August 16, 2021

Weak Cold Front Brings Cooler Temperatures, Smoke Relief, But Potential Fire Danger over the Eastern Slopes

 We have just begun a major weather transition for the region....and you will be much more comfortable because of it.

A weak cold front is now moving through the region, bringing clouds to the west and much cooler temperatures.  As shown in this morning's visible satellite image, low clouds dominate the offshore waters and have pushed across the western Washington lowlands and southern BC.  With the switch to westerly (from the west) winds aloft, much of the region is smoke-free, except for locations downwind of localized fires.


Thus, green colors (good air quality) dominated the regional AIRNOW map, particularly west of the Cascade crest. 


 Over the next week temperatures will remain in the 70s in the west and 80s in the east, and relative humidity will rise, lessening fire danger in general.

But there is a wildfire risk today as cool air streams into the west.  The cooler, denser air will result in increased pressure west of the Cascade crest and the development of a large pressure difference (gradient) over the Cascades.   Aa s result, winds will be enhanced over the eastern slopes of the Cascades.   The hot-dry-wind fire weather index (shown below) indicates high values later this afternoon, particularly over the lower Columbia Basin and northern California.


The essential cause of this major weather change is the loss of the big upper-level ridge (which results in warming) and the approach of a well-defined upper-level trough (see the plot of the heights at 500 hPa below--think of this as the pressure at around 18,000 ft).


The upper-level pattern for the next week will be dominated by an upper ridge offshore and cool northerly (from the north) flow over the Northwest (see below).  Every once in a while an upper trough will bring more clouds and a chance of showers.


We have only a month left for the potential of major heatwaves and substantial fires in our region, and the next week will be near normal.

Although there has been considerable talk about this fire season being much greater than normal, the statistics don't really show that.   

Below are the latest numbers from the WA Department of Natural Resources for year-to-date fires.   The burned area (right panel) is only a little above the 10-year mean and I suspect that with cooler weather, we will slide down to the 10-years mean by the end of the month.  We have had more fires than normal (left panel), but many of those are refuse fires from overeager cleanups during spring....and most of these fires were very small.






12 comments:

  1. Hey Cliff -- a friendly request: you often use the term "our region" or "the pacific northwest" and it's not always clear if you mean northwest WA, WA, WA+OR+NorCal, WA+OR+NorCal+ID+MT, or the overall American West. I think this might be a source of some of the push-back you get on posts, where people forget or don't know or can't tell or can't be bothered to check what the scope of the blog or blog post is; e.g. you make a claim about wildfires or average temperatures and cite a study, and people think you are confused or disingenuous because they don't realize your claims are restricted to a certain region. Perhaps that is on them, and they should be more diligent before reacting, but I often find myself unable to deduce which "region" we're talking about in your posts. I think it might help your messages get out if that was more clear. Just a thought, thanks.

    (Also: I'm not saying this is the case for this post.)

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  2. Some relief in the Rogue Valley of southern Oregon would be greatly appreciated. Since June 17, our daily high has averaged 98F, about 10 degrees above normal. On top of that, there have been many days with smoke-filled skies. A Summer from Hell it has been.

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  3. The average fire season figure is not accurate. If thats just dnr land. Total acreage burned in FS land is about double a normal fire year.

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    1. It is accurate. But this is DNR land. Do you have a source to view the USDA forest service land YTD versus climatology....

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    2. The National Interagency Fire Center generates statistics for each state, which are summarized nicely on the Wikipedia page (.../List_of_Washington_wildfires#Year-by-year_statistics). Since the year isn't over, it is hard to tell at this point. The agency does have stats for the fires that are currently burning, which is pretty much all of the ones that generated significant smoke. We have about 286,789 burning, which is relatively small, but again, that doesn't count those that came and went.

      I think overall the assessment by Cliff Mass is correct. Other than the range fires (which don't generate much smoke) it really wasn't a big year for fires in Washington State (although that can change). However, it was a big year for BC fires, and the Northern California/Southern Oregon area, both of which have effected us.

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    3. The number I referenced was acres. Sorry for leaving out such a key word.

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  4. I think it is the case for this post. If I’m not mistaken, BC, OR, and northern CA are all having historically large wildfire seasons. Certainly BC and CA. At least Oregon and BC are sometimes to occasionally talked about on this blog. So to only choose statistics for WA for this post is a bit perplexing. A take of “WA is an island of normal fire season in a sea of above-average fire seasons” seems more accurate for the wider context.

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  5. This comment has been removed by the author.

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  6. How is the smoke situation looking for this week?

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    Replies
    1. I'm wondering about that too. NWS forecast mentions the possibility of BC smoke entering our area.

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  7. I appreciate Professor Mass helping me stay abreast of current weather. After sealing my asphalt driveway during last weeks heatwave. I noticed very dry crisp leaves started to fall over the weekend. Now I am looking forward to some showers.

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  8. I'm so confused by that chart you posted. What is it supposed to include and why don't you link to the info online? If you search for Washington total acres burned in 2020 news sources say around 800k while your chart shows 14k?

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