textproduct: Portland

SYNOPSIS

Quick-moving fronts will bring chances for showers across the area through Saturday. There is a 15-20% chance for thunderstorms Friday and Saturday afternoon, mainly from Lincoln City/Salem northward. Any passing thunderstorms may produce lightning, heavy downpours, and erratic winds. Colder temperatures will also bring snow levels down to 3500-4500 ft and lead to light snow along the Cascade passes Friday night to Saturday night. Drier and warmer weather returns Sunday through next week as high pressure re-builds.

DISCUSSION

Radar imagery as of early Thursday afternoon depicts light rain moving into the north Oregon coast as a shortwave trough brushes the Pacific Northwest. Widespread cloud cover and chances for light showers continue through this evening, with the highest chances along the coast and across southwest Washington. Interior valley locations south of the Portland/Vancouver Metro will see drier conditions as the bulk of the precipitation tracks northward. Tonight into Friday morning, mostly dry conditions return before another shortwave trough dips down from the Gulf of Alaska and swings another quick-moving front through the area Friday afternoon through Saturday. This will return widespread chances for precipitation, with the highest chances over the Cascades due to orographic enhancement. West-southwesterly winds will also increase across the area with gusts up to 20 mph along the I-5 corridor and higher gusts of 25-40 mph across higher terrain and through the central Columbia River Gorge.

Across the Cascades, snow levels are forecast to drop to 3500-4500 feet Friday night to Saturday night behind the frontal passage. This will lead to light snow along the Cascade passes. Chances of exceeding 2 inches of snow in a 48-hour period ending 11 PM Sunday are 40-60% along the passes, with the highest chances along Santiam Pass. 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.

While rain amounts are expected to be light and non-impactful, there is also a 15-20% chance for short-lived thunderstorms Friday and Saturday afternoon, mainly from Lincoln City/Salem northward. Colder air aloft filtering in from these passing troughs will gradually increase atmospheric instability, especially in the afternoon. Model soundings are also showing inverted-V profiles, suggesting that these thunderstorms would be elevated rather than surface-based as the lower atmosphere remains drier. Increased cloud cover would also prevent surface- based initiation of thunderstorms. Any thunderstorms that do develop these next two afternoons may produce lightning, brief heavy downpours, and erratic winds. Temperatures aloft look a little too warm for small hail, but can't rule it out.

By Sunday, ensemble guidance is in agreement that the aforementioned trough will shift eastward into the Rockies while upper-level ridging builds over the northeast Pacific. Monday through Wednesday, most ensemble members (80%) suggest that this ridge will gradually shift over the Pacific Northwest, leading to above-average 500 mb heights and thus warmer, drier, and sunnier weather. -10

AVIATION

Largely VFR conditions in place inland and MVFR along the coast as a weak front traverses the region through the next 6-9 hours. Light rain showers continue to spread inland to Portland-area and Willamette Valley terminals. Along the coast, MVFR cigs are favored (60-80% chance), while low-end VFR cigs are more likely inland (10-25% chance of MVFR). Restricted vis is most likely along the coast, but may briefly occur within any heavier rain shower. Farther to the south/east near KEUG, chances for rain showers and any terminal impacts are lower. Behind the boundary, skies should trend clearer while cloud bases trend upward, with VFR conditions expected at all terminals after 04-06z Fri. Winds throughout the period remain out of the southwest to west at 5-10 kt, however 10-15 kt winds with gusts to 20 kt may occur during the frontal passage.

KPDX AND APPROACHES...Low-end VFR cigs near 4 kft expected to persist as a weak front passes overhead this afternoon. Light rain showers are most likely to continue through 04z Fri, but vis restrictions are unlikely and chances for MVFR cigs are only 10-15%. Southwest to west winds at 5-10 kt continue throughout the period. -36

MARINE

Southwesterly winds will turn northwesterly this evening behind a frontal boundary moving onshore. Wind gusts may reach 20 kt beyond 40 NM, but widespread or frequent gusts near shore are not anticipated. The front is bringing rain showers with increasing coverage to the north, but impacts to visibility are minimal and showers will diminish by midnight tonight.

Behind the boundary, a west-northwesterly swell will build to near 10 ft by early Saturday morning while wind gusts again reach near 20 kt as another front approaches. Small Craft Advisories therefore remain in effect from late Friday through late Saturday when overall seas reach 10-12 ft at 12-14 seconds, initially north of Cape Foulweather and beyond 10 NM, spreading toward the coast and southward through Friday night. Additionally, very strong ebb currents across the Columbia River Bar early Friday and Saturday mornings will shorten the wave period and result in steeper seas.

Northerly to northwesterly winds then continue into early next week while seas ease to 5-10 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. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.

WA...None.

PZ...Small Craft Advisory from 1 AM to 7 AM PDT Friday for PZZ210.

Small Craft Advisory from 2 AM Saturday to 2 AM PDT Sunday for PZZ210.

Small Craft Advisory from 8 PM Friday to 2 AM PDT Sunday for PZZ251-271-272.

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