115 PM PDT SAT DEC 01 2007 WEATHER SYNOPSIS FOR SATURDAY AND SUNDAY Following a weak upper level disturbance that moved mostly southward offshore and brought occasional light snow or flurries to many areas Friday but mainly trace accumulations, a second and stronger upper level trough now lies in a northeast-southwest alignment about 200 miles west of Forks, WA in a continuing northerly flow aloft down the BC coast. The parent upper low should gradually carve out further to the west under the old high amplitude upper ridge during the next 24 hours while this upper trough slowly deepens and slides eastward over the region. Increasing flow with this upper trough and an associated surface low should force increasing moisture over the region, with increasing light snow reaching most areas mid-day and early afternoon. As the circulation increases mid-late Saturday afternoon and night, snowfall rates should increase with moderate snow likely spreading into most areas by late afternoon or early evening, and heavier snowfall arriving Saturday night. This precipitation should be accompanied by generally strengthening winds both aloft and through the Cascade passes, but only very slight warming, mainly in the south. The undercutting of the recent blocking ridge should be completed on Sunday as the first in a series of strong southern branch frontal systems breaks through under the closing upper high over Alaska. Associated frontal precipitation and energy should be swept into the circulation of the northern branch low and enhance moderate to heavy precipitation and substantially strengthening winds later Sunday morning into Sunday night. Although gradual warming aloft is expected with this second disturbance, and this may produce some rain or freezing rain at lower elevations in the Olympics and Cascade west slopes, an increasingly strong and continued cool easterly surface flow should ensure that precipitation remains as snow in the Cascade passes and along the Cascade east slopes until Sunday evening. However, as the associated surface low passes by to the north later Sunday evening, this should weaken the offshore gradients and may allow for some downward mixing of warm air aloft, with the possibility of some freezing rain or rain spreading briefly toward the passes. Such a warming trend in the passes should be quite brief though, with a very strong and continued cool easterly flow redeveloping later Sunday night and early Monday ahead of an even stronger third disturbance expected on Monday. In areas not protected by the cooler easterly pass flow, a transition to mixed snow and rain or straight rain may occur later Sunday afternoon and night as the freezing levels rise to around 5000 feet in the north and 7500 ft in the south. WEATHER FORECAST FOR SATURDAY AND SUNDAY * OLYMPICS- Occasional light snow increasing and becoming light to moderate Saturday afternoon and moderate to heavy later Saturday afternoon and night. Moderate to heavy snow increasing Sunday morning. Periods of heavy snow later Sunday morning through Sunday afternoon with local freezing rain lower valleys. Moderate to occasionally heavy snow or rain Sunday night becoming rain or mixed rain and snow most locations early Monday. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- Occasional light snow or snow showers increasing in the north mid-day and in the south early-mid Saturday afternoon. Light snow further increasing and becoming moderate later Saturday afternoon and moderate to heavy Saturday night. Moderate to occasionally heavy snow Sunday morning increasing later Sunday morning. Heavy snow becoming mixed with rain or freezing rain lower elevations Sunday afternoon, with mixed rain and snow or freezing rain slowly spreading to progressively higher terrain later Sunday afternoon and night. * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Occasional light snow or snow showers increasing in the north mid-day and in the south early-mid Saturday afternoon. Light snow increasing and becoming moderate later Saturday afternoon and moderate to heavy Saturday night. Moderate to occasionally heavy snow increasing later Sunday morning. Heavy snow Sunday afternoon, with some freezing rain developing lower approaches to the passes on the west slopes. Mixed rain and snow or freezing rain slowly spreading toward the passes later Sunday afternoon and evening, especially in the south. Heavy snow Sunday night, with freezing rain or rain continuing lower approaches to the passes west slopes. * EAST SLOPES WASHINGTON CASCADES- Occasional light snow or snow showers increasing mid-late Saturday afternoon and becoming moderate Saturday night. Periods of moderate snow Sunday morning. Moderate to occasionally heavy snow Sunday afternoon, with some freezing rain developing lower terrain in the south. Mixed rain and snow or freezing rain slowly spreading northward later Sunday afternoon and night, but precipitation remaining mostly snow higher terrain, especially north. * MT HOOD AREA- Occasional light snow increasing and becoming moderate mid-late Saturday afternoon and moderate to heavy Saturday night. Moderate to heavy snow increasing later Sunday morning. Heavy snow becoming mixed with rain or freezing rain lower elevations and near the Gorge