130 PM PST WED DEC 26 2007 WEATHER SYNOPSIS FOR WEDNESDAY AND THURSDAY A moderate upper trough over western Washington Wednesday morning is slowly moving east of the area mid-day and early Wednesday afternoon. As the trough moves further east and weak upper ridging briefly builds offshore, an associated moderate westerly flow should slowly decrease and turn more northwesterly across most of Washington during the afternoon and the Mt Hood area late Wednesday afternoon and evening. This should allow moderate snow or snow showers in most areas near and west of the Cascade crest Wednesday morning to slowly decrease in the northern Washington Cascades mid- day, in the central Cascades Wednesday afternoon, and in the Mt Hood area late Wednesday, along with gradually diminishing ridge top winds and continued low and lowering freezing levels. Although a Puget Sound convergence helped enhance showers in the central Washington Cascades through much of the morning and mid-day, decreasing pressure gradients should allow the convergence related snowfall to slowly diminish Wednesday afternoon near or just south of Snoqualmie Pass. As the weak upper ridge moves across the area overnight and early Thursday, further decreasing showers are likely along with some partial clearing. The next moderate to strong Pacific frontal system should move onshore mid-late Thursday morning, with light snow developing during the morning increasing Thursday afternoon and becoming moderate to heavy later Thursday afternoon and evening. This frontal precipitation should be accompanied by increasing moderate to strong ridgetop and pass winds but only very little warming. Following the frontal passage Wednesday evening, moderate westerly flow should produce moderate to heavy orographic showers Thursday night into early Friday. WEATHER FORECAST FOR WEDNESDAY AND THURSDAY * OLYMPICS- Light to moderate snow showers, heaviest west slopes, gradually decreasing Wednesday afternoon. Light to occasionally moderate snow showers decreasing further Wednesday night with some brief partial clearing. Increasing clouds Thursday morning with light snow redeveloping. Light to moderate snow increasing and becoming moderate to heavy Thursday afternoon. Snow decreasing Thursday evening, but moderate to occasionally heavy showers likely later Thursday night into early Friday, heaviest west slopes. * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Light to moderate snow showers north and moderate to occasionally heavy snow showers central and south mid- day, with convergence near Stevens Pass shifting toward Snoqualmie Pass early afternoon. Showers further decreasing in the north Wednesday afternoon, in the central Cascades mid-late Wednesday afternoon, and in the south late Wednesday afternoon and evening. Light to moderate showers further decreasing Wednesday night with some brief partial clearing. Increasing clouds Thursday morning with light snow redeveloping later Thursday morning. Light to moderate snow increasing and becoming moderate to heavy snow Thursday afternoon, decreasing and becoming more showery Thursday evening. Moderate to occasionally heavy snow showers likely Thursday night into early Friday. * EAST SLOPES WASHINGTON CASCADES- Cloudy with light showers north and moderate showers central and south mid-day. Showers decreasing in the north early Wednesday afternoon, in the central Cascades mid-late Wednesday afternoon and in the south late Wednesday afternoon and evening, with showers primarily confined to higher terrain near the crest. Light showers decreasing overnight and early Thursday morning with brief clearing trend, especially lower elevations. Increasing clouds mid-late Thursday morning with light snow redeveloping. Light snow increasing Thursday afternoon and becoming moderate mid-late Thursday afternoon and evening. Snow decreasing and becoming light to moderate showers Thursday night into early Friday, heaviest higher terrain near the crest. * MT HOOD AREA- Moderate to occasionally heavy snow showers gradually decreasing later Wednesday afternoon and evening and becoming light to moderate. Showers further decreasing Wednesday night with some brief partial clearing later Wednesday night and early Thursday. Increasing clouds Thursday morning with light snow redeveloping. Light to moderate snow increasing and becoming moderate to heavy Thursday afternoon. Snow becoming more showery Thursday