FXUS66 KPDT 262325 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 425 PM PDT Fri Jul 26 2024

Updated aviation discussion

SHORT TERM
Tonight through Sunday night...Satellite imagery reveals widespread smoke and haze across most of eastern Oregon and eastern Washington, the result of regional wildfires which continue to burn. Have extended smoke and haze in the forecast through Sunday morning based primarily on HRRR smoke output.
While smoke and haze will likely linger across the region, especially in the vicinity of ongoing wildfires, have opted to exclude mention in the gridded forecast due to uncertainty in the evolution of fires and smoke output.

Weak troughing over the Pacific Northwest will keep near-normal temperatures and locally breezy westerly gap winds in place this weekend. Saturday, a weak disturbance tracking across southeast Oregon will bring non-zero, but low (<5%) chances of showers or thunderstorms across the southern Blue Mountains during the afternoon. A concurrent shortwave tracking across northern Washington appears mostly dry with the main consequence being the wind.

LONG TERM
Monday through Friday...Models continue to be in good agreement in having a trough over the Pacific Northwest with a ridge over the central CONUS Monday and Tuesday. Tuesday afternoon into Wednesday, a front moves through the area and carries the trough eastward while closed low develops out in the eastern Pacific. Models show general agreement in having a westerly zonal flow overhead Wednesday though about 40 percent of model ensemble cluster members begin having the ridge building into our area.
Thursday and Friday, model ensemble members all have the ridge building and strengthening over our area with temperatures warming well above normal to around 100 degrees by Friday.

The Extreme Forecast Index highlights Precipitation amounts along the Washington Cascades Monday and Tuesday with values of 0.72-0.82 and some shift of tails values indicating a possibility of heavier rain. It also highlights breezy winds Monday afternoon in central Oregon and portions of the Columbia Basin with a value 0.73. Cooler than normal temperatures are highlighted Monday and especially Tuesday due to cooler air with the front.

Monday will see the trough coming ashore with a slight chance to chance of showers developing the Cascades in the late morning and afternoon then spreading into the rest of the area Monday night.
The Cascade crest could get up to a half of rain but amounts will be fairly light away from the crest. Models are not showing enough instability for thunderstorms and the timing may be too late (evening) for convection in the eastern mountains, so just have showers. With the front, winds will increase to 10 to 20 mph over most of the area. Model probabilities show a 25-55 percent chance of 20 mph sustained winds, but chances of 25 mph sustained winds drop to below 15 percent and that is just in the Columbia Gorge and Kittitas Valley. Temperatures Monday will be similar to Sunday with highs in the mid 80s to around 90 in the lower elevations.

Tuesday will see the trough overhead and then moving off to the east Tuesday night. This will be a more favorable day for showers and a slight chance of thunderstorms in the eastern mountains though amounts will be just a few hundredths of an inch for the most part. Showers and thunderstorms will end quickly in the early evening. The Columbia Basin will again have 10 to 20 mph winds but winds will be lighter in central Oregon. Temperatures will drop a few degrees to the lower to mid 80s with 70s in the mountains.

With ridging beginning to develop Wednesday, foresee dry weather and light winds with temperatures warming to near normal in the upper 80s to lower 90s. The ridge will strengthen further Thursday with temperatures rising to the mid to upper 90s and in the mid 80s to lower 90s in the mountains. NBM probabilities show a 20-35 percent chance of reaching 100 degrees in the Columbia Basin, central Oregon and in the John Day valley.

On Friday, the cutoff low that has been parked out in the eastern Pacific moves closer to the coast and our flow turns more southerly and unstable due to a possible ridge breakdown. There is some chance that this situation could pull some moisture into the area and PWATs are over an inch in the GFS, but at this time models are holding off on any showers or thunderstorms. It looks more favorable next Saturday but that is past the scope of the long term forecast. This will have to be watched during the course of next week. Temperatures will warm further with highs rising to 97-105 in the lower elevations and mainly in the 90s in the mountains. NBM probabilities give a 40-70 percent chance of reaching 100 degrees, but the chances of 110 degrees are less than 5 percent even in the warmest locations. Perry/83
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