GENERAL WEATHER SUMMARY
January 1, 2000
Water year 2000 is starting out as a very abnormal year. Spring like temperatures were observed during December and dry conditions prevailed basin wide for the last half of December. However earlier Fall precipitation provided above average seasonal precipitation for British Columbia, Washington, Montana and northern Idaho with below average seasonal precipitation in other areas. Warm temperatures have retarded the average snow pack accumulation for most of the basin. Consequently, expected seasonal runoff is below average for the Snake River area and near to slightly above average for the rest of the basin.
Precipitation for the month of December was generally slightly below average except for pockets of above average precipitation in western Washington, in northeast Oregon and in the Clearwater basin in Idaho. The driest conditions were in southern Idaho and southern Oregon. Seasonal precipitation for the Columbia River above The Dalles is near average.
Mountain snow as reported by the British Columbia Ministry of Environment and the Natural Resource Conservation Service is below average for most of the basin. The exceptions are British Columbia, Montana, northern Idaho and southwest Washington were above average January 1st snow conditions exist. The lowest January 1st snow conditions are in southern Oregon and southern Idaho where the snow pack is near 50% of average. Fall runoff has been above average in Washington, British Columbia and in the Clearwater River area in Idaho. The above average runoff in northern areas reflects more rainfall runoff due to above average temperatures and indicates that soil moisture storage in these areas will be above average.
The January 1st volume forecast for the January - July period at The Dalles is 105.million acre feet or 99 percent of average. This compares to a runoff of 124.1 million last year.
December was generally drier and warmer than normal. Pacific northwest mean temperatures departed +4.4 degrees from normal relative to 1961-1990 normals (31 stations). Mean temperature departures ranged from -3.9 to 12.6 degrees.
Frequent periods of light precipitation, which were focused mainly across west side basins as well as central and northern Idaho, were common early in the month as a series of frontal systems affected the region. During the middle of the month, moderate to locally heavy precipitation occurred across the Olympic peninsula, southwest Washington, and far northwest Oregon. This happened in response to several weather disturbances which were able to tap abundant moisture from the subtropics. The prevalent southwest flow led to above normal temperatures. During the end of the month, a strong ridge of high pressure maintained generally dry weather and near normal temperatures across the region. Temperature inversions and lack of significant mixing in lower levels of the atmosphere produced ideal conditions for persistent low clouds, fog, and patchy drizzle or snow flurries across some valley locations.
Maximum temperature records were broken at Great Falls (58 deg on the 28th), Astoria (59 deg on the 22nd). Monthly minimum temperature was broken at Astoria (42.1 deg). Astoria exceeded daily precipitation records, 2.15 inches on the 14th, 2.07 inches on the 15th.
For December, precipitation was 90 percent of normal (1961-1990) at Columbia above Coulee; 93 percent of normal at the Snake River above Ice harbor; and 93 percent at Columbia above The Dalles.