textproduct: Pendleton

KEY MESSAGES

- Light showers linger today.

- Breezy winds Saturday. *Wind Advisory Issued*

- Showers return late Saturday, continuing through the week.

DISCUSSION

Current radar and visible satellite imagery shows spotty, light returns across the Washington Cascade and Northern Blue Mountains under partly to mostly cloudy skies. This is in response to a weak shortwave passing through the area, which brought minimal rain amounts of around 0.01" across the east Cascade slopes and Columbia Basin of Washington. Another weak shortwave will pass across south-central British Columbia this evening to allow for rain chances to return across these same areas after 4PM. Similar rain amounts of around 0.01" are likely with this second wave of showers, which should dissipate by 4AM Saturday morning as a ridge to the southeast starts to build. This transient ridge, coupled with low level moisture from light rain overnight, will likely allow areas of fog to develop across the Yakima and Kittitas Valley on Saturday morning. Confidence is fairly high as the REFS suggests a 60-70% chance of fog forming, with the NBM hinting at a 10-20% chance of visibilities of 1 mile or less across both valleys. This may warrant the issuance of a Dense Fog Advisory Saturday morning, so check the forecast and road conditions if travel is planned in these areas.

The upper level ridge will continue to strengthen as it departs east Saturday morning, just as an upper level trough approaches the Washington coast and enhances southwest flow aloft. These synoptic features will allow for a pressure gradient to develop over the Blue Mountains, attributing to breezy south winds across Central and Eastern Oregon - with highest winds over the Grande Ronde Valley. The GFS has consistently forecasted pressure gradients of 3-4mb between Baker and Meacham over the past 48 hours, which is down from 4-5mb earlier in the week. In the last 24, timing has also trended much earlier in the day, instead of aligning more with incoming showers late Saturday into Sunday morning. As a result, a Wind Advisory has been issued across the Grande Ronde Valley between 9AM and 8PM Saturday with south winds of 20-30 mph with gusts of 45-50 mph. Confidence is high in these values as both the REFS and NBM advertise a 55-75% chance of wind gusts of 45 mph or greater. This may not seem very "high", but the presence of the earlier departing ridge of high pressure will lead to cold pooling over the valley, which is vaguely showcased by HRRR guidance of a north-south temperature difference of 2-5 degrees.

The approaching trough brings light showers to the Cascades Saturday evening, the east slopes Saturday night, and the Columbia Basin early Sunday. Conditions will be drying out late Sunday morning, with overnight/morning rain accumulations of 0.20-0.30" above 4500 feet and 0.10" or less along the east slopes of the Cascades and northern Blue Mountains/foothills. Another transient upper level ridge moves into the area Sunday evening ahead of an upper level shortwave that approaches the coast Monday afternoon, returning showers along the Cascades, east slopes, and northern Blue mountains late Monday through Tuesday morning. Another 0.20-0.40" of rain is expected at elevations of 4500 feet and above, with 0.10" or less elsewhere along the east slopes and northern Blue Mountains. Guidance is in fairly good agreement with a ridge of high pressure building across the region late Tuesday, but uncertainty arises in its strength and the depth of an upper level trough that approaches the coast. Currently, ensemble members are split related to which of these two features is stronger. If the trough is the stronger one, there will be a better potential for rain amounts across the Columbia Basin versus being confined more along the Cascade and Blue Mountains if the ridge is the stronger synoptic feature. Regardless of which scenario pans out, southwest flow aloft will keep temperatures above normal and snow levels elevated to above 5000 feet - potentially reaching around 8500 feet midweek. 75

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/

VFR conditions with at least slight chances of rain (5-15%) through the afternoon at most sites (except BDN and RDM). DLS and PSC will see slight higher PoPs (20-30% chance) through the late afternoon and early evening. Could see a chance (5-15% chance) of temporary MVFR conditions developing in places with the heavier showers. Otherwise, winds in general will be light with no CIG or VIS issues.

Preliminary Point Temps/PoPs

PDT 40 65 44 56 / 10 0 10 30 ALW 48 66 48 56 / 10 0 10 40 PSC 41 59 40 61 / 30 0 10 0 YKM 41 59 40 58 / 30 0 20 0 HRI 40 61 40 59 / 10 0 10 10 ELN 37 55 34 50 / 60 0 40 30 RDM 35 69 38 51 / 0 0 10 10 LGD 37 59 42 52 / 0 0 0 20 GCD 37 67 39 55 / 0 0 0 0 DLS 45 62 45 57 / 40 0 70 10

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Wind Advisory from 9 AM to 8 PM PST Saturday for ORZ049.