Reposted from the Cliff Mass Weather Blog
Monday 23 November 2020
One reason research is so fun is that every now and then you learn something important that is unexpected. A new and very important finding.
I recently had such a “wow” experience about the September forest fires that caused so much damage and smoke on the western slopes of the Cascades.
I currently have a research grant from the National Science Foundation and smaller grants from the USDA Forest Service and WA DNR to work on forest fire / weather issues. Recently, our group completed three papers on the intersection of weather / climate and forest fires: the Wine Country Fires of 2017, the Camp Fire of 2019 and the Diablo Downslope winds in northern California.
I was working on a paper on the meteorology of the great wildfires in western Oregon / Washington when the great wildfire siege happened on Labor Day Oregon. So of course my group turned around to understand the event.
Why did this major event happen? How did it develop? Was something unusual going on? How well was it forecasted? These and other questions were on the table.
We knew that the strong easterly winds (from the east) during the event were critical to starting and / or spreading the fires. In fact, my early work indicated that ALL major fires on the western slopes of the Oregon and Washington Cascades were associated with strong easterly winds.
I asked research meteorologist David Ovens to take a look at the upper air weather observatories in the region, places where weather stations (radiosondes) are launched twice a day to bring us winds and other weather variables up.
Of particular interest was the radiosonde data from Salem, Oregon, at the first standard altitude above the surface (925 hPa – about 800 meters above the surface). This altitude is very relevant to winds observed over the nearby western slopes of the Oregon Cascades.
The Salem record goes back 64 years to 1956, long enough to tell us a lot about how unusual the situation was that September. It didn't take long for Dave to send me a character with the information I requested and I had my wow moment.
Below is the picture. Let me explain
I asked him to only record the winds at 925 hPa (again about 800 m above sea level) and to limit the analysis to August and September, since these are the months of major historical fires on the western slopes of the cascades. Every observation during the 64 years during these months was recorded with the associated wind direction indicated by the x-axis and the wind speed indicated by the y-axis.
During these late summer months, when the wind is strong, you will notice two main peaks: (1) north to north-northeast and (2) south to southwest. The north wind peak occurs when high pressure builds over the eastern Pacific, and the southwest winds occur when a strong trough or system of low pressure approaches the coast. The southwest winds are the strongest (up to 51 knots!), But they are associated with clouds and rain, so little fire hazard from them.
I asked Dave to mark the observations made during the Oregon firestorm period with red dots – and then the wow moment came. Look at the red dot for September 8 at 1200 UTC – 5 a.m. (2020-09-08 12z) – just as the fires over western Oregon were accelerating (indicated by a red arrow).
Simply extraordinary. The winds at this time were the strongest ever observed at the site for any wind direction from north, east, southeast or northwest during these months. The stronger the winds, the greater the potential for rapid fire growth and the greater the potential for fire ignition from failure of electrical infrastructure and other causes. The important thing is that these were by far the strongest winds from the east and thus slope on the western slopes of the Oregon Cascades. Downhill winds from the east are inevitably very dry and the longer the air blows from that direction the drier it gets.
Why were the east winds so strong?
Our research identified the reason: unusually cold air and accompanying high pressure moving east of the cascades.
The pressure analysis on September 8th at 5:00 AM is shown below, showing a measure of the difference from normal by colors. The high pressure was concentrated in Idaho and extended to eastern Oregon. The colors indicate that the print was VERY unusual – up to 4-5 standard deviations from the mean for that date. To give you an idea of how unusual a deviation from 4.5 would be, this would indicate an event that would occur once in 147,000 cases.
The air was so cold it brought record chill and snow to the Front Range of Colorado and the surrounding area.
So the whole situation is ironic and interesting: record cold in the east brought record fires in the west.___________________________________