A cooler and drier than normal April and early May has kept, for the most part, the Columbia River basin's water supply bottled up in mountain snowpack, but forecasters say the region will eventually be blessed with an average spring-summer outpouring.

The Northwest River Forecast Center's May 7 "final" monthly prediction is that 96.3 million acre feet will flow past the lower Columbia's The Dalles Dam during the April-September period, which would be 98 percent of the 1971-2000 average.

Most of the Columbia-Snake river basin's snowpack runoff funnels past The Dalles.

The new forecast represents a decrease from the April 7 final prediction, which was 101 percent of average. Most of the basin saw below normal precipitation during April, some less than 50 percent. Wednesday's forecast assumes 95 percent of average precipitation for the basin for the first half of May and normal precipitation for the rest of the season.

The Climate Impacts Group says in its April 30 climate outlook update that April temperatures (through the 28th) were at least 2 degrees F cooler than the 1971-2000 mean throughout the Pacific Northwest, with cold departures in excess of 4 degrees in Idaho, and eastern Oregon and Washington. That continued a pattern of colder than normal temperatures that was observed in March and to a lesser extent for the entire winter season, according to the University of Washington's CIG.

A couple areas – such as central Oregon and in the Snake River's upper reaches in southeast Idaho, the average April temperature was more than 6 degrees below normal, according to the NWRFC.

A slow warmup has taken place in recent days, starting to fill basin tributaries that have mostly have below average flows for this time of year.

The Natural Resources Conservation Council's SNO-TEL monitoring stations show snowpacks' snow-water equivalents well above average for this point in time.

As an example, the Lower Columbia-Hood River drainages' mid- to low elevation snowpack sits at 541 percent of normal as of May 7, a time when much of its snowpack would be melted. That SNO-TEL reading is qualified by the NRCS, which says the "analysis may not provide a valid measure of conditions." The electronically monitored sites have not been checked manually.

The exceptions are the Big and Little Wood drainages in south-central Idaho at 91 percent of normal. SNO-TEL measurements for the Columbia above the Methow River in central Washington show a snow-water equivalent of 103 percent. That area ha received only 92 percent of its seasonal precipitation.

The NWRFC's May final forecast says the runoff past Lower Granite Dam on the lower Snake River in southeast Washington should be 100 percent of normal, 24.1 maf.

The Clearwater River, which flows into the Snake just above Lower Granite, is expected to provide 113 percent of its normal runoff. In-flows to Dworshak reservoir on the North Fork of the Clearwater are expected to be 111 percent of normal for the April-July period.

The meltdown is beginning. Dworshak in-flows steadily climbed from 8,900 cubic feet per second to 16.1 kcfs from May 1 through May 7.

"We're filling a little bit," the U.S. Army Corps of Engineers' Jim Adams told members of the Implementation Team Thursday. Operations at Dworshak Dam shifted Wednesday, dropping outflows from full powerhouse capacity of about 10.4 kcfs to about 7.5 kcfs. The plan drop outflows to 5.5 kcfs to slowly lift the reservoir elevation from this week's low of 1,475 to full pool, 1,600 feet, by early July.

The runoff has started to pickup across the basin. Inflows into Libby Dam's reservoir, from 7.1 kcfs May 3 to 14.1 kcfs Wednesday. The latest NWRFC forecast predicts Libby inflows will be 95 percent of average for that April-September period. Libby Dam is in northwest Montana.

NOAA Weather Service's Climate Prediction Center outlook for May-June-July temperature in western Washington is for April's cooler than normal temperatures to continue (greater than a 33 percent chance). For the remainder of the region, the CPC projects a greater than 33 percent chance of above normal May-June-July temperatures in southern and eastern Oregon and central Idaho, and a greater than 40 percent chance for the same in southeast Oregon and southern Idaho.

The precipitation forecast is for a continuation of April's dry conditions throughout the region (greater than a 33 percent throughout the Northwest and exceeding a 40 percent chance for the same in eastern Oregon and southern Idaho).

According to the CIG, the forecasts should be interpreted as the tilting of odds towards general categories of conditions, and should not be viewed as a guarantee that the specified conditions will be realized.

"The forecasts tend to have most skill in years of significant warm or cold ENSO conditions, like this one," according to the CIG outlook.