textproduct: Pendleton

DISCUSSION

Key Messages:

1. Widespread precipitation today, freezing rain possible. *Winter Weather Advisories Active*

2. Mountain precipitation Friday, returning Sunday and Monday.

3. Warming trend through Saturday and staying above normal.

Current radar and infrared satellite imagery shows light returns extending across the east slopes of the Cascades and into the Columbia Basin under mostly cloudy skies. This is in response to an incoming shortwave and weak frontal system that is flattening the upper level ridge that developed across the area on Wednesday. The previously positioned ridge has allowed temperatures to drop to near or slightly below freezing across portions of the Lower Columbia Basin and the Yakima/Kittitas Valleys, which will provide the potential for freezing rain to occur as the weak warm front passes through the area this morning. The best chance for freezing rain to occur and result in ice accumulations of between 0.03- 0.10" will be across the Kittitas Valley (60-70% chance), with much lower chances extending into the Yakima Valley (20-40% chance), Tri-Cities (10-20% chance) and the Hanford (15-25% chance) area. Thus, Winter Weather Advisories have been issued across the Kittitas Valley and the Upper/Lower east slopes of the Washington Cascades until noon. Slick roads are expected through the morning hours, so use caution and provide additional time if traveling.

Due to the passing warm front today, snow levels will be increasing from 4500-5500 feet this morning to 5500-6500 feet tonight, leading to snowfall at higher elevations across the Cascade and Blue Mountains and pass-level rainfall. Moisture is lacking with this system as the upper level ridge continues to build during the shortwave's ride through the area, with expected snow amounts of 1-2 inches at higher terrain and lower elevation rainfall of around 0.05" through the Columbia Basin, Blue Mountain foothills, and the Grande Ronde Valley. Slightly higher rain amounts of 0.10-0.15" are likely through the Columbia Gorge and the Yakima/Kittitas Valley through the day today.

The upper level ridge rebuilds briefly this evening before shifting east as another weak shortwave approaches the area and passes along the Canadian border. This will bring another round of precipitation to our mountain zones as snow levels hover between 6000-6500 feet. These higher elevations are forecast to pick up an inch of additional snowfall, at best. Lower elevations between 2500-5500 feet along the east slopes of the Washington Cascades will likely experience rain amounts of between 0.05-0.25", with trace amounts possible over the Simcoe Highlands and lower elevations of the Columbia Basin, Blue Mountain foothills, and the Yakima/Kittitas Valleys.

Ensembles showcase another transient upper level ridge moving across the Pacific Northwest on Saturday ahead of another progressive pattern of shortwaves Sunday and Monday before a stronger upper level ridge sets up over the area Tuesday onward. Ensemble members all highlight another round of widespread precipitation on Sunday as lower elevations of the Columbia Basin and Blue Mountain foothills are expected to see 0.01-0.05" through the day. More uncertainty develops on Monday as only 52% of ensemble members keep precipitation confined to our mountain zones. Snow levels look to decrease to between 5000-5500 feet Sunday and 4000-5000 feet on Monday, but the lack of overall moisture due to the synoptic pattern being more dominated by ridging will lead to snow accumulations of less than an inch both days. Ensembles then align with a substantial upper level ridge building over the Pacific Northwest on Tuesday and lingering through Thursday.

The overall meridional pattern of rebuilding upper level ridges and transient shortwave troughs leads to persistent west- southwest flow aloft, which keeps above normal temperatures in the forecast through the weekend and into next week. The only elongated tenure of an upper level ridge coupled with surface high pressure looks to occur midweek next week. This should keep any developing cold pools or inversions to a minimum until then. 75

AVIATION...12Z TAFs

Rain will create minor impacts at most of the terminals This Morning. RDM and BDN look to remain largely rain shadowed with VFR cigs, while DLS will see rain persisting through the morning with the MVFR visibility. Th3e rain at YKM is close to freezing, at 33 degrees, however it looks like the temperatures should remain stable and not switch over to freezing rain. A TEMPO period for rain after about 15z is also in store for PSC/ALW and PDT.

PRELIMINARY POINT TEMPS/POPS

PDT 52 38 50 36 / 50 20 0 10 ALW 51 39 50 37 / 60 20 10 20 PSC 49 34 47 33 / 60 30 10 10 YKM 44 34 45 33 / 80 40 10 10 HRI 50 34 48 34 / 50 20 0 10 ELN 41 33 40 32 / 90 50 30 20 RDM 51 34 55 32 / 10 10 0 0 LGD 45 35 47 35 / 60 10 0 10 GCD 48 34 52 35 / 40 10 0 0 DLS 47 38 49 39 / 80 40 20 20

PDT WATCHES/WARNINGS/ADVISORIES

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