textproduct: Pendleton

KEY MESSAGES

- Moderate to heavy mountain snow through the day *Winter Storm Warnings Active*

- Breezy winds today and Saturday *Wind Advisory Active*

- Above normal temperatures expected next week

DISCUSSION

Current radar and infrared satellite imagery shows moderate to heavy returns across the Washington Cascades and the Northern Blue Mountains as snow continues to fall under mostly to partly cloudy skies. These conditions are associated with the passing of an embedded upper level trough supplied by a weak atmospheric river that is tapping subtropical moisture. This has provided ample snowfall and periods of heavy snow across the Washington Cascades, as 12 inches of snowfall has accumulated at White Pass and 10 inches has fallen over Snoqualmie Pass in the last 24 hours. Additional snowfall of 1-2 feet is expected to occur through the day today over the Washington Cascade passes. Confidence in these snow amounts are high as both the NBM and HREF advertise an 80-95% chance of 12 inches or more and a 45-65% chance of 18 inches or more snowfall through 5 AM Saturday morning across the Washington Cascade Passes. Snotel sites across the Northern Blue Mountains have picked up 5-7 inches of snowfall over the last 24 hours, with an additional 3-5 expected through the day today. Confidence in these amounts are more marginal as the NBM and HREF suggest a 50-80% chance of 4 inches of snowfall through the day today. These snow accumulations, coupled with expected additional amounts through today, warrant the continuation of active Winter Storm Warnings across the Washington Cascades until 5 PM today and until 5 AM Saturday over the Northern Blue Mountains.

The weak atmospheric river will also lead to widespread rainfall across lower elevations through the day today, and dropping further south as the day progresses. Rain chances will peak tonight into early Saturday morning as rain amounts of 0.25- 0.50" are expected across lower elevations of the Columbia Basin and between 0.75-1.25" over the Northern Blue Mountain foothills. Limited rainfall is anticipated across Central Oregon with between 0.01-0.07" likely. The HREF and NBM suggest a 30-50% chance of 0.50" or more rainfall for the Tri- Cities and Hermiston areas, and a 50-90% chance across the Northern Blue Mountain foothills through 5 AM Saturday. Precipitation will become more confined to mountainous terrain as Saturday morning progresses, with dry conditions expected for the Columbia Basin, Central Oregon, and Blue Mountain foothills by noon. Mountain snowfall will also dissipate through Saturday evening as an upper level ridge builds in from the coast and surface high pressure traverses the Canadian border.

A stationary front will camp out across the Oregon/Washington border today, which will allow surface low pressure to develop over the Columbia Basin, as a surface high pressure infiltrates into Central Oregon. These two features will create a pressure gradient along the southern Blue Mountains, which will lead to breezy winds along the Oregon Cascade crest, Central Oregon, North-Central Oregon, Simcoe Highlands, and most focused across the Southern Blue Mountain foothills as gusts of up to 50 mph will be possible. Thus, a Wind Advisory has been issued for the Southern Blue Mountains through 5 PM today. Confidence in advisory level winds (45 mph or greater) being met is high as both the HREF and NBM highlight a 70-90% chance of occurrence. Another area that may be impacted by breezy, advisory-level winds is along the Highway 97 corridor between Shaniko and Grass Valley. This area will need to be further monitored as gusts of 25-35 mph have already been observed this morning.

The approaching embedded upper level trough will pass overnight tonight into Saturday morning, coupled with a cold front that will pass through the area from northwest to southeast. This frontal system will allow for another pressure gradient to develop along the Blue Mountains as a surface high pressure follows in its wake. As a result, another breezy day is expected Saturday across the Blue Mountain foothills and valleys. Currently, the NBM advertises a 40-70% chance of advisory-level winds across the Northern/Southern Blue Mountain foothills, Wallowa County, and the Grande Ronde Valley. At this time confidence is lacking, but a wind product may be warranted if guidance continues to suggest a stronger gradient associated with the passing frontal boundary.

The upper level ridge that builds along the coast on Sunday will slowly move over the Pacific Northwest Monday before slowly continuing east through the beginning part of the week. A less substantial upper level low will develop over the Gulf of Alaska and deepen the backside of the upper level ridge, pushing it into the subtropics and developing a moderate atmospheric river as indicated by the GEFS. The ECMWF categorizes the atmospheric river as weak, but both indicate a Integrated Vapor Transport Magnitude (IVT) of between 400-650 kg/m*s. This event does seem to focus more along the British Columbia and Washington coasts, but looks to sag south as it weakens on Tuesday and lingers through Thursday. Ensembles are in good agreement with the overall pattern, with minor differences in the strength of the Gulf of Alaska upper low. This primarily relates to precipitationchances extending into the Columbia Basin Tuesday and Wednesday, as well as high temperatures breaking into the low to mid-70s across lower elevations Wednesday and Thursday. 97% of ensembles suggest lower elevation rain Tuesday and Wednesday (albeit light accumulations of less than 0.05"), but only 36% of members advertise high temperatures breaking into the 70s both Wednesday and Thursday. Snow levels will also be on the rise starting on Sunday, reaching 7000 feet Monday before climbing to above 8000 feet on Tuesday. 75

AVIATION /06Z TAFS THROUGH 06Z SATURDAY/

VFR conditions currently across all sites, with the exception of MVFR at KPSC/KYKM due to reduced ceilings of 2000-2500 feet. These conditions are expected to persist through the morning hours as stationary front lingers over the area, which will also keep light rain over all terminal through much of the day. The only exception will be across KRDM/KBDN, which is expected to arrive toward the end of the period. Breezy winds are also anticipated for KRDM/KBDN/KPDT with gusts of 20-30kts. 75

Preliminary Point Temps/PoPs

PDT 57 36 47 28 / 80 90 60 10 ALW 53 37 48 30 / 90 100 70 20 PSC 55 36 54 29 / 80 80 30 0 YKM 53 30 49 28 / 90 70 10 0 HRI 58 38 53 29 / 70 80 40 0 ELN 44 28 43 24 / 90 70 10 0 RDM 58 36 49 24 / 10 60 30 0 LGD 56 37 47 26 / 80 100 90 30 GCD 58 39 47 26 / 20 70 80 10 DLS 58 38 53 33 / 90 90 30 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Storm Warning until 5 PM PDT this afternoon for WAZ030- 523. Winter Storm Warning until 5 AM PDT Saturday for WAZ522. OR...Winter Storm Warning until 5 PM PDT this afternoon for ORZ502. Wind Advisory until 5 PM PDT this afternoon for ORZ508.