textproduct: Pendleton

KEY MESSAGES

- Light to Moderate snow accumulations continue in the Cascades this morning, with another round along the Oregon Cascade crest and in the Washington Blue Mountains tonight through Wednesday.

- Atmospheric river will produce heavy snow accumulations in the Washington Cascades late Tuesday through Thursday night.

- Strong winds may develop across the Columbia Basin and high mountain ridges/peaks Wednesday night into Thursday as cold front approaches and low level jet develops.

DISCUSSION

Today through Friday night: The PacNW will be situated under an active, near zonal flow aloft through the rest of the workweek. Today through tonight, light to locally moderate snow bands will develop along the Cascade crest and the northern Blues as a shortwave trough to the north clips the region. Snow levels will be well below pass level across the Cascades and Blues, allowing snow accumulations in these areas. However, hi-res ensembles show that the bulk of the precipitation in the Blues will be focused in the WA portion of the mountain range. Winter weather advisories have been issued for the OR Cascade east slopes and the WA Northern Blues through tomorrow afternoon and evening.

By tomorrow, the first wave of an Atmospheric River 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 a moderate plume of moisture into the region Thursday, resulting in more light to moderate precipitation rates across the mountain zone. But by Friday, a second wave in the AR will result in another round of moderate to heavy precip in the mountains throughout Friday. At the beginning of the AR period, snow levels will start off below pass level across the region, but will rise to at least above 4.5kft late Wednesday as warm air advects into the region with the arrival of surface warm front and strong low-level jet. A cold front sagging south into WA will bring snow levels back down to 2.5kft to 4kft across the central WA Cascade crest 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 over a three day period ending Friday morning. As for how heavy are accumulations, the NBM is indicating a 85-95% chance of snow accumulations greater than 2 feet at Snoqualmie and White pass through Friday morning, with chances of 3 feet around 40-60%. As for Santiam Pass and the northern Blues, chances of seeing snow accumulations of at least 12 inches through a three day period ending Friday morning are only 40-60%, though the best chance will be through Wednesday afternoon when snow levels are still at or below the surface.

In the lower elevations, snow levels will generally be above the surface, resulting in light rain accumulations throughout the week. Rain shadowing on the lee side of the Cascades will limit the amount of precipitation most areas will see (0.05 to 0.15 inches), but portions of the Blue mountain foothills will see 0.25 to 0.45 inches of rain through Thursday night, with an additional 0.1 to 0.2 inches Friday (confidence 60-80%).

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 late Wednesday 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 (55-75%) 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. A High Wind Watch is in effect for these areas Wednesday evening through Thursday morning.

Saturday through Monday: 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 through Monday, persistent light rain/snow will redevelop across the Cascade crest and northern Blues as a weak AR rounds the top of the offshore ridge(confidence 45-65%). Lawhorn/82

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Mostly VFR conditions to prevail through the period. Light rain will develop at site DLS after 00Z, with CIGs potentially lowering to MVFR after 03Z-06Z. Rain will also be possible at KALW/KPDT/KRDM/KBDN after 05Z, KYKM after 12Z and KPSC after 15Z, Have indicated the likeliest times using PROB30. Winds will increase to 12-17kts with gusts 20-25kts after 20Z. Winds will weaken at sites DLS/YKM to less than 10kts this evening, with winds continuing at 10kts to 15kts at sites PDT/RDM/BDN/ALW/PSC through 18Z tomorrow. Gusts to around 21kts will persist at site BDN after 04Z. Perry/83

Preliminary Point Temps/PoPs

PDT 46 34 57 47 / 20 40 50 50 ALW 46 37 55 47 / 30 50 70 70 PSC 52 37 58 49 / 0 20 20 40 YKM 48 28 54 41 / 20 40 60 80 HRI 50 36 59 49 / 10 30 30 40 ELN 44 27 45 36 / 30 50 70 80 RDM 41 28 54 39 / 10 20 30 30 LGD 40 29 48 42 / 50 50 80 80 GCD 40 28 52 43 / 40 20 40 30 DLS 49 36 53 47 / 50 80 80 80

PDT WATCHES/WARNINGS/ADVISORIES

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