textproduct: Pendleton

UPDATED AVIATION

PREV DISCUSSION

/issued 245 PM PST Sat Dec 13 2025/

DISCUSSION...Today through Sunday: A ridge off high pressure has moved over the PacNW today, which has resulted in dry conditions across the area with mostly clear skies. Most river sections have fallen below flood stage today, however the gauge for the Yakima River at Kiona still shows this section of the river experiencing Major flooding. The Yakima river at Kiona is forecast to drop below flood stage by Monday morning as the last of runoff in the Cascades filters through this portion of the river and into the Columbia.

Tomorrow, a weak frontal system and upper shortwave will bring light rain back to the WA Cascade east slopes, while the remainder of the forecast area will remain dry. Snow levels will remain well above pass level (6.5kft+) with the passage of this system, meaning most of the WA Cascade crest will see light rain. Otherwise, expect increasing cloud cover across the remainder of the forecast area with locally breezy winds early Sunday.

Monday through Friday: Ensemble model guidance continues to remain in great agreement that a potent low pressure system in the Gulf of Alaska will guide a series of shortwave troughs and frontal systems across the PacNW beginning Monday, and persisting through Friday. That said, disagreement grows towards the middle of the week in regards to precipitation amounts and timing of each system, with confidence only moderate (45-65%) in precipitation amounts in the afternoon forecast package. The first in the series of systems will arrive to the PacNW early Monday morning with another weak to moderate atmospheric river (IVT values 250-600 kg/m/s), with widespread chances of rain across the region, and moderate to heavy rain along the Cascade crest/east slopes and northern Blues. NBM 24-hr probabilities for 1.5 inches of rain through Tuesday morning have trended down slightly since yesterday to 45-75% (yesterday was 60-80%), indicating a growing uncertainty in amounts for the Cascades; chances of 1 inch for the OR Cascade crest is only 30-50%, and <15% for the northern Blues(similar to yesterday). Confidence is mod-high (65-85%) that this next round of rain will lead to rises on the Yakima and Naches rivers again, with the rivers forecast to reach minor flood stage as early as late Tuesday night.

Tuesday, a more potent cold front and shortwave trough will impact the PacNW and will continue to be aided by a weak to moderate AR. That said, 500mb heights will be dropping and a cold air advection will be taking place into the PacNW as early as Monday night, with snow levels along the WA Cascade crest dropping to around 4.5kft-5.5kft by Tuesday morning. This will result in moderate to heavy snow developing along the crest and east slopes of the WA Cascades. By Wednesday, ensemble guidance is in good agreement of the arrival of a much colder airmass that will drop snow levels to around 2.5kft to 4kft (lower to the north, higher in the south), allowing snow to develop across our mountain areas, including portions of central OR by Wednesday. That said, there is moderate confidence (40-60%) that snow levels will rise above pass levels across our Oregon zones by Thursday, allowing a rain/snow mix to develop; precip will then turn to snow again as snow levels lower to around 3.5kft to 4kft Friday. Otherwise, disagreement amongst ensemble members Tuesday and beyond does decrease confidence in exact snow amounts by the end of Thursday. But to provide an idea of how much snow could be expected, the NBM continues to show a 70-90% chance of 12 inches of snowfall across the WA Cascade crest and east slopes by the end of Thursday, with chances now 55-75% along the OR Cascade crest and a 25-50% chance in the northern Blues.

Besides precipitation, ensemble guidance shows increasing strong winds early next week. Starting with Monday, there is a 40-70% chance of wind gusts reaching 45mph across our wind prone areas in the lower elevations and across exposed ridges along the Cascade east slopes. The cold front approach Tuesday will result in the strongest winds of the period, with widespread 50-80% chance of seeing wind gusts at or above 45 mph in the western half of the forecast area, then increasing in coverage across the remainder of the forecast area Tuesday night into Wednesday. Overall, forecaster confidence in the strong winds developing is mod-high (60-85%) for Tuesday afternoon into Wednesday, given good agreement in the synoptic pattern amongst ensemble members. Lawhorn/82

AVIATION... 06Z TAFs...VFR conditions across all TAF sites except DLS. DLS is hovering between 1/4-1/2SM due to FG and low CIGS of 100ft. Guidance suggests his to be the case through at least 17Z, however, timing on when it will clear out is low (30%). Otherwise, all other TAF sites will remain VFR through the period with CIGs varying between SCT-FEW250. Winds will be primarily diurnally driven with sustained winds below 5kts. 90

PRELIMINARY POINT TEMPS/POPS

PDT 36 56 42 64 / 0 0 0 70 ALW 40 56 46 61 / 0 10 10 80 PSC 35 54 41 62 / 0 10 10 60 YKM 36 51 41 60 / 0 10 20 80 HRI 35 54 41 62 / 0 10 10 60 ELN 35 49 38 54 / 0 20 40 90 RDM 29 57 38 63 / 0 0 0 50 LGD 37 52 39 51 / 0 0 0 80 GCD 33 56 37 56 / 0 0 0 60 DLS 41 52 44 58 / 0 20 30 90

PDT WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None.