textproduct: Pendleton

DISCUSSION

The low stratus layer remains lingering over the Columbia Basin of WA/OR and Yakima/Kittitas Valleys with few to scattered high clouds passing through, observed from current satellite imagery. While majority of the fog has lifted since this morning, road cams and surface observations are still showing localized patchy fog along I-84 near Cabbage Hill and Poverty Flats. Some parts of Cabbage Hill may have improved visibilities, but expect oscillating visibilities as this fog remains lingering across most of that area through these afternoon hours or early evening if continues sloshing around.

As this high pressure system continues through Saturday with quiet weather, fog will remain as the main concern along with the low stratus layer. Based on surface observations for the past days, visibilities has been trending towards less than one half a mile across the Columbia Basin of WA, Yakima/Kittitas Valleys, Foothills- Northern and Southern Blue Mountains of OR extending to Central Oregon with patchy/areas of dense fog during overnight and early morning hours. The HRW_ARW, HRW NSSL, HRRR and NAM CONUS Nest models show visibilities at a quarter mile or less slowly returning later this evening and then overnight hours for the Columbia Basin of WA/OR including Yakima/Kittitas Valleys. Can't exactly rule out on how much the spread and strength of this fog development could be, especially with the models (NBM and HREF) struggling to keep up. High temperatures may stay mostly in the 40s for the Columbia Basin of WA/OR where they will struggle to increase further due to the low stratus. The remaining forecast area will be mainly in the 50s. Redmond and Bend has reached to the upper 60s and low 70s, respectively yesterday when Redmond had broken the old record of 67 degrees in 2017 after exceeding to 70 degrees. The HRRR model show the temps for Redmond and Bend to be in the low to mid 60s today, which was more reasonable since they have been receiving more surface daytime heating after the low stratus lifted this morning. As relative humidities remain elevated around the Columbia Basin, this will keep the light yet ongoing moist air for the low stratus layer to last longer.

Sunday through Monday, the ridging pattern will then start to slowly erode with the approaching shortwave and frontal system. Widespread precip will develop with a 40-60% probability of 0.05 inch and 20- 40% prob for 0.10 inch, based on the NBM. Light mountain snow might accumulate up to 1 inch of snowfall Sunday over the WA/OR Cascades and southern parts of Wallowa County (15-30% probab). Snow levels for Sunday will be around 4.5-7 kft at the pass levels. Same for portions of eastern mountains (Grande Ronde Valley, Southern Blue Mountains-OR, and John Day Highlands) and Northwest Blues, but with less than 0.5 inch. As this system continues Monday, snow chances will slightly increase for those areas with snow levels dropping to 2.9-3.4 kft. Sustained southwesterly winds will become breezy Sunday afternoon into early Monday morning at 10-15 mph with gusts up to 25 mph from the tighten surface pressure gradients (>50% confidence). Feaster/97

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/

Variable conditions currently across all sites, with VFR at KBND/KRDM, MVFR at KDLS/KRDM/KYKM/KYKM/KPSC due to reduced ceilings of 1500-2500 feet. These conditions are expected to improve slightly into the early evening to VFR as ceilings lift to 15-25kft. LIFR/IFR conditions are expected to return tonight and through the overnight period for the majority of terminals, but KRDM/KBDN will drop to LIFT overnight as fog develops and visibilities drop to 1/2SM. KYKM will also degrade to LIFR, but in relation to ceilings of 300-400 feet. IFR expected at KPDT/KALW due to visibilities of 2SM with mist. Light rain also expected at KDLS/KYKM toward the end of the period. Winds will stay light at below 10 kts at all terminals due to lingering high pressure. 75

Preliminary Point Temps/PoPs

PDT 34 53 43 53 / 0 20 60 80 ALW 37 53 45 52 / 0 20 70 90 PSC 35 50 41 52 / 0 30 50 80 YKM 34 47 37 47 / 0 40 50 60 HRI 34 49 42 52 / 0 20 60 80 ELN 33 43 34 42 / 0 60 50 60 RDM 31 56 39 52 / 0 10 40 90 LGD 35 57 42 50 / 0 10 60 90 GCD 34 58 42 52 / 0 10 40 90 DLS 38 48 44 49 / 0 60 80 90

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Air Quality Alert until noon PST Saturday for ORZ506-509.