145 PM PST THU DEC 20 2007 WEATHER SYNOPSIS FOR THURSDAY AND FRIDAY A moderate upper trough moved over much of the region Thursday morning, maintaining significant shower activity in most areas, especially in the Mt Hood area and south and central Cascades where strongest westerly flow occurred. Also a moderate Puget Sound convergence helped enhance moderate to locally heavy showers near Stevens and Snoqualmie Passes Thursday morning. As pressure gradients relax Thursday afternoon and upper flow turns briefly more northwesterly, this should allow the convergence to slowly drift southward and diminish during the afternoon. Although the main upper trough should move mostly to the east of the area during the afternoon, a secondary weaker surface and upper trough should help maintain at least some light to moderate shower activity in most areas near and west of the crest for much of the afternoon. As moderate upper ridging briefly builds offshore late Thursday and approaches the coast early Friday, a briefly drier northwesterly upper flow should allow for decreasing shower activity overnight into early Friday. Although a weakening moderate upper ridge should move onshore Friday morning and reach the Cascades mid-day and early Friday afternoon, considerable moisture associated with a strong disturbance passing mostly north of the area on Friday is expected to spill through and over the ridge, and this should affect much of the region. As a result, after some partial clearing early Friday morning, increasing clouds are expected to reach the Olympics Friday morning and the Cascades during the afternoon along with some occasional light snow or snow showers, heaviest from about Snoqualmie Pass northward, with partly cloudy skies and more scattered light showers further south. Following passage of this first weather disturbance, the upper ridge should rebuild very briefly Friday night and early Saturday ahead of a second weather disturbance aimed more toward the Northwest. However, considerable cloudiness and light shower activity should persist in most areas into early Saturday, especially in the north-central Cascade and Olympics. WEATHER FORECAST FOR THURSDAY AND FRIDAY * OLYMPICS- Light to occasionally moderate snow showers slowly decreasing Thursday afternoon and becoming scattered light showers Thursday evening. Partly cloudy with scattered light showers Thursday night and early Friday. Increasing clouds later Friday morning with occasional light snow developing. Light snow at times Friday afternoon and evening, slowly increasing Friday night. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Light showers north and moderate to occasionally heavy snow showers central and south early Thursday afternoon. Showers decreasing slowly mid-late Thursday afternoon and evening. Light snow showers with some partial clearing Thursday night becoming partly cloudy with widely scattered showers early Friday. Increasing clouds later Friday morning with occasional light snow or snow showers redeveloping mid-day and Friday afternoon, especially from about Snoqualmie Pass northward. Otherwise partly to mostly cloudy with scattered light showers south. Clouds and light showers continuing Friday night north, but briefly decreasing in the south. Increasing snow all areas early Saturday. * EAST SLOPES WASHINGTON CASCADES- Light to occasionally moderate snow showers early Thursday afternoon, heaviest higher terrain in the south and in the central Cascades in convergence. Showers gradually decreasing mid-late Thursday afternoon to light showers with some partial clearing, especially lower terrain away from the crest. Light showers becoming scattered Thursday night with further brief clearing trend. Partly sunny early Friday. Increasing clouds mid- day and Friday afternoon becoming mostly cloudy north and partly cloudy south with increasing chance showers in the north. Showers briefly decreasing Friday night, especially in the south. * MT HOOD AREA- Moderate to occasionally heavy snow or snow showers slowly decreasing later Thursday morning. Mostly cloudy with light to moderate showers Thursday afternoon. Partly cloudy with scattered light showers Thursday night and Friday morning. Increasing mid and