2 PM PST SAT DEC 15 2007 WEATHER SYNOPSIS FOR SATURDAY AND SUNDAY A moderate frontal system moved through the area overnight and early Saturday, bringing increased winds, slight and brief warming, and increasing moderate snow to most locations, with 24-hour snowfall amounts ranging from about 3 to 10 inches. Behind the front, satellite pictures continue to indicate significant instability showers in the following cool air mass. These showers have been and should continue to be primarily directed over the west slopes of the Olympics and Washington Cascades by a moderate westerly flow aloft. The associated upper trough brought a surge of shower activity Saturday morning with a weaker secondary trough expected to slightly and briefly enhance showers mid- late Saturday afternoon. Weak upper ridging should move onshore overnight and early Sunday ahead of a strong frontal system, but considerable warm frontal moisture is already spilling over the ridge and this should reach the coast this evening and spread over the Cascades overnight. With this moisture arriving, light to moderate showers should increase to more steady precipitation overnight and early Sunday, mainly in the Olympics and north-central Cascades as much of the warm frontal moisture is expected to slide northeastward into southern BC. As the warm frontal moisture swings northward and the upper ridge moves weakly over the region, this should produce some decrease in precipitation in most areas early-mid Sunday morning. However, with imminent arrival of the strong cold front later Sunday morning and mid-day, generally increasing snow should spread into the Olympics mid-late Sunday morning and the Cascades around mid-day. While some limited warming is expected Sunday, an increasingly strong and cool lower level easterly flow across the Cascade passes should ensure that precipitation near the passes remains as snow, and other non-pass locations, snow is likely above about 2000 ft in the north and 3500 ft in the south. At upper elevations, strengthening south to southwesterly winds should accompany the increasing moderate to heavy snowfall likely moving into most locations Sunday afternoon, with moderate to heavy snow and strong winds expected to continue Sunday night before decreasing and becoming more showery early Monday. WEATHER FORECAST FOR SATURDAY AND SUNDAY * OLYMPICS- Moderate to occasionally heavy snow showers slightly decreasing early Saturday afternoon, but increasing again mid-late Saturday afternoon. Showers changing to light to occasionally moderate snow Saturday night and early Sunday, heaviest south and southwest slopes. Light to occasionally moderate snow increasing mid-late Sunday morning with increasing winds. Windy with moderate to heavy snow Sunday afternoon and night, heaviest south and southwest slopes. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST FROM MT RAINIER NORTHWARD- * CASCADE PASSES FROM MT RAINIER NORTHWARD, INCLUDING STEVENS AND SNOQUALMIE PASSES- Moderate to heavy showers slightly decreasing early Saturday afternoon, increasing again mid-late Saturday afternoon. Light to moderate showers becoming light to moderate snow later Saturday night and early Sunday, briefly decreasing early-mid Sunday morning. Snow generally increasing late Sunday morning and mid-day. Windy with snow increasing Sunday afternoon and becoming moderate to heavy mid-late Sunday afternoon and night. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST SOUTH OF MT RAINIER- * CASCADE PASSES SOUTH OF MT RAINIER, INCLUDING WHITE PASS- Moderate showers slightly increasing mid-late Saturday afternoon, decreasing Saturday evening. Light snow redeveloping later Saturday night, but decreasing early- mid Sunday morning. Light to moderate snow increasing later Sunday morning and mid-day. Windy with moderate snow increasing Sunday afternoon, becoming moderate to heavy mid-late Sunday afternoon and night. * EAST SLOPES WASHINGTON CASCADES- Light to moderate showers decreasing early Saturday afternoon, slightly increasing again later Saturday afternoon and evening, heaviest higher terrain near the crest and in the north and central Cascades. Showers becoming light snow north and central and occasional light snow south later Saturday night and early Sunday, briefly decreasing Sunday morning. Light to occasionally moderate snow re-developing again late Sunday morning north and mid-day south. Moderate snow Sunday afternoon and night, decreasing early Monday. * MT HOOD AREA- Light to moderate showers Saturday afternoon and evening, decreasing later Saturday night and early Sunday with some brief partial clearing. Light snow re-developing mid-late Sunday morning. Windy with moderate snow increasing and becoming moderate to heavy Sunday afternoon. Windy with moderate to heavy snow Sunday night. SNOW LEVELS-CASCADE MTNS 1500 ft N, 2000 