textproduct: Pendleton

DISCUSSION

A shortwave trough and attendant surface cold front, currently offshore in the Pacific, will swing across the forecast area overnight into early Wednesday morning. During this period, 12Z HREF guidance suggests a low-level jet of 50-70 kts coupled with a surface pressure difference of 10-18 mb between PDX (Portland) and GEG (Spokane). This will drive a high wind event across the majority of our forecast area, and High Wind Warnings and Wind Advisories remain in effect from late this evening through Wednesday. Strongest wind gusts of 70-80 mph are expected along wind-prone portions of the Blue Mountain foothills and slopes of the northern Blue Mountains. Of note, the Storm Prediction Center (SPC) has placed the majority of our forecast area under a Marginal risk of severe thunderstorms due to the potential for severe outflow from low-topped convection with frontal passage.

After frontal passage Wednesday morning, windy conditions are forecast to continue through the afternoon hours, gradually tapering off by late afternoon and evening as winds aloft weaken and surface pressure gradients relax.

Along with the high winds, the cold front will facilitate a transition from rain to snow for the mountains. Have opted to issue additional Winter Weather Advisories for the Oregon Cascades, northern and southern Blue Mountains, and lower east slopes of the Washington Cascades due to the potential for near- blizzard conditions characterized by moderate to heavy snowfall rates coupled with blowing snow and low visibility with frontal passage.

Looking ahead, another high wind event is possible Thursday as winds aloft increase, but confidence is currently low-medium (20-50 percent) in reaching High Wind criteria. That said, NBM probabilities indicate better chances (40-70 percent) in reaching Wind Advisory thresholds across most of our low-elevation zones.

Another concern Wednesday night through Friday is the potential for winter weather and widespread precipitation as an atmospheric river is directed into the Pacific Northwest. Will note there is still ensemble variance in the axis and magnitude of that precipitation, but the ECMWF EFI is highlighting most of central to eastern Oregon with values of 0.8-1.0 for QPF, which indicates good ensemble agreement in a climatologically significant event for December. Low-elevation snowfall is possible with this system (30-50 percent chance of measurable snow in central Oregon and the Kittitas Valley, with higher chances of 70-90 percent in the Grande Ronde Valley and John Day Basin), but current forecast snow levels from the NBM quickly rise to 6-8 kft in Oregon and 3-5 kft in Washington Thursday in the wake of a warm frontal passage which would limit duration of snowfall.

River Flood Watches and Flood Watches remain in effect due to anticipated rising river levels through the week.

AVIATION...00Z TAFs

VFR conditions prevail with a cold front approaching the area overnight tonight. This next set of unsettled weather will bring high winds across the area, bringing some sites up to 50 knot gusts in the early morning hours, then decreasing by the late morning hours to early afternoon, albeit winds will still be 25 to 35 mph in most sites. Embedded with this system will be precipitation mainly in the form of rain, although VIS and CIG conditions will remain in VFR. Turbulence is possible in the overnight hours with wind shear guidance showing 40 to 60 knot shear across most sites. A small chance (5 to 15 percent chance) remains for some LLWS across all sites. An Airport Weather Warning remains in effect for PSC from 1 AM to 5 AM Wednesday due to elevated winds.

PRELIMINARY POINT TEMPS/POPS

PDT 40 50 35 55 / 90 50 20 100 ALW 41 52 38 55 / 100 60 20 100 PSC 42 53 37 54 / 90 20 20 100 YKM 38 50 33 45 / 90 30 30 100 HRI 42 52 36 55 / 90 30 30 100 ELN 33 43 30 40 / 100 50 40 100 RDM 32 44 28 56 / 100 50 50 100 LGD 36 48 31 45 / 100 90 30 100 GCD 36 49 30 50 / 100 80 50 100 DLS 43 49 38 49 / 100 60 70 100

PDT WATCHES/WARNINGS/ADVISORIES

OR...High Wind Warning from 10 PM this evening to 4 PM PST Wednesday for ORZ041-044-507-508-510-511.

Wind Advisory from 10 PM this evening to 4 PM PST Wednesday for ORZ049-050-505.

Winter Weather Advisory from 4 AM to 10 AM PST Wednesday for ORZ502-503.

Winter Weather Advisory from 1 AM to 4 PM PST Wednesday for ORZ509.

WA...High Wind Warning from 10 PM this evening to 4 PM PST Wednesday for WAZ024-026>029-521.

Flood Watch through Friday afternoon for WAZ522-523.

Winter Weather Advisory until 10 PM PST Wednesday for WAZ522.

Winter Weather Advisory from 1 AM to 10 PM PST Wednesday for WAZ523.