Sourdough Fire Rises After Rainfall; Smoke Impacts Focused on Westside through Tuesday

 A critical week of fire weather lies ahead 

After widespread wetting rains fell across Washington early last week, it was discouraging to see the Sourdough Fire start producing smoke again yesterday. Last Monday was a great reminder that the damp greyness of Autumn is on the horizon, but fire season is still upon us and will be for the better part of the next month and a half. A mid-August heatwave begins today across the Pacific Northwest, with very hot and dry conditions expected through the workweek. The Northwest Coordination Center raised their preparedness level to 4 (out of 5) this weekend, indicating that an increase in fire starts is likely and that firefighting resources are starting to be stretched a bit too thin. It is crucial that we all work together to limit any new sparks this week! 

Fire danger will evolve in two stages this week: First, areas west of the Cascades will see a peak in fire danger Sunday-Wednesday morning. After Wednesday, onshore flow will return west of the Cascades and critical fire danger will shift east of the Cascades. This also mirrors smoke impacts for the week. Red Flag Warnings are flying for the west slopes of the Cascades through at least Monday evening. 

As of Sunday morning, the only fire-of-concern from a smoke perspective in Washington is the Sourdough Fire burning above Diablo Lake in the North Cascades. 

Prevailing winds through Tuesday evening will be out of the northeast across the Cascades and out of the north through the Puget Lowlands. The poorest air quality will be in the morning hours in the immediate vicinity of the fire and southeastward through the Skagit Valley. AQI values each morning will range from Very Unhealthy near the fire in Newhalem to Unhealthy for Sensitive Groups in Marblemount to Moderate in Concrete. 

Ceilometer data from Puget Sound Clean Air shows that the smoke exiting the Skagit Valley and spreading to the east and south in the Puget Lowlands is primarily above 3,000 feet, which will limit air quality concerns to Moderate in the I-5 corridor north of Seattle, though smoke will likely be visible in Whatcom, Skagit, and Snohomish counties.

During the afternoon hours when the fire is producing the most smoke, mid-level winds will transport elevated smoke southwestward toward Lake Chelan and the Methow Valley. Through Tuesday evening, the most significant impacts east of the fire will be for Stehekin, where AQ is likely to reach Unhealthy for Sensitive Groups. Two fires burning just north of the Canadian border will transport primarily elevated smoke across far NE Okanogan County and the Methow Valley through the middle of the week. 

The model image below shows total smoke transport through Sunday evening, with similar conditions expected through Tuesday evening. 

Unfortunately, with critical fire expected west of the Cascades through the midweek period, new fire starts are also likely. Smoke from any new fires will generally be transported southward and westward across western Washington. By Wednesday morning, westerly winds will return across Washington and smoke concerns will shift to eastern Washington. The Methow Valley will begin to see more significant smoke impacts from the Sourdough Fire by Tuesday night, with westerly winds continuing into the late-week period. There will be an update to the Smoke Blog on Tuesday to provide more details and decipher impacts from any new fire starts! 

Matthew Dehr

Wildfire Meteorologist

Washington DNR