Overshadowed by the mighty EF4 and EF3 tornadoes that tore across south-central Oklahoma last Monday, May 9, another less damaging twister--from the same thunderstorm--has left seasoned scientists both astonished and fascinated. This tornado, which struck near Roff, OK, was rated an unexceptional EF1 on the Enhanced Fujita Tornado Damage Scale. However, the Roff tornado stands out in several other ways.
—It was an anticyclonic tornado, meaning that it rotated clockwise rather than counterclockwise. A few such twisters probably occur each year, but they account for only perhaps 1% of all U.S. tornadoes—and that’s a very rough estimate, according to tornado climatology expert Harold Brooks (NOAA National Severe Storms Laboratory). In fact, the Storm Data procedures maintained by NOAA include no specific requirement or methodology for reporting whether or not a tornado is anticyclonic.
—It developed in the “forward flank” part of the storm, enveloped in rain-cooled air. “This is the first time I’ve ever heard of a well-developed anticyclonic tornado buried inside the forward-flank core,” said Roger Edwards (NOAA Storm Prediction Center), who has predicted and observed tornadoes for more than 25 years.
—Its damage path was initially estimated to be as much as 13 miles long, as documented in a preliminary storm survey conducted by the National Weather Service in Norman, OK. If confirmed, this path length will likely be a record; I have been unable to find evidence of any other anticyclonic tornado with a path this long. The estimated duration of 35 minutes might also end up as a record-setter for anticyclonic tornadoes. However, reanalysis of the damage could reduce the estimated path length and duration, as the radar signature of the tornado was shorter-lived, according to Gabriel Garfield (University of Oklahoma/CIMMS/NWS), who participated in the storm survey.
Figure 1. An example of the relatively light damage inflicted by the anticyclonic tornado near Roff, OK, on May 9, 2016. Photo credit: Courtesy Gabriel Garfield, OU/CIMMS/NWS.
Figure 2. Doppler radar image of the supercell thunderstorm that contained a large EF3 cyclonic tornado (the Sulphur tornado) and a weaker EF1 anticyclonic tornado (the Roff tornado). The reflectivity image (top, with reds indicating heavy precipitation) shows the classic hook echo in connection with the EF3 tornado (labeled in the velocity image at bottom). The anticyclonic tornado (labeled in the bottom image) is firmly embedded in the heavy rain core along the storm’s forward flank, as shown in the top image. Image credit: Courtesy Roger Edwards and his Weather or Not blog.
The scarcity of anticyclonic tornadoes is not a direct result of the Coriolis effect, which makes tropical cyclones spin cyclonically. The Coriolis force actually has little direct influence on circulations as small as tornadoes (not to mention toilets or other drains that happen to cross the equator). What appears to be the main driver is the wind shear that produces rotating supercell thunderstorms. In the Northern Hemisphere, a blossoming supercell will often split in two, with one cell angling to the right of the mean upper-level wind, spinning cyclonically, and the other angling leftward and rotating anticyclonically. Rightward-moving, cyclonically-spinning cells are the ones better positioned to ingest warm, moist air and grow more vigorously. Thus, most tornadoes are cyclonic, spawned by mesocyclones within cyclonically rotating supercells. (This Wikipedia page includes a nice conceptual diagram of a supercell thunderstorm.) Toward the outer edge of a storm’s rear-flank gust front, there can be anticyclonically rotating features; very rarely, one of these will spin up an anticyclonic twister.
What happened in Oklahoma last week doesn’t quite fit the classic picture: the anticyclonic Roff tornado developed right in the rain-cooled heart of the storm rather than on its periphery. The Roff tornado was embedded in rain for most or all of its lifespan. “We saw no evidence of it in the gray murk of heavy rain to our [west], nor any suspicious wind shifts,” reported Edwards, who was on the same storm photographing the nearby Sulphur tornado. As for what caused the anticyclonic Roff tornado, “we can offer only speculation and conjecture at this point,” said Edwards in a blog post. The smoking gun could end up being a stray left-moving storm, evident on radar, that zipped northward and became embedded in the larger, stronger supercell that produced the Roff and Sulphur tornadoes. It’s conceivable that this left-moving cell injected some anticyclonic spin into the forward flank of the storm.
Deducing what happened will take some doing, as this storm was more than 50 miles from the Oklahoma City NEXRAD radar. Data from mobile radar, including the RaXPol unit operated by Howard Bluestein and colleagues (University of Oklahoma), may shed some light on the oddly positioned Roff twister. “Having a tornado in that location is worrisome for my team,” said Doppler on Wheels chief scientist Joshua Wurman in an email, “because we count on being able to transect cores safely. That's not where we expect to find danger.”
Edwards mused: “The odds are extraordinarily tiny of ever seeing another anticyclonic tornado entombed in the forward-flank core of even a violently tornadic supercell. Yet now we know it’s possible, and we must be vigilant of that.”
