200 PM PST TUE DEC 18 2007 WEATHER SYNOPSIS FOR TUESDAY AND WEDNESDAY An upper level trough and the associated surface low pressure system are moving onshore across the central Washington coast Tuesday afternoon. The associated frontal feature has caused periods of moderate to heavy precipitation at relatively low snow levels throughout the day Tuesday. Easterly surface pressure gradients across the Cascades are relaxing as the low moves eastward. As the low tracks inland this evening, the loss of easterly winds at the passes may allow a brief change to rain at Snoqualmie Pass early Tuesday night. Snow levels should lower behind the low as cooler air and moderate moist westerly flow maintain orographic precipitation along the west slopes Tuesday evening and night. This should maintain moderate showers at lowering snow levels overnight. A short wave disturbance offshore is expected to develop early Wednesday and spread increasing frontal moisture into the region early Wednesday. This should renew precipitation across the forecast area as well as causing increasing and very strong crest level winds through the day Wednesday. The strong frontal passage with Wednesdays system is expected to cross the Cascades Wednesday afternoon. This should bring a period of heavy precipitation and lowering snow levels with a shift to strong westerly flow behind the front. The strong flow late Wednesday and Wednesday night should maintain moderate showers, mainly along the west slope areas at further lowering snow levels. WEATHER FORECAST FOR TUESDAY AND WEDNESDAY * OLYMPICS- Tuesday afternoon: Moderate rain or snow along the south and east slopes and light to moderate snow elsewhere. Tuesday night: Diminishing light to moderate snow showers heaviest west Olympics. Wednesday: Increasing moderate rain or snow early Wednesday becoming moderate to heavy mid-day. Wednesday night: Light to moderate snow showers, heaviest west part. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST-CASCADE PASSES INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Tuesday afternoon: Rain or snow changing to moderate showers. Tuesday night: Diminishing light to moderate snow showers. Wednesday: Increasing moderate rain or snow early Wednesday becoming moderate to heavy mid-day. Wednesday night: Light to moderate snow showers. * EAST SLOPES WASHINGTON CASCADES- Tuesday afternoon: Light to moderate snow showers. Tuesday night: Diminishing light snow showers. Wednesday: Increasing light to moderate snow early Wednesday becoming moderate. Wednesday night: Light snow showers gradually diminishing. * MT HOOD AREA- Tuesday afternoon: Moderate rain or snow showers and gradually diminishing. Tuesday night: Light to moderate snow showers gradually decreasing. Wednesday morning: Increasing moderate snow and very windy. Wednesday afternoon and evening: Moderate to heavy snow and very windy. SNOW LEVELS-CASCADE MTNS 2500 ft N, 4000 ft S Tue aft Lowering Tue night 1500 ft N, 2500 ft S early Wed morn 2000 ft N, 2500 ft C, 3000 ft S late Wed morn 1500 ft N, 2500 ft S, Wed aft 1000 ft N and S Wed night ...except snow level at the surface Cascade Passes and east slopes early Tuesday afternoon, rising to free air levels Tuesday evening. Snow level again lowering to surface passes and east slopes Wednesday rising to free air levels late Wednesday afternoon and night. SNOW LEVELS-OLYMPIC MTNS 3500 ft Tue aft Lowering Tue night 2000 ft Wed morn 2500 ft Wed aft 1000 ft Wed 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 WED THU * HURRICANE RIDGE .25-.5 .5 * MT BAKER 1-1.5 1.5-2 * WASHINGTON PASS .25-.5 .75-1 * STEVENS PASS .5 .75-1 * SNOQUALMIE PASS .75-1 1-1.5 * MISSION RIDGE .5 .25-.5 * CRYSTAL MTN 1 1 * PARADISE .5-.75 1-1.5 * WHITE PASS .5 .75-1 * MT HOOD 1 1-1.5 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS E 5-10 Tue aft W 10-20 Tue night E 0-10 Wed W 10-20 Wed aft W 5-15 Wed night * FREE WINDS AT 5000 FT S-SW 25-40 Tue aft strongest south SW 30-40 Tue night strongest south S-SW 35-50 Wed S-SW 35-50 N, W-SW 35-50 S Wed aft SW 25-35 N, W 25-40 S Wed night * FREE WINDS AT 9000 FT W-SW 30-40 N, 35-50 S Tue aft and eve SW 40-60 Wed W-SW 30-40 Wed night EXTENDED WEATHER SYNOPSIS FOR THURSDAY THROUGH SATURDAY Continued cool and moist westerly flow in the wake of a frontal passage Wednesday should maintain showers along the west slope areas Thursday at low snow levels. Showers should diminish as a brief upper ridge of high pressure builds over the region. A transitory high pressure ridge is expected to move across the area Friday to allow for a period of light winds cool temperatures and partly cloudy skies. Another frontal system is expected to approach the area Saturday morning spreading increasing cloud and light precipitation over the forecast region during the morning hours. The front should reach the Cascades by afternoon causing increasing winds and moderate precipitation at gradually rising snow levels. Yet another and stronger system is expected to move rapidly into the area Saturday night and into Sunday. This system is currently expected to cause very windy conditions with moderate to heavy precipitation and significantly rising snow levels by Sunday. EXTENDED FORECAST FOR THURSDAY THROUGH SATURDAY Thursday: Light orographic snow showers west slope areas with moderate showers central Cascades in convergence. Friday: Partly cloudy. Saturday: Increasing light to moderate rain or snow early. Becoming moderate rain or snow late. * SNOW LEVELS 5-1500 ft N, 1-2000 ft S Thu Near sea level N, 1-2000 ft S Fri 2000 ft N, 4000 ft S Sat 2500 ft N, 4500 ft S Sat night