Drenchings Trump Twisters in Tornado Alley

Despite a few high-profile tornadoes on Saturday, damage and injuries were minimal, and millions of people across the nation’s heartland experienced a more garden-variety weekend of spring showers and thunderstorms, some bearing extremely heavy rain. The unusually wet pattern will continue over the Plains this week, with severe weather mostly restricted to Texas until at least Friday. The most impressive supercell storms on Saturday were across northwest Texas and southwest Oklahoma, with the most prominent tornado traveling alnog a 35-mile-long path from near Elmer to near Snyder. Hitting mostly rural areas, this large tornado earned a preliminary rating of EF2 on the Enhanced Fujita Tornado Damage Scale, based on initial surveys coordinated by the National Weather Service office in Norman, OK. It’s possible that considerably stronger winds existed but failed to strike any of the objects (“damage indicators”) that are used in storm surveys to derive an EF rating. Storm-chaser video from near Elmer showed rapid motion around a wedge-shaped tornado that at one point appeared several times wider than it was tall (see Video 1 below). This is largely a function of the very low cloud base produced by high relative humidity near the ground.


Figure 1. A panoramic view of the storm in southwest Oklahoma that generated the tornado shown in Video 1 and Figure 2. Image credit: Daphne LaDue.


Video 1. Impressive footage from the Elmer-Tipton tornado. The dramatic width of the tornado during its “wedge phase” is clearly evident. If embedded clip is not visible, the video can be viewed directly at YouTube. Video credit: Jeff Snyder.


Figure 2. The “stovepipe phase” of the long-lived tornado in southwest Oklahoma on Saturday, May 16. This photo was taken near the town of Tipton. Image credit: James LaDue.


The University of Oklahoma’s RaXpol radar, a mobile, high-resolution dual-polarization system, gathered data from the Elmer-Snyder tornado. Although the radar was located roughly 10 to 12 miles from the tornado, the data will help clarify wind speeds just above the surface in and near the twister. (Note that radar-derived winds are averaged over a three-dimensional volume around the beam, rather than derived at a single point.) From an initial scan of the data, Howie Bluestein (University of Oklahoma) estimates that peak winds from RaXpol were around 150 mph in the lowest few hundred feet, with evidence of debris in the dual-polarization data at altitudes at least as high as 25,000 feet. “There was a classic debris signature aloft,” says Bluestein. Researchers are pondering ways to rate tornadoes that could employ the growing availability of high-resolution radar estimates of tornado winds without compromising the consistency of the nation’s tornado database, in which most events cannot be linked to such radar data. The El Reno tornado of May 31, 2013, was officially rated EF3, based on the available damage indicators, while RaXpol detected EF5-strength winds of at at least 290 mph over a volume centered less than 60 feet above ground level. (See this Weather and Forecasting journal article for more details on the 2013 RaXpol data.)

Oklahoma was on high alert Saturday night after the highly visible tornadoes appeared on local and national TV. Fortunately, the initial supercells congealed into a solid line before reaching central Oklahoma, which reduced their ability to form significant tornadoes, but a new broken line of supercells formed a bit further east. A late-night tornado (11:30 pm CDT) produced EF2 damage along a path at least 12 miles long near the Tulsa suburb of Broken Arrow. In all, Saturday produced 45 tornado reports across 9 states, with about half occurring in Texas and Oklahoma. As was the case on the previous Saturday (May 8), the day’s severe weather was split into two focal points by a large area of morning showers and thunderstorms that developed early in the day over northern Oklahoma and Kansas, keeping the atmosphere in those areas too cool and stable for supercell activity. The other hot spot on Saturday was in central Minnesota, where 9 tornadoes were reported, including one that struck a farmstead near Montevideo.


Figure 3. The 0- and 84-hour forecasts at 500 mb from the NAM model, valid at 1200 GMT Monday, May 18, and 0000 GMT Friday, May 22, are remarkably similar, reflecting the persistence in the current pattern. Image credit: NCAR/RAL Real-Time Weather Data.


Another wet week ahead
Prodigious rains are emerging as the big weather story this month—not a surprise, given that an unseasonably early El Niño event is now strengthening. Persistent, recurrent upper lows in the Southwest (see Figure 3) have channeled deep moisture from the Gulf of Mexico across the central states and provided jet-stream energy to lift the moist air. It appears quite possible that this month will eclipse June 1989 as the wettest month in Oklahoma City history, in records going back to late 1890. With 12.85” reported as of Monday morning, only 1.82” is needed to break the 1989 record, with more rain in the forecast throughout this week. Other cities are also in the running for monthly rain records, including San Diego, where 2.35” has fallen this month; the city’s May record is 2.54” (1921), and light showers are expected late this week. Phoenix scored 0.93” of rain last Thursday, the most observed on any May day on record. The current monthly total of 1.17” in Phoenix makes it the second-wettest May on record, behind only May 1930 (1.31"). A wet weekend pushed Jamestown, ND, up to 8.29” for the month, the highest May total in records going back to 1893.


Figure 4. The 7-day precipitation outlook from NOAA’s Weather Prediction Center for the period 1200 GMT Monday, May 18 - 25. Image credit: NOAA/NCEP Weather Prediction Center.

The zone of rich moisture that’s covered the Plains much of this month will be pushed southward early this week by a chilly-for-May surface air mass. The next chance for widespread severe weather will likely be on Friday or Saturday, as yet another upper low in the Southwest (see Figure 3, above) moves across the Plains. However, it’s again possible that early-morning showers and storms will shunt the most unstable air away from the zone of strongest wind shear, limiting the area at risk of significant tornadoes or other severe weather. Heavy rains appear to be a safe bet (see Figure 4, above), again putting parts of the Southern Plains at risk of flash flooding.

Bob Henson


Figure 5. A “ground-to-cloud” flash lights up the sky near Bixby, OK, on Saturday, May 16. Upward-directed lightning, which typically occurs near transmission towers, wind turbines, and other large objects, has recently been linked to thundersnow events. Image credit: wunderphotographer mrwing13.