|Above: A tornado spins south of Wynnewood, Oklahoma, as viewed from the west at 4:18 pm CST on May 9, 2016. Image credit: Courtesy James LaDue.|
Even as La Niña begins moving out of the picture this spring, it could be priming the atmosphere for a pattern that favors U.S. severe weather. A forecast group based at Columbia University has issued an outlook for the period March through May calling for enhanced odds of tornadoes and hail across much of the nation, especially over the Southern Plains.
Produced by Columbia researcher Chiara Lepore, the outlook is based on research showing that the state of the El Niño/Southern Oscillation (ENSO) during winter provides some skill in predicting large-scale severe weather patterns across the U.S. the following spring.
La Niña has prevailed in the tropical Pacific for the second straight winter. This winter’s weak to near-moderate La Niña event was more potent than the marginal La Niña episode of 2016-17. In its latest monthly ENSO update, issued Thursday, NOAA’s Climate Prediction Center calls for the current La Niña event to weaken through the spring. CPC also gives an 80% chance that either El Niño or neutral conditions will be in place by the end of 2018.
In line with previous research, the Columbia team has shown that severe weather is more likely over large parts of the nation after a La Niña winter than after an El Niño winter. Their outlook issued for March-May 2018 is quite similar to the example shown for weak-to-moderate La Niña conditions in a 2017 Geophysical Research Letters paper by Lepore and colleagues Michael Tippett (Columbia) and John Allen (Central Michigan University). Since the state of ENSO for a given winter can often be predicted fairly well by the prior October (as was the case this year), the group’s technique can provide an outlook for the next spring’s severe weather prospects with a few months of lead time.
|Figure 1. Probabilistic outlook for severe weather across the United States, averaged for the period March through May 2018. The 2018 outlook is based on the weak to near-moderate La Niña conditions (Oceanic Niño Index near -0.9) that prevailed from December 2017 to February 2018. The study uses three brackets of probability—above, below, and near normal—which are set at 35%, 35%, and 30% in the absence of El Niño or La Niña influence. Values in green show where the odds of severe weather (tornadoes and hail) are boosted above normal. Northeast Texas and southeast Oklahoma have a greater-than-50% chance of above-normal activity, which implies a less-than-20% chance of below-normal activity. Image credit: Courtesy Chiara Lepore, Columbia University, based on techniques described in Lepore et al., “ENSO-based probabilistic forecasts of March–May U.S. tornado and hail activity,” Geophysical Research Letters 2017, DOI:10.1002/2017GL074781.|
Clues from the Trans-Niño pattern
Other scientists have found hints that the Southern Plains may be at higher risk of tornado outbreaks when the atmosphere is moving away from a second winter of La Niña toward El Niño. Dubbed a transitioning La Niña pattern, or a positive Trans-Niño event, this pattern features increasing sea surface temperatures off the coast of South America (the Niño1+2 region) together with SSTs in the central Pacific that are rising but still cooler than average. The current setup points in this direction, as SSTs have now climbed above average in the Niño1+2 area while remaining close to 1°C below average in the benchmark Niño3.4 region.
Sang-Ki Lee (NOAA/University of Miami) and colleagues analyzed the influence of ENSO transitions on severe weather in a 2016 Environmental Research Letters paper. They found that a transitioning La Niña tends to favor tornado outbreaks across the Southern Plains, especially during mid-spring (April), whereas a resurgent La Niña—one that returns the next winter—is more closely associated with tornado outbreaks further north and east, from the Upper Midwest to the Deep South. Two other modes of ENSO behavior—a continuing El Niño and a departing El Niño—were less likely to yield springtime tornado outbreaks.
