textproduct: Pendleton

KEY MESSAGES

- Snow showers continue with Winter Weather Headlines in effect through Thursday AM

- Drier conditions Friday into the weekend.

- Active weather returns late weekend into next week.

DISCUSSION

An upper-level trough of low pressure is overhead this morning, and snow showers continue across the region. Showers have been locally heavy with visibility of one-quarter mile or less in snow. While atmospheric conditions continue to be marginally supportive of cellular snow squalls, observations have largely lacked the wind component (i.e. winds have not gusted more than 10-20 kts based on surface observations). Heaviest snow showers should continue to diminish into the morning hours across the Blue Mountains region, though daytime heating will facilitate another round of shallow, diurnally driven convective showers later this morning through afternoon. While the lower elevations of the Columbia Basin/Gorge and adjacent lowlands will see a mix of rain and snow, snow will be the dominant precipitation type for the mountains and higher basins of our forecast area. Winter weather headlines remain in effect for all of the Blue Mountains region for the anticipated showers, and were expanded to cover the Ochoco-John Day Highlands zone. Farther west, opted to issue a new Winter Weather Advisory for the East Slopes of the Oregon Cascades, valid 10 AM today until 10 AM Thursday, for additional snow showers as the shortwave that is currently visible in water vapor imagery off the BC coast dives down into the Pacific Northwest and drives additional snowfall.

Thursday night into Friday, the last in a series of shortwaves is expected (95 percent confidence) to exit the Pacific Northwest to the east leaving drier conditions in its wake. An upper-level ridge is then very likely (95 percent confidence) to build over the Rockies while a large closed low from the Gulf of Alaska slides down the coast and retrogrades offshore of the Pacific Northwest. This would leave warmer, predominantly dry, and breezy to windy weather on tap for the weekend, especially east of the Cascades. The pattern supports windy conditions for our wind-prone areas such as the base/slopes of the Blue Mountains and the Grande Ronde Valley. The ECMWF EFI for wind is highlighting these areas with values of 0.5-0.8 which indicates decent ensemble agreement in windy conditions relative to the model's climatology. Will note that ensemble clusters show the main source of variance among ensemble members' solutions of the 500-mb height field to be the longitudinal placement of the aforementioned anomalously deep closed low. Solutions that place the low closer to the coast would result in better chances of precipitation for the Cascades -- and to a lesser extent some inland locations -- while solutions that show the low farther offshore would result in drier conditions for all.

By early next week, ensemble clusters show the closed low gradually filling and splitting with shortwaves moving inland. It's too soon to talk with much confidence about snow totals, but Monday into Tuesday will likely (80 percent chance) see a return of active weather across the forecast area. 86

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

All TAF sites except ALW are VFR. ALW is MVFR due to low clouds and is expected to remain MVFR through 20Z before the deck lifts to VFR. Rain and snow showers will continue across the region during the next 24 hours, though chances at any TAF site are low (>30%). Mostly VFR conditions will persist through the forecasted period with >30% probabilities of VIS and CIGs reaching MVFR for YKM around 9Z. Otherwise, all TAF sites will see VIS of 6SM and CIGs. 90

Preliminary Point Temps/PoPs

PDT 24 38 24 40 / 10 20 20 10 ALW 26 38 26 40 / 10 20 30 30 PSC 26 42 26 45 / 10 10 10 0 YKM 26 39 23 40 / 20 20 10 0 HRI 26 41 25 44 / 10 10 10 0 ELN 23 34 19 36 / 30 30 10 10 RDM 21 36 17 38 / 20 30 10 0 LGD 18 37 20 38 / 10 40 40 30 GCD 19 36 19 36 / 20 50 40 10 DLS 31 42 28 44 / 40 40 10 10

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Storm Warning until 3 PM PST this afternoon for WAZ030. OR...Winter Weather Advisory until 3 PM PST this afternoon for ORZ049-050-503-506. Winter Storm Warning until 3 PM PST this afternoon for ORZ502. Winter Weather Advisory until 10 AM PST Thursday for ORZ509.