130 PM PST WED DEC 5 2007 WEATHER SYNOPSIS FOR WEDNESDAY AND THURSDAY An upper level ridge of high pressure is gradually building northward into the Gulf of Alaska Wednesday afternoon. A developing upper and surface low pressure system is forming about 400 miles west of the northern Oregon coast Wednesday afternoon. Currently the Pacific Northwest is under generally weakening westerly flow producing partly cloudy conditions with a few isolated light showers, mainly in the north Washington Cascades Wednesday afternoon in a gradual cooling trend. The westerly flow should continue to diminish allowing for decreasing showers later Wednesday and Wednesday night as an upper ridge builds offshore. The disturbance well offshore of Oregon is expected to track east and slightly southward through Thursday to spread some high and mid clouds over the central and southern areas late Wednesday through Thursday. Precipitation associated with this system should be heaviest and more prominent well south of the forecast area; however periods of light to moderate precipitation remain possible over the Mt Hood area Wednesday night and early Thursday with lessening affects towards the north. As the upper ridge gains strength offshore, the Pacific Northwest should become under the influence of a cool and dry north to northeast flow aloft over the next several days. By Thursday generally fair skies and cool weak northerly flow are expected for much of the mountain areas of Washington, especially the north and central areas. WEATHER FORECAST FOR WEDNESDAY AND THURSDAY * OLYMPICS- Wednesday afternoon and night: Occasional light snow showers ending late with gradual clearing and cool. Thursday: Mostly sunny and cool with light winds. Thursday night: Fair skies, cold with light winds. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- Wednesday afternoon and night: Occasional light snow showers ending late with gradual clearing and cool. Thursday: Mostly sunny and cool with light winds. Thursday night: Fair skies, cold with light winds. * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Wednesday afternoon and night: Occasional light snow showers ending late with gradual clearing and cool. Thursday: Mostly sunny and cool with light winds. Thursday night: Fair skies, cold with light winds. * EAST SLOPES WASHINGTON CASCADES- Wednesday afternoon and night: Snow showers ending with clearing and cool. Thursday: Mostly sunny and cool with light winds. Thursday night: Fair skies, cold with light winds. * MT HOOD AREA- Wednesday afternoon: Mostly cloudy with occasional light rain or snow showers. Wednesday night: Briefly increasing light to moderate rain or snow showers, mainly late night and early Thursday morning. Thursday: Decreasing light rain or snow showers becoming partly cloudy Thursday afternoon. Thursday night: Fair skies, cold with light winds. SNOW LEVELS-CASCADE MTNS 2000 ft N, 2500 ft C, 4000 ft S Wednesday aft 1000 ft N, 2000 ft C, 4000 ft S Wednesday night 500 ft N, 2000 ft C, 4500 ft S Thursday morning 500 ft N, 2500 ft S Thursday night SNOW LEVELS-OLYMPIC MTNS 2000 ft Wednesday aft 1000 ft Wednesday night 500 ft Thursday and Thursday night Cascade Snow/Freezing Levels refer to the northern Washington Cascades (N) through Mt Hood area (S). Central Washington Cascade snow levels (typically near Snoqualmie Pass) are normally midway between indicated N and S levels. Note that surface snow/freezing levels are common near the passes during easterly pass flow and may result in multiple snow/freezing levels. 24 HOUR FORECAST OF PRECIPITATION IN INCHES OF WATER EQUIVALENT ENDING AT 4AM THU FRI * HURRICANE RIDGE LT .10 NR 0 * MT BAKER .25-.5 NR 0 * WASHINGTON PASS LT .25 NR 0 * STEVENS PASS LT .25 NR 0 * SNOQUALMIE PASS LT .25 NR 0 * MISSION RIDGE LT .10 NR 0 * CRYSTAL MTN LT .25 NR 0 * PARADISE LT .25 NR 0 * WHITE PASS LT .25 LT .10 * MT HOOD .25 .25 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS W 5-15 Wednesday aft W 0-10 Wednesday night E 0-10 Thursday and Thursday night * FREE WINDS AT 5000 FT W-NW 10-20 Wednesday aft W-NW 5-15 Wednesday night E-NE 5-15 Thursday N-NE 5-15 Thursday night * FREE WINDS AT 9000 FT NW 15-25 Wed aft N-NW 5-15 Wed night NE 5-15 Thursday NE 10-20 Thursday night EXTENDED WEATHER SYNOPSIS FOR FRIDAY THROUGH SUNDAY A strong upper level ridge of high pressure should dominate the eastern north Pacific through the extended period. This should cause a cool and dry northerly flow aloft over the region with easterly surface flow across the Cascade passes producing generally clear skies, cool temperatures and light winds. The ridge is expected to extend inland over B.C. Friday and Saturday causing cold and dry northeast flow aloft over the Pacific Northwest. Winds through the extended period should remain relatively light with freezing levels lowering through the period to near sea level. A weak disturbance moving through the ridge may spread increasing clouds over the area Sunday. EXTENDED FORECAST FOR FRIDAY THROUGH SUNDAY Mostly fair and dry with light winds Friday through Saturday with increasing high clouds mainly Sunday. * FREEZING LEVELS Near sea level1000 ft N, 2-3000 ft S Friday Near sea level500 ft N, 1-2000 ft S Saturday and Sunday