textproduct: Pendleton

KEY MESSAGES

- Warming trend peaks today, cooling through remainder of week.

- Mountain showers and isolated thunderstorms Wednesday.

- Breezy afternoon winds Wednesday through Saturday.

DISCUSSION

Current radar and infrared satellite imagery shows dry conditions as high level clouds linger across the Columbia Basin. These clouds are associated with the remnants of an upper level low pressure currently over southern Saskatchewan, which will weaken and move east as a weak transient ridge passes over the Pacific Northwest. This will provide clearing skies through the morning as afternoon temperatures reach into the low to mid-80s across the Columbia Basin, Central Oregon, Columbia Gorge, Blue Mountain foothills, and the Yakima/Kittitas Valleys. These high temperatures are about 5 degrees above normal for this time of year. Several systems are poised to pass through the area over the weekend, cooling highs to near normal values Thursday and below normal over the weekend.

An incoming upper level shortwave, extending from a strong upper level low pressure system located in the Gulf of Alaska, will allow for increasing cloud cover tonight ahead of a 25-45% chance of afternoon showers across Cascades, John Day Highlands, Blue Mountains, and Wallowas - with best chances residing across the Washington Cascades and east slopes. The incoming shortwave does erode the weak, transient ridge to allow for a 10-20% chance of isolated afternoon thunderstorms across Grant, Crook, Union, Wallowa, and southern Umatilla counties. The HREF highlights mean surface CAPE of 200-400 J/kg, as multiple members suggest isolated storm initiation around 1 PM. Low level shear is favorable between 30-35 kts, but lapse rates are expected to decrease to below 6 C/km by mid/late afternoon, which should keep any developing storm cells sub-severe and isolated in nature.

The initial passing of the shortwave trough Wednesday will allow for a pressure gradient to develop along the Cascades, causing breezy afternoon/evening conditions through the Simcoe Highlands, Columbia Gorge, north-central Oregon, Blue Mountain foothills, and the Yakima/Kittitas valleys. Sustained winds of 20-25 mph and gusts of up to 40 mph will be possible across these areas. The NBM advertises a 60-80% chance of 40 mph gusts or more across the Simcoe Highlands and north-central Oregon, with a 25-35% chance through the Kittitas Valley. The parent upper level low over the Gulf of Alaska will slowly meander south along the coast of British Columbia, reaching the Washington coast Friday before passing through the area on Saturday. This is expected to produce an even stronger pressure gradient across the area, potentially leading to advisory-level (45+ mph) wind gusts across the Simcoe Highlands and north- central Oregon as the NBM suggests a 60-80% chance of occurrence. However, uncertainty is present in the low's strength and timing, which is indicated by a slight majority (51%) of ensemble members which show a weaker or slower incoming system. This outcome would lead to a more relaxed pressure gradient developing along the Cascades, resulting in lower wind potential both Friday and Saturday. 75

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

VFR conditions are forecast at all sites for the next 24 hours. Mostly clear skies today will transition to mostly cloudy to overcast skies consisting of mid/high cloud by Wednesday morning. Sustained winds of 12 kts or less are forecast though the period, with higher gusts (especially at DLS Wednesday morning).

Preliminary Point Temps/PoPs

PDT 81 51 78 48 / 0 0 10 10 ALW 81 55 80 54 / 0 0 10 10 PSC 85 54 83 52 / 0 0 10 10 YKM 84 56 80 48 / 0 0 30 0 HRI 84 53 81 51 / 0 0 10 0 ELN 79 53 74 46 / 0 0 50 10 RDM 82 50 74 38 / 0 0 10 0 LGD 79 47 76 45 / 0 0 30 20 GCD 83 47 77 42 / 0 0 40 10 DLS 87 63 78 54 / 0 0 10 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...None.