - July flows above average in most areas.
The June 1st volume forecast for the Columbia River above The Dalles is 123 million acre feet a drop of 1 percent from the May 1st forecast. This compares with a January - July volume runoff in 1998 of 104.1 million acre-feet.
Precipitation Summary
Overall, below normal temperatures and precipitation prevailed in the basin during the month of May, near to above normal precipitation west of the Cascades.
The beginning of May began with more of April. A series of Gulf of Alaska disturbances dominated the region resulting in what seemed like a perpetual broad trough of low pressure a loft with moist unstable air circulating at the surface. This pattern brought rain to the valleys and snow to the mountains. May 17th marked a change. A shift to warmer southwest flow began to infiltrate the basin. By late May a high pressure ridge took hold of the basin. Above normal temperatures on the east side of the Cascades during the latter part of May helped to spur the main runoff in the Snake River basin. Flood stages were reached in the Upper Snake area as well as along the Imnaha River. Elsewhere, moderate stream flow rises were experienced in late May in the Upper Columbia, Pend Oreille, East Kootenay, Spokane, and Okanogan areas.
Pacific Northwest mean temperatures departed -2.7 degrees from normal relative to 1961-1990 normals (31 stations). Mean temperature departures ranged from -4.9 to -0.3 degrees.
Daily record minimum temperatures were broken at Missoula (22 deg on the 6th... 21 deg on the 8th)... tied at Eugene (32 deg on the 9th)... tied at Portland (37 deg on the 10th).
Daily record maximum temperatures were broken at Astoria (85 deg on the 23rd)... Medford (94 deg on the 23rd).
A daily rainfall record of .99 inches was reported at Astoria on may 2nd.
For May, precipitation was 70 percent of normal (1961-1990) at Columbia above Coulee; 95 percent of normal at the Snake River above Ice Harbor; and 79 percent at Columbia above the Dalles.