130 PM PDT FRIDAY NOV 30 2007 WEATHER SYNOPSIS FOR FRIDAY AND SATURDAY The Northwest continues to remain under the primary influence of a rather cool and slightly moist northerly flow down the backside of high amplitude upper ridging that lies about 7-800 miles offshore. Several weak disturbances imbedded in this flow have brought intermittent light to occasionally moderate snow to most areas, with heaviest snowfall in the south where a slightly stronger westerly developed as the disturbance moved southward. Currently the most recent disturbance now lies in northern California with a weaker disturbance slowly sliding southward off the south- central Washington coast early Friday afternoon. However, associated weak lift with this disturbance has combined with considerable low level moisture to spread occasional light snow or snow flurries southward. A third and stronger short wave in the northerly flow is expected to become much more active as it carves out considerable overwater trajectory in its track just off the southern BC coast early Saturday. This trough should begin to undercut the recently blocking ridge to its west as it develops a significant circulation. Moisture rotating around this rapidly developing upper low should begin to affect the region late Saturday morning in the Olympics, with more widespread and increasing winds, low freezing levels and light to moderate snowfall expected to reach most areas early-mid Saturday afternoon. By late Saturday and Saturday night, further strengthening winds and moderate to heavy snowfall are expected in most areas, especially near and west of the Cascade crest. WEATHER FORECAST FOR FRIDAY AND SATURDAY * OLYMPICS- Partly cloudy with occasional light snow showers Friday afternoon. Showers decreasing Friday night and early Saturday. Light snow increasing later Saturday morning. Moderate snow increasing mid-late Saturday afternoon and night, heaviest west and southwest slopes. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Partly to mostly cloudy Friday afternoon with occasional light snow showers or flurries north and central and occasional light snow south. Flurries or light showers decreasing in the north mid-late Friday afternoon and in the south Friday night. Partly to mostly cloudy early Saturday morning with increasing clouds later Saturday morning and mid-day and light snow redeveloping. Light to moderate snow increasing later Saturday afternoon, becoming moderate to heavy Saturday night. * EAST SLOPES WASHINGTON CASCADES- Mostly cloudy Friday afternoon with flurries or light snow showers north and central and occasional light snow or snow showers south. Flurries or light showers decreasing Friday night. Mostly cloudy Saturday morning with a few flurries. Occasional light snow redeveloping mid-day, gradually increasing Saturday afternoon and becoming moderate late Saturday afternoon and night, heaviest higher terrain near the crest. * MT HOOD AREA- Mostly cloudy with occasional light snow Friday afternoon, briefly decreasing Friday night and Saturday morning and becoming scattered light showers. Showers increasing mid-day and early Saturday afternoon, becoming moderate snow late Saturday afternoon and moderate to heavy snow Saturday night. SNOW LEVELS-CASCADE MTNS 500 ft N, 1000 ft S Friday afternoon 500 ft N and S Friday night Sea level N, 500 ft S Saturday Sea level N, 1000 ft S late Saturday, except snow levels at the surface Cascade east slopes and passes Saturday afternoon and night SNOW LEVELS-OLYMPIC MTNS 500 ft Friday Sea level Friday night and Saturday 1000 ft late 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 SAT SUN * HURRICANE RIDGE LT .10 .25-.5 * MT BAKER LT .10 .75 * WASHINGTON PASS LT .10 .5 * STEVENS PASS LT .10 .5 * SNOQUALMIE PASS LT .10 .5-.75 * MISSION RIDGE LT .10 .25 * CRYSTAL MTN LT .10 .5-.75 * PARADISE LT .25 .75 * WHITE PASS LT .25 .5-.75 * MT HOOD LT .25 .5-.75 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS E 0-10 Friday afternoon E 5-15 Friday night and Saturday morning E 10-15 Saturday afternoon and night * FREE WINDS AT 5000 FT S-SE 5-15 Cascades, NE 5-15 Olympics Friday mid-day and afternoon S-SE 5-15 