Updated September 27,2017
So what was that storm that hit us on May 4th, 2017 ?” Was it a record-breaking severe thunderstorm, a tornado, a microburst or something else? The answer to that question is yes to all but the tornado part. Depending on where you live, you experienced all or some of these nasty effects from a series of storms that dozed over us on May 4th. The first and most powerful storm approached the area southeast of Olympia and tracked toward Dupont. An area about ten miles wide experienced what were likely 60 to 80 mile per hour winds and truly record breaking rainfall. The storm produced over 600 lightning strikes from Chehalis through DuPont. Some people reported hail the size of half dollars just south of Lacey.
If you were in Lacey you likely saw all of these things and were wondering if perhaps a tornado didn’t touch down because of the severe damage that occurred in some areas. The National Weather Services has reviewed the evidence and they have determine that it was not a tornado but rather a Derecho or a Straight Line Wind event. This is a downdraft produced by major thunderstorms that causes air currents to rush outward below a thunderstorm. Most of the damage patterns suggest that this is the case. Snapped off trees and power poles indicate that this downdraft was a potent blast and very rare for our area.
The amount of lightning was incredible. There were multiple reports of ground strikes all over the area. The rainfall was also extraordinary. The highest rainfall rates were calculated by King 5 Doppler radar at 14.36 in/hr.! Luckily the intense rain only lasted 10 minutes or we would have had very serious flooding issues and possibly some very dangerous
The storm covered a large area but its effects were very different in Olympia than they were further east toward Lacey and southeast of Lacey. The city of Olympia saw an incredible lightning show and received very heavy rainfall but did not experience winds.
The scientific explanation is as follows:
On May 4, 2017 an unusual weather anomaly came together which included a tropical southerly flow from the south, combined with a high resident humidity, surface temperatures in the upper 70s and the key, and ideal wind shear in the upper atmosphere caused by the jet stream. These factors led to an unusually high convective index, or potential energy available to move moisture and energy from the surface to the upper reaches of a cloud.
The towering thunderstorms reached well into the Stratosphere, over 40,000 feet above the ground. That is a very rare event for Western Washington where most thunderstorms are truncated by westerly shear winds before they can explode above 30,000 feet.
That was the setting for what came next: One of the most severe thunderstorm events in most peoples’ memory.
Officially what happened was called a “Microburst” or “Straight Line Winds” or “Derechos”. These are very strong downward plumes of air that descend from high up in the thunderstorm. They are generally cold so they accelerate until the hit the ground then they spread out in arcs in a straight line below the cloud – hence the name. These can pack a punch as dangerous as a tornado. This is why many people thought tornados touched down. There is some anecdotal evidence that there were possibly some funnel clouds that may have briefly touched down but it was not substantiated by the National Weather Service who conducted an investigation.
The winds and other effects created by these storms are distinct in their pathway across the land. They have several characteristics that identify them as Straight Line winds:
- They have a distinct pattern of damage that indicates wind was directional (as opposed to a tornado which has a swirl pattern with debris being scattered in every direction equally. The direction of downed trees and powerlines indicate a series of focused blasts of wind blowing from the southwest to the northeast. This is very consistent throughout the damage areas along the path of the storm.
- Indication of violent directional shear winds. Evidence for this is the snapping of trees and power poles many feet off the ground as if there was a hand grabbing a tree and forcing it down and to the side strong enough to snap the trunk. This was clearly evident throughout the path of destruction between Tumwater and Nisqually.
In Lacey along Yelm Highway several old growth trees mainline transmission poles were snapped and toppled. - Radar Data indicating directional winds moving at high speeds or plumes dropping quickly from the storm clouds. These Storms can and did generate directional winds approaching one hundred miles per hour. There is also a clear direction of the storm’s movement. The May 4 storm encompassed an areas about 10 miles wide and about 20 miles long (around 200 sq miles!), from south of Tumwater to Tenino and travelled Northeast through parts of Lacey and Nisqually.
The winds come from a conveyor belt-type convection mechanism. Warm and moist air accelerates up the front or back side of the thunderstorm high up into the storm’s core and emerges in the upper atmosphere where it cools very rapidly and begins to fall back to the surface. Its moisture freezes into ice and rubs against surrounding particles creating a huge static electric field which results in the fierce lightning generated by this storm. It’ like the lightning balls (Van de Graaff generators) you may have seen in school that create a static charge when the elastic belt is running. They discharge the electricity from the metal ball at the top in an arc several inches or up to a foot long. I used to love those things in science class.
It is the same principle that generates lightning within thunderstorms but on a huge scale, using ice and wind as the ingredients to generate electrical charges. When the charge gets large enough it leaps to an area in the cloud or to the ground to equal out the charge. That is lightning.
In the graphic below you can see the general structure of the type of storm we saw on May 4, 2017. The conveyor belt of wind moves the moisture (rain and ice) rapidly up in the cloud and this is whet generates the Van de Graff type of static generator. The air is then cooled after it rises very high in the sky then it sinks rapidly back to the earth because of gravity. It blasts out of the base of the storm as a straight line wind event.
