2 PM PST SUN DEC 30 2007 WEATHER SYNOPSIS FOR SUNDAY AND MONDAY A very strong zonal-westerly flow across the eastern Pacific, with a jetstream of 140-160 knots, is continuing to direct abundant orographic precipitation over much of the Northwest early Sunday afternoon after producing increasingly heavy snowfall overnight through Sunday morning. Along with strong westerly surface flow, the heavy snowfall combined with wind gusts exceeding 50-70 mph continues blizzard like conditions and very poor visibility in many areas. This strong flow should continue through Sunday afternoon into Sunday evening before upper and surface ridging slowly building offshore bring gradually decreasing and more northwesterly winds later Sunday night and early Monday. However, weak convergence should help maintain heavier showers in the central Cascades into Sunday night before gradually decreasing after midnight. As brief surface and upper ridging move over the area on Monday, the last day of 2007, this should finally bring a break from the almost month long onslaught of intense weather in the Northwest. This ridging should bring decreasing winds and diminishing light to occasionally moderate showers early Monday morning with showers ending by mid-day. Further decreasing clouds should allow for at least partly cloudy skies Monday afternoon with some further brief clearing in the Cascades late Monday. However, as the ridge moves slowly east of the area overnight, this should allow for increasing clouds from a weak warm front to move over the Olympics and northern Washington Cascades later Monday night and early Tuesday, along with rising freezing levels and slowly increasing winds. Although this should also increase the possibility of some very light rain or snow in the Olympics and north-central Cascades, it should be brief with the associated strong cold front not likely to move onshore and over the Olympics and northern Washington Cascades until later Tuesday and not reaching the southern Cascades and Mt Hood area until early Wednesday. WEATHER FORECAST FOR SUNDAY AND MONDAY * OLYMPICS- Heavy snow or snow showers Sunday afternoon, heaviest west slopes. Moderate to heavy snow or snow showers slowly decreasing Sunday evening. Moderate snow showers Sunday night, decreasing Monday morning and becoming light. Showers ending later Monday morning. Partly cloudy mid-day becoming partly to mostly sunny Monday afternoon. Increasing clouds Monday night with light rain or snow showers developing later Monday night. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Heavy snow Sunday afternoon and evening with locally heavy accumulations likely. Moderate to heavy snow showers decreasing and becoming moderate Sunday night, heaviest near Stevens and Snoqualmie Passes in convergence. Moderate snow showers decreasing early Monday morning. Light to occasionally moderate showers mostly ending late morning with partial clearing. Partly sunny Monday afternoon. Variable high clouds Monday night, mainly north, otherwise mostly fair with increasing winds, especially Cascade passes. * EAST SLOPES WASHINGTON CASCADES- Moderate to heavy snow or snow showers Sunday afternoon higher terrain near the crest, with light to moderate snow or snow showers lower terrain. Showers slowly decreasing Sunday evening, becoming light to moderate showers Sunday night, mainly higher terrain near the crest with partial clearing lower elevations. Partly cloudy with light showers ending Monday morning. Partly to mostly sunny Monday afternoon. Patchy low clouds or fog developing lower valleys overnight, otherwise mostly fair mid and higher elevations. * MT HOOD AREA- Heavy snow or snow showers Sunday afternoon with locally heavy accumulations likely. Moderate to heavy snow or snow showers decreasing Sunday evening. Moderate to occasionally heavy snow showers decreasing and becoming moderate Sunday night. Light to moderate snow showers decreasing early Monday morning and ending late morning with partial clearing. Partly to mostly sunny Monday afternoon. Few high clouds Monday night, otherwise mostly fair with increasing winds lower terrain near the Gorge and Cascade passes. SNOW LEVELS-CASCADE MTNS 500 ft N, 1000 ft S Sunday afternoon 500 ft N and S Sunday night Sea level N, 500 ft S Monday morning 500 ft N, 1000 ft S Monday afternoon 1000 ft N, 3000 ft C, 7000 ft S Monday night 4500 ft N, 6000 ft C, 8000 ft S early Tuesday, except snow levels lowering to near the surface Cascade passes and east slopes Monday afternoon and evening SNOW LEVELS-OLYMPIC MTNS 1000 ft Sunday afternoon 500 ft Sunday night and Monday 4000 ft Monday night 6000 ft early Tuesday 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 MON TUE * HURRICANE RIDGE .75 LT .25 * MT BAKER .75-1 LT .25 * WASHINGTON PASS .5-.75 LT .10 * STEVENS PASS 1 LT .25 * SNOQUALMIE PASS 1 LT .25 * MISSION RIDGE .25 LT .10 * CRYSTAL MTN .75 LT .10 * PARADISE 