high clouds Friday afternoon and night with some light showers developing late Friday. SNOW LEVELS-CASCADE MTNS 1000 ft N, 1500 ft S Thursday afternoon 500 ft N, 1000 ft S Thursday night and Friday morning 1000 ft N, 1500 ft S Friday night 1000 ft N and S Friday night 2500 ft N, 3000 ft C, 5000 ft S early Saturday, except snow levels lowering to near the surface Cascade passes and east slopes Friday night SNOW LEVELS-OLYMPIC MTNS 1000 ft Thursday afternoon and night 500 ft Friday morning 1000 ft Friday afternoon 1500 ft Friday night 2500 ft early Saturday 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 .25 .25 * MT BAKER LT .25 .25-.5 * WASHINGTON PASS LT .25 LT .25 * STEVENS PASS .5 .25 * SNOQUALMIE PASS .25-.5 .25 * MISSION RIDGE LT .25 LT .10 * CRYSTAL MTN LT .25 LT .25 * PARADISE .25-.5 .25 * WHITE PASS .25-.5 LT .25 * MT HOOD .5-.75 LT .25 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS W 5-15 Thursday afternoon W 0-10 Thursday night Variable 0-10 Friday morning E 0-10 Friday afternoon E 10-15 Friday night E 10-20 with higher gusts early Saturday * FREE WINDS AT 5000 FT W 10-20 N, 15-30 C, 20-40 S and NW 20-40 Olympics Thursday afternoon W-NW 10-20 N, 15-30 C, 20-40 S Thursday night W 10-20 N, 15-30 S Friday morning SW 10-20 Cascades, 20-35 Olympics Friday afternoon S-SW 15-25 Cascades, 30-45 Olympics Friday night S 20-30 Cascades, 40-60 Olympics early Saturday * FREE WINDS AT 9000 FT W-NW 20-40 N, 30-50 S Thursday afternoon NW 15-30 N, 20-40 C, 30-50 S Thursday night NW 15-30 N, 20-35 S Friday morning W 15-30 N and C, W-NW 10-20 S Friday afternoon W-SW 20-40 N, 15-25 S Friday night SW 20-40 N Cascades, 30-50 Olympics early Saturday EXTENDED WEATHER SYNOPSIS FOR SATURDAY THROUGH MONDAY An active eastern Pacific jetstream is expected to direct several moderate to strong frontal systems and associated upper troughs through the region Saturday through Monday, with yet another strong disturbance expected to grace the area later on Christmas Day. In the short term, a moderate surface front should spread increasing snow, strong winds and briefly rising freezing levels onshore early-mid Saturday, with lowering freezing levels and moderate to heavy snow likely mid-day and Saturday afternoon before turning more showery later Saturday. Weak upper ridging should produce decreasing showers later Saturday night and Sunday morning before the second and stronger weather disturbance moves onshore Sunday afternoon. This should produce strong winds, a brief and limited rise in freezing levels and increasingly heavy snow in most areas mid-late Sunday, with significant cooling, strong winds and moderate to heavy orographic snow showers likely overnight and Monday morning with locally heavy accumulations likely. While weak upper ridging should allow for gradually decreasing showers mid-late Monday into Tuesday morning, another strong frontal system is expected to move over the region mid-late on Christmas Day. Overall, a very active week ahead...fortunately one in which any sustained or substantial warming trends are not expected, and even with the brief warming expected, a cool easterly surface flow should help insulated the lower Cascade passes from most of the warming aloft. EXTENDED FORECAST FOR SATURDAY THROUGH MONDAY Increasing winds with light to moderate snow increasing early-mid Saturday. Moderate to heavy snow Saturday afternoon, decreasing and becoming more showery later Saturday. Moderate showers decreasing late Saturday and early-mid Sunday. Moderate snow redeveloping Sunday afternoon, becoming heavy late Sunday. Moderate to heavy snow or snow showers later Sunday night and early-mid Monday with locally heavy accumulations likely. Moderate showers decreasing mid-late Monday. * SNOW LEVELS 2-4000 ft N, 4-6000 ft S early Saturday 1-3000 ft N, 2-4000 ft S mid-late Saturday 1-2000 ft N, 1-3000 ft S early-mid Sunday 1-3000 ft N, 3-5000 ft S later Sunday 1-2000 ft N and S early-mid Monday 500-1500 ft N and S late mid-late Monday, except snow levels at the surface Cascade passes and east slopes Saturday morning, rising to near free air levels mid-late Saturday afternoon, lowering briefly to near the surface again early-mid Sunday and rising to near free air levels late Sunday