Some clockwise pioneers There are extremely rare cases where a left-moving, anticyclonically rotating thunderstorm generates an “anti-mesocyclone” that spawns an anticyclonic twister, as occurred near Sunnyvale, CA, on May 4, 1998. However, the best-known examples of anticyclonic twisters occur in conjunction with more powerful cyclonic twisters as part of a single supercell thunderstorm, as was the case in Oklahoma last week. Sometimes these are weak, short-lived satellite twisters very close to the companion cyclonic tornado; others are stronger and more separated. Wurman unraveled DOW data for several of these in a 2013 Weather Analysis and Forecasting paper. “Our paper does not include any observations of [forward-flank] anticyclonic tornadoes, so this current one is certainly an outlier,” Wurman told me.
One classic case—probably the first anticyclonic tornado ever filmed—unfolded just west of Ames, IA, on June 13, 1976. In this storm, a powerful F3 anticyclonic tornado (the strongest anticyclonic tornado documented to date) closely paralleled the path of an F5 that ripped through the town of Jordan and the nearby countryside. The twisters were only about a mile apart as they moved largely in sync (see Figure 3). The YouTube clip at bottom shows both tornadoes.
A few years later, Ted Fujita and Roger Wakimoto examined the mammoth, nearly stationary supercell that tormented Grand Island, NE, for nearly three hours on June 3, 1980. (This is the storm that inspired the book and movie “Night of the Twisters”). Of the seven tornadoes that emerged that night, three were found to be anticyclonic. “This storm produced the most complex damage patterns imaginable,” wrote James McDonald (Texas Tech University) in the Bulletin of the American Meteorological Society. “No one but Ted Fujita could have sorted them out.”
Figure 3. Damage paths from the tornadoes that struck just west of Ames, Iowa, on June 13, 1976. The F5 tornado is in purple, the anticyclonic EF3 tornado is in green, and a cyclonic F2 satellite tornado that rotated around the F5 is in yellow. Rows of dots are separated by one mile each. Image credit: NWS/Des Moines, IA.
Doppler radar sharpens the picture, but the process remains fuzzy The growth of mobile radar over the last few years has yielded much more data on the evolution of anticyclonic tornadoes, although plenty of questions remain about how and why they form. A group led by Bluestein examined four such events in a paper published in April in Monthly Weather Review.
Perhaps the most notorious case was the El Reno, OK, storm of May 31, 2013, the one that killed storm researchers/photographers Tim Samaras, Carl Young, and Paul Samaras. That mammoth EF3 tornado, which lasted 41 minutes, was accompanied for 12 minutes by an anticyclonic EF2 tornado, several miles to its east, whose path arced southeastward (see Figure 5 below).
All four cases analyzed by Bluestein and colleagues involve anticyclonic tornadoes that developed on the rear, or trailing, end of a supercell’s rear-flank gust front. Each case also featured a mesocyclone producing a companion cyclonic tornado that was either weakening or had just dissipated. However, there was little consistency in the timing or in other aspects of these four cases, and "no characteristic stands out as being unusual,” the authors wrote. Multiple mechanisms could be at work in producing such anticyclonic tornadoes, they added. High-resolution computer simulations may tell us more about these clockwise mavericks in the years to come.
Figure 6. WU depiction of convective outlooks issued early Monday morning, May 16, 2016, for Monday (left) and Tuesday (right).
A localized severe threat on Monday and Tuesday There’s no rest for the storm-weary in Oklahoma and Texas, as another week will begin with a round of potentially severe weather. This time the threat will be focused in the eastern Texas Panhandle into northwest Oklahoma, where a warm front will be lifting north and a dry line pushing east. Storms may cover a fairly broad swath along the warm front and dry line, but the strongest will likely be close to the intersection point, where NOAA/SPC has placed an enhanced risk area. A few tornadoes are possible, especially as low-level winds crank up toward late afternoon and early evening; the storms will likely congeal into a southeastward-moving complex late Monday. The pattern should repeat itself in south Texas on Tuesday, though tornadoes may be less likely, with upper-level energy increasingly separated from an advancing cold front. This cool, dry air mass will sweep across most of the eastern U.S. by midweek, pinching off the prospects of widespread severe weather through at least the end of the week.
We'll be back on Wednesday with our next post.
Figure 7. One of the best illustrations I’ve seen of the life-saving value of tornado safety practice, drawn from the Sulphur tornado pictured above. From a Facebook post by the NWS office in Norman, OK: “We ran across this young man about 5 miles northwest of Sulphur, OK who was home alone at the time of the tornado. He was able to survive by doing exactly what we always tell people to do—he went to a small room in the center of his home away from outside walls and windows. He walked away without a scratch. There are no guarantees when it comes to tornado safety, but in most cases, the advice works!” Image credit: NWS/Norman.
Video 1. This mini-documentary on Theodore “Ted” Fujita’s research into the Iowa tornadoes of June 13, 1976, includes commentary from Fujita and high-quality (for the era) film footage of both the cyclonic and anticylonic tornadoes. The relevant segment extends from about 3:25 to 5:20.