115 PM PST THU JAN 17 2008 WEATHER SYNOPSIS FOR THURSDAY AND FRIDAY A strong upper level high pressure circulation is centered about 500 miles off the southern Oregon coast early Thursday morning. An upper ridge extends from the offshore high well north across southeast Alaska. This is producing a strong northerly flow aloft over the Pacific Northwest with mid and high clouds spilling over the ridge and affecting mainly the north part of the forecast area. The upper ridge should dominate the area Thursday and Friday to produce generally fair weather in the south and some cloudiness over the north and Olympics Thursday. By later Friday a developing disturbance moving over the ridge should begin to turn southward on the downstream side of the ridge. This disturbance should spread increasing high clouds over the region Friday afternoon spreading southward as the disturbance approaches from the north. This weak frontal system is expected to move across the region early Saturday and should cause a little light precipitation beginning in the north Washington Cascades and Olympics Friday night spreading to the central areas by early Saturday. Precipitation amounts with this system are expected to be generally light as it moves across the area, with the greatest amounts expected in the north. WEATHER FORECAST FOR THURSDAY AND FRIDAY * OLYMPICS- Thursday aft and evening: Becoming mostly sunny and warmer with some high clouds. Thursday night: Mostly fair. Friday morning: Increasing high clouds becoming mostly cloudy. Friday aft and evening: Cloudy with clouds lowering and thickening. Friday night: Cloudy with light rain or snow. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST NORTH OF STEVENS PASS- Thursday aft and evening: Becoming mostly sunny with some high clouds. Thursday night: Mostly fair. Friday morning: Increasing high clouds becoming mostly cloudy. Friday aft and evening: Cloudy with clouds lowering and thickening. Friday night: Cloudy with light snow. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST STEVENS PASS SOUTHWARD- * CASCADE PASSES, INCLUDING STEVENS PASS, SNOQUALMIE PASS AND WHITE PASS- * EAST SLOPES WASHINGTON CASCADES- Thursday aft and evening: Becoming mostly sunny with a few high clouds. Thursday night: Mostly fair except for patchy fog low elevations. Friday morning: Increasing high clouds becoming mostly cloudy. Friday aft and evening: Cloudy with clouds lowering and thickening. Friday night: Cloudy with a chance of light snow late and mainly north. * MT HOOD AREA- Thursday aft and evening: Mostly sunny with a few high clouds. Thursday night: Mostly fair. Friday morning: Mostly fair with a few high clouds. Friday aft and evening: Becoming mostly cloudy with mainly high level clouds. Friday night: Cloudy with clouds gradually lowering. FREEZING LEVELS-CASCADE MTNS 1500 ft N, 3000 ft C, 8000 ft S Thursday afternoon 3000 ft N, 5000 ft C, 8000 ft S Thursday night 2000 ft N, 4000 ft C, 8500 ft S Friday SNOW LEVELS-CASCADE MTNS 2000 ft N, 2500 ft C, 7500 ft S Friday night FREEZING LEVELS-OLYMPIC MTNS 7500 ft Thursday afternoon 6500 ft Friday SNOW LEVELS-OLYMPIC MTNS 5500 ft early Friday night 3500 ft late Friday 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 FRI SAT * HURRICANE RIDGE 0 LT .25 * MT BAKER 0 LT .25 * WASHINGTON PASS 0 LT .10 * STEVENS PASS 0 LT .10 * SNOQUALMIE PASS 0 LT .10 * MISSION RIDGE 0 0 * CRYSTAL MTN 0 0 * PARADISE 0 0 * WHITE PASS 0 0 * MT HOOD 0 0 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS W 0-10 Thursday Variable 0-10 Thursday night and early Friday W 0-10 later Friday morning W 5-15 Friday afternoon and night * FREE WINDS AT 5000 FT N 10-20 Cascades, 20-30 Olympics Thursday afternoon N-NE 10-20 Cascades, 15-25 Olympics Thursday night NW 10-20 Friday morning NW 15-30 Friday afternoon, strongest Olympics NW 15-30 Friday night, strongest Olympics and Mt Hood * FREE WINDS AT 9000 FT N 25-35 N and Olympics, 20-30 S Thursday afternoon N-NW 20-30 Thursday night NW 20-30 N and Olympics, N 20-30 S Friday morning NW 35-45 N and Olympics, N-NW 30-40 S Friday afternoon and night EXTENDED WEATHER SYNOPSIS FOR SATURDAY THROUGH MONDAY The dominate ridge of high pressure should remain offshore through the extended period with a few variations, the first being a weak frontal system to move southward across the area Saturday. This system should cause increasing northwest crest level flow and a period of generally light precipitation beginning in the north Friday night and shifting across the central areas mid morning Saturday and to Mt Hood area Saturday afternoon. The ridge should gain strength offshore behind the disturbance Sunday as the ridge builds into B.C. A strong surface high pressure center is expected to develop over southern B.C. on Sunday. This should cause increasing offshore flow and a northeast flow aloft producing a cool and dry weather pattern. A modified arctic front may reach the area early Sunday to cause a little low level moisture and a chance of a few flurries. Strong and cool offshore flow should maintain fair and cool weather Monday with some strong east winds across the passes and over Mt Hood. EXTENDED FORECAST FOR SATURDAY THROUGH MONDAY Saturday morning: Light to briefly moderate snow north part and light snow developing central and cloudy south. Saturday midday and afternoon: Light snow. Saturday night: Light snow showers mainly south part with partial clearing north. Sunday: partly cloudy with a chance of a few light snow flurries, mainly north and central areas. Monday: becoming mostly sunny, cool and windy in areas, mainly Mt Hood. * SNOW LEVELS 1-3000 ft N and S, early Saturday 1-2000 ft N and C, 1-3000 ft S mid Saturday Sea level to 1000 ft N, 1-2000 ft S late Saturday Sea level N and S Sunday and Monday