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Bob Henson • July 30, 2013 | Just as forecasters now peg the odds of a busy Atlantic hurricane season months in advance, we might soon have outlooks that assess the risk of an active tornado season weeks or even months ahead of time.
Such a system isn’t yet on the drawing board, but researchers are exploring how it might work. A white paper prepared earlier this year by a team of NOAA and university scientists is now making the rounds, and a testbed could be in place as soon as the spring of 2014.
“We’d like to move toward something probabilistic, much like the hurricane outlooks,” says NOAA’s Scott Weaver. He’s the lead author of the white paper, which includes the testbed idea as well as a variety of other recommendations.
The white paper emerged from workshops in May 2012 and March 2013 (see writeup and video clips here) that included tornado specialists, climate scientists, and representatives from the insurance industry.
“We’re interested in learning the best ways to make this useful to decision makers,” says Weaver.
The push to explore extended (two- to six-week) and seasonal (three-month) tornado prediction has gained momentum due to several factors.
Currently, NOAA’s Storm Prediction Center (SPC) issues convective outlooks that highlight parts of the country that can expect tornadoes and other severe weather up to eight days in advance. There’s no system in place for predictions going beyond that eight-day window.
In contrast, hurricane outlooks that extend as far as a year ahead are issued for the North Atlantic and Northwest Pacific several times each year by Colorado State University, NOAA’s Climate Prediction Center, the private firm Tropical Storm Risk, and other groups (see this PDF summary of 2013 outlooks from Risk Management Solutions). Forecast skill is generally modest, especially for major hurricanes and for the longest-range periods, but the outlooks are followed closely and given wide coverage in the media.
It’s important to note that while these outlooks sometimes indicate which parts of the U.S. coast are most vulnerable to a landfalling tropical cyclone in a given year (e.g., the Gulf of Mexico or the mid-Atlantic), they make no effort to predict individual hurricanes.
Along these lines, it’s possible that a long-range tornado outlook might specify the odds of enhanced severe weather weeks to months in advance across broad regions of the country (e.g., the upper Midwest, Southeast, or southern Great Plains) without trying to forecast when or where a specific tornado outbreak might strike.
What might such a forecast actually look like? Greg Carbin (SPC) and Michael Tippett (International Research Institute for Climate and Society) are experimenting with techniques for converting climate-model output into monthly tornado probabilities and presenting those odds graphically.
Are today’s models up to the task of long-range tornado outlooks? There are indications that climate models such as the CFS can sometimes detect the large-scale circulation trends conducive to severe weather weeks or even months out. Researchers are now working to quantify just how skillful these projections are.
The trick is that even when large-scale patterns are favorable, tornadic storms don’t always materialize. Local and regional subtleties often get in the way, such as the formation of clouds that reduce instability or the emergence of “capping” layers of very warm air a mile or two above ground. These factors can be challenging to pin down even a day or two in advance. Still, knowing that storm-friendly large-scale patterns are more likely might provide at least some measure of value to emergency managers, utilities, insurance companies, and others hungry for any morsel of guidance.
Not all experts are convinced of the value of seasonal tornado outlooks, including NCAR’s Morris Weisman. He was one of the coordinators of a thunderstorm-related field project this spring called MPEX (the Mesoscale Predictability Experiment), which examined how to make 6- to 24-hour forecasts of severe storms more precise in time and space. The project’s 18 days of field operations included the two disastrous events in central Oklahoma on May 20 and May 31.
MPEX data suggest that expanded observations could add skill in some cases to severe weather prediction within the one-day window, says Weisman. As for the notion of longer-range outlooks, he wonders how useful these can be, given the huge importance of the short-range factors noted above.
“So many times I’ve seen big events be completely misforecast beyond five days,” says Weisman. “The details change dramatically.”
Even though seasonal forecasts wouldn’t be aimed at predicting specific severe weather episodes, Weisman believes the signals embedded in El Niño and other climate shapers are still too weak for seasonal forecasts to provide a great deal of value. “The methodology being proposed is fine,” he says, “but the assumption is that we do have some level of skill at those time scales, and I submit we don’t—yet.”
Purdue’s Jeff Trapp, who was also a PI on MPEX, agrees up to a point. “Although we and others have basic methodologies that can be applied to this problem, we still need to demonstrate that the predictions can be skillful,” says Trapp. “Such a demonstration needs to happen before we can begin to talk about a timeline for implementation.”
Still, he adds, “I do think it will be plausible to identify, with relatively long lead time, regions where relevant environmental conditions will be anomalously—and persistently—large. The outstanding question is whether that potential lead time is two weeks, or four weeks, or twelve weeks.”
Has 2013 been a bad year for tornadoes? The answer depends on your perspective. Late May brought a multi-day barrage of violent storms to central Oklahoma, with tornadoes on May 19, 20, and 31 that killed at least 33 people. The EF5 twister that struck Moore, Oklahoma, on May 20 destroyed or damaged thousands of structures and delivered an estimated $2 billion in damage, making it one of the costliest tornadoes in U.S. history.
Yet outside of those few devastating days, it’s been an unusually tranquil year for U.S. tornadoes. As of July 21, the “inflation adjusted” tally of U.S. tornadoes for 2013 was 564, which puts it near the lowest values for any year since records began in 1950. Weak tornadoes are spotted much more frequently now than in past decades, so NOAA’s inflation-adjustment analysis removes this linear upward trend (see details here). This allows for an apples-to-apples comparison from the 1950s into the 2010s.
Even when looking at raw tornado counts for the last nine years to date, this year (664 tornadoes as of July 21) falls well below 2005, its nearest competitor (817 tornadoes as of July 21). And 2013 is running far behind 2008, which had notched 1773 twisters by this point.
For seasonal prediction, it turns out that the number of days with tornadoes in a given year may be easier to predict than the total number of tornadoes. However, researchers are well aware that a “quiet” season, with few tornado days, can still include a few catastrophic events, so they’re pondering ways to address the issue. It’s not unlike the phenomenon of a relatively placid hurricane season that produces a single monster storm. That was the case in 1992: the year’s only U.S. landfalling hurricane was Andrew—one of the most destructive on record.