The main water producing areas of the Columbia River in British Columbia and Montana continue in a cool wet pattern. June 1st snow water equivalents and monthly precipitation are both in the 130 to160 percent range. As a result, northern basins and the main stem of the Columbia River down to The Dalles have seen a 1 to 8 percent increase in streamflow volume from May 1st forecasts.
A cool northwest flow dominated through much of the month and kept temperatures below normal levels across most areas. Upper level low pressure systems brought mainly light precipitation as they moved southeast from the Gulf of Alaska into British Columbia and the Pacific Northwest. Above normal precipitation was mainly confined to parts of Southern British Columbia and Northwest Montana.
PRECIPITATION SUMMARY
May, precipitation was 129 percent of normal (1971-2000) at Columbia above Coulee, 55 percent of normal at the Snake River above Ice Harbor, and 94 percent at Columbia above The Dalles.
Water supply season, precipitation was 102 percent of normal (1971-2000) at Columbia above Coulee, 90 percent of normal at the Snake River above Ice Harbor, and 98 percent at Columbia above The Dalles.
Daily precipitation records established in May occurred on the 27th. They included 0.40 inches at Pendleton Airport, 0.51 inches at Sandpoint ID, and 0.52 inches at Whitman Mission, WA.
TEMPERATURE SUMMARY
The 31 station temperature index for the Pacific Northwest departed -1.7 degrees from normal relative to the 1971-2000 normals. Mean temperature departures ranged from -3.9 to 0.8 degrees.
New high temperature records established in May included 87 at Kalispell and 91 at Missoula on the 19th and 81 at Jackson and 91 at Pocatello on the 30th.
There were many new low temperature records established in May. They included 26 at Olympia, 27 at Yakima, 30 at Hoquiam, and 32 at Walla Walla on the 7th; 25 at Pocatello, 25(tie) at Meacham, 28 at Boise, 29 at Olympia, 31(tie) at Salem, 33(tie) at Pendleton Airport, and 36 at Hoquiam and Walla Walla on the 8th; 16 at Jackson, 20 at Pocatello, 22 at Kalispell, 23 at Missoula, and 30 at Quillayute on the 9th; 24 at Kalispell, 31(tie) at Olympia, and 31 at Quillayute on the 10th; 27 at Kalispell on the 15th; and 29(tie) at Meacham and 38(tie) at Pendleton Airport on the 24th.
SNOW SUMMARY
In northern portions of the basin June 1st snow water equivalents are 130 to 160 percent of average. Some mid-Columbia and Snake River tributaries such as the Yakima and the Clearwater rivers have average snow water equivalents on June 1st. In contrast most middle and upper Snake basins have lost their snowpacks with only the highest areas (>8000 feet) reporting snow.
STREAMFLOW SUMMARY
May streamflow has been below average reflecting continued cool conditions especially in the northern areas. Streamflow for the northern rivers is 70 to 90 percent of average. Only the Spokane river observed above average streamflow during May. All Snake river tributaries and the main stem of the Snake river had low streamflow (40 to 80 percent). May streamflow for the Columbia river at The Dalles was 87 percent of the 1971 to 2000 30-year average.
Streamflow volume forecasts improved 1 to 15 percent in northern areas of the Columbia river basin due to the wet cool conditions. On the Snake river dry weather produced drops in streamflow volumes of 3 to 15 percent. The June 1st forecast for the January-July period on the Columbia river at The Dalles is 100 milion acre-feet (MAF) or 93 percent of the 1971 to 2000 30-year average. This compares to a streamflow runoff of 58.2 MAF during water year 2001.