outflow Sunday afternoon. Heavy snow, mixed rain and snow or freezing rain slowly spreading to progressively higher terrain later Sunday afternoon and night. SNOW LEVELS-CASCADE MTNS Sea level N and S Saturday afternoon Sea level N, 1000 ft S late Saturday afternoon Sea level N, 500 ft C, 2000 ft S Saturday night and Sunday morning 3000 ft N, 3500 ft C, 4000 ft S Sunday afternoon 4000 ft N, 4500 ft C, 6000 ft S Sunday night 5500 ft N and C, 8000 ft S early Monday, except snow levels at the surface Cascade passes and east slopes Saturday and remaining at the surface Sunday, rising slightly Sunday evening (as easterly surface flow decreases), lowering back to the surface again later Sunday night SNOW LEVELS-OLYMPIC MTNS Sea level Saturday afternoon 1000 ft Saturday night 500 ft Sunday morning 1500 ft mid-day 3500 ft Sunday afternoon 5000 ft Sunday evening 7000 ft later Sunday night and early Monday, except snow levels locally near the surface Saturday afternoon, and remaining at the surface lower valleys Saturday night and Sunday morning before rising to near free air levels Sunday afternoon 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 .5-.75 1-1.5 * MT BAKER .5-.75 2-3 * WASHINGTON PASS .5 1 * STEVENS PASS .5 1-1.5 * SNOQUALMIE PASS .5-.75 1.5 * MISSION RIDGE .25-.5 .5-.75 * CRYSTAL MTN .75 2 * PARADISE .75-1 2 * WHITE PASS .75 1.5 * MT HOOD .5-.75 1.5-2 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS E 5-15 early Saturday afternoon E 10-20 later Saturday afternoon E 10-20 with higher gusts Saturday night E 5-10 early Sunday morning E 15-25 with higher gusts later Sunday morning and afternoon E 0-10 Sunday evening E 10-20 with higher gusts late Sunday night and early Monday * FREE WINDS AT 5000 FT S-SE 10-15 Cascades, S-SW 15-25 Olympics early Saturday afternoon S-SE 15-30 Cascades, SW 20-35 Olympics later Saturday afternoon S 20-40 N, S-SW 25-40 S, and W-SW 20-40 Olympics Saturday night SW 20-35 N, 25-40 S Sunday morning S 15-30 N, 20-40 C, 40-60 S mid-day SW 20-40 N, 30-50 C, 40-60 S Sunday afternoon S-SW 15-30 N, 30-50 C, 40-60 S Sunday night S 40-60 Cascades, 80-100 Olympics early Monday * FREE WINDS AT 9000 FT S-SW 10-20 Cascades, 15-25 Olympics early Saturday afternoon SW 20-40 N, 30-50 C and S later Saturday afternoon SW 30-50 N, 40-60 S Saturday night W-SW 40-60 N, 50-70 S Sunday morning SW 30-50 N, 40-60 C, 60-80 S mid-day W-SW 50-70 N, 60-80 C, 70-90 S Sunday afternoon W-SW 40-60 N, 60-80 C and S, and 25-40 Olympics Sunday night SW 70-90 N, 80-100 C and S late Sunday night and early Monday EXTENDED WEATHER SYNOPSIS FOR MONDAY THROUGH WEDNESDAY The third and strongest weather system in the current series should move into the region Monday morning, ushering in even stronger winds, further warming and heavy to very heavy precipitation, with 24-hour precipitation amounts likely reaching the 3 to 5 inch range and beyond. Winds in some exposed ridgetop locations, carried into the region by a 160-180 knot jetstream, may well exceed 100 mph averages with higher gusts likely. And although strong and initially cool easterly surface flow expected to redevelop later Sunday night should continue most of Monday until a pass wind shift Monday night or early Tuesday, the substantial warming aloft should nevertheless mix downward and eastward mid-day and Monday afternoon. This should allow rain or freezing rain to reach most areas below about 6500 feet in the north and 8500 feet in the south. Following frontal passage Monday night, freezing levels should slowly lower, bringing an end to the freezing rain and allowing snow to slowly spread to progressively lower elevations. This cooling trend should continue on Tuesday as a secondary trough moves slowly over the area, helping to maintain periods of light to moderate rain or snow at lowering freezing levels and slowly diminishing winds. Weak upper and surface ridging should then produce decreasing and more showery precipitation late Tuesday through early Wednesday before a splitting upper trough moves mainly across Oregon mid-late Wednesday. This should bring further cooling along with some moderate rain or snow primarily to the Mt Hood area and extreme southern Washington Cascades but only slightly increasing shower activity further north. EXTENDED FORECAST FOR MONDAY THROUGH WEDNESDAY Warmer and very windy with periods of heavy to very heavy rain or freezing rain spreading to most locations Monday. Rain or freezing rain decreasing and becoming mostly rain or snow showers later Monday night and early Tuesday. Moderate rain or snow showers Tuesday, heaviest north, decreasing later Tuesday. Light showers early-mid Wednesday with some partial clearing. Moderate rain or snow redeveloping in the southern Washington Cascades and Mt Hood area mid-late Wednesday with increasing light showers north. * SNOW LEVELS 4-6000 ft N, 7-9000 ft S early Monday 5-7000 ft N, 8-10,000 ft S mid-day and Monday afternoon 4-6000 ft N, 6-8000 ft S Monday night 4-6000 ft N, 5-7000 ft S Tuesday morning 3-5000 ft N, 4-6000 ft S Tuesday afternoon and night 2-4000 ft N, 3-5000 ft S early-mid Wednesday 1-3000 ft N, 2-4000 ft S later Wednesday, except snow levels at the surface Cascade passes and east slopes Monday, rising to near free air levels late Monday or early Tuesday