230 PM PST FRI DEC 21 2007 WEATHER SYNOPSIS FOR FRIDAY AND SATURDAY Avalanche Bells (with apologies to Silver Bells) Avalanche, Avalanche, soon it will all slide away. Quiet valleys, windswept ridgetops On lee slopes theres a feeling of tension. Scary snowpack, we do not lack, Surface fluff masks deeper slab potential. Analyze, no surprise, its caution time in the mountains. Shooting cracks, some whomping sounds Show weak layers and facets near the ground. Be aware, take some care, its your life in the balance. A big slide, no fun to ride You dont want to go where its going. Avalanche, Avalanche, dont let it carry you away. Adding more load, is a bad road And may not bring just holiday cheer. Avalanche, Avalanche, soon it will all slide away. While brief and weak upper and surface ridging brought generally decreased showers and some periods of brief partial clearing to some areas overnight and early Friday, considerable moisture streaming through and over the offshore ridge helped maintain partly to mostly cloudy skies in most locations through early Friday afternoon along with mainly scattered light shower activity. With satellite pictures continuing to indicate further moisture oozing through the ridge and the weak southern remnants of a disturbance moving mostly north of the region adding additional clouds and light showers, mostly cloudy skies are expected mid-late Friday afternoon and evening along with occasional light snow or light snow showers and relatively light winds. A second and stronger weather system should follow closely on the heels of the northern disturbance as it exits to the northeast late Friday, and this should spread increasing light snow onshore and over the Olympics Friday night, with light snow reaching the north-central Cascades early Saturday. Increasing light to moderate snow should spread to most areas mid-late Saturday morning, becoming moderate to occasionally heavy Saturday afternoon before decreasing and becoming more showery late Saturday afternoon and evening. This snowfall should be accompanied by increasingly strong winds Saturday along with a limited and relatively brief warming trend. As the associated upper trough moves east of the area late Saturday and Saturday night and brief upper ridging produces a drier and cool northwesterly flow, moderate showers should slowly decrease and become light Saturday night and early Sunday. WEATHER FORECAST FOR FRIDAY AND SATURDAY * OLYMPICS- Mostly cloudy with occasional light snow Friday afternoon. Light snow at times Friday night, increasing late Friday night and early Saturday. Light to moderate snow becoming moderate later Saturday morning, and moderate to occasionally heavy mid-day and early Saturday afternoon. Snow decreasing and becoming more showery mid- late Saturday afternoon. Light to moderate showers, heaviest west slopes, slowly decreasing Saturday night with some brief partial clearing. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Mostly cloudy with occasional light snow or flurries Friday afternoon and night. Light snow increasing late Friday night and early Saturday from about Mt Rainier northward. Light to moderate snow spreading to most areas and increasing later Saturday morning. Moderate to occasionally heavy snow Saturday afternoon, decreasing and becoming more showery late Saturday afternoon and evening. Moderate showers slowly decreasing Saturday night with some brief partial clearing. * EAST SLOPES WASHINGTON CASCADES- Partly to mostly cloudy higher terrain Friday afternoon and night with few flurries or light showers higher terrain and partial clearing lower terrain. Increasing clouds Friday night with light snow developing north. Light snow increasing and spreading southward Saturday morning. Light to occasionally moderate snow Saturday afternoon, decreasing and becoming light to moderate showers late Saturday afternoon and evening. Light showers Saturday night, mainly higher terrain near the crest. * MT HOOD AREA- Partly to mostly cloudy with scattered light snow showers Friday afternoon and evening. Mostly cloudy later Friday night with light snow redeveloping Saturday morning. Light to moderate snow mid-day, increasing and becoming moderate to heavy Saturday afternoon. Moderate showers gradually decreasing Saturday night. SNOW LEVELS-CASCADE MTNS 500 ft N, 1500 ft C and S Friday afternoon 1000 ft N, 2000 ft C, 2500 ft S Friday night 1500 ft N, 2500 ft C, 4000 ft S Saturday morning 2000 ft N, 2500 ft C and S mid-day 1500 ft N, 2500 ft C, 3500 ft S Saturday afternoon 1500 ft N, 2000 ft C, 3000 ft S Saturday night, except snow levels lowering to near the surface Cascade passes and east slopes Friday night, remaining at the surface Saturday morning and rising to near free air levels mid- late Saturday afternoon SNOW