textproduct: Portland

SYNOPSIS

Upper level troughing will maintain showers with seasonably cool temperatures today, as well as a 10-20% chance of thunderstorms this afternoon. Cooler temps aloft have also allowed snow levels to fall to around 3500-4000 feet. A Winter Weather Advisory is in place through this evening for the Cascade passes. Drier and warmer weather returns Sunday through next week as high pressure re-builds.

DISCUSSION...Today through Friday

Radar and satellite imagery this morning show widespread to scattered showers moving east across the area as low pressure near the Olympic peninsula maintains moist, onshore flow. Broad upper level troughing over western Canada will continue to gradually drop over the Pacific NW today maintaining cooler temperatures aloft. A shortwave trough within the northwesterly flow is likely to enhance shower activity this afternoon, along with a 10-20% chance of brief thunderstorms for northern portions of the forecast area. Any sunbreaks this afternoon could allow daytime heating to enhance the instability to the upper end amount (300 J/kg), but forecast soundings do show warmer air in the midlevels creating a cap at around 500 mb which would keep cloud top heights limited in growth.

The cooler temperatures aloft have also allowed snow levels to come back down to around 3500-4500 ft overnight. Webcams at Cascade passes do show some snow covered roads at Government Camp where the temperature is around 32-33 degrees, and blowing snow at Santiam and Willamette Passes where the temperature also sits at around 32 degrees. The probability for exceeding 6 inches of snowfall through this evening remains at around 30-60%, though total snowfall amounts are likely to vary due to the showery nature of the precipitation. Highest chances for accumulating snowfall remain near Mt Hood. With a higher sun angle in May, any daytime sunshine will likely be more efficient in melting snow on the roads, which should help limit impacts to drivers. But, make sure to refer to ODOT/WSDOT for the latest road conditions. A Winter Weather Advisory remains in place for the Cascades above 4000 ft through this evening. Total snowfall amounts between 4 to 8 inches are possible, highest amounts near Mt Hood. Up to 12 to 18 inches of snow is possible above 5000 to 5500 feet.

After another day of seasonably cool temperatures, expect a warming trend to begin Sunday and continue through the middle of next week. As the upper level trough shifts east, high pressure returns over the region. Ensemble guidance is is general agreement that upper level ridging builds into the NE Pacific. Latest guidance shows more confidence in the ridge progressing eastward over the Pacific NW. The farther east the ridge does shift over the PacNW, temperatures are more likely to see higher end amounts, with afternoon highs reaching the 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 maintain mostly dry conditions for next week. 12/DH

AVIATION

Increasing northwest flow aloft today as an upper level trough drops over the region. Predominately VFR conditions expected as scattered showers continue across the area through this evening, with a 10-20% chance of brief MVFR conditions occurring with passing showers. 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. 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 likely beginning around 18-22Z Sat for most terminals.

KPDX AND APPROACHES...Predominately VFR with scattered rain showers through the evening. This will maintain a 10-20% chance of brief MVFR conditions. Southwest winds around 6-8 kt become westerly and increase to 9-12 kt by 20Z Sat and then shift northwesterly by 00Z Sunday. /DH

MARINE

Breezy northwest winds continue today as surface low pressure to the north weakens and pushes inland into western Washington. Scattered showers continue through this evening, with erratic and gusty winds possible near stronger showers. A mid- period northwest swell will build seas to around 10 to 12 ft today, then gradually subsiding back below 10 ft by early Sunday. The Small Craft Advisory remains in place for all waters, including the Columbia River Bar, through this evening.

North to northwesterly winds return Sunday into early next week while seas ease to around 4 to 8 ft as high pressure builds offshore. Winds will be strongest across the central coastal waters each afternoon and evening. 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. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 11 PM PDT this evening for ORZ126>128.

WA...Winter Weather Advisory until 11 PM PDT this evening for WAZ211.

PZ...Small Craft Advisory until 2 AM PDT Sunday for PZZ210-251>253- 271>273.