NZUS96 KOTX 212248 LLLOTX IDZ001>004-026-027-WAZ031>038-041-043-044-047>049-221048- Public Information Statement National Weather Service Spokane WA 248 PM PST Thu Nov 21 2024 Thursday night through Friday: The Inland Northwest will remain in a broad southwest flow aloft into the weekend as an upper level trough remains nearly stationary off the offshore. The surface low that brought the significant weather to western and central Washington continues to weaken and drift to the northwest as weak waves bring showery weather into the region. With an upper level ridge centered over the Aleutians, this low pressure will remain within the confines of the broad trough . Meanwhile, the next system expected to move through the Pacific Northwest has started to rotate through the base of the upper level low and will approach the coast this evening. As the showers this afternoon near their demise, the next round of precipitation will quickly begin its entrance into eastern Washington and north Idaho around midnight. A wave that is currently located about 40N, 135W will strengthen over the next couple hours as it approaches the northern California coast. By this evening, models show this low traveling up the Pacific Northwest coast through Friday morning and remaining offshore Vancouver Island through the weekend. While this low travels up the Pacific Northwest coast overnight tonight, it tap into the moisture plume that has been aimed into northern California and transports more warm, moist air into the region. Current PWAT Values across the Inland Northwest in the 0.45 to 0.60 inch range will increase to 0.75 to 1.20 inches as this plume surges northward. This will bring widespread moderate to heavy precipitation tonight through Friday. Similar to the previous low pressure system, this low will strengthen the easterly pressure gradient across Washington and initially focus strong upslope flow into the east slope of the Cascades until the flow shifts back to the southwest by early Friday afternoon. By Friday afternoon and evening, precipitation will continue into far eastern Washington and north Idaho through much of Saturday. Given several days of warm southwest flow into the region now, snow levels have come up above the valley floor for most locations. The exception will be for the Methow Valley which typically hangs on to colder air much longer than any other valleys in our area. With that said, the Methow Valley Airport is currently sitting at 35F/34F right now. Model soundings suggest precipitation to begin as a rain/snow mix and transition to a moderate to heavy snow between 5/6AM and 10AM on Friday. Given the temperature profile, this snow will likely be very heavy and dense before transitioning to rain by the late morning. Rain Amounts/Impacts: With this next swath of precipitation, the ECMWF Extreme Forecast Index is highlighting central Washington (including the Columbia Basin) as an anomalous event with regards to QPF during this timeframe. Current probabilities for over a half inch of liquid through Friday is about a 40 to 50 percent chance. Further east, the NBM is giving far eastern Washington and north Idaho a 80 to 100 percent chance for over a half inch. For over an inch, far eastern Washington (Spokane, Pullman) have a 10 percent chance and northern Idaho has 20 to 30 percent chance. Following the wet period, this additional precipitation will result in steep rises over small creeks and streams. Places near steep terrain will be susceptible for rock and mud slides. Saturday: Precipitation will gradually decrease from west to east as a frontal boundary pushes across the region early Saturday, except for over north Idaho where moist upslope flow into the Idaho Panhandle support showers through the entire day. As the front moves through, snow levels will fall to 3000-3500 feet and transition precipitation to snow across the mountains. Snow will struggle to accumulate on roads initially, but wintry driving conditions may be experienced over the higher mountain passes through the day. /vmt