Smoke season starts tomorrow

It was only a matter of time. With a number of moderately-sized wildfires present across the region and no widespread rain in sight, major population centers west of the Cascades have been reliant on onshore flow to keep smoke intrusions to a minimum.

The overnight marine push signaled a change in the weather pattern that will end the run of largely clean air across western Washington and Oregon (excluding local spots close to fires that have already been experiencing “smoke season” for weeks).

The upper-air pattern as of Friday afternoon shows broad troughing over the PNW and an area of low pressure offshore of southern California. At first glance, this pattern does not look favorable for a smoke intrusion, although stronger winds associated with last night’s marine push have helped fan the flames of existing fires.

By Saturday afternoon, the trough over the PNW is expected to progress westward, leaving the PNW in a weak flow regime. Meanwhile, hurricane Hilary will began transitioning into a potent extratropical cyclone that will cause serious and potentially unprecedented flooding issues in southern California.

Within the quiet zone of the PNW, low- and mid-level flow will turn northeasterly just long enough to pull smoke down from southern British Columbia and into the PNW. Here’s the HRRR’s depiction of the evolution from late Friday evening through Saturday night.

Unfortunately, this smoke will be in the lower levels of the atmosphere, so air quality will be impacted immediately upon arrival, by Saturday afternoon.

How long will the smoke be around?

The forecast beyond this weekend doesn’t look much better. The PNW is expected to remain under weak steering flow throughout next week. A few rain showers are possible as well. As of right now, any precipitation originating from the remnants of Hurricane Hilary is not expected to get any closer than Idaho and far eastern Oregon.

On Monday, an area of low pressure will approach the region and bring some temporary relief, especially to coastal areas. However, the low is expected to park in place for a few days, pulling smoke into its circulation and spreading it offshore across the coastal waters of the Pacific and eventually back around onto land.

NASA’s Global Model and Assimilation Office (GMAO) runs the GEOS-Chem chemistry model on top of the GFS, allowing us to get a rough idea of what the smoke situation might look like next week. I’ve found that the CO BB (Carbon Monoxide Biomass Burning) product is a decent proxy for vertically integrated smoke.

By Monday, the smoke situation looks to improve west of the Cascades as another marine push moves in. However, low pressure centered around Vancouver Island is also expected to pull more smoke from BC westward over the Pacific.

By Tuesday, the smoke continues to travel counter-clockwise around the low pressure system, approaching the coast from the west:

Next Wednesday looks the worst of all, with smoke fully entrenched just about everywhere.

These longer-range forecasts should be considered very rough approximations, but it’s hard to see the smoke going away with weak steering currents expected to remain in place through the end of August.

In past years, we’ve seen that smoke is extremely difficult to clear out once it gets entrenched under the marine inversion on the west side of the Cascades.