ft S Saturday afternoon through early Sunday 2000 ft N and C, 3500 ft S later Sunday morning 1500 ft N, 2500 ft S Sunday afternoon 1500 ft N and C, 3000 ft Sunday night 1000 ft N, 1500 ft S early Monday, except snow levels lowering to near the surface Cascade passes and east slopes Sunday morning, remaining at the surface Sunday afternoon and evening, and rising to near free air levels around midnight Sunday SNOW LEVELS-OLYMPIC MTNS 2500 ft Saturday afternoon and night 2000 ft Sunday 1500 ft Sunday 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 SUN MON * HURRICANE RIDGE .75-1 .75 * MT BAKER 1-1.5 1 * WASHINGTON PASS .5 .5 * STEVENS PASS .75 .5-.75 * SNOQUALMIE PASS .75-1 .75 * MISSION RIDGE LT .25 .25-.5 * CRYSTAL MTN .5 .75-1 * PARADISE .75 1 * WHITE PASS .5-.75 .75 * MT HOOD .25 .75-1 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS W 5-15 with occasional higher gusts Saturday afternoon Variable mostly E 0-10 Saturday night E 5-15 early Sunday morning E 10-20 with higher gusts later Sunday morning and afternoon E 5-15 Sunday evening W 5-15 with higher gusts after midnight, increasing early Monday * FREE WINDS AT 5000 FT W-SW 15-30 N, 20-40 S Saturday afternoon SW 20-30 Cascades, 20-40 Olympics Saturday night S-SW 20-40 Cascades, 30-50 Olympics Sunday morning S-SE 30-50 Cascades, 40-60 Olympics mid-day and early Sunday afternoon S 30-50 Cascades, 20-40 Olympics later Sunday afternoon S 30-50 Cascades, 20-30 Olympics Sunday night, decreasing early Monday * FREE WINDS AT 9000 FT W 20-40 N, 15-30 S Saturday afternoon SW 15-30 N, 10-20 S, 20-40 Olympics Saturday night SW 20-40 Cascades, 30-45 Olympics early Sunday S-SW 30-50 Cascades, 40-60 Olympics later Sunday morning SW 40-60 N and S Sunday afternoon SW 20-40 N, 30-50 S Sunday evening W-SW 15-30 N, 20-40 C, 30-50 S and Olympics late Sunday night EXTENDED WEATHER SYNOPSIS FOR MONDAY THROUGH WEDNESDAY A series of disturbances in a broad upper trough covering much of the eastern Pacific should rotate through much of the west coast Monday through Wednesday in a generally cool and moderate to strong westerly flow aloft. Following the strong disturbance passing to the east of the area early Monday, a moderate westerly flow and associated short wave trough should combine with significant shower activity to produce moderate to heavy orographic snowfall in most locations near and west of the Cascade crest early to mid Monday along with moderate winds and continued low freezing levels. While gradually decreasing showers are expected behind the trough Monday afternoon, the next splitting and high amplitude trough is expected to approach the area Monday night and early Tuesday. After slowly decreasing showers Monday afternoon, heaviest in the south, this front should spread increasing moderate snow northward over most areas Monday night and early Tuesday morning. Moderate to heavy showers and strong winds should follow Tuesday afternoon and night with the upper trough, with further heavy snow accumulations likely. Although slightly rising freezing levels are expected in the south, most precipitation should remain as snow above about 3500 to 4000 feet in the south with little or no warming expected further north. Also this storm should favor some locally heavy snowfall along the Cascade east slopes late Tuesday. Moderate to heavy showers should slowly and very briefly decrease late Tuesday night and early Wednesday ahead of yet another moderate to strong frontal system rotating out of the now resident Gulf of Alaska low. This disturbance should spread increasing moderate to heavy snow northward along both sides of the crest mid-late Wednesday, with moderate to heavy snow likely in the south and moderate snow in the north at continued low freezing levels. All in all, the next week promises to be a very productive and interesting one, both weather and avalanche-wise. EXTENDED FORECAST FOR MONDAY THROUGH WEDNESDAY Moderate to heavy snow showers early-mid Monday, slightly decreasing Monday afternoon. Increasing snow spreading northward Monday night and early-mid Tuesday. Moderate to heavy snow or snow showers likely mid-late Tuesday with heavy accumulations likely. Moderate showers briefly decreasing early Wednesday, but increasing moderate to heavy snow spreading northward mid-late Wednesday. * SNOW LEVELS 500 to 1500 ft N, 1-2000 ft S Monday through early Tuesday 500 to 1500 ft N, 2-4000 ft S later Tuesday morning and afternoon Sea level to 1000 ft N, 1-2000 ft S Tuesday night and Wednesday 500-1500 ft N, 1-3000 ft S late Wednesday, lowering early Thursday...except snow levels at the surface Cascade passes and east slopes late Monday through mid-day Tuesday, rising to near free air levels Tuesday afternoon, lowering to near the surface again mid-late Wednesday and rising to near free air levels late Wednesday night and early Thursday