textproduct: Pendleton

KEY MESSAGES

- Additional accumulations between 1 to 2 feet of snow is expected along the Upper East Slopes of the WA Cascades through Friday night.

- Additional snow accumulations of 7 to 15 inches are also expected across the Lower East Slopes of the WA Cascades, and above 4kft in the Northern Blues.

- Strong winds will gradually decrease today, but breezy conditions will persist into the weekend.

DISCUSSION

Today through Friday night: The PacNW will continue to be situated under a near zonal flow aloft that will be accompanied by an Atmospheric River (AR). Good agreement amongst ensemble AR products that the axis of the AR will continue to be aligned with the OR/WA border through Friday night, and then will effectively "shut-off" by late Saturday morning.

Snow levels have risen above 5-6kft tonight across the forecast area, which has resulted in much of the precipitation along the Cascade crest to transition into rain and a rain/snow mix across the passes. Currently, the bulk of precipitation has sagged just south of the Columbia River, west of the Cascades. East of the Cascade crest, strong rain shadowing effects are preventing precipitation from developing across the forecast area, especially the usual favorable areas along the Northern Blues.

An AR will continue to push IVT values of 250-500 kg/m/s into the region through Friday, resulting in persistent moderate to locally heavy precipitation rates across the Cascade crest and Northern Blues. While snow levels are at or above 5kft at the moment, a cold front sagging south into WA will bring snow levels back down to 2.5kft to 4kft north of the Columbia River Thursday through Friday, while snow levels will generally be above 4kft to 5.5kft across the remainder of the forecast area. The AR combined with the low snow levels across the WA Cascades and the Northern Blues will at least result in heavy snow accumulations through Friday morning, with the upper slopes of the WA Cascades persisting into Friday night.

As for how heavy additional accumulations will be, the NBM is indicating a 80-95% chance of snow accumulations greater than 2 feet at Snoqualmie and White pass through Friday night. In the lower slopes of the WA Cascades, chances for an additional 8 inches are 75-90%, including Blewett Pass. In the northern Blues, areas above 4kft will see a 65-85% chance of greater than 6 inches, while these chances drop to around 55-80% for 10 inches. While probabilities are generally lower along the northern Blues, this AR event has already shown the potential for heavy snow accumulations to reach this area and produce winter impacts, so have opted to issue a Winter Storm Warning through Friday afternoon.

In the lower elevations, snow levels will generally be above the surface, resulting in light to moderate rain accumulations through Friday night. Rain shadowing on the lee side of the Cascades will limit the amount of precipitation most areas will see (0.1 to 0.3 inches) through Friday night, but portions of the Blue mountain foothills are still favored to see between 0.5-1 inch of precip with locally higher amounts closer to the foot of the Blues through Saturday morning.

The aforementioned cold front and it's parent surface low passing to the north will continue to bring breezy conditions across the lower elevations, with windy conditions persisting across portions of the Columbia Basin. Isolated areas have seen strong low-level jet winds mix down to the surface throughout tonight, especially along exposed ridges. This trend will continue through the morning hours, but winds are expected to gradually decrease starting late this morning and through the remainder of the day.

Saturday through Wednesday: There is good agreement amongst ensemble cluster guidance that a ridge building offshore will push the AR north of the region, bringing a momentary respite from heavy precipitation in the mountains. Sunday into early next week, there remains good agreement that the upper ridge will shift inland, bringing warming temperatures and rising snow levels. Rounding the top of the ridge, a weak AR will clip the region, with persistent light rain/snow developing across the Cascade crest and northern Blues Sunday, then transitioning to mostly rain into the middle of next week (confidence 65-80%).

Lastly, the combination of heavy rain/snow in the mountains/foothills followed by well above normal temperatures early next week, will result in rises in rivers and streams starting today. At this time, river forecasts indicate only rises into Action stage over the next 24-72 hours along portions of the Naches, Klickitat, Walla Walla, Umatilla, and Grande Ronde rivers. Lawhorn/82

AVIATION /18Z TAFS THROUGH 18Z FRIDAY/

VFR conditions are generally expected through the TAF period, though some local MVFR is possible if rain is heavy enough.

Winds will remian gusty through the afternoon, and will begin to decrease this evening at DLS, PDT, YKM, ALW and PSC late this afternoon or during the evening. Wind gusts should be in the 20 to 25 kt range, and winds should decrease to around 10 to 15 kts.

Preliminary Point Temps/PoPs

PDT 53 40 52 37 / 80 90 90 90 ALW 53 40 50 37 / 80 90 90 90 PSC 57 40 53 37 / 40 70 70 70 YKM 52 34 52 30 / 60 70 70 70 HRI 56 41 53 40 / 60 80 80 80 ELN 43 32 44 28 / 60 70 70 60 RDM 55 38 57 38 / 40 30 20 60 LGD 48 41 53 39 / 100 90 90 100 GCD 52 43 57 42 / 70 50 40 70 DLS 51 41 56 39 / 90 90 90 90

PDT WATCHES/WARNINGS/ADVISORIES

WA...High Wind Warning until 11 AM PDT this morning for WAZ026-028- 029-521. Winter Storm Warning until 5 PM PDT Friday for WAZ030-523. Winter Storm Warning until 5 AM PDT Saturday for WAZ522. OR...High Wind Warning until 11 AM PDT this morning for ORZ044-507- 508-510. Winter Storm Warning until 5 PM PDT Friday for ORZ502.