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Northern Columbia Basin Snowpacks Improved; Spring, Summer Water Supply Predicted Normal
Posted on Friday, February 11, 2011 (PST)

The latest Northwest River Forecast Center forecast predicts that the Columbia River basin will enjoy a “normal” water supply this spring and summer fed by mountain snowpacks that are in relatively good shape.


The center’s monthly “final” forecast issued Wednesday says that water volume running past the lower Columbia’s The Dalles Dam is expected to be 99 percent of the 1971-2000 average for the April-August (92.5 million acre feet) and April-September (98 MAF) periods. That’s up from a Jan. 7 final forecast of 97 percent of average.


The latest forecast for runoff past The Dalles for the January through July period is higher as a percentage of average -- 103 percent (110 MAF), due in large part to a wet, warm early January that resulted in high early runoff in portions of the region. The forecast is also up from the early January forecast of 97 percent of average.


January saw a flipflop with the northern portion of the basin getting strafed with a much higher level of storm activity than the southern half of the region.


During the early part of the winter, the southern part of the basin received the most rain down low and snow at mid and higher elevations.


January precipitation was 144 percent of normal in the part of the basin above central Washington’s Grand Coulee Dam, according to data posted by the NWRFC. That lifted precipitation totals for the water year (Oct. 1 through Jan. 31) to 111 percent of normal after a slow start. Basin precipitation upstream of Castlegar, British Columbia was 149 percent of normal in January and through Jan. 31 was at 99 percent of normal for the water year.


The northern portions of the Columbia-Snake drainage collected substantial new snow-water equivalent during January while most of the Snake basin saw a decrease, according to a NWRFC narrative. Because of the improvement in the northern tier the water volume forecasts for the upper reaches basin jumped from 5 to 9 percent.


Above average precipitation coupled with average to below average temperatures overall in January allowed the Columbia-Kootenai and western Montana drainages, as well as other Canadian drainages, to accumulate SWE at an above average rate.


That helped push up the runoff forecasts. Canadian measuring locations jumped from 90-93 percent of normal forecasts in early January to 97-102 percent of average in the Feb. 9 forecast. The Kootenai (in-flows to Libby Reservoir) forecast rose from 93 percent to 98 percent of normal and the Flathead River (at Columbia Falls, Mont.) prediction jumped from 105 to 112 percent.


Precipitation during January above southeast Washington’s Ice Harbor Dam was 94 percent of normal. The precipitation total for the water year in the upper Snake is now at 124 percent of normal.


In the southern parts of the Columbia-Snake basin the Feb. 1 SWEs were 5 to 30 percent lower than on Jan. 1, according to the NWRFC. The Feb. 1 SWEs range from 110 to 115 percent in the Upper Snake and Pend Oreille drainages and 80-100 percent in most other areas. The lowest Feb. 1 SWEs were in the northwest Washingon Cascades and in western Oregon at 60-75 percent.


The Feb. 9 NWRFC forecast is based on the full month of January observed precipitation and the assumption that precipitation would be 110 percent of normal during the first two weeks of February and normal for the rest of the winter season.


The NWRFC narrative says that the month of January started out cold throughout the basin. But by the second week of the month, high pressure began to build over the southern half of the basin. That, along with southerly mid-level flows, helped bring warmer than normal, and mainly dry conditions, to the area. Those conditions, which continued to the end of the month, melted much of the low-level snowpack.


Also during that time period the jet stream set up over the Puget Sound and southern British Columbia, bringing near normal temperatures and heavy precipitation at times, the narrative says.


La Niña climatic conditions in place since last summer are expected to weaken during the next several months, with ENSO-neutral or La Niña conditions equally likely during May-June 2011, according to the “El Nino/Southern Oscillation (ENSO) Diagnostic Discussion issued” by the National Oceanic and Atmospheric Administration’s Climate Prediction Center Thursday.


An El Niño is characterized by higher than average sea surface temperatures in the central and eastern equatorial Pacific Ocean, reduced strength of the easterly trade winds in the Tropical Pacific, and an eastward shift in the region of intense tropical rainfall. Such conditions can affect weather worldwide.


A La Niña is characterized by the opposite conditions, and can tip the odds toward wetter, cooler weather in the winter in the Northwest with the potential for above normal snowpack and streamflow. The ENSO influence on Pacific Northwest climate is strongest from October to March.


“Expected La Niña impacts during February-April 2011 include suppressed convection over the west-central tropical Pacific Ocean, and enhanced convection over Indonesia,” according to the CPC discussion. “Potential impacts in the United States include an enhanced chance of above-average precipitation in the Northern Rockies and western regions of the Northern Plains (along with a concomitant increase in snowfall), Great Lakes, and Ohio Valley.”


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