LEVELS-OLYMPIC MTNS 1500 ft Friday afternoon and night 1000 ft Saturday morning 3500 ft mid-day and early Saturday afternoon 2500 ft later Saturday afternoon 2000 ft Saturday 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 SAT SUN * HURRICANE RIDGE .25 .5 * MT BAKER LT .25 .75-1 * WASHINGTON PASS LT .10 .5 * STEVENS PASS LT .25 .5-.75 * SNOQUALMIE PASS LT .25 .5-.75 * MISSION RIDGE LT .10 .25 * CRYSTAL MTN LT .10 .5 * PARADISE LT .25 .75 * WHITE PASS LT .10 .5 * MT HOOD LT .10 .75 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS Variable 0-10 early Friday afternoon E 0-10 later Friday afternoon E 5-15 Friday night E 10-20 Saturday morning and early afternoon W 5-15 late Saturday afternoon W 10-15 Saturday night, decreasing late * FREE WINDS AT 5000 FT SW 10-15 Cascades, 15-25 Olympics Friday afternoon S-SW 15-25 Cascades, 20-40 Olympics Friday night S 20-40 N, 15-30 S and 30-50 Olympics Saturday morning SW 30-50 N and S later Saturday morning and early afternoon W 15-30 N, 25-40 S later Saturday afternoon W 10-20 N, 15-30 S Saturday night, decreasing late * FREE WINDS AT 9000 FT W 15-25 N, NW 10-20 S Friday afternoon W-SW 15-30 N, 10-20 S and 20-40 Olympics Friday night W-SW 20-35 Cascades, 30-45 Olympics Saturday morning W-SW 30-45 N and S mid-day and early Saturday afternoon W 25-40 N, 30-50 S later Saturday afternoon W 20-40 N, 20-30 S Saturday night, decreasing late EXTENDED WEATHER SYNOPSIS FOR SUNDAY THROUGH TUESDAY After weak upper ridging provides a brief break from precipitation early-mid Sunday morning, the most recent forecast models now indicate that increasing clouds and increasing light snow should reach the Olympics later Sunday morning, with increasing moderate rain or snow, strong winds and significant warming now expected Sunday afternoon, especially in the south. Although a cool easterly surface flow across the Cascade passes should ensure that most precipitation remains as snow near the passes, some freezing rain is possible on the west approaches to the passes late Sunday along with a possibility of some mixed snow and rain near the lower passes when pass winds shift to westerly. The strong front should produce moderate to heavy snowor rain in the south-later Sunday afternoon and evening. This should be followed by moderate to heavy orographic snow showers at rapidly lowering freezing levels and moderate winds Sunday night into Monday morning as the associated moderate upper trough moves through the region. Also a moderate Puget Sound convergence should enhance showers in the central Washington Cascades. Weak upper ridging should then move onshore mid- late Monday, bringing decreasing light showers and winds along with relatively low and lowering freezing levels. After another weak ridge brings further decreasing showers and some partial clearing later Monday into Tuesday morning, a stronger frontal system is expected to reach the area mid- late on Christmas Day. This should spread increasing light snow over most areas mid-day and early Christmas afternoon, with moderate to heavy snow, strong winds and brief and limited warming later Christmas afternoon and night. It should also be noted that longer range models are consistent in indicating a very strong and relatively cool zonal flow developing late next week and continuing into New Years eve. If this proves correct, December of 2007 will certainly be a month to remember, for a variety of reasons. EXTENDED FORECAST FOR SUNDAY THROUGH TUESDAY Partly cloudy with scattered light showers early Sunday. Increasing clouds with light snow developing in the Olympics later Sunday morning. Light to moderate snow spreading southward mid-day and early Sunday afternoon, with moderate to heavy snow or rain likely later Sunday afternoon and evening, along with some local freezing rain along the west approaches to the passes. Much cooler with moderate to heavy snow or snow showers Sunday night and Monday morning with some locally heavy accumulations likely. Moderate showers decreasing Monday afternoon becoming light Monday night with partial clearing. Partly cloudy Christmas morning. Increasing clouds mid-day with light snow increasing and spreading southward Tuesday afternoon. Moderate to heavy snow later Tuesday afternoon and night. * SNOW LEVELS 1-2000 ft N, 2-4000 ft S early Sunday 1-3000 ft N, 3-5000 ft C and 5-7000 ft S mid-late Sunday 1-2000 ft N, 2-4000 ft C and S late Sunday night and early Monday 500-1500 ft N and S later Monday through early Tuesday 500-1500 ft N, 1-3000 ft S mid-late Tuesday, lowering early Wednesday, except snow levels lowering to near the surface Cascade passes later Sunday morning, rising to near free air levels Sunday evening, lowering to near the surface again Christmas morning, and rising to near free air levels late Christmas Day