100 PM PST THU DEC 13 2007 WEATHER SYNOPSIS FOR THURSDAY AND FRIDAY A weak disturbance is tracking through western Washington Thursday afternoon is a cool and moist northwest flow aloft. This disturbance is producing generally light precipitation over the Olympics and from about Mt Baker southward to Mt Rainier along the west slopes Thursday afternoon. This system should gradually weaken and south and eastward overnight Thursday while maintaining periods of light snowfall extending to about the White Pass area. Little precipitation is expected to reach the Mt Hood area from this feature. This pattern of cool northwest flow is expected to continue through early Friday with diminishing light showers early Friday. A change in the overall weather pattern is expected to begin Friday as the upper ridge that has recently been centered over the eastern north Pacific gradually moves inland. This should allow strong weather disturbances to move across the region producing more significant precipitation and strong winds. The flow should switch to southwesterly Friday and increase late in the day as an offshore frontal system approaches. South to southwest flow should become strong overnight Friday and precipitation increase along with briefly rising freezing levels. However, easterly surface flow across the Cascade passes should maintain precipitation as snow at the lower passes Friday night. WEATHER FORECAST FOR THURSDAY AND FRIDAY * OLYMPICS- Thursday afternoon and evening: Increasing light to occasionally moderate snow showers. Thursday night: Light snow showers gradually diminishing. Friday: Increasing light to occasionally moderate rain or snow, heaviest west and south parts. Friday night: Light to moderate rain or snow becoming moderate snow late. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- CASCADE PASSES- INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Thursday afternoon and evening: Light to occasionally moderate snow showers, heaviest north part. Thursday night: Light snow showers gradually diminishing. Friday: Light rain or snow showers at times otherwise mostly cloudy. Friday night: Increasing light to moderate rain or snow becoming moderate late night. * EAST SLOPES WASHINGTON CASCADES- Thursday afternoon and evening: Cloudy with occasional light snow showers mainly north and central. Thursday night: Snow showers ending with partial clearing. Friday: Increasing clouds Friday. Friday night: Light snow developing late. * MT HOOD AREA- Thursday afternoon and evening: Mainly mid and high level cloudiness gradually increasing. Thursday night: Cloudy with a chance of light snow showers. Friday: Increasing clouds. Friday night: Light rain or snow developing Friday evening, increasing becoming light to moderate late. SNOW LEVELS-CASCADE MTNS 1000 ft N, 1500 ft C, 2000 ft S Thursday afternoon 1000 ft N, 2000 ft C, 2500 ft S Thursday evening and night 1500 ft N, 2500 ft C, 4000 ft S Friday 2000 ft N, 3500 ft C, 4000 ft S Friday night, except at the surface Cascade east slopes and passes Friday and Friday night. SNOW LEVELS-OLYMPIC MTNS 2500 ft Thursday afternoon 3000 ft Thursday night 4000 ft Friday 3000 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 LT .10 .25 * MT BAKER LT .25 .5-.75 * WASHINGTON PASS LT .25 .25 * STEVENS PASS LT .25 .5 * SNOQUALMIE PASS LT .25 .5 * MISSION RIDGE LT .10 LT .25 * CRYSTAL MTN LT .25 .5 * PARADISE LT .25 .5 * WHITE PASS LT .10 .25-.5 * MT HOOD LT .10 .5 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS W 5-10 Thursday afternoon and night E 0-10 Friday E 5-15 Friday night * FREE WINDS AT 5000 FT W 5-15 Thursday afternoon and night SW 10-20 Friday S-SW 15-30 Friday afternoon S-SW 25-40 Friday night * FREE WINDS AT 9000 FT W-NW 20-30 Thursday afternoon NW 15-25 Thursday night and early Friday SW 15-30 Friday afternoon SW 35-50 Friday night EXTENDED WEATHER SYNOPSIS FOR SATURDAY THROUGH MONDAY A cold frontal passage should move across the Olympics and Cascades early Saturday morning causing moderate to briefly heavy precipitation at lowering snow levels and with strong south to southwest crest level winds. Flow shifting to strong westerly behind the front later Saturday morning should maintain orographic snow showers at low snow levels along the west slopes of the Olympics and Cascades along with a possible convergence zone mid-morning Saturday affecting the central Cascades with enhanced showers. Moist westerly flow should maintain snow showers along the west slopes much of Saturday afternoon before a brief decrease in precipitation late Saturday ahead of the next disturbance. Mostly cloudy conditions with increasing southwesterly flow is expected Sunday ahead of another approaching upper level trough and surface frontal system. Precipitation from the approaching system should again increase Sunday night with a strong frontal passage expected Monday morning across the Cascades. Strong winds with moderate to briefly heavy precipitation is expected early Monday at continued low snow levels. Strong westerly flow behind this second front should maintain showers and possible convergence zone enhancement along the west slopes mid-morning Monday through late Monday. EXTENDED FORECAST FOR SATURDAY THROUGH MONDAY Saturday morning: Moderate to briefly heavy rain or snow early Saturday. Snow changing to moderate snow showers, mainly along the west slope areas later Saturday morning through afternoon. Saturday night: Decreasing showers west slopes and becoming partly cloudy. Sunday: Partly cloudy early. Increasing light snow north part and Olympics by afternoon. Sunday night: Increasing moderate snow Olympics and west slope areas with light snow east slopes. Monday: Moderate to briefly heavy snow early Monday. Snow changing to moderate snow showers, mainly along the west slope areas later Monday morning through afternoon. Gradually decreasing snow showers Monday night. * SNOW LEVELS 15-2500 ft N and C, 2-3000 ft S Saturday thru Monday