The heat wave is forecast to slowly come to an end over the next couple of days but fire danger will increase as it makes its way out. This has prompted the National Weather Service to issue a Red Flag Warning in effect from 12:00 Monday to 21:00 Tuesday for the combination of low humidity and breezy conditions in the Columbia Basin. The Excessive Heat Warning remains in effect until 23:00 Monday.
To the south, red flag warnings extend from the Blue Mountains to Northern California for dry thunderstorms. Monsoon moisture from the Desert Southwest is making it all the way to the Pacific Northwest. Flooding has occurred in several locations, including Flagstaff, Arizona, but by the time it makes it to the Pacific Northwest enough water has been dropped that the thunderstorms are much drier. A few thunderstorms may stray into low elevation areas, but if this occurs it should be very isolated.
The McKinney Fire in Northern California was likely started by thunderstorm activity late last week. Since Friday, it has spread to over 50,000 acres and blanketed Southern Oregon in dense smoke. Some of this smoke as been wafted northward, causing haze in the Tri-Cities area. This can be expected to continue for the next several days but should remain well above the surface.
As noted above, the fire danger for the Columbia Basin comes from the combination of wind and low relative humidity. The ridge of high pressure driving the heat over the last week is forecast to weaken, leading to cooler temperatures west of the Cascades. This will lead to a cross-Cascade pressure gradient bringing strong winds through the mountains. It won’t be an exceptional wind storm, but gusts to around 30 mph with relative humidity below 20% can be expected.
The strongest wind speeds are likely in and downwind of major mountain gaps, such as the Columbia River Gorge and Kittitas Valley. As temperatures cool into the overnight hours, humidity will increase and wind will slowly decrease to lower the fire danger but things ramp up again Tuesday afternoon.
This diurnal wind pattern of breezy spots in the afternoon and most folks fairly calm in the morning, which is a major part of wind energy generation in the region this time of year, is likely to continue most of this week. Fire danger should decrease despite this as highs in the 90s Wednesday and beyond will allow for slightly higher relative humidity values and a bit less wind.
Be sure to use caution, especially around dry grasses and sagebrush. Don’t park in tall grass, don’t throw cigarette butts on the ground, and postpone outdoor burning until later this week.
