Severely stubborn smoke situation starts to subdue in some spots

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It wasn’t supposed to be this way.

Space Needle webcam at noon Tuesday 9/15/2020

Despite repeated predictions of improvement in the smoke, it remains as thick as ever near the surface over almost all of Washington State.

Many of the more optimistic smoke forecasts were based on the HRRR-smoke model, which continues to paint an unrealistic picture of the near-surface smoke situation.

A very bad portrayal of the current near-surface smoke from the HRRR-smoke model.

The PurpleAir map shows that the AQI is still poor in most places.

PurpleAir PM 2.5 AQI (10-min average) over WA state at 12:30 PM PDT Tuesday 9/15

But not all places! There is some noticeable improvement in a few spots–the AQI at Lake Quinault is down to 61 and a number of sites on the east slopes of the Cascades have moved into red and orange territory.

What is happening? Since last week, a strong inversion has been present in the lowest ~5,000 ft of the atmosphere. Inversions are stable layers (warm air above cold air), so they are resistant to vertical mixing. The situation is further exacerbated by the sun heating the top of the smoke layer, which only strengthens the inversion.

However, as the low pressure system offshore moves closer, the upper level winds have brought in cooler and cleaner air and the inversion has weakened.

On Tuesday afternoon one could see blue sky at Hurricane Ridge on the Olympic Peninsula. The stubborn smoke layer can be clearly seen in the valleys.

Hurricane Ridge view around noon Tuesday. Blue sky is visible with smoke in the valleys.

Unfortunately, this cleaner air has struggled to mix down to the surface. A big part of the problem is that the low pressure system bringing us the scattered clouds/rain is still sitting offshore, inducing a weak offshore-directed pressure gradient that is preventing any cleaner air from moving in from the Pacific Ocean. The air is actually quite clean just offshore and the AQI has improved quite a bit on portions of the CA/OR coast. But without an onshore-directed pressure gradient it can’t move inland.

Visible satellite image on Tues afternoon–note the cyclone offshore.

However, there is another pathway to cleaner air, which is occurring this afternoon. With partial cleaning over the mountains, the sun appears to have broken through the inversion and started to mix out the smoke. This is only happening over the mountains, which extend above the lower smoke layer (see Hurricane Ridge webcam).

These terrain-induced circulations have started to improve the air quality at sensors near mountain valleys. Lake Quinault on the SW side of the Olympic Mountains is the best example. The AQI is down to 60 (moderate)!

Much-improved air quality at a PurpleAir site on Lake Quinault. However, there is no improvement just a few miles away on US 101.

Unfortunately, the air quality has not improved at all just a few miles away on US-101, or at Hoquiam on the coast, where the wind is still directed offshore.

So we have an escape, at least in spots, and at least for a few hours.

This process isn’t helping much in the lowlands away from mountains. And after the sun sets this evening the solar-induced mixing will come to an end and the smoke will settle back into the valleys. Even worse, another wave of upper-level smoke will move in from the south by Wednesday morning.

HRRR-smoke forecast for early Wednesday morning

So no widespread relief is expected yet. But eventually the low pressure system will move toward us and the flow will shift to westerly. Hopefully by the weekend. For real this time.

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