May 1, 1999
A dry month with less than average snow accumulations has caused a general drop in water supply volumes. The only exception to this trend is on tributaries in southern Idaho near the Idaho-Nevada border and in the far Upper Snake where some increases in volume were observed.
Precipitation for April ranged from 40 to 60 percent for most basins. The only area with above average monthly precipitation was on the Southern Snake basins and in the Upper Snake. The seasonal precipitation for The Columbia River above The Dalles is at 110 percent.
The snow water equivalents decreased in most areas during April. However, west of the Cascades in Oregon and Washington, cool temperatures have caused most precipitation to fall in the form of snow at higher elevations. This produced much above average snow water equivalents for the May 1st accumulation date. East of the Cascades, snow water equivalents generally decreased and now range from 100-135 percent for most of the basin. The exceptions to this are a small area in the lower Kootenai in Canada and a small area on the Idaho-Nevada border where snow water equivalents are greater than 150 percent. Runoff for the month of April was below average on the Upper Columbia, Pend Oreille and Spokane rivers and average to slightly above average in other areas.
May 1st volume forecasts range from 130 percent in the Cascades in Washington and in Central Idaho to near average on the Clark Fork and Flathead rivers. The January - July forecast for The Columbia River above The Dalles is 124 million acre feet or 117 percent or average. This compares with a runoff of 104 million acre feet in 1998.
Precipitation Summary
Mid-late April warming and snowmelt occasionally reached northward beyond the Middle-Upper Snake into the Upper Columbia. Yet despite these transient high pressure ridging and interior warm southerly advection effects, probably the most frequent question on everyone's mind was "when will summer ever get here?"
Mean temperatures departed -1.5 degrees from normal for the Pacific Northwest relative to 1961-1990 normals (31 stations). Mean temperature departures ranged from -5.1 to 0.8 degrees.
No record daily rainfall reported, with most information indicating generally widespread below normal precipitation. Daily record maximum temperature records were broken at Portland (79 deg on the 24th) Astoria (77 deg on the 15th and 83 deg on the 16th ). Record lows were established at Eugene (31 deg on the16th) ...Missoula (20 deg on the 10th...18 deg on the 11th...and 17 deg on the 15th)...and Kalispell (17 deg on the 15th...and 19 deg on the 16th).
Surprisingly similar to March, early April frequent Pacific winter-like frontal disturbances with lingering unsettled post frontal conditions produced more valley rain with diminishing but still surprisingly low mountain snows focused largely into western Washington and northwestern Oregon. Occasionally locally moderate precipitation also reached into The Blues...Idaho...western Montana...and Wyoming. After mid-April there was a persistent tendency for pesky cut-off great basin lows to wrap showery valley rain and mountain snow northward into Idaho, western Wyoming and western Montana. Resulting warm southerly advection into the interior resulted in noticeable mid-late April Upper Snake snowmelt rises...with occasional minor to moderate rises also reaching northward into Upper Columbia and Kootenay drainages. Mid-April westside high pressure ridging and warming also resulted in some minor west Cascade snowmelt runoff. However, despite this evident erosion of mid-elevation snow, more widespread high elevation snow
accumulations occurred in both early and late April.
For April...precipitation was 62 percent of normal (1961-1990) at Columbia above Coulee; 83 percent of normal at the Snake River above Ice Harbor; and 63 percent at Columbia above The Dalles.