The combined effects of this storm: The lightning, the high winds and heavy rains contributed to the destruction of numerous homes and structures and the downing of hundreds of large old growth trees. It is estimated that the cost of the storm in Thurston County alone was over $20 million.
Climate Change and Wild Weather
Many people are asking if this type of storm is related to climate change because of the rarity and severity of the storm. It is the first documented event of this type that I could find in Thurston County. The scientific community is split on whether this type of storm is something we can expect as the climate conditions change. The reason is it difficult to say is because these types of storms depend on a lot of factors, the major one being differences in temperature and humidity vertically (from the ground to the top of the cloud) in the parent storm. This temperature gradient is expected to increase, however, it will do so not only at the surface but also higher in the atmosphere. The net result is the same gradient will exist but at slightly higher temperature so these storms should not be any more likely than they are now. This is a very simple way of looking at this type of complex storm because with increases in temperature and humidity there are changes that occur across the climate spectrum; from shifting Jetstream patterns and sea currents to changes in boundary layers in the atmosphere.
Since this has happened only once as far as I could find in the weather records, we will have to wait and see if there is another episode, or if this was truly an isolated event. I personally never want to see it happen over my home again as you will see below from my personal experience with this beast if you read on.
This is the type of event that could get extra energy from any small contribution of a changing climate pattern as noted in many studies and computer models…..
My personal story started out in Olympia and as I drove home from the office I could tell this was going to be an experience like no other storm I had seen in this area. The sky was black with areas of a greenish blue glow between the darker pillows in the clouds. I have never seen such an eerie site in the clouds. I have since read that this glow is not uncommon in very large, high altitude thunderstorms. It is probably refracted sunlight coming from high up in the cloud tops much like a fiber optic cable can transmit light in a glass fiber. In this case the rain or ice acts as a fiber cable guiding the light into the cloud where it is diffused and causes a glow. In any event it was something I have never seen before. So I changed my plans to stop on my way home because ominous sky was getting closer and darker until the street lights began to turn on. It was 4:30 in the afternoon in early May.
The lightning became intense with flashes every thirty seconds or so I really changed my plans and changed my route to get home quicker. I was able to begin taking video of a truly frightening cloud formation at the base of this monster. It was moving and twisting and changing shape. My first thought was “this is a tornado that I am filming so I filmed some more and drove a bit faster. At this point I was in the path of whatever was coming so I made it to my house and into the garage in time to get a bit more video of this cloud formation dancing and surging above my house. By this time it was so dark out that my yard lights came on and I had to turn the headlights on to see the driveway. It was huge and changing shape very quickly like smoke coming off of a fire. Within thirty seconds it was directly over me and the wind went from calm and silent to violent in a matter of seconds. I could hear branches snapping and the air was filled with flying pieces of debris. The waterfall of rain then began and the lightning was flashing like strobe lights in the dark.
I made it into the house and the roar of wind and rain, and growling and crackling of thunder made a truly sinister chorus. I was actually become very nervous looking up through the glass roof of our solarium and watching the trees being tortured in the wind. Then lightning struck a home nearby and made a tremendous thunder clap and shaking the house even more. By this time the rain had overflown the gutters and was cascading off the roof as if there were no gutters at all. The power went out and I could hear powerlines cutting through branches as they struggled to saw through wood with electric arcs. They actually won and the power came back on. Then off. Then on again. This went on for about fifteen minutes before it began to slowly calm as the thunder got a little more distant and the rain and wind began to subside. Within 20 minutes the sun was shining again.
The aftermath was immediately apparent. The water was finally back in the gutters and draining normally and the sun actually began to shine from behind the back of the beast that just attacked us from above. There was a little too much sun and that is when I noticed the tree line was no longer a line but looked like a broken saw tooth. Several teeth were missing. Two large trees had been toppled and a smaller tree was also down across the access road to my house and several other homes. So out came the saw and I was joined by several neighbors as we cut away debris until the next lightning storm came over and the thunder became louder than the groaning saws and the lightning was brighter than daylight. We had to give up to avoid potentially being hit by lightning. By that time we had been sawing for a couple of hours and had the road opened.
By eight that night another thunderstorm put on yet another extraordinary lightning show with large arcs leaping every minute or so and again torrential rain filling the gutter to the brim. Nothing like the first storm and by this time I was used to the light show.
That was a storm like I have never seen in Western Washington before. Much more violent than any lightning storm previously had visited upon us. The national weather service actually issued eight severe thunderstorm warnings on May 4. That was more than they have issued in the previous 40 years put together. Truly one for the record books for our area – a story I personally do not want to read again.
I know if you are from the Midwest and other parts of the country this was not unusual but you must realize for this part of the country it was highly abnormal and we are not accustomed to it, just like snowstorms, which locals are aware you find amusing as well. The atmospheric conditions that create these storms very rarely occur here because our topography and other factors prevent large scale thunderstorms from forming.
Climate Matters 