textproduct: Pendleton

DISCUSSION

Key Messages:

1. Stagnant conditions persist through week. *Dense Fog Advisory Active* *Air Stagnation Advisory Active*

2. Dry conditions and cooling trend into next week.

Current radar and visible satellite imagery shows dry conditions and clear skies, with the exception of lingering fog and low stratus across the Yakima and Kittitas Valleys. This has resulting in the issuance of a Dense Fog Advisory over these valleys earlier in the day, with the Kittitas Valley's advisory extending until noon Thursday due to the continuing visibilities as low as one quarter of a mile. Fog is likely to return tonight into Thursday morning across the Yakima Valley (50-60% chance), and even potentially across the Simcoe Highlands, Columbia Gorge, Lower Columbia Basin, and the northern Blue Mountain foothills (15-25% chance) as a strong upper level ridge and surface high pressure resides across the Pacific Northwest, allowing inversions to be present. This is evident as current temperatures at lower elevations of the Columbia Basin are in the low to mid-40s, whereas higher elevations across the Northern Blue Mountains and the Cascades are in the mid-to upper 50s. This pooling of cold air at lower elevations will inhibit vertical mixing, keeping air and particulates close to the ground surface. This has warranted the issuance of an Air Stagnation Advisory across the Lower Columbia Basin, Columbia Gorge, Blue Mountain foothills, Central and North Central Oregon, and the Kittitas/Yakima Valleys until 10 AM Friday. This may lead to further degradation of air quality, with current values in the moderate category with Air Quality Index (AQI) values between 50-80 across the aforementioned areas.

The upper level ridge will shift just offshore late today and elongate up the British Columbia and Southern Alaska coast through the remainder of the week, enhancing north-northwest flow aloft across the Pacific Northwest and advecting a cooler airmass into the area. This will lead to temperatures cooling through the weekend, bottoming-out Sunday and Monday with high temperatures in the mid-to upper 30s across the Columbia Basin and Blue Mountain foothills with lows dropping in the low to mid-20s. Ensembles suggest the upper level ridge will break down Monday into Tuesday as an embedded shortwave rides over and suppresses the ridge. This will bring the next chance (15-25%) for precipitation Tuesday evening into Wednesday. There are considerable uncertainty regarding timing and rain amounts related to overall strength of the embedded shortwave trough and how quickly the ridge breaks down, but 56% of ensemble members suggest precipitation arriving along the Cascades in the early morning hours on Wednesday. All ensemble members highlight mountain precipitation across the Cascade and Blue Mountains Wednesday, but only 19% of members show widespread precipitation reaching lower elevations of the Columbia Basin. Snow levels are forecast to be between 3500-4500 feet Wednesday, which means any precipitation will fall as snow across our mountainous terrain. However, the amounts are expected to be rather low as 88% of ensemble members keep 24 hour snow totals through Thursday afternoon below 1 inch. 75

AVIATION...00Z TAFs

LIFR conditions continue for YKM at the beginning of the period. Expecting the low level cloud deck to persist through 02Z but will keep monitoring for potential longevity beyond. Otherwise expecting some more fog to drop to a half mile in the DLS in the early morning hours lasting until around 18Z, with ALW in a TEMPO group around the same time for half mile fog. Otherwise, continuous VFR conditions are expected with small chances for areas to develop temporary fog in IFR or worse (5-15 percent chance) going through the overnight hours.

PRELIMINARY POINT TEMPS/POPS

PDT 30 43 26 39 / 0 0 0 0 ALW 32 42 29 39 / 0 0 0 0 PSC 29 43 29 41 / 0 0 0 0 YKM 31 47 28 44 / 0 0 0 0 HRI 30 42 28 40 / 0 0 0 0 ELN 31 43 26 40 / 0 0 0 0 RDM 27 50 21 42 / 0 0 0 0 LGD 32 50 28 46 / 0 0 0 0 GCD 35 54 30 52 / 0 0 0 0 DLS 33 45 31 44 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until 10 AM PST Friday for ORZ041-044- 507-508-510-511.

WA...Air Stagnation Advisory until 10 AM PST Friday for WAZ024- 026>029-521.

Dense Fog Advisory until 2 PM PST this afternoon for WAZ026-027.