Marine Weather and Tide Forecast for Pasco, WA

FXUS66 KPDT 221140 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 440 AM PDT Thu May 22 2025

Updated Aviation Discussion

SHORT TERM
Today through Saturday...Nighttime satellite and radar imagery shows a shortwave trough lifting a west-east oriented band of showers across far northern OR and southern WA, while partly cloudy skies have developed across the southern third of the forecast area.

A shortwave trough will continue to slide across the PacNW today while splitting into two distinct waves, with the stronger vort max associated with this system skirting just south of the forecast area; the weaker of the two shortwaves will meander across the southern half of WA. The current band of showers will continue to gradually lift north across the forecast area this morning, but will take on a SW to NE orientation as the shortwave trough pushes further inland into the early afternoon. CAMs are favoring some potential clearing across portions of the Blues, Strawberrys, and Wallowa county early in the afternoon, which would allow for weak instability to develop in these areas. This will result in a slight chance (15-20%) of isolated thunderstorms to develop across the aforementioned areas this afternoon into the early evening. Both shortwaves will exit the forecast area late this evening, with shower activity area-wide coming to an end before midnight.

Friday, a relatively weaker and drier shortwave trough will move across the western CONUS, with the main shortwave energy associated with this system staying well to the south of the PacNW. Nonetheless, increasing surface instability into the afternoon will combine with weak lift from the passing shortwave to result in slight chances (15-25%) of light showers and isolated thunderstorms. As daytime heating comes to a close and the shortwave trough exits the region into the evening hours, shower activity will diminish area-wide. Through Friday evening, the best chances for accumulating precipitation will be confined to the mountain areas, where there is a 55-70% chance that the Cascade crest and eastern mountains will see at least 0.1 inches of rainfall in a 48-hour period.

Saturday, upper level ridging will build into the PacNW with dry and quiet conditions. The region will see temperatures about 5 to 10 degrees warmer than Friday, with a 70-80% chance that temperatures will exceed 80 degrees in the Columbia Basin/Gorge, Yakima valley, and the John Day Basin. Lawhorn/82

LONG TERM
Sunday through Wednesday...Sensible weather concerns through the extended forecast will revolve around the passage of an upper shortwave trough that will trigger shower and isolated thunderstorm activity across the region late Sunday through Monday. Confidence in forecast beyond Monday wanes as disagreement amongst ensemble members grows in regards to potential ridging/troughing influencing the region.

Upper level ridging will persist across the PacNW into Sunday, with the ridge axis pushed into the northern Rockies by an upper trough offshore. A warming trend will continue through Sunday afternoon as well, with temperatures in the mid to upper 80s in the lower elevations and a 45-55% chance that the Columbia Basin warms into the 90s. Increased daytime heating and a developing southwest flow aloft will help in increasing surface instability across much of central and northeastern OR as well as along the northern Blue mountains/foothills in WA for Sunday afternoon. As a result, a slight chance (15-25%) of isolated thunderstorms will be possible across the aforementioned areas Sunday afternoon into the evening. Ensemble cluster guidance is in decent agreement that shower activity will continue overnight Sunday into Monday as the shortwave trough lifts across the PacNW. About 60% of ensemble members do favor weak CAPE values (100-250 J/kg)
redeveloping over the eastern mountains and portions of central OR Monday afternoon, providing another afternoon/early evening of isolated thunderstorm chances (10-20%).

Tuesday into the middle of next week, ensemble guidance comes into disagreement on whether upper level ridging building back into the region will keep shortwave trough features offshore or not.
Light convective showers and isolated thunderstorms are still favored each afternoon over the eastern mountains and OR Cascade crest, but ~40% of ensemble members indicate more widespread shower chances into Wednesday with a trough passage. At the moment, overall confidence in forecast beyond Monday is low (15-25%), though low-mod confidence (25-35%) that isolated thunderstorm/shower activity will develop in the afternoons over mountain zones. Lawhorn/82

AVIATION
12Z TAFs...VFR conditions to prevail through the period. CIGs of bkn-ovc to remain AOA 7kft to 12kft AGL through the period, with periodic few-sct as low 4kft AGL through the late morning (confidence 60-80%). Light rain showers will periodically impact sites DLS/YKM through 16Z, and site PSC through 13Z. Winds will be 12kts or less through the period. Lawhorn/82
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