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.