Synopsis

Dry weather is expected through Wednesday, with variable cloud cover and mild temperatures. Cooler weather and increasing precipitation chances arrive Thursday through the weekend.

Discussion

Monday night through Wednesday: An upper-level low currently off the PNW coast around 130W is currently providing the Inland Northwest with widespread middle and high level clouds as of Monday afternoon. This low will dig southward off the California coast Monday night into Tuesday morning, allowing subtle ridging to build into the Inland Northwest. Temperatures trend warmer on Tuesday with less cloud cover. North winds will increase through the Okanogan Valley Monday night through Tuesday morning as high pressure builds into British Columbia and strengthens the CYLW-KPSC pressure gradient to +4-6 mb. This will support sustained winds 15-20 mph and gusts 25-30 mph down the Okanogan Valley Tuesday morning into the early afternoon. Winds will also increase, though not as strong, out of the northeast through the Purcell Trench in north Idaho with sustained winds 12 to 18 mph. Temperatures on Wednesday will be similar to Tuesday, but with lighter winds as the north-south pressure gradient decreases.

Thursday through Sunday: Changes in the weather pattern arrive Thursday as the closed low over California begins to eject northeastward and a frontal system with a trailing shortwave approaches from the west. While the frontal system looks mostly dry, moisture and precipitation associated with the upper-level low moving into far eastern Washington and north Idaho will bring a 30- 50% chance of rain across far eastern Washington and a 50-80% chance across north Idaho. Additionally, winds will be breezy to gusty from the northwest to west with the cold front. With the KPDX-KGPI increasing to +12-17 mb Friday morning, there is high confidence for wind gusts greater than 30 mph across central Washington and southeast Washington (60-80% chance) with a 30-50% chance across the Spokane area. Blowing dust is not anticipated to be a significant hazard given the lack of dust generation during recent comparable wind events. However, this will mark the trend toward cooler daytime high temperatures.

By the weekend, confidence in the weather pattern significantly decreases regarding the strength and track of the next approaching weather system. Ensemble analysis reveals considerable variance: 42 percent of global ensemble members project a deeper trough influencing the Inland Northwest with cooler, wetter conditions. Conversely, 25 percent maintain an offshore closed low, and 33 percent depict a flat ridge with a weaker system traversing northern British Columbia. Both of these latter scenarios favor a warmer, drier sensible weather outcome. AI ensembles provide little consensus, as the GEFS AI ensemble favors the cooler and wetter solution (which is the opposite of the physics-based models) while the ECMWF AIFS ensemble indicates only subtle mean troughing, highlighting the uncertainty for the weekend. /vmt