textproduct: Pendleton

UPDATE

*Dense Fog Advisory* Issued across the northern Blue Mountain Foothills of Washington until 6 AM as visibilities of one quarter to one half mile are being observed via observations and area webcams. A temperature inversion is already setting up behind the lifting warm front, which continues to bring very light showers and low level moisture to the area. These conditions are expected to persist through the early morning hours.

*High Wind Warnings* have also been issued between 7 AM and 2 PM Wednesday along the Grande Ronde Valley, Wallowa County, and the Blue Mountain foothills of Washington and Oregon. South-southeast winds of 30-40 mph with gusts up to 65 mph will be possible, as downsloping winds resulting from an incoming cold front from the west will enhance the pressure gradient along the eastern mountains. This gradient, coupled with morning temperature inversions near the height of terrain, are expected to duct downsloping winds to create super critical flow and high winds as the air rushes down Blue Mountain foothills. The passing cold front mid-afternoon will alleviate the pressure gradient, halting quick downsloping winds. 75

AVIATION...06Z TAFS

Variable conditions currently across sites, with VFR at KPSC/KYKM/KRDM/KDLS, MVFR at KPDT/KBDN, and LIFR at KALW due to reduced visibilities of 1/4 to 1/2SM associated with a lifting warm front bringing low level moisture and surface high pressure. These conditions are expected to persist through the early morning hours. KBDN/KRDM are also expected to drop to LIFR conditions overnight as a result of reduced ceilings of 400ft. IFR conditions are likely at KPDT overnight as well, associated with ceilings around 500ft. MVFR conditions over KYKM and KDLS also overnight due to reduced ceilings of 1.5kft. These ceilings will lift slightly through mid-morning as winds pick up for KPDT/KALW/KRDM/KBDN ahead of a moderate cold front passing the area during the late morning through early afternoon timeframe. Conditions look to improve toward the end of the period in the wake of the departing system. 75

PREV DISCUSSION

/issued 218 PM PST Tue Dec 23 2025/

KEY POINTS...

1. Widespread low elevation rain today through afternoon through Wednesday morning

2. Mountain Pass Snow through Christmas Eve *SPS in affect*

3. Breezy winds Christmas eve *SPS in affect 1 AM through 4 PM Wednesday*

DISCUSSION...Current satellite shows mostly skies across the CWA. Radar shows a few returns moving across the counties through the center of the CWA with ground observations showing 0.01- 0.04 inches of rain along the lower elevations and near 0.20 inches along some of the higher elevations through the Blues. Rain is expected to continue to linger through tomorrow afternoon.

Models show an active weather pattern with an associated atmospheric river (AR) is expected to impact the Cascades, most notably over the Washington Cascades. Moderate rain favored, with amounts between 0.5- 1 inch (60-80% confidence) at the Cascade crest with lesser amounts found at the lower elevations. Lower elevations will see up to 0.05 inches of rain overnight and again into Wednesday with 80-90% confidence. Models show the upper level low with the AR to move just enough to push the band of precipitation across the OR portions of the CWA moving form the southeast to the northwest before becoming mostly focused over the Cascades by tonight.

Snow levels will increase to near 4500-5000 feet today as the models show the atmospheric river to be a bit warmer than originally anticipated. Short term models do show a swath of snow accumulations across the WA Cascades through the evening with 60-80% probabilities of up to an inch of snowfall. However, a shot of cooler air will come into the areas Wednesday night bringing the snow levels back down to near 3000-3500 with another accumulation of near 1 inch (30- 50%). Highest amounts have since shifted to Christmas night into Friday morning with 70-90% probabilities of 2-3 inches of snowfall.

Models continue to show a surface level low tightening the pressure gradients across the region, especially through central OR eastward. Pressure gradient models show a tightening of the gradients of near 15 mb from central OR along the John-Day Ochoco Highlands and through the eastern mountains. This will lead to breezy conditions across the aforementioned areas. This surface low will bring south winds of 15 to 25 mph with gusts up to 30-40 mph expected with higher winds nearing 45 mph along the higher terrain (70-90% confidence). The south winds will create a cross wind along the I-84 corridor so high profile trucks should heed caution when driving the pass.

Saturday onwards, models are in decent agreement showing a persistent ridge moving in over the region. Dry conditions will return beginning with seasonal temperatures. By the end of the period, temperatures could begin to crest to nearly 10 degrees above seasonal average along the higher terrain and through central OR and the John-Day Basin (60-70% confidence). With that said, pattern recognition for this time of year suggests that with the upper level ridge lingering for a few day, winter time fog will settle over the region later in the period. 90

PRELIMINARY POINT TEMPS/POPS

PDT 34 55 35 49 / 40 30 20 30 ALW 37 54 39 49 / 50 40 30 50 PSC 35 52 32 46 / 60 30 10 40 YKM 33 46 29 43 / 90 70 10 40 HRI 33 53 33 46 / 60 30 20 30 ELN 30 42 27 39 / 80 80 10 60 RDM 30 51 29 50 / 40 40 10 10 LGD 38 51 38 49 / 20 40 40 50 GCD 39 52 36 50 / 10 50 40 60 DLS 38 51 36 45 / 80 70 30 50

PDT WATCHES/WARNINGS/ADVISORIES

OR...High Wind Warning from 7 AM to 2 PM PST Wednesday for ORZ049-050- 507-508.

WA...High Wind Warning from 7 AM to 2 PM PST Wednesday for WAZ029.

Dense Fog Advisory until 6 AM PST Wednesday for WAZ029.