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UPDATE

This evening, model guidance has trended towards greater amounts of total snow accumulation in the Cascades and at the Cascade passes tonight through Saturday night. There is now a 50-70% chance for 6+ inches of total snow accumulation from 11 PM tonight to 11 PM Saturday for elevations above 4000 ft. As a result, a Winter Weather Advisory has been issued for the South Washington Cascades and the Northern and Central Cascades of Oregon above 4000 feet from 11 PM tonight to 11 PM Saturday. Total snow accumulations between 4 to 8 inches, highest amounts at Government Camp. 12 to 18 inches of snow expected above 5000 to 5500 feet.~12

SYNOPSIS

Showers continue through this evening as a weak frontal system moves across the area. Upper level troughing will maintain showers with seasonably cool temperatures through Saturday, as well as a 10-25% chance of afternoon thunderstorms. Cooler temps will also bring snow levels down to 3500-4000 feet, allowing for light snow along the Cascade passes tonight through Saturday night. Drier and warmer weather returns Sunday through next week as high pressure re-builds.

DISCUSSION...Today through Thursday

Radar imagery and surface observations as of early Friday afternoon depicts widespread rain moving through southwest Washington and far northwest Oregon (mainly from Lincoln City/Salem northward) as an upper- level shortwave trough and associated surface front swings through the Pacific Northwest. West-southwesterly winds have also picked up with this system with gusts up to 20-25 mph along the I-5 corridor and 25-40 mph across higher terrain and exposed ridgetops.

Looking at satellite imagery, it appears we'll transition post- frontal conditions by the late afternoon as cloud cover appears the break up behind the front just offshore. Colder air aloft behind the front and potential surface heating from cloud breaks will increase atmospheric instability this afternoon and evening. Therefore, there is a 10-25% chance for thunderstorms through 11 PM this evening across northwest Oregon and southwest Washington, with the highest chances along the south Washington coast and Willapa Hills. Any passing thunderstorms may produce lightning, heavy downpours, erratic winds, and/or small hail.

Showery conditions continue through Saturday as broad upper- level troughing progresses through the Pacific Northwest. Another impact from this system will be snow levels falling to 3500-4500 feet (lowest across the southwest Washington Cascades). Orographics of the Coast Range and Cascades will likely enhance precipitation as west to northwesterly flow persists. This may lead to some accumulating snow, especially above 3500 feet including the Cascade passes. Latest guidance suggests there is anywhere from a 30-50% chance of exceeding 6 inches of snow in a 24-hr period ending 5 AM Sunday along the Cascade passes, highest near Government Camp. If planning on traveling to the Cascades, make sure to check the latest forecast, refer to ODOT/WSDOT for the latest road conditions, and pack an emergency supply kit. Considering many of the forest roads are free of snow from the early melt-out and access to unmaintained roads is growing, the lower snow levels may catch people off guard if they are unprepared for cold, wet, and even snowy conditions.

After seasonably cool temperatures today and Saturday, expect a warming trend to return Sunday through early next week. As the upper level trough shifts east, expect high pressure to return back over the region. Ensemble guidance is is general agreement that upper level ridging builds into the NE Pacific. But, there remains uncertainty with how far east the ridge progresses toward the PacNW. The farther east the ridge does shift over the Pacific NW, temperatures are more likely to see higher end amounts, mid to upper 70s by the middle of next week. If the region stays under more northwesterly flow aloft, afternoon temps would be more likely to remain in the upper 60s to lower 70s. The majority of ensemble guidance does return to mostly dry conditions for next week. -10/DH

AVIATION

As of 04Z Sat, VFR conditions at most terminals, rain showers linger mostly north of KSLE, and southwesterly to southeasterly winds generally under 8 kt. Through the TAF period, expect rain showers and mostly VFR conditions, with a 10-20% chance of MVFR conditions.

For coastal terminals, expect generally westerly winds around 8-10 kt through 16-18Z Sat, then shifting northwesterly and increasing to around 10-14 kt. Winds will likely gust to 20 kt through the TAF period, with weakened gusts possible between 12-18Z Sat.

For inland terminals, expect southwesterly winds around 6-8 kt through 16-18Z Sat, then increasing to around 9-12 kt and gradually shifting northwesterly towards 00Z Sun. Wind gusts up to 20 kt will become possible beginning around 18-22Z Sat for most terminals.

KPDX AND APPROACHES...VFR conditions and rain showers will continue through at least 06Z Sun, with a 10-20% chance of MVFR conditions. Southwest winds around 6-8 kt will increase to 9-12 kt by 18Z Sat and then shift northwesterly towards 00Z Sun. Wind gusts up to 20 kt possible beginning around 19-21Z Sat. ~12

MARINE

An upper level trough continues to move across the area, supporting low pressure offshore along with a weak cold front. Westerly flow with scattered rain showers expected to persist through Saturday. Winds expected to increase with the approaching front, supporting gusts up to 25 kt across all of the waters from Cape Shoalwater WA to Florence OR, including the Columbia River Bar. A Small Craft Advisory remains in effect for all the waters until 2 AM Sunday. A westerly swell will increase seas to 10-12 feet at 11-13 seconds tonight. Very strong ebb currents across the Columbia River Bar early Saturday morning will shorten the wave period and result in steeper seas.

Northerly to northwesterly winds return Sunday into early next week while seas ease to around 4 to 8 ft as high pressure builds offshore. Strong currents during morning ebb tides will continue daily through Thursday, potentially yielding steep and hazardous seas for small craft crossing area bars through much of the week. ~12/19/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 11 PM PDT Saturday for ORZ126>128. WA...Winter Weather Advisory until 11 PM PDT Saturday for WAZ211. PZ...Small Craft Advisory until 2 AM PDT Sunday for PZZ210-251>253- 271>273.