The Green Church shook with wind on Tuesday night while Reno National Weather Service (NWS) weather officer Chris Smallcomb delivered the fourth SNARL lecture of the season on the Devils Windstorm of 2011.
Smallcomb opened the talk with a sobering reminder of another devastating wind event, the 1.3 mile-wide, category EF5 tornado that plowed through Oklahoma on Monday afternoon. The tornado tore through 17 miles of ground in over 50 minutes, reaching speeds of 210 miles per hour, and killed 24, destroying hundreds of homes in the process. Nevertheless, “It’s amazing that with 15-30 minutes warning, people were able to take action,” Smallcomb said.
He then strove to lighten the mood with a brief introduction to the National Weather Service, beginning with a list of humorous questions received annually by the NWS, including “what’s the weather going to be like on my wedding day next year?” and “how big was that earthquake?” The NWS, a Federal government agency under the Department of Commerce, provides 122 local forecast offices for weather prediction across the country, and issues all official storm warnings, said Smallcomb.
“Sometimes our forecasts are a little iffy,” he admitted, “but we’re getting better and better.” A photograph of NWS Reno members ‘predicting’ weather using an eight ball and dice elicited appreciative laughs from the audience.
Smallcomb then segued into his presentation on extreme weather, which includes events like winter storms, flooding, fire, and downslope windstorms. “Computer models used to forecast weather are getting better and better, such that we can start seeing the signs [of extreme weather] seven to ten days out,” he said. When he began his career in weather service 13 years ago, he added, “You’d be lucky if you predicted these storms three days out.”
But while NWS Reno saw early signs of a wind event in the Mammoth area in late November 2011, no one could have predicted the ferocity of the ‘Devils Windstorm’ in the Devils Postpile monument area. This wasn’t the first intense wind event in the Eastern Sierra by any means, Smallcomb said; a downslope windstorm in December 2008, with wind gusts of up to 150 miles per hour, destroyed a NWS Reno radar dome on Virginia Peak.
During the Devils Windstorm of 2011, however, the Mammoth Mountain summit observation site clocked sustained winds, not just gusts, of 125-150 miles per hour. Smallcomb explained that the observation site equipment could only measure up to 150 miles per hour; therefore a 1.3-1.5% multiplier must be applied to sustained wind to calculate the speed of wind gusts during that time. “We’re pretty confident there were 200 mile per hour gusts,” he said, “at least on the summit of Mammoth Mountain.”
With no observation site in the Devils Postpile monument area, NWS Reno scientists had to gage wind speeds by damage. The windstorm felled trees not only in great numbers in the same direction, suggesting high, sustained winds, but also snapped trees like toothpicks, Smallcomb said. This latter detail, which is reminiscent of damage recorded after category EF2 tornadoes, suggested that wind speeds must have been between 111-135 miles per hour. “This windstorm probably would have sounded like a tornado bearing down on you,” Smallcomb said.
The Devils Windstorm was unusual not only because of the speed of the wind, but also because of the sustained period of the event, which lasted for 24 hours as opposed to a maximum of 15 hours recorded during previous downslope windstorms. The windstorm was also unusual because the wind came from the northeast, when typically wind blows from the southwest. Trees had no defenses built up against this wind direction, and that, coupled with lack of snowfall to secure root systems because of the drought year, ended in the destruction of huge swaths of forest.
The Devils Windstorm was part of a much larger wind event, Smallcomb said. At the same time as the Devils Windstorm, “We saw widespread downslope winds on the western slopes of the Sierra; a major Santa Ana wind event in Los Angeles, with 80 mile per our gusts and massive power outages; and a wind storm in Salt Lake City, with a 102 mile per hour gust inside the city,” he said.
This historic wind event was the result of a potent mix of ingredients, Smallcomb explained. The first ingredient was a big difference in pressure between strong low pressure over Arizona and high pressure over the Pacific Ocean, which created strong northeast winds over the Sierra. The second ingredient was an unusual NE-SW oriented jet stream located over the Rockies and California. “Pieces of the jet stream wind did hit the mountain tops,” he said, “adding to the tremendous wind speeds.” Third, the terrain channeled the wind through a small gap between Mammoth Mountain and its neighbor to the north. “Forcing a ton of air into a really small area, air has no choice but to speed up,” Smallcomb said. That, coupled with the steep leeward slope in the Devils Postpile monument area, created a favorable environment for an extreme downslope windstorm.
The final ingredient for the Devils Windstorm was a stable atmosphere right at mountain top level. A stable air mass over the Postpile area directed wind back down to the ground, whereas an unstable air mass would have allowed the wind to continue upward, likely creating a thunderstorm, Smallcomb explained.
NWS Reno saw many of these signs of an impending wind event, Smallcomb said, “but wind magnitudes were under-forecast.” He added, “The exact strength of wind gusts will always be the biggest challenge [to predict]. It would be gutsy to call for gusts of 200 miles per hour.”
The likelihood of an event like this occurring again is low, Smallcomb said, “but obviously it happened once, so it could happen again.” When asked by an audience member whether such high winds could ever hit the town of Mammoth, Smallcomb offered some reassurance. “All the ingredients would have to come together in just the right proportions, intensities, and timing,” he said. “These ingredients include wind speeds above 100 miles per hour at mountain top levels, wind direction that would result in a downslope wind storm east of the mountains, and atmospheric stability that would result in the most intense downslope winds crashing into the town rather than well downwind of the Sierra. Even just slight adjustments to any of these ingredients would limit how intense the wind gets in town.”
While the effect of these slight adjustments also makes predicting such an event with certainty challenging, Smallcomb believed chances of such a windstorm finding its way into town were slim. “It’s not out of the question to see extreme winds in Mammoth,” he said, “but it’s unlikely.”
Next week’s lecture (May 28): Lahonton Cutthroat Trout Recovery by Dawne Becker. Tuesday at 7 p.m., Green Church.