130 PM PDT SUN DEC 09 2007 WEATHER SYNOPSIS FOR SNDAY AND MONDAY With general upper ridging still dominating the Northwest weather scene early Sunday, a weak to moderate upper trough and associated frontal system are expected to pass by mostly north and east of the region Sunday afternoon and evening with only the weak and dying southern part of the frontal band moving over the area, mainly in the north. The associated northwesterly flow aloft spread increasing clouds southward over most areas Sunday morning along with some light snow or snow showers in the northern Washington Cascades and Olympics and light showers in the south-central Cascades and Mt Hood area. Behind the upper trough and dying front, decreasing light showers are likely late Sunday with slowly clearing skies expected later Sunday night and early Monday. Mostly sunny and cool weather should follow in most areas later Monday morning and afternoon in a generally dry northerly flow ahead of briefly rebuilding upper ridging just offshore. As the next Pacific disturbance begins to flatten and move the ridge eastward late Monday, increasing mid and high clouds ahead of a moderate warm front moving into southern BC should spread over the ridge and brush the Olympics and northern Washington Cascades later Monday night. This should bring slightly increasing ridgetop winds and slow warming aloft, mainly in the Olympics, however snow levels should drop to near the surface over the Cascade passes and along the Cascade east slopes as a cold air dome over eastern Washington and associated cool easterly flow insulates these areas from the gradually and briefly rising freezing levels. WEATHER FORECAST FOR SUNDAY AND MONDAY * OLYMPICS- Occasional light snow Sunday afternoon decreasing and becoming light snow showers Sunday night. Partly cloudy with light snow showers ending Monday morning. Partly to mostly sunny Monday afternoon. Increasing clouds later Monday night and early Tuesday. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Light snow slowly decreasing Sunday afternoon north and central with light snow showers slowly decreasing in the south. Partly cloudy with light showers decreasing all areas Sunday night. Partly cloudy north with scattered light showers ending Monday morning, otherwise partly cloudy central and south. Decreasing clouds becoming mostly sunny all areas Monday afternoon. Mostly fair south and central Monday night, but increasing clouds in the north later Monday night and early Tuesday. * EAST SLOPES WASHINGTON CASCADES- Occasional light snow higher terrain north and light showers central and south early Sunday afternoon. Clouds and light snow or showers slowly decreasing later Sunday afternoon and evening. Partly cloudy in the north Sunday night with clouds decreasing in the south. Partly cloudy north and central early Monday, otherwise decreasing clouds becoming partly to mostly sunny south Monday morning. Mostly sunny all areas Monday afternoon. Mostly fair Monday night with areas of fog developing lower valleys. Increasing clouds in the north early Tuesday. * MT HOOD AREA- Cloudy with light snow showers decreasing later Sunday afternoon. Decreasing clouds with scattered light showers decreasing further late Sunday and Sunday night. Partly sunny early Monday. Partly to mostly sunny later Monday morning. Mostly sunny Monday afternoon. Mostly fair Monday night. SNOW LEVELS-CASCADE MTNS Sea level N and C, 1500 ft S Sunday afternoon Sea level N and C, 1000 ft Sunday night and Monday morning 500 ft N, 1500 ft S mid-day 1000 ft N, 1500 ft S Monday afternoon 1000 ft N, 2000 ft S late Monday, except snow levels lowering to near the surface Cascade passes and east slopes late Monday SNOW LEVELS-OLYMPIC MTNS 1500 ft Sunday afternoon 2500 ft Sunday night and Monday morning 3500 ft Monday afternoon 5500 ft Monday night except snow levels locally near the surface lower valleys Monday, rising to near free air levels 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 LT .10 0 * MT BAKER LT .25 LT .10 * WASHINGTON PASS LT .10 0 * STEVENS PASS LT .10 0 * SNOQUALMIE PASS LT .10 0 * MISSION RIDGE LT .10 0 * CRYSTAL MTN LT .10 0 * PARADISE LT .10 0 * WHITE PASS LT .10 0 * MT HOOD LT .10 0 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS W 5-15 Sunday afternoon W 0-10 Sunday night Variable 0-10 Monday morning E 0-10 Monday afternoon E 5-15 Monday night * FREE WINDS AT 5000 FT W-NW 10-20 Cascades, NW 15-25 Olympics Sunday afternoon N-NW 10-20 Sunday night NE 5-15 Monday morning SE 5-15 mid-day S-SE 5-15 Monday afternoon S-SW 10-20 Monday night * FREE WINDS AT 9000 FT N-NW 20-40 mid-day and Sunday afternoon N-NW 30-50 N, 20-40 C, 15-30 S Sunday night N-NW 30-50 N, 25-40 C, 15-30 S Monday morning N 15-30 N, 20-40 S mid-day N 15-25 Monday afternoon N 10-20 Monday night EXTENDED WEATHER SYNOPSIS FOR TUESDAY THROUGH THURSDAY Most of the warm frontal moisture from the next developing Pacific frontal system should be diverted north of the area across south-central BC early Tuesday, and although the following occlusion should weaken as it moves southeastward over the area Tuesday afternoon and night, it should still bring increasing light snow to the Olympics and northern Washington Cascades, occasional light snow to the central southern Washington Cascades, and increasing clouds and light showers to the Mt Hood area, mainly Tuesday night into early Wednesday. Although the front and associated upper trough should move well to the south and east by later Wednesday morning, and rebuilding upper ridging should once again reestablish in the eastern Gulf of Alaska, a cool and somewhat moist north-northwesterly flow aloft should help maintain partly to mostly cloudy skies and occasional light snow or snow showers in most areas mid-late Wednesday and Thursday along with intermittent periods of partial clearing. Longer range forecast models indicate that the first significant weather system in some time should move onshore late Thursday and Friday. If models are correct, this disturbance should bring moderate to heavy snow at relatively low and lowering freezing levels and strong winds to most areas Friday, with moderate orographic snow or snow showers likely extending into early-mid Saturday. EXTENDED FORECAST FOR TUESDAY THROUGH THURSDAY Increasing clouds spreading southward early-mid Tuesday. Light snow developing in the north and Olympics Tuesday afternoon. Light snow north and central, occasional light snow south and mostly cloudy with light showers developing Mt Hood area Tuesday night. Light snow north and central, occasional light snow south and light snow or snow showers Mt Hood area early Wednesday. Light snow showers decreasing late Wednesday and early Thursday. Partly sunny Thursday. Increasing clouds late Thursday with light snow redeveloping in the north and Olympics Thursday night. Snow increasing early Friday. * SNOW LEVELS 1-2000 ft N, 3-5000 ft S early Tuesday 3-5000 ft N, 5-7000 ft C, 6-8000 ft S mid-day 2-4000 ft N, 4-6000 ft C, 5-7000 ft S Tuesday afternoon 1-2000 ft N, 1-3000 ft C, 4-6000 ft S Tuesday night Sea level to 1000 ft N, 1-2000 ft C, 3-5000 ft S early Wednesday Sea level to 1000 ft N and C, 1-3000 ft S mid-late Wednesday Sea level to 1000 ft N, 1-2000 ft S Thursday, except snow levels at the surface Cascade east slopes and passes Tuesday, rising to near free air levels late Tuesday, near free air levels Wednesday through early Thursday, and lowering to near the surface again later Thursday