textproduct: Portland

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

Mostly VFR conditions across the airspace though there are signs of a downward trend as the cold front creeps closer to the forecast area. Areas of MVFR conditions have developed along the coast, mainly KAST, and are expected to persists well into the evening. Chances for MVFR conditions inland increase to 20-40% for any given hour after 00z Saturday. Scattered showers are expected through the majority of the TAF period. Any showers that passe directly over a terminal could drop conditions below VFR briefly. Westerly winds along the coast and southwesterly inland with gusts around 20 kt possible through 03-06z Saturday.

KPDX AND APPROACHES...Predominately VFR conditions are expected through the period, though chances for MVFR increase this evening to around 20-30% as rain showers increase across the area. Southwest winds around 8-11 kt with gusts up to 20 kt through late this evening. -19

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 PZZ271, PZZ251, PZZ272 and the Columbia River Bar. Wind gusts expand to the remaining marine zones (PZZ252, PZZ273, and PZZ253) overnight. A Small Craft Advisory begins this afternoon for the first round of zones and expands to the rest of the southern zones at 5 am on Saturday.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. -19/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 2 PM this afternoon to 2 AM PDT Sunday for PZZ210-251-271-272.

Small Craft Advisory from 5 AM Saturday to 2 AM PDT Sunday for PZZ252-253-273.