textproduct: Pendleton

KEY MESSAGES

- Mountain snowfall continues across Cascade and Blue Mountains. *Active Winter Storm Warning & Advisory*

- Hydro concerns through Sunday, potentially returning midweek.

- Mountain precipitation and warming temperatures through week.

DISCUSSION

Current radar and infrared satellite imagery shows moderate to heavy returns across the Cascades, Blue Mountains, and the Columbia Basin under cloudy skies. This is in response to an incoming upper level shortwave trough, surface cold front, and weak to moderate atmospheric river that continues to impact the area through much of the day. The cold front is poised to pass through the area early this morning, advecting cold air into the region and plummeting snow levels to around 2000 feet by mid- morning. This will usher in a transition from rain to snow over the Northern Blue Mountains early this morning as the atmospheric river slowly sags south. There should be enough moisture to provide an additional 6 to 9 inches of snowfall through the afternoon as temperatures will be slow to warm in the wake of the departing cold front. Thus, a Winter Weather Advisory has been issued across the Northern Blue Mountains until 8 PM this evening for elevations above 4000 feet.

Light snowfall is still occurring over the Washington Cascades, and is beginning to intensify as the Puget Sound Convergence Zone has developed and extends over the Washington Cascade crest. This will keep light to moderate snow over Washington Cascade passes through the morning, which has warranted the extension of the Winter Storm Warning until 11 AM today as an additional 4 inches will be possible. Currently, the Snoqualmie Pass/I-90 corridor is closed in both directions due to multiple spinouts and collisions.

The passing upper level shortwave, coupled with the surface high pressure pushing inland over the Washington coast later this morning, will allow a pressure gradient to develop along the Cascades and Blue Mountains to enhance winds along the east slopes of the Washington Cascades, Blue Mountain foothills, and Blue Mountain valleys as gusts of up to 40 mph will be possible. Both the HREF and NBM highlight areas of the Southern Blue Mountain foothills as experiencing the best chances of gusts reaching 40 mph with a 70-90% chance of occurrence. Peak winds are expected to coincide with the passing of the cold front between 6AM and 9AM this morning.

The substantial moisture related to the atmospheric river, and the rain-on-snow occurring ahead of the cold front passage has lead to river rises on some area rivers. Most notably is the Umatilla River near Gibbon and the Walla Walla River near Touchet, which are anticipated to crest in minor flood stage later today. As a result, a River Flood Warning has been issued until 11 PM today for the Umatilla River near Gibbon and a River Flood Watch has been issued between Saturday afternoon through Sunday afternoon for the Walla Walla River near Touchet. Other river reaches are forecast to rise above action/bankfull stage that include the Grande Ronde River at Troy and the Naches River near Naches later today. Make sure to use caution in low-lying areas near these rivers and find an alternate route if encountering flooded roadways.

The upper level shortwave continues east and deepens as an upper level ridge moves onshore and over the area on Monday. This will bring drier conditions over lower elevations of the Columbia Basin and Central Oregon, but weak embedded shortwaves will keep light mountain precipitation through the early part of the week as snow levels increase from 1000-3000 feet Sunday afternoon to around 8500 feet by Monday afternoon. Ensembles are in good agreement with the development of an upper level low pressure off the Gulf of Alaska, which enhances the backside of the upper level ridge to allow a weak to moderate atmospheric river event to unfold through much of next week. Both the GEFS and ECMWF suggest a weak atmospheric river developing Monday before enhancing to a weak to moderate event Wednesday to Thursday. The initial trajectory looks to focus more along the southern British Columbia and northern Washington coasts early in the week before sagging south Wednesday. Differences do arise related to the strength of the upper ridge and low off the coast of Alaska, with the result leading to more widespread and lower elevation rainfall associated with a weaker upper level low and warmer temperatures with a stronger upper level low. Currently, 51% of ensembles hint at a weaker low and more widespread rainfall. Even with this marginal discrepancy, periods of light lower elevation showers are likely midweek onward as high temperatures break into the low to mid-70s across lower elevations of the Columbia Basin and Central Oregon. 75

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

VFR conditions currently across the majority of sites, with the exception of MVFR conditions at KALW and IFR conditions at KPSC due to reduced ceilings of 2500 feet and 700 feet respectively. These conditions are expected to slowly improve through the morning as a cold front passes and departs to our southeast. Ceilings are expected to lift during this time, but widespread breezy winds of 20-30kts will occur across all sites through much of the day. Winds will start to subside this evening as high pressure begins to move onshore. 75

Preliminary Point Temps/PoPs

PDT 49 26 51 39 / 60 10 0 20 ALW 48 29 50 41 / 80 10 0 40 PSC 53 28 53 38 / 20 0 0 10 YKM 48 26 48 33 / 0 0 0 20 HRI 53 28 53 39 / 20 0 0 10 ELN 41 23 42 31 / 10 0 0 40 RDM 48 23 55 35 / 40 0 0 0 LGD 47 25 49 38 / 100 20 0 50 GCD 47 25 52 38 / 80 0 0 20 DLS 51 32 50 38 / 0 0 0 20

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 8 PM PDT this evening for WAZ030. Winter Storm Warning until 11 AM PDT this morning for WAZ522. OR...Winter Weather Advisory until 8 PM PDT this evening for ORZ502.