200 PM PST THU FEB 28 2008 WEATHER SYNOPSIS FOR THURSDAY AND FRIDAY An upper ridge of high pressure is slowly transiting the Pacific Northwest through Thursday night. The ridge allowed skies to clear in the mountains early Thursday with warming temperatures providing another spring-like day. Winds should remain light with further slowly rising freezing levels through early Friday under the influence of the high pressure. The ridge should move east of the area by early Friday with a shift to southwest flow aloft as an approaching frontal system begins to affect the area. High clouds should spread over the mountain areas during the day and gradually lower and thicken during the day with strengthening southwest crest level winds. The frontal precipitation should begin in the Olympics and northwest Washington Cascades early Friday afternoon and spread southward by late Friday afternoon or evening. Winds should become strong by Friday afternoon with freezing levels gradually lowering. The frontal passage is expected across the Olympics late Friday afternoon and to the Cascades Friday evening or early Friday night in the south area. Freezing levels lower rapidly Friday evening with the front along with a period of heavy precipitation and continued strong winds. Strong westerly flow is expected behind the front Friday night and this should maintain moderate orographic precipitation along the west slope areas at further lowering freezing levels. This pattern is likely to produce a convergence zone late Friday night to early Saturday to enhance precipitation over the central Cascade passes. WEATHER FORECAST FOR THURSDAY AND FRIDAY OLYMPICS- Thursday afternoon: Mostly sunny with variable high clouds. Thursday night: Mostly fair with high clouds. Friday: Increasing high clouds becoming mostly cloudy. Friday afternoon and evening: Moderate to heavy rain or snow, heaviest along the south and west parts. Friday night: Light to moderate snow showers. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Thursday afternoon: Partly sunny with variable high clouds. Thursday night: Mostly fair with high clouds. Friday: Increasing high clouds morning. Clouds lowering early afternoon. Friday afternoon and evening: Light rain or snow developing in the north during the afternoon spreading south by evening. Friday night: Moderate to heavy rain or snow becoming mostly snow overnight. * EAST SLOPES WASHINGTON CASCADES- Thursday afternoon: Mostly sunny with variable high clouds. Thursday night: Mostly fair with high clouds. Friday: Increasing high clouds becoming mostly cloudy. Friday afternoon and evening: Light rain or snow developing in the north during the late afternoon spreading south by evening. Friday night: Light to moderate snow. * MT HOOD AREA- Thursday afternoon: Mostly sunny with variable high clouds. Thursday night: Mostly fair with high clouds. Friday: Increasing high clouds becoming mostly cloudy. Friday afternoon and evening: Clouds lowering afternoon with light rain or snow developing by evening. Friday night: Moderate to heavy rain or snow becoming mostly snow overnight. FREEZING LEVELS-CASCADE MTNS 6500 ft N, 8000 ft C, 10,000 ft S Thursday aft and night 7500 ft N, 10,000 ft S Friday morning SNOW LEVELS-CASCADE MTNS 4500 ft N and C, 5000 ft S Friday afternoon 2000 ft N and S Friday night FREEZING LEVELS-OLYMPIC MTNS 7500 ft Thursday aft and night 8000 ft Friday morning SNOW LEVELS-OLYMPIC MTNS 5500 ft Friday midday 4000 ft Friday afternoon 3000 ft Friday evening 2000 ft 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 .5 * MT BAKER 0 1 * WASHINGTON PASS 0 .5-.75 * STEVENS PASS 0 .75 * SNOQUALMIE PASS 0 .75-1 * MISSION RIDGE 0 .25-.5 * CRYSTAL MTN 0 .5-.75 * PARADISE 0 .75 * WHITE PASS 0 .5-.75 * MT HOOD 0 .75 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS W 0-10 Thur midday Var 0-10 Thur aft E 0-10 Thur night E 5-15 Fri morn and aft W 10-20 Fri eve W 15-25 Fri night * FREE WINDS AT 5000 FT W-SW 5-15 Thur aft and eve S-SW 5-15 Thur night S-SW 10-20 Fri morn S-SW 30-40 Fri aft and eve W-SW 25-35 Fri night * FREE WINDS AT 9000 FT W 10-20 Thur aft W-SW 15-25 Thur night SW 15-25 Fri morn SW 40-60 Fri aft and eve W-SW 35-50 Fri night EXTENDED WEATHER SYNOPSIS FOR SATURDAY THROUGH MONDAY Strong and moist westerly flow following a frontal passage Friday evening should maintain orographic precipitation along the west slopes at low freezing levels on Saturday. A convergence zone should form Saturday to affect mainly the Snoqualmie Pass area Saturday. The east slopes should be blocked under this pattern. A rebuilding upper ridge offshore on Sunday should shift the upper flow to northwesterly as precipitation ends Sunday morning. The upper ridge moves across the area later Sunday to allow for clearing and light winds. The brief break Sunday should be replaced with another strong frontal system for Monday. The next front should spread increasing snow at low freezing levels across the region during the day Monday. This system should produce moderate to heavy precipitation and very strong winds as well. EXTENDED FORECAST FOR SATURDAY THROUGH MONDAY Saturday through Saturday night: Renewed moderate to heavy snow mainly near and west of the crest with further lowering snow levels, especially Snoqualmie Pass in convergence. Sunday morning: Decreasing light snow showers ending. Sunday afternoon and night: Becoming partly cloudy or mostly fair. Monday: Increasing moderate to heavy snow and windy. Decreasing snow showers Monday night. * SNOW LEVELS Saturday thru Sunday: 1-3000 ft N and S Monday morn: 1-2000 ft N, 4-6000 ft S Monday late morn thru night: 1-3000 ft N and S