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Statewide Smoke Forecast through Friday Nov. 16, 2018

Forecast issued: Nov. 15, 2018
Forecaster: James Miller, USDA Forest Service

Air quality across Oregon has been impaired over the past 24 to 36 hours. The smoke is from local sources such as wood stoves and from wildfire smoke from fires burning in California. Determining the exact source can be tricky. As of 11 a.m. on Thursday, Nov. 15, 2018, about three-quarters of the air quality monitoring sites across Oregon were reporting moderate air quality, including most of the Willamette Valley from Eugene north to Portland, most of Southern Oregon from Medford east to Lakeview and Burns, and the Bend area east into the Blue Mountains region. Pockets of unhealthy air quality were reported at six locations, including Baker City, Cave Junction, Salem, parts of metro Portland and Roseburg. Only about 10 percent of monitoring sites were reporting good air quality.

Figure 1. The Air Quality Index at 11 a.m. on Thursday, Nov. 15, 2018.  See the map above for current conditions.

The first question one might ask is, “Why are we seeing widespread smoke impacts in mid-November here in Oregon?” It’s not all from fires burning in the state, rather our air quality is currently impaired partly by smoke from the devastating Camp Fire burning in northern California. High pressure centered over the Intermountain West created off-shore (northeasterly to easterly winds) across much of southern to central California. During the late night hours on Tuesday and early morning hours on Wednesday, winds along the northern California coast and up through Oregon were from the south/southwest, bringing the wildfire smoke from interior California into Oregon. The shorter days of November coupled with the lower sun angle reduce the amount of atmospheric mixing we get in the daytime, thus smoke that infiltrates into valley locations can be stubborn to clear out, as we’ve seen over the past 24 to 36 hours.

Figure 2. Winds at the 850 mb level (approximately 1 mile above the surface) across the western United States on Tuesday, Nov. 13, 2018 at 4 p.m. were from the east and northeast across southern to central California. Winds along the Pacific coastline were mainly from the south to southwest, as indicated by the large red arrows. This weather pattern allowed smoke from California to transport north into Oregon throughout the late night hours on Tuesday into the morning hours on Wednesday.

The next question one might have is, “When will the smoke clear out?” This is a challenging one to answer definitively because much of it depends on the lower to mid-level wind patterns that develop over the next 24 to 48 hours coupled with how much afternoon mixing each specific valley location receives. Based on the smoke and weather forecast models, air quality will generally remain in the moderate category throughout the state through at least Friday afternoon. Late in the day on Friday, an Arctic high pressure system will slide down the eastern slopes of the Rocky Mountains and should produce easterly to northeasterly winds for most of the state which we believe will help scour out the smoke that’s settled into many valley locations. Thus, air quality should begin to improve late in the day on Friday into Saturday morning, though smoke impacts from Pendleton to Baker City and in southwestern Oregon may persist longer than in the Willamette Valley. A key uncertainty is how strong the low-level wind flow will be in more sheltered valley locations.

Figure 3. Near-surface smoke forecast from the AIRPACT-5 model for Friday, Nov. 16, 2018 at 6 a.m.

Figure 4. Near-surface smoke forecast from the AIRPACT-5 model for Friday, Nov.16, 2018 at noon. Smoke impacts are expected to continue through Friday afternoon, especially in western valley locations.