textproduct: Pendleton

KEY MESSAGES

- Long-duration atmospheric river event will bring heavy snow to the WA Cascades through Friday, and high winds to the interior Northwest late tomorrow.

- As much as 1-3 feet of snow along the WA Cascade Crest and portions of the east slopes through Friday.

- Strong wind gusts 50-60 mph anticipated this evening through Thursday morning across the lower elevations and foothills.

- OR Cascades, lower east slopes of the WA Cascades, and northern Blues will see moderate to locally heavy snow of 6 inches and more rest of today.

DISCUSSION

Today through Friday night: The PacNW will continue to be situated under a near zonal flow aloft that will be accompanied by a long duration Atmospheric River (AR). Good agreement amongst ensemble AR products that the axis of the AR will aligned with the OR/WA border throughout its duration.

Today, the peak of the first wave of an AR will reach the PacNW, increasing precipitation along the Cascade crest, with persistent light to moderate precip along the Northern Blues and Eagle Caps. The 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. Beginning this morning, snow levels will start off below pass level across the region, but will rise to at least above 4.5kft late this afternoon as warm air advects into the region with the arrival of surface warm front and strong low-level jet. Snow levels will be low enough for long enough duration for portions of the northern Blues, OR Cascade crest, and Lower East Slopes of the WA Cascades to see moderate to locally high snow accumulations through this evening. 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 will result in heavy snow accumulations through Friday morning. As for how heavy the accumulations will be, the NBM is indicating a 70-85% chance of snow accumulations greater than 3 feet at Snoqualmie and White pass through the end of Friday, with chances of 2 feet 90-100%. Looking through the duration of the AR, Santiam Pass will see it's best chance of snow accumulations through this afternoon, with a less than 30% chance of 0.1 inches Thursday through Friday. In the northern Blues, the best chances (60-80%) of another 6 inches will mainly be focused north of the OR/WA border.

In the lower elevations, snow levels will generally be above the surface, resulting in light to moderate rain accumulations throughout the week. 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 could see between 0.5-1 inch of precip with locally higher amounts closer to the foot of the Blues.

The aforementioned cold front and it's parent surface low will also bring the potential for strong winds across the Columbia Basin and adjacent valleys/foothills this evening through Thursday morning. The cold front passage will also coincide with a strong 850mb jet (55-65kts) developing over the Columbia Basin Wednesday night. There is a chance (60-90%) that these strong winds will mix down into the lower elevations, especially in wind prone areas across north central OR, Simcoe Highlands, and the Columbia Basin. As a result, High Wind Warnings are in effect across these areas this evening through late Thursday morning.

Saturday through Tuesday: 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 through early next week (confidence 50-65%). Lawhorn/82

AVIATION /12Z TAFS THROUGH 12Z THURSDAY/

VFR conditions to prevail through the period. Atmospheric river impacting the region will bring rain impacts to sites DLS/PDT/YKM/ALW, with rain ending later this evening at sites PDT/ALW. Winds will increase at all sites during the period, with peak winds 20-30kts and gusts 30-40kts at sites PDT/RDM/BDN/ALW/PSC later this evening. Winds at sites DLS/YKM will be 15-20kts with gusts up to 30kts, developing after 23Z and persisting through the end of the period. There is a chance (20-35%) that low level wind shear may impact all sites later this evening and overnight. That said, there was high enough confidence to include wind shear at site ALW after 03Z. Lawhorn/82

Preliminary Point Temps/PoPs

PDT 57 43 54 41 / 50 50 70 90 ALW 56 44 54 42 / 80 80 80 90 PSC 59 46 59 42 / 40 40 40 60 YKM 54 39 53 34 / 70 80 60 70 HRI 60 46 57 43 / 40 40 50 70 ELN 45 35 45 33 / 80 90 70 70 RDM 54 39 56 38 / 30 40 40 30 LGD 48 41 49 43 / 90 90 90 90 GCD 53 43 53 43 / 40 30 60 50 DLS 54 43 52 42 / 90 90 90 90

PDT WATCHES/WARNINGS/ADVISORIES

WA...High Wind Warning from 8 PM this evening to 11 AM PDT Thursday for WAZ026-028-029-521. Winter Weather Advisory until 8 PM PDT this evening for WAZ030. Winter Storm Warning until 5 AM PDT Friday for WAZ522. Winter Weather Advisory from 8 AM this morning to 8 PM PDT this evening for WAZ523. OR...High Wind Warning from 8 PM this evening to 11 AM PDT Thursday for ORZ044-507-508-510. Winter Weather Advisory until 5 PM PDT this afternoon for ORZ509.