130 PM PST THU DEC 27 2007 WEATHER SYNOPSIS FOR THURSDAY AND FRIDAY Yet another in the seemingly and quite literally endless train of weather systems is bearing down on the Olympics and Cascades Thursday afternoon. This system is spreading increasing precipitation and strengthening winds over most mountain stations Thursday afternoon. The strong zonal westerly flow across the north east Pacific is limiting the usual warming associated with these typical frontal systems in our region. Thus freezing levels remain quite low and should continue to be for the next several days. Frontal precipitation should further increase from Thursday afternoon through evening and into Thursday night with only very limited warming. The front is expected to move across the Olympics Thursday evening and the Cascades before midnight. The heaviest precipitation should be near the frontal passage and behind it along the west slopes. A strong and moist westerly flow behind the front early Friday should maintain heavy orographic precipitation along the west slope areas as well as enhanced precipitation in the Stevens to Snoqualmie Pass areas in a likely convergence zone that should last through midday Friday. A gradual decrease in precipitation is expected Friday afternoon and night in the Washington Cascades and Olympics between systems. A surface low pressure center is expected to move inland across the central Oregon coast Friday afternoon and continue east Friday night to maintain heavier precipitation over the Mt Hood area through Friday night before diminishing early Saturday. WEATHER FORECAST FOR THURSDAY AND FRIDAY * OLYMPICS- Increasing snow Thursday afternoon through evening becoming moderate to heavy. Snow changing to showers Thursday evening. Light to moderate snow showers Thursday night except moderate to heavy along the west slopes. Continued light to moderate snow showers Friday morning, but moderate to occasionally heavy showers likely west slopes. Snow showers decreasing midday Friday becoming light. Cloudy Friday afternoon and night with scattered snow showers. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Increasing light snow Thursday afternoon. Further increasing snow Thursday evening and early Thursday night becoming moderate to heavy. Snow changing to showers late Thursday night. Moderate to occasionally heavy snow showers Thursday night through Friday morning. Snow showers decreasing midday Friday becoming light. Cloudy Friday afternoon and night with scattered snow showers. * EAST SLOPES WASHINGTON CASCADES- Light snow developing Thursday afternoon, increasing Thursday evening and night becoming light to moderate, heaviest near the crest. Snow changing to light snow showers early Friday morning. Decreasing light snow showers later Friday morning. Scattered light snow showers Friday afternoon and night with partial clearing. * MT HOOD AREA- Increasing snow Thursday afternoon becoming moderate. Snow further increasing becoming moderate to heavy Thursday evening and early Thursday night. Snow changing to showers late Thursday night. Moderate to occasionally heavy snow showers Thursday night through Friday morning. Snow showers decreasing later Friday morning and midday. Snow increasing again Friday afternoon and night becoming light to moderate. SNOW LEVELS-CASCADE MTNS 500 ft N, 1500 ft S Thursday afternoon Sea level N, 1500 ft S Thursday night 500 ft N, 1500 ft C and S, early Friday 1000 ft N, 1500 ft C, 2000 ft S Friday afternoon 500 ft N, 1000 ft C, 2500 ft S Friday night SNOW LEVELS-OLYMPIC MTNS 1000 ft Thursday afternoon 1500 ft Thursday night through 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 .25-.5 .25 * MT BAKER .75 .25-5 * WASHINGTON PASS .5 .25 * STEVENS PASS .5-.75 .5 * SNOQUALMIE PASS .75-1 .5-.75 * MISSION RIDGE .25 LT .25 * CRYSTAL MTN .5-.75 .25-.5 * PARADISE .75-1 .5 * WHITE PASS .5-.75 .25-.5 * MT HOOD .75-1 .75 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS E 10-20 with occasional higher gusts Thursday afternoon and early night W 5-15 early Friday morning W 10-20 with higher gusts late Friday morning W 5-15 Friday midday W 0-10 Friday afternoon E 0-10 Friday night * FREE WINDS AT 5000 FT S 30-45 Thursday afternoon and evening S 20-30 N, SW 25-40 C, W 25-40 S Thursday night W-SW 15-25 N, W 20-30 C, W 25-35 S early Friday SW 10-20 N, W-SW 15-25 C, W-SW 2-30 S mid-late Friday SW 10-20 Friday night * FREE WINDS AT 9000 FT W-SW 25-40 Thursday afternoon W 25-35 S Thursday night through Friday afternoon W 30-40 Friday night EXTENDED WEATHER SYNOPSIS FOR SATURDAY THROUGH MONDAY Very strong zonal flow should maintain an active weather pattern across the region Saturday and Sunday before a ridge of high pressure build over the area Monday to produce the first break in this extended stormy pattern for some time. A westerly jet stream expected to exceed 150 kts and centered along the central Washington coast Saturday morning should help to usher in another strong frontal system through the day Saturday. Heavy precipitation and very strong winds should again increase through the day Saturday across the Olympics and Cascades with a frontal passage expected late in the day Saturday. By Sunday morning the westerly jet stream over southwest Washington and northwest Oregon is expected to exceed 180 kts. This very strong westerly flow should extend to lower in the atmosphere Saturday afternoon into Sunday with very strong westerly flow maintaining windy conditions with heavy precipitation along the west slopes of the Olympics and Cascades. A strong convergence zone should also form late Saturday and Sunday. Heavy precipitation should continue through Sunday as moist and very strong westerly flow persists. Gradually diminishing flow late Sunday night should allow for decreasing winds and precipitation. Freezing levels should remain very low behind the front Saturday night through Sunday with only very limited warming late Saturday ahead of the front. A ridge of high pressure should begin building over the area Monday to allow for a break in precipitation and some partial clearing. EXTENDED FORECAST FOR SATURDAY THROUGH MONDAY Increasing light to moderate snow early Saturday becoming heavy later Saturday morning. Windy with heavy to very heavy snow Saturday afternoon through Sunday afternoon, with significant accumulations likely. Snow gradually decreasing late Sunday. Mostly cloudy Monday with warming expected Monday night but remaining dry. * SNOW LEVELS 500-1500 ft N, 2-3000 ft S Saturday Sea level to 1000 ft N, 1-2000 ft S Saturday night and Sunday Sea level to 1000 ft N and S late Sunday and Monday 4-5000 ft N, 7-8000 ft S late Monday night