1 LT .25 * WHITE PASS .75 LT .10 * MT HOOD .75-1 LT .10 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS W 15-30 with higher gusts Sunday afternoon W 15-25 with occasional higher gusts Sunday evening W 10-20 Sunday night W 5-15 early Monday Variable 0-10 later Monday morning E 5-15 Monday afternoon E 10-20 with higher gusts Monday night * FREE WINDS AT 5000 FT W 15-30 N, 25-40 C and S Sunday afternoon W-NW 15-30 N, 20-40 C and S Sunday night W 5-15 N, NW 10-20 S early Monday Variable 5-10 Cascades, S 5-15 Olympics later Monday morning and mid-day S-SE 15-25 Monday afternoon S-SE 15-30 Cascades, 25-40 Olympics Monday night SW 15-30 Cascades, 30-50 Olympics early Tuesday * FREE WINDS AT 9000 FT W 25-40 N, 30-50 C and S Sunday afternoon W-NW 25-40 N, 30-50 C and S Sunday night NW 25-40 early Monday morning NW 15-30 later Monday morning and mid-day N-NW 10-20 Monday afternoon W 10-20 Cascades, SW 15-25 Olympics Monday night SW 15-30 Cascades, 30-50 Olympics early Tuesday EXTENDED WEATHER SYNOPSIS FOR TUESDAY THROUGH THURSDAY After a weak warm front brushes the region late Monday through early Tuesday and brings in the New Year with some clouds and occasional light rain or snow in the Olympics and northern Washington Cascades, briefly decreasing showers are expected Tuesday morning in the north and Olympics with partly to mostly sunny skies further south. Although this should be accompanied by rising freezing levels aloft on Tuesday, an increasingly strong and cold easterly surface flow across the Cascade passes should insulate the pass areas and Cascade east slopes from the warming aloft. As a large and deep offshore trough further deepens on Tuesday, several waves moving along the back edge of the associated strong cold front arcing from southern BC, across the Olympics and to the south-southwest should slow or briefly halt its eastward motion. This should maintain periods of light to moderate rain or snow in the Olympics later Tuesday morning and afternoon, but mainly mid and high clouds in the north and variable high clouds with considerable filtered sunshine further south. Unfortunately, despite the lack of precipitation, this situation should also be accompanied by increasingly strong winds in most locations. As the last of the frontal waves ripples northward late Tuesday, the cold front is expected to slowly edge eastward overnight and Wednesday morning, with increasing light to moderate rain or snow spreading southward along with continued moderate to strong winds and lowering freezing levels. During this freezing level transition, some local freezing rain is possible near the west approaches to the Cascade passes. Moderate rain or snow should decrease and become more showery mid-late Wednesday. However, this front should open the door for another active weather week opening up the New Year as several more strong disturbances rotating around the deep upper low just offshore move over much of the west coast. Although the strongest flow and associated heaviest precipitation should shift primarily southward over much of California later Wednesday through the end of the week, the Northwest should remain under the influence of a moderate and very moist south to southwesterly flow. After decreasing showers late Wednesday into early Thursday, the northern part of another strong storm moving into California should spread increasing moderate rain or snow northward mid-late Thursday. This should produce moderate precipitation northward along both sides of the Cascade crest, with snow likely above about 3000 ft in the north and 4-5000 ft in the south. However, a continuing cool easterly flow across the Cascades should help maintain mostly snow near the passes EXTENDED FORECAST FOR TUESDAY THROUGH THURSDAY Cloudy Olympics and north-central Washington Cascades early Tuesday with occasional light rain or snow, otherwise partly to mostly sunny south-central Cascades and Mt Hood area. Continued mostly cloudy Olympics with intermittent light rain or snow Tuesday morning, increasing Tuesday afternoon; however, clouds decreasing in the north Cascades later Tuesday morning and afternoon becoming partly cloudy north and mostly sunny south. Light to moderate rain or snow Olympics spreading slowly southward in the Cascades overnight and early Wednesday. Chance local freezing rain west approaches to the Cascade passes early Wednesday. Moderate rain or snow early-mid Wednesday, decreasing and becoming more showery later Wednesday through early Thursday. Increasing light to moderate rain or snow spreading northward mid-late Thursday. * SNOW LEVELS 4-6000 ft N, 6-8000 ft S Tuesday 3-5000 ft N, 5-7000 ft S late Tuesday 2-4000 ft N, 3-5000 ft S early Wednesday 1-3000 ft N, 2-4000 ft S mid-late Wednesday 2-4000 ft N, 3-5000 ft S early Thursday, slightly rising late Thursday...except snow levels at the surface Cascade passes and east slopes Tuesday through Thursday