textproduct: Portland

SYNOPSIS

A cool and unsettled pattern remains in place across northwest Oregon and southwest Washington this afternoon and evening. Scattered showers and mountain snow continue mainly in southwest Washington. A few slower responding rivers continue to have river flooding impacts. Winter travel conditions may linger across the Cascade passes with occasional mountain snow expected beyond this weekend. An active and progressive pattern continues into next week, with periodic chances for rain and breezy conditions, and a very low chance for lowland snow at the end of next week.

DISCUSSION...Now through Friday

Scattered showers continue across southwest Washington and north Oregon coast and Coast Range this afternoon as the area remains within a cool post- frontal airmass. Snow levels remain roughly 2,500 to 3500 feet. Snow showers are ongoing across the southwest Washington Cascades, keeping winter driving conditions in place at pass elevations. With that said, the Winter Weather Advisory has ended due to reduced snow amounts. Travelers should remain prepared for changing conditions through the evening and overnight hours.

Shower coverage continues to decrease through the day and into tonight as a weak ridge aloft moves through the area. Intermittent showers may still linger overnight across southwest Washington and higher terrain. Rainfall amounts at lower elevations will generally remain light, limiting the potential for additional hydrologic impacts. However, river levels will stay elevated as runoff from earlier systems continues to drain through area watersheds. Additional information on river conditions can be found in the Hydrology section below.

By Sunday, flow aloft turns more southwesterly ahead of an approaching Pacific trough. This will bring an increase in shower coverage during the day, with the most organized precipitation expected Sunday evening into Sunday night as the trough moves inland. While this system is not expected to produce widespread lowland impacts, renewed snowfall across the Cascade passes is likely to create additional travel challenges. With current guidance indicating decent snow accumulation possible at pass elevations (50-60% chance for more than 6 inches of snow over 24 hours), a Winter Weather Advisory has been issued from 4 AM Sunday to 4 AM Monday. Additionally, guidance suggests a 5-15% chance for more than 12 inches of snow over 24 hours from 4 AM Sunday to 4 AM Monday.

An active pattern continues into early next week as broad upper-level troughing remains anchored over the northeastern Pacific. Embedded shortwave disturbances will move through the region in succession with brief breaks between each, maintaining periodic chances for showers across northwest Oregon and southwest Washington. One of these disturbances is expected to arrive around Monday, accompanied by increasing south winds and locally breezy conditions. Ensemble guidance suggests coastal areas will be most susceptible to stronger wind gusts with this system (5-20% chance of gusts above 50 mph), while inland locations should generally see more modest winds (20-40% chance of gusts above 35 mph). Given saturated soils, even moderate wind gusts may be enough to result in isolated tree damage, particularly along the coast and in exposed areas.

Forecast uncertainty increases by the middle of the week as ensemble guidance diverges on the evolution of a closed low to the south. Most ensemble solutions keep the strongest winds and heaviest precipitation displaced well to the south. However, a smaller subset of ensemble members suggests the low will swing farther north, bringing additional rain and wind impacts into the area. Additionally, snow levels may lower towards the end of next week, bringing the potential for lowland snow. Specifically, guidance suggests a 1-5% chance for 0.1 inch of snow over 24 hour on Friday for lowland locations north of Salem. While chances for snow are low, a snow/rain mix is forecasted for Friday to reflect the uncertainty between models while bringing awareness to this possibility. While confidence in specific impacts next week remains low at this time, the overall pattern favors continued unsettled conditions before a potential drying trend develops late next week and into next weekend. ~12

AVIATION

Showers continue to dissipate across the airspace and is resulting in VFR conditions across the airspace. While showers will continue to slow diminish, there remains a 10-20% chance for thunderstorms through 12Z Sun along the coast. Any passing thunderstorms may bring lightning, heavy rain, erratic gusty winds, and small hail. Heavy rain could briefly drop VIS/CIGs to IFR thresholds or lower.

Another system approaching from the south will push showers from south to north starting around 09Z Sun. This will bring showery precipitation as well as a 25-45% chance for MVFR CIGs across Willamette Valley and coastal terminals between 12Z-18Z Sunday, increasing to a 45-70% chance after 18Z Sunday. Southwesterly winds will gradually weaken below 10 kt across the area tonight.

KPDX AND APPROACHES...Predominately VFR conditions through around 17Z Sunday. Showers are expected to approach from the south and will bring MVFR conditions around 20Z Sunday. Southwesterly winds under 10 kt. /42

MARINE

A weak front will maintain breezy conditions through tomorrow, but winds should begin to weaken for the waters south of Cape Foulweather tonight into Sunday morning as this front is progressing further north. Small Craft Advisories remain in effect for zones PZZ271, PZZ251, PZZ272, PZZ152 and the Columbia River Bar through 1 PM Sunday. Seas subside towards 7-9 ft at 10 sec on Sunday. There is also a 15-20% chance for thunderstorms across the waters including the Columbia River Bar through Sunday afternoon. Any thunderstorms may produce lightning, brief erratic winds, and heavy rain which could reduce visibility.

The next robust frontal system arrives on Monday, returning breezy southwesterly winds and building seas. A Gale Watch has been issued for all waters including the Columbia River Bar between 7 AM to 7 PM Monday for southwesterly wind gusts up 40 kt. Guidance currently shows a 40-60% chance for Gale force wind gusts on Monday, with the highest chances across the inner waters out 10 NM. Seas also build to 12-14 ft at 10-11 sec on Monday as a westerly swell moves in. Guidance suggests a 20-40% chance that seas rise above 15 ft. Marine conditions briefly settle down Tuesday to Wednesday before more active weather returns at the end of the week. /42-10

HYDROLOGY

While heavy rain has ended over the area and we have transitioned to showers, minor river flooding continues for a few rivers. As of this afternoon, the Luckiamute River near Suver and Pudding River at Aurora remains at minor flood stage. Expect most rivers to fall below flood stage by Sunday morning, with the Pudding River at Aurora expected to fall below flood stage on Tuesday. The latest river forecasts across the region can be found at: www.water.noaa.gov/wfo/pqr. Be sure to never drive through a flooded road and heed any remaining road closures.

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory from 4 AM Sunday to 4 AM PST Monday for ORZ126>128.

WA...Winter Weather Advisory from 4 AM Sunday to 4 AM PST Monday for WAZ211.

PZ...Small Craft Advisory until 6 PM PST Sunday for PZZ210.

Gale Watch from Monday morning through Monday evening for PZZ210- 251>253-271>273.

Small Craft Advisory until 1 PM PST Sunday for PZZ251-252-271- 272.