One year ago, on December 14, Aumsville, Oregon saw a rare weather occurrence. An EF-2 tornado tore through the eastern part of town, damaging dozens of homes. Around 50 homes received damage, of those 10 were unlivable. Luckily, only two people were injured, and they were minor cuts and bruises.
The tornado marked only the fourth EF-2 in the history of Oregon. Only 114 tornadoes have been recorded in the state since 1887. However, one of those 114 also set its sights on Aumsville back in March of 1960.
The tornado brought together a community. Dumpsters were set out for debris collection, neighbors helped neighbors pick up the pieces, and the Marion County Jail’s 48 inmates headed straight out to help.
The track of the tornado was about 5 miles long; however, it is not likely it stayed on the ground the whole time. Besides the damage in the city limits of Aumsville, a few more reports were received from an area northeast of the town.
One year later, damage has been repaired and life continues in Aumsville. However, the town will always have the memory of that day that destroyed dozens of homes but brought a community together.
Tornado Ingredients: So why Aumsville? Generally in Oregon, we have some of the ingredients needed to form a tornado. However, we hardly ever have all the ingredients at once, or over a large area. You need energy or heat to create instability (generally measured by Convective Available Potential Energy or CAPE), moisture (something Oregon generally has plenty of), and something to cause the storm to spin such as a front or collision of air masses of some sort.
Conditions on December 14, 2010: Over the few days preceding the Aumsville tornado, we built up warm, moist air near the surface. We also caught a few peaks of sunshine Tuesday morning, before the deck of clouds rolled through. Those components likely fueled the energy. To add even more energy, we had a cold front bring in a cold pool of air aloft. That temperature difference between the surface and the upper levels, caused air to rise quickly, thus creating thunderstorms. In fact the Lapse Rates that day were at 8 C/Km, where we are normally at 5 or 6 C/Km. However, the layer in which most of this happened was pretty thin. That being said, a lot of areas had several of the requirements to have thunderstorms, but most of them fizzled out due to lacking one of the ingredients.
Moisture and instability really were not a problem. I think the major igniter of this storm was wind shear, which created the spin. Lots of things in Oregon can create wind shear. We have topographical features which can aid in trapping winds and causing velocity changes. However, in Aumsville’s case, I think it was how near the strong low pressure system it was. Aumsville was just far enough north to catch stronger wind shear, but just far enough south to catch the heat from the weekend’s ridge. Wind shear value that day in Salem (west of Aumsville) was around 40 knots between 11:00 a.m. and noon that day.
However CAPE cannot be ruled out as a contributor. While the rule is generally 1000 J/Kg of CAPE to support tornado formation, the Aumsville tornado occurred in an environment with around 500 J/Kg. While energy was not the main cause of the tornado, I think it just added to the other variables that lined up perfectly for only a few minutes. The fact that these ingredients came together at the same time is amazing and highly unlikely for this time of year in this part of the country.
Why Not the Southern Valley or Umpqua Basin? While areas further south of Aumsville were warmer, the key component was the wind shear. Areas such as Eugene, Cottage Grove, and Roseburg, were just too far away from the center of low pressure. The coastal region has plenty of moisture and strong winds, but lacked energy to get the storms to lift. That doesn’t mean some of the areas went without significant weather. The mountains picked up significant snowfall, the northern coastal areas had gusty winds into the 40 mph range, and several areas in the Willamette Valley picked up hail.
The Enhanced Fujita Scale: Here is the EF scale and how it matches up with wind speeds:
0 65-85 mph
5 Over 200
For more information on the criteria for EF Scale ratings, click here.
Final Thoughts: While the Aumsville tornado was not the biggest, baddest tornado ever seen, the fact that it even happened is amazing in itself. While Oregon has only seen a few more than 100 reported tornadoes since 1887 and that two of them happened in Aumsville is also amazing. From someone who has seen the “textbook” supercells in Tornado Alley, this tiny storm with hardly any classic support was hardly a suspect to drop a life-threatening tornado. It goes to show you that it doesn’t matter where you are and what time of year, if Mother Nature provides the opportunity, severe weather could strike.
While Aumsville is slightly out of our coverage area, we tried to capture the storm not only weather-wise, but as it impacted the community. To see some of our live coverage of the storm the night after it happened, click here.
To see some of the stories our reporters put together about the challenges the community faced during clean up, click here. Use the links at the bottom of that page to view more stories.
For more information about tornadoes in generally and some safety tips, click here. Remember it is always a good idea to be prepared, no matter where you live and what time of year. Aumsville proves it can happen anytime, anywhere.
— Meteorologist, Megan Taylor
If you have questions or comments about tornadoes or severe weather in general, the weather team would love to talk to you. Comment below on this story or visit us on Facebook or Twitter. You can also send weather questions to our email.