Return of Smoke, Fog, and the Inversion Layer

FastCast—Wednesday, Oct. 12 to Saturday, Oct. 15:

The much-needed clean air will come to an end across the Puget Sound area on Wednesday and Thursday. High pressure will return, bringing offshore flow, the return of spotty surface smoke, and likely the return of an inversion, likely bringing back fog and persistent low clouds. With an inversion in the forecast, high temperatures can vary wildly from the forecast, as much as 5-15 degrees cooler than the forecast. So, keep that in mind while reading the FastCast. The inversion signal (details below) is weaker on Wednesday and quite strong on Thursday and Friday, so there’s a higher chance of fog to end the work week. Not factoring in the inversion, expect Wednesday’s highs in the upper 60s to low 70s. Thursday through Saturday will be warmer, with highs reaching the low to mid 70s. Lows through Saturday (regardless of the inversion) will be in the mid to upper 40s, a sign that it’s fall. Surface smoke is also expected, with light smoke spreading across most of the area by Thursday. Be prepared to take precautionary measures with degraded AQI possible. At the end of the blog, there is some good news for rain-lovers!

———————————————————

Special Weather Note: Wednesday, October 12th, marks the 60th Anniversary of the 1962 Columbus Day Storm, by far the worst storm ever to impact the Pacific Northwest, and one of the strongest non-tropical cyclones to ever hit the United States. UW Atmospheric Sciences professor Cliff Mass has a very informative blog about the storm, linked below. It is worth a read to understand this historic event.

Cliff Mass Weather Blog: The 60th Anniversary of the Northwest's Biggest Storm of the Last Century: The Columbus Day Storm

———————————————————

Continue reading the full blog below!

Tuesday was the first day in over a week with air quality in the “good” category across the lowlands. Unfortunately, that won’t last much longer. As a high pressure ridge rebuilds over the region, flow will slowly turn more easterly, with AQI likely degrading as smoke slowly moves into the area. 

Surface smoke will be more prevalent north of Seattle on Wednesday, and will extend through most of Puget Sound by Thursday evening (see HRRR forecast below).

This will likely bring AQI into the “moderate” or “unhealthy for sensitive groups” categories. Also notice the localized areas of heavy surface smoke in the Cascades, Eastern Washington, and now near Vancouver, WA. A new wildfire, called the Nakia Creek Fire, is burning in very steep terrain approximately 9 miles NE of Camas, WA. This fire will continue to grow from its current 250 acres. 

With the return of the high pressure ridge, an inversion layer is likely again across the lowlands. For a refresher, here is a very helpful graphic from a NC State University study explains the inversion layer.

Back in the FastCast, I mentioned that inversions can wildly alter high temperatures. This graphic shows why. When an inversion is persistent and ends up lasting most of the day, the clouds separating the warm air (inversion layer) from the cooler air below keep temperatures 5-15º below the forecast.

So…how can meteorologists predict inversions? It all comes down to soundings. Soundings are graphical cross-sections of the atmosphere, showing everything from temperature and dewpoint to key factors in predicting thunderstorms and tornadoes. For this blog, just pay attention to the yellow box.

The sounding below is from the European model (via Pivotal Weather), and it is for 8 AM Thursday. 

Inside the yellow box, notice the red and green lines. Green represents dewpoint and red represents temperature. At the bottom of the graph, red and green are very close together, indicating that temperature and dewpoint are very close to each other. This represents the “cooler air” layer near the surface on the inversion graphic.

Now, as we go up the graph, look at how the red and green lines rapidly separate. That is the clue to seeing an inversion layer. Where the lines rapidly separate is the clue to the actual inversion layer of warm air. 

Below the inversion layer, potentially on Wednesday but likely on Thursday and Friday, expect fog and low clouds, with dense fog at times.

Now, let’s take a look at the extended outlooks from the NWS Climate Prediction Center. Below is the temperature outlook for October 17 (Monday) to October 21 (next Friday).

Most of Western Washington is under the 70-80% probability of above average temperatures, with Eastern WA, Eastern OR, and Western ID under the 80-90% probability. 

The precipitation outlook for the same period (October 17-21) is below.

Western Washington has a 33-40% probability of below normal precipitation, still not great, but much better than recent outlooks. Eastern Washington has a 40-50% probability of below normal rain. 

Now…for the good news promised in the FastCast. Below is the precipitation outlook for October 19 (next Wednesday) to October 25 (the following Tuesday).

For the first time in months, there is a 40-50% chance of above average precipitation over much of Washington state!! This signals a shift, present in long-range forecast models, toward a cooler and more typical fall pattern. While details are much to fuzzy to be certain, this is good news for putting out fires and returning to normal weather.

Stay tuned!