textproduct: Pendleton

DISCUSSION

Key Messages:

1. Showers and mountain snow persists through Friday afternoon.

2. Windy conditions Friday afternoon through Saturday morning. *Wind Advisories Issued*

3. Widespread rain returns Sunday, workweek river flood concerns. *Hydrologic Outlook Issued*

Current radar and visible satellite imagery showing light to moderate showers across the Columbia Basin, foothills, Blue Mountains, and the Cascades under overcast skies. This is in response an upper level shortwave and associated frontal system that will slowly pass through the Pacific Northwest over the next 24 to 36 hours. An additional 0.25-0.50" of rainfall is anticipated this evening across the Columbia Basin, Blue Mountain foothills, Wallowa county, and the John Day Basin, with less than 0.05" through Central Oregon, Eastern Gorge, and the Yakima/Kittitas Valleys. Snow levels will hover between 4500-5500 feet through the remainder of the day, leading to an additional 2-6 inches of snowfall over the northern Blue Mountains, 1-2 inches for the Southern Blue Mountains, and less than an inch over the Washington Cascades above 4000 feet. An incoming cold front Friday will increase rain showers across the region through the morning and early afternoon before becoming more confined along the Cascade and Blue Mountains in the evening into Saturday morning. Friday's rain amounts are expected to reach 0.25-0.50" along the east slopes of the Cascades, Grande Ronde Valley, and Wallowa County, 0.10-0.20" along the Blue Mountain foothills and the John Day Basin, and less than 0.10" through the Columbia Basin, Central Oregon, and the Yakima/Kittitas Valleys. Snow levels will be on the rise and into the 7500-8500 foot range through Friday afternoon/evening, leading to only an additional 1 to 2 inches of snowfall over higher terrain across the Cascades and northern Blue Mountains.

The primary concern in the short-term resides with windy conditions associated with the passage of a stout cold front Friday evening into Saturday morning. As a result, a 10-11.5 mb pressure gradient will develop across the Lower Columbia Basin (between PDX and GEG) as advertised by the GFS, NAM, and SREF. These values relate to sustained west winds of 20-30 mph with gusts of up to 55 mph possible. Confidence in these values is moderate to high as the NBM and HREF highlight a 55-75% chance of wind gusts reaching 45 mph or greater and a 40-70% chance of gusts reaching 50 mph or greater. Thus, a Wind Advisory has been issued from 4 PM Friday through 10 AM Saturday for the Eastern Columbia River Gorge, Simcoe Highlands, Blue Mountain foothills of Oregon, North-Central Oregon, and the Lower Columbia Basin of Oregon; and from 7 PM Friday through 7 AM Saturday for the Kittitas Valley and the Blue Mountain foothills of Washington. Peak winds are expected to occur overnight between 1 AM and 4 AM Saturday, with highest winds likely across the Simcoe Highlands.

Ensemble guidance is in agreement with a substantial Atmospheric River (AR) event impacting the Pacific Northwest early to mid- week, allowing for significant, persistent rainfall Monday evening through Wednesday evening and increasing snow levels of between 6000-8000 feet. Confidence in specific rain amounts is lacking, but the current forecast calls for 2-4 inches across the Cascades, 1-2 inches over the Blue Mountains, and 0.20-0.50" over the Lower Columbia Basin, Central Oregon, John Day Basin, and the Blue Mountain foothills during that 48 hour period. These wet and warm conditions will lead to river flooding concerns, primarily over the Naches and Yakima Rivers as both are forecast to reach action stage either Tuesday or Wednesday. Even though ensemble guidance is in agreement with the overall pattern of an incoming AR event, the location and overall amount of moisture is still in question as guidance struggles with how much the upper level ridge located off the coast of California suppresses. The more it suppresses and pushes further south, the more south and over the area the AR moisture plume gets directed. 53% of ensemble members suggest slightly more precipitation/moisture over the Washington Cascades, which would provide additional confidence in the Yakima and Naches Rivers reaching action stage. The Northwest River Forecast Center is currently advertising a 31%-63% chance of the Naches River reaching Action Stage and a 21%-28% chance of the Yakima River reaching Action Stage. At this time, there is a 5-9% chance of either river reaching minor flood stage. 75

AVIATION...00Z TAFs

The rain has helped a few TAF sites clear out from the fog. Currently DLS/RDM/YKM are all VFR. PSC/YKM are fluxing between VFR-MVFR due to CIGs sloshing around. PDT is LIFR due to a dense fog bank has just rolled in, ALW is MVFR due to both low CIGs and VSBY less than 5SM. This will be the case through much of the period as the CIGs and VSBY will fluctuate. Winds will be of no assistance in pushing the fog out either as may sites will remain below 6kts except DLS who will be gusty and VFR through 04Z before slowing.

PRELIMINARY POINT TEMPS/POPS

PDT 38 55 41 51 / 80 90 40 40 ALW 38 51 42 51 / 90 100 60 50 PSC 36 54 41 55 / 60 80 10 10 YKM 32 51 38 55 / 40 70 40 20 HRI 38 55 42 55 / 70 90 20 20 ELN 34 46 35 48 / 50 80 60 40 RDM 37 54 33 51 / 50 90 30 10 LGD 35 48 38 46 / 100 100 80 70 GCD 37 49 36 46 / 100 100 50 40 DLS 43 57 47 55 / 70 90 80 60

PDT WATCHES/WARNINGS/ADVISORIES

OR...Wind Advisory from 4 PM Friday to 10 AM PST Saturday for ORZ041- 044-507-508-510.

WA...Wind Advisory from 4 PM Friday to 10 AM PST Saturday for WAZ024- 521.

Wind Advisory from 7 PM Friday to 7 AM PST Saturday for WAZ026- 029.