evening, but moderate to heavy showers likely Thursday night into early Friday. SNOW LEVELS-CASCADE MTNS Sea level N, 1000 ft S mid-day through Thursday morning 500 ft N, 1500 ft S Thursday afternoon Sea level N, 1000 ft S Thursday night, except snow levels lowering to near the surface Cascade passes and east slopes Thursday morning, rising to near free air levels later Thursday night SNOW LEVELS-OLYMPIC MTNS 1000 ft Wednesday afternoon 500 ft Wednesday night and Thursday morning 1000 ft Thursday afternoon and 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 THU FRI * HURRICANE RIDGE .25 .5 * MT BAKER .25-.5 .75 * WASHINGTON PASS .25 .5 * STEVENS PASS .5 .5-.75 * SNOQUALMIE PASS .5-.75 .5-.75 * MISSION RIDGE LT .25 .25 * CRYSTAL MTN .25-.5 .5-.75 * PARADISE .5-.75 .75-1 * WHITE PASS .25-.5 .5-.75 * MT HOOD .5-.75 .75-1 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS W 5-15 Wednesday afternoon and evening Variable 0-10 Wednesday night and early Thursday E 0-10 mid-late Thursday morning E 10-20 with occasional higher gusts Thursday afternoon and evening W 5-15 Thursday night W 10-20 with higher gusts late Thursday night * FREE WINDS AT 5000 FT W-NW 10-20 N, 15-30 S Wednesday afternoon W 5-15 N, 10-20 S Wednesday night W-SW 10-20 N, 15-25 S Thursday morning S-SW 15-30 N, 20-40 S and Olympics mid-day S 30-45 Thursday afternoon S 20-30 N, W 25-40 S and W 30-50 Thursday evening W 15-25 N, 30-50 S later Thursday night * FREE WINDS AT 9000 FT W-NW 15-25 N, 20-40 S Wednesday afternoon W 10-20 N, 15-30 S Wednesday night W 15-25 N, 20-40 S Thursday morning W-SW 20-30 N, 20-40 C, 30-50 S and Olympics Thursday afternoon W-SW 15-30 N, 20-40 C, 30-40 S Thursday night EXTENDED WEATHER SYNOPSIS FOR FRIDAY THROUGH SUNDAY A secondary upper trough expected to move over the area later Friday morning should enhance moderate to heavy showers at continued low freezing levels with moderate to heavy snow or snow showers expected to persist for much of Friday before slightly and briefly decreasing later Friday night into early Saturday, mainly in the north. However, very strong zonal flow with the next deep upper low moving toward the area from the eastern Pacific should continue to produce periods of moderate to heavy snowfall over much of the region Saturday and Sunday. Although the most recent model runs now indicate that heaviest snowfall should extend from south-central Washington throughout much of Oregon this weekend, several impulses racing over the area in a 140-160 knot jetstream should provide enhanced snowfall throughout much of the forecast area before slow ridging occurs late Sunday and Monday. In any case, some 2-4 feet or more of snowfall is possible across much of the Cascades near and west of the crest from Friday through Sunday afternoon, especially in the south. Longer range forecast models are indicating that after a moderate ridge brings decreasing showers and some partial clearing on the Monday, the 31st, a significant warming trend appears likely New Years Eve and New Years Day. Fortunately, the most recent model runs indicate that deepening of the associated trough may shunt heaviest rainfall to the north of the region for much of New Years Day with a brief period of moderate to heavy precipitation possibly delayed until late on New Years Day or perhaps the 2nd. In any case, the weather ahead promises an interesting end to a very intense La Nina December and an exciting start to the New Year. EXTENDED FORECAST FOR FRIDAY THROUGH SUNDAY Cool with moderate to heavy snow or snow showers Friday, especially west slopes Olympics, Mt Hood area and Washington Cascades near and west of the crest, with lighter showers Cascade east slopes. Showers slightly decreasing late Friday. Increasing light to moderate snow early Saturday becoming heavy later Saturday morning. Windy with heavy to very heavy snow Saturday afternoon through Sunday afternoon, especially in the south, with heavy accumulations likely. Snow decreasing late Sunday. * SNOW LEVELS Sea level to 1000 ft N, 1-2000 ft S Friday Sea level to 1000 ft N, 1-3000 ft S Friday night 500-1500 ft N, 2-4000 ft S early-mid Saturday 500-1500 ft N, 1-3000 ft S mid-late Saturday Sea level to 1000 ft N, 1-2000 ft S Saturday night and Sunday Sea level to 1000 ft N and S late Sunday, except snow levels lowering to near the surface Cascade passes and east slopes Saturday morning, rising to near free air levels mid-day Saturday