The authors stress that “a seasonal outlook cannot pinpoint exactly when, where and how many tornadoes may strike. Instead, the goal of a seasonal outlook is to predict in terms of probability which regions are more likely to experience a widespread outbreak of tornadoes.”
|Figure 2. A path of debris remained in Pratt City section of Birmingham, Alabama, on April 29, 2011. Two days earlier, the city was struck by an EF4 tornado as part of the catastrophic 2011 Super Outbreak, one of the nation’s two worst tornado swarms on record. This outbreak occurred following a La Niña winter, as did the only other comparable event on record—the 1974 Super Outbreak (April 3). Image credit: Kevin C. Cox/Getty Images.|
Late March into early April: a time to watch
Up through early March, the U.S. severe weather picture has been slightly calmer than usual. This year’s “inflation-adjusted” tornado total (which factors out the higher frequency of observations in recent years) had reached 54 through March 7, compared to an average value of 74. We’ve seen a parade of large-scale storms bringing multiple shots of winter weather from the Deep South to New England. However, these strong, fast-moving systems haven’t allowed enough time for sufficiently moist, unstable air to reach the U.S. from the Caribbean and the Gulf of Mexico and become juxtaposed with vertical wind shear.
Yet another large-scale storm system heading toward the mid-Atlantic might produce one or more tornadoes after dark on Saturday night, March 10, across the lower Mississippi Valley, according to the Day 2 outlook issued at midday Friday by the NOAA/NWS Storm Prediction Center. Once again, instability will be limited, so a major outbreak is not expected.
|Figure 3. A slight risk of severe weather is expected on Saturday afternoon and evening, March 10, 2018, according to the Day 2 outlook issued at midday Friday, March 9. Image credit: NOAA/NWS/SPC.|
Things may get more active by late March, according to Victor Gensini (Northern Illinois University). This week, Gensini and colleagues launched another spring of ERTAF (Extended Range Tornado Activity Forecasts). The idea behind ERTAF is to predict whether U.S. severe weather outbreaks are more or less likely than average during weeklong periods going out to 3 weeks. That’s too far out for useful local forecasts, but the ERTAF group has found broad predictive value in long-range model forecasts of atmospheric angular momentum—basically, how fast the atmosphere is spinning relative to Earth’s rotation.
The first ERTAF forecast, issued on Sunday, March 4, called for below average activity for Week 2 (March 11-17) but above average activity on Week 3 (March 18-24). By that point, long-range models suggest a weakening of the upper-level trough over the U.S. East and development of troughing over the Southwest, which could favor severe weather east of the Rockies. Very warm SSTs now prevail across the Gulf of Mexico; if these persist, they could help juice the atmosphere and boost the odds of severe weather further. Update: In its forecast issued on March 11, ERTAF included its first-ever areal outlook, highlighting an region from the Southern Plains to the Ohio Valley for heightened tornado potential during the week of March 18-24.
All it takes is one
Gensini pointed out that seasonal forecasts based on ENSO and its subcategories are prone to the hazards of small sample sizes, because only a few events of each ENSO type exist in the long-term record. Even so, he told me, “I would agree that a majority of areas east of the Continental Divide are at an increased risk for tornadic activity due to the current general circulation pattern.” This doesn’t imply day after day of big tornadoes, noted Gensini. Even during horrific tornado seasons like 1974 and 2011, the worst severe weather tends to be concentrated on just a handful of outbreak days.
“The contribution to annual activity from outbreaks can really mask otherwise quiet conditions," noted Gensini. "All it takes is one tornado for somebody to think this year was ‘above average’!”
If there is indeed enhanced severe weather over the Southern Plains, it might not help the bone-dry Southern High Plains, where extreme to exceptional drought has set in. During such conditions, said CMU’s Allen, “severe events become more dependent on strong moisture advection rather than mesoscale-driven processes, particularly for tornadoes. Hence, we tend to favor fewer events, but when they do come together they are somewhat larger in magnitude in the form of outbreaks—and a strong progressive dry line that surges into eastern Oklahoma. It also means west of about 100°W [the western border of the body of Oklahoma], we are probably going to see a quieter-than-average season.”