Wildfires are often as unpredictable as the weather. In fact, weather is the primary factor that makes the fires unpredictable.
The Washington state wildfires that are still burning – only partially under control – have moved erratically and frustrated firefighters.
The Background On The Washington Fires
The popular perception is that Washington receives a lot of rain — and it does. But not ALL of Washington.
Moisture-laden air sweeps in off the Pacific Ocean on prevailing westerly winds; Aberdeen, on the coast, measures about 67 inches of rain per year.
Seattle, on the east side of the coastal mountains receives about 37 inches. To the east of Seattle are the higher Cascade Mountains, and further east Spokane gets only 16 inches of rain per year.
The current wildfires are just east of the Cascades, in the ‘rain shadow’ of the mountains which have squeezed out much of the moisture on their western, windward side. The area is a virtual desert with rainfall around 13 inches per year. Furthermore, most of the rain in all of Washington state falls in the winter.
Little rain falls east of the Cascades in the summer, and vegetation becomes combustibly dry.
Washington’s Recent Heat Wave
As Decoded Science has often noted, sea surface temperatures well above normal in the Gulf of Alaska have contributed to a persistent warm high pressure area over the Pacific coast for almost a year. Periodically the high drifts far enough east to bring very hot weather to the interior west, including interior Washington state.
The last ten days have comprised a heat wave with temperatures daily in the 90s along with low humidity; this set the stage for the wildfires.
The Hot Air Is Also Dry
It isn’t always the case that low rainfall totals mean the air is dry, but there is a correlation. In the case in which air is flowing over a mountain, most of the moisture has been lost on the windward side, and the air on the lee side is very dry. Last week’s dew points were in the 40s
Current Washington Fires: The Particulars
Two weeks of very hot and dry weather created a tinderbox in central Washington and Oregon on the east side of the Cascade Mountains. Any spark would have set things ablaze, and the sources were natural (lightning) and man-made (a carelessly thrown cigarette)
The variable terrain, with hills,valleys, and canyons, produces locally variable and unpredictable wind currents. In the past few days, gusty winds, variable in direction, have kept firefighters guessing which way the fires would spread.
On Friday and Saturday, a wind-shift line came through that was the remnant of an old front. There was no rain, and most of the temperature contrast had been lost in the mountains. All that was left was a strong pressure gradient (change of pressure with distance). Since wind speed is proportional to pressure gradient, the winds increased.
Where hills and canyons create bottlenecks, the wind can form local currents that can be gusty and unpredictable.
Better Firefighting Weather Is On Tap
The winds have died down and a new cold front is approaching the fire area, this one with cooler temperatures behind it; there also could be some helpful rain, especially on Wednesday.
But even this front is a double-edged sword: The rain could be accompanied by lightning, which could set more fires, and there will be gusty winds again.
When This Fire Is Extinguished, Will Fire Season Be Over?
Probably not. Longer term forecasts indicate a return to the warm high pressure, with temperatures again soaring into the 90s and low humidity.