textproduct: Pendleton

UPDATED AVIATION...18Z TAFS

VFR conditions are expected through the majority of the TAF period. After 08Z for BDN/RDM there is a 30% probability that these two sites will see CIGs and VSBY decrease to MVFR due to RASN. However, temperatures will play a roll in whether the precipitation falls as rain or snow or a combination of the two, therefor we cannot rule out the possibility of BDN/RDM seeing sub-MVFR. All other sites have a 30% probability of MVFR after 13Z due to CIGs and VSBY as low as 2500ft and 4SM due to RA and BR.

Winds will mainly be 10 kts or less. However, there could be some gusts to around 20 kts through the morning before they decrease to 10 kts or less. 90

PREV DISCUSSION

/issued 323 AM PST Sat Dec 20 2025/

DISCUSSION...

Key Messages:

1. Significant mountain snowfall through today. *Active Winter Storm Warnings*

2. Hydro concerns linger. *Active Flood Warning*

3. Mountain snow and lower elevation rain chances Sunday.

4. Widespread precipitation Tuesday, basin snow chances midweek.

Current radar and infrared satellite imagery shows light to moderate snowfall across the Cascades with dry conditions across the Columbia Basin and Central Oregon under mostly clear skies. This is in response to a departing upper level shortwave embedded in zonal flow across the Pacific Northwest. Flow aloft will shift slightly from the west to more from the southwest through the day today, keeping upslope snowfall occurring across the Washington and Oregon Cascades that slowly tapers off through the afternoon and evening. An additional 5-10 inches of snow is expected to fall through the day today, which is on top of about 8 inches within the previous 24 hours as reported by the Northwest Avalanche Center (NWAC) over Snoqualmie Pass. Also, an additional 2-3 inches of snowfall is also anticipated along the Oregon Cascades above 3500 feet on top of 4-6 inches over the last 24 hours (McKenzie/Hogg Pass Snotel). As a result, Winter Storm Warnings continue to be active for the east slopes of the Oregon Cascades until 10 AM this morning and the upper east slopes of the Washington Cascades until 4 PM today.

Hydro concerns are still present for some area river reaches, primarily across the Yakima Valley and the Lower Columbia Basin of Washington. Currently, the only river in minor flood stage is the Naches River at Naches, which crested early Friday morning and is forecast to drop out of flood stage later this morning. Thus, a River Flood Warning is active until noon today. There are also several rivers in action stage including the Yakima River at Kiona and the Walla Walla River at Touchet, which are expected to drop below action stage in the next 48 hours. The Naches River at Cliffdell, which is also in action stage, is currently forecast to linger in action stage through Friday, December 26th. Even with widespread precipitation returning Sunday and again Tuesday onward, river levels are expected to continue to decline.

An upper level trough will close into a low pressure off the coast of British Columbia this evening into Sunday as a sharp shortwave spins off and moves onshore and across the region Sunday. This synoptic feature provides additional mountain snowfall and lower elevations rain on Sunday. Precipitation intensity peaks through the afternoon and evening as the associated weak cold front from the northwest meets a lifting surface low from northern Nevada. Cascade passes can expected 5-10 inches of snowfall, with 1-3 inches over the northern Blue Mountains above 4000 feet. The amounts over the Cascades may warrant a Winter Weather Advisory as the NBM suggests a 50-70% chance of 5 inches or greater across Snoqualmie, White, and Santiam Passes through Monday morning. Periods of light rainfall is also likely across lower elevations Sunday, with best chances (40- 70%) after 4 PM. Rain amounts will stay below 0.10", with lower elevations of the Yakima Valley and the Lower Columbia Basin of Washington potentially staying dry.

Ensembles are in fairly good agreement with the overall pattern of an upper level trough that drops from the British Columbia coast south over the eastern Pacific Tuesday and extending through midweek. Several shortwave troughs look to spin off the parent system through this timeframe, introducing moisture across the region while cooler air is advected Wednesday through Friday associated with northwest flow aloft. This may lead to low elevation snow accumulations over portions of the Yakima Valley, Lower Columbia Basin, and Blue Mountain foothills overnight Wednesday through Friday. Currently, there is a 10-20% chance of measurable snowfall (0.01" or greater) over the aforementioned areas. 59% of ensemble members are currently leaning toward less than 10% chance for measurable snowfall overnight into Thursday morning. However, 80% of members suggest a better chance for the foothills, 58% of members suggest a better chance over Tri-Cities, and 49% of members advertise a better chance over Yakima Thursday night into Friday morning. This better chance would be on the higher end of the earlier mentioned range from 10-20%, which relates to ensembles struggling with timing and strength of the passing shortwaves Tuesday onward and the overall low pressure system off the west coast. 75

A

PRELIMINARY POINT TEMPS/POPS

PDT 48 32 45 34 / 10 10 60 60 ALW 48 35 45 36 / 20 10 60 70 PSC 50 32 44 33 / 10 10 40 40 YKM 45 28 40 26 / 20 10 30 40 HRI 50 32 44 33 / 10 10 50 50 ELN 38 24 36 24 / 40 30 30 60 RDM 43 25 47 28 / 0 20 70 60 LGD 39 27 42 31 / 10 20 80 80 GCD 42 30 46 31 / 0 50 90 70 DLS 46 35 43 34 / 60 20 60 90

PDT WATCHES/WARNINGS/ADVISORIES

OR...Winter Storm Warning until 10 AM PST this morning for ORZ509.

WA...Winter Storm Warning until 4 PM PST this afternoon for WAZ522.