Cascades, N 5-15 Olympics Friday night S 5-15 Cascades, S 10-25 Olympics Saturday morning S-SW 10-20 Cascades, S-SW 25-40 Olympics mid-day S-SW 20-40 Cascades, SW 20-35 Olympics Saturday afternoon W-SW 15-30 N, 20-40 C, 30-45 S Saturday night * FREE WINDS AT 9000 FT Variable 5-15 N and C, W 5-15 S, N 10-20 Olympics mid-day W-SW 5-15 N and C, 10-20 S, N 10-20 Olympics Friday afternoon SW 5-15 Cascades, NW 5-15 Olympics Friday night W-SW 10-20 N and S Saturday morning W-SW 25-40 N, 30-50 S Saturday afternoon W-SW 30-45 N, 40-60 S Saturday night EXTENDED WEATHER SYNOPSIS FOR SUNDAY THROUGH TUESDAY As the previous northern branch low continues to carve further west, circulation around this low should slowly merge with an increasingly strong southern branch of westerlies breaking through under the recent strong blocking and now closing upper high over the northern Gulf of Alaska. This should open the door for a series of increasingly strong weather disturbances to transit over the region Sunday through early Tuesday as increasing circulation around several deep upper lows sends abundant moisture, strong winds and gradual warming to the area. The first such strong disturbance should help maintain or increase periods of moderate to heavy snowfall in most areas on Sunday along with strong to very strong ridgetop and pass winds. While slow warming is also expected within the increased westerly flow on Sunday, a very strong and cool easterly pass flow at the surface should help ensure most precipitation as snow near and east of the Cascade passes, although some freezing rain may develop at lower elevations along the west approaches to the Cascade passes. However, the transition to a generally (and hopefully briefly) warmer weather regime Sunday night and Monday should provide some ugly weather conditions in many areas as a second and perhaps stronger weather system moves onshore. Heavy snow late Sunday should begin to transition to rain or freezing rain in the Olympics and some Cascade west slope locations Sunday night and early Monday as increasing warm air moves in aloft. Copious moisture and very strong winds should accompany further warming on Monday, with rain or freezing rain slowly spreading toward the Cascade passes and east slopes mid-late Monday. After several waves ripple along the back edge of the frontal band over the Northwest Monday night, the front should slowly exit the area late Monday night or early Tuesday, when a pass wind shift to westerly is likely. This should leave behind slowly lowering freezing levels, decreasing winds and more showery precipitation by later Tuesday morning. As a final secondary trough moves east of the area Tuesday afternoon and moderate upper ridging rebuilds briefly offshore, decreasing light to moderate showers, lighter winds and further slight cooling are expected through Tuesday night. EXTENDED FORECAST FOR SUNDAY THROUGH TUESDAY Increasing moderate to heavy snow and strong winds Sunday with limited but slow warming. Heavy precipitation, very strong winds and significant warming aloft late Sunday through Monday, with rain or freezing rain reaching higher elevations in the Olympics and at Cascade west slope locations late Sunday night and Monday morning and rain or freezing rain slowly spreading toward the Cascade passes mid-late Monday. Moderate to heavy rain, snow or freezing rain decreasing and becoming mostly rain or snow later Monday night and early Tuesday. Moderate rain or snow showers decreasing later Tuesday morning. Light to moderate showers decreasing later Tuesday. * SNOW LEVELS Sea level to 500 ft N, Sea level to 1000 ft S early Sunday Sea level to 1000 ft N, 1-3000 ft S Sunday afternoon 2-4000 ft N, 4-6000 ft C and S Sunday evening 4-6000 ft N, 6-8000 ft C and S late Sunday night or early Monday 5-7000 ft N, 7-9000 ft S later Monday morning 6-8000 ft N, 9-11,000 ft S Monday afternoon 4-6000 ft N, 6-8000 ft S Monday night 3-5000 ft N, 5-7000 ft S early Tuesday 3-5000 ft N, 4-6000 ft S late Tuesday, except snow levels at the surface Cascade passes and east slopes Sunday through Monday morning, slowly rising to near free air levels Monday afternoon and evening