GENERAL OUTLOOK
Snow packs have depleted sharply basin wide. In northern areas the June 1st snow is about 70 percent of average, while southern portions of the basin have lost most of their snow. May runoff was slightly below average in the Upper Columbia, Flathead, on the North Cascades in Washington and in the Upper Snake area. Other portions of the basin had May runoff from 40 to 70 percent of average.
A reduced snow pack and slightly below average May precipitation has caused a drop in forecast volumes. Forecasts dropped 2-6 percent in most basins. The only increases in volume occurred west of the Cascades in Washington and Oregon from rainfall runoff. The June 1st January - July forecast for the Columbia River at The Dalles is 102.0 MAF or 96 percent, down 3 percent from last month. This compares to a runoff of 124.1 MAF in 1999.
Temperatures were below normal across southern British Columbia and northern Washington, near normal across southern Washington, northern Oregon, northern Idaho and western Montana; and, above normal across southern Oregon, southern Idaho and northwest Wyoming. Precipitation was above normal west of the Cascades and across southern British Columbia, northwest Wyoming and extreme Southeast Idaho. Precipitation was near or below normal elsewhere across the Pacific Northwest.
During the beginning and end of the month, a trough of low pressure along the Pacific Northwest coast brought cool and showery conditions to the west side and northern tier basins. During the middle of the month, far southeast basins received precipitation from an upper level low pressure system which slowly tracked from the gulf of Alaska into Northern California, then into the Great Basin.
Pacific Northwest mean temperatures departed +0.3 degrees from normal relative to 1961-1990 normals (31 stations). Mean temperature departures ranged from -1.7 to 6.6 degrees.
Record high temperatures reported during the month of may included:
80 at Butte on the 1st (tie) and 72 at Astoria on the 14th (tie).
Record low temperatures reported during the month of may included:
24 at Kalispell on the 7th and 33 at Quillayute on the 24th.
Several daily precipitation records were broken during May. On the 9th .93 inches were reported at Olympia and .74 inches were observed at Seattle. On the 10th, Portland set a record with .42 inches on the 30th, .83 inches was measured at Spokane. On the 31st, Great Falls picked up 1.20 inches of precipitation and 3.4 inches of snow.
For May, precipitation is:
93 percent of normal (1961-1990) at Columbia above Coulee,
93 percent of normal at the Snake River above Ice Harbor,
and 101 percent at Columbia above The Dalles.
For the water supply season, precipitation is:
109 percent of normal (1961-1990) at Columbia above Coulee,
93 percent of normal at the Snake River above Ice Harbor,
and 102 percent at Columbia above The Dalles.