A thunderstorm squall line moving at freight-train speeds ripped through the eastern Texas Panhandle and across Oklahoma on Sunday night, Feb. 26. Unseasonably potent conditions for late winter led to a swarm of damaging-wind reports and a preliminary total of at least nine fast-moving tornadoes. One death was reported, from a tornado that struck the tiny western Oklahoma town of Cheyenne.
Sunday’s most extensive damage came from a twister that tore across parts of Norman, Oklahoma, passing roughly a mile south of the National Weather Center, which houses the NOAA/NWS Storm Prediction Center. The video below, taken by longtime tornado and wind researcher James LaDue, depicts the tornado’s formation from a vantage point next to the National Weather Center, where LaDue works.
Numerous streets were closed in southern and eastern parts of Norman because of downed trees and power lines, and more than a dozen homes were damaged. At least 12 injuries were reported, none serious, according to police reports relayed by the Washington Post.
Storm surveys conducted on Monday assigned the twister, and another one just to its northeast, a preliminary EF2 rating on the Enhanced Fujita scale. Three other twisters in central Oklahoma were rated EF1.
For the period through 6 a.m. CST Monday, the Storm Prediction Center has cataloged more than 130 preliminary reports of high wind from the storm complex. At first glance, the wind swath appeared to be large, strong, and dense enough to qualify as a derecho – a prolonged, intense corridor of thunderstorm downdrafts that is typically at least 60 miles wide and 250 miles long. The Storm Prediction Center had predicted the severe weather episode days in advance, and on Sunday morning, the center declared: “A derecho is forecast with widespread damaging winds and embedded swaths of significant severe gusts from 80-110 mph.”
A derecho out of season?
One or two derechos typically sweep across the Midwest each spring and summer, including the $11 billion catastrophe that hammered Iowa on Aug. 10, 2020. February is not immune to derechos: One swept across central Alabama on Feb. 16, 2001. However, it’s rare to see a derecho of Sunday’s magnitude occurring in February as far north and west as Oklahoma, which is well away from the favored location of wintertime severe weather in the Deep South.
One recent cold-season analog that comes to mind is the widespread, billion-dollar derecho that slammed the Northern Plains and Upper Midwest on Dec. 15, 2021, and also produced the largest single-day tornado outbreak in December history. As was the case in Norman on Sunday, brief embedded tornadoes often spin up along the quasi-linear squall lines associated with derechos. This is especially the case for serial derechos – those associated with north-south cold fronts – as opposed to the progressive derechos that sweep along east-west frontal zones.
Given that only about 11% of all U.S. derechos occur from December to February (or very roughly one per decade), getting two noteworthy derechos in the Great Plains in consecutive meteorological winters is another hint that our warming climate may allow severe weather season to extend farther north during winter than we’re used to.
From blizzard to blowing dust to derecho
Driving Sunday’s derecho was an exceptionally strong band of upper-level winds kicking into the Southern Plains ahead of a frigid upper-level low. On Friday, that upper low led to a once-in-a-generation snow blitz in Southern California, with blizzard conditions and multiple feet of snow recorded in the mountains north and east of Los Angeles and torrential rains at lower levels. Top snow amounts reported from NWS/Los Angeles included 93 inches at Mountain High (7,000 feet) and 48-72 inches at Mount Pinos (8,500 feet). Rainfall totals of 6 to 7 inches were widespread at lower elevations, including Pasadena (8.11 inches), Burbank (6.88 inches), and Beverly Hills (6.64 inches).
As the surface cold front barreled across the Texas Panhandle on Sunday, massive amounts of blowing dust filled the skies. Thunderstorms erupted just ahead of the front, bringing downburst winds of more than 100 mph.
Meanwhile, a surge of moist, unstable air swept into Oklahoma from the Gulf of Mexico. In fact, the amount of precipitable water (water vapor in an imaginary column) above central Oklahoma as the squall line approached – between 1.1 and 1.2 inches – was close to the record value for February of 1.28 inches, and far above the monthly average of around 0.4 inches.
The balmy surface air provided just enough instability to generate thunderstorms that in turn could take advantage of eye-popping wind shear. A special sounding (instrumented weather balloon) launched from Norman at 9 p.m. CST Sunday about 15 minutes before the tornado showed winds at the 850-millibar level (about a mile above the surface) were raging at 75 knots, or about 85 miles per hour. Out of the 60,000-plus standard soundings collected in the Oklahoma City area roughly twice a day since 1945, just one sounding has measured winds that strong at that height. Note that this isn’t quite an apples-to-apples comparison, since those standard soundings are collected at 6 a.m. and 6 p.m. CST – as opposed to 9 p.m., when a low-level jet stream can be rapidly intensifying a mile or so above Oklahoma – but it still illustrates the unusually volatile nature of Sunday night’s setup.
Those fierce winds were also pulling in warm, dry air above the surface that would normally have “capped” the balmy surface air and suppressed thunderstorm development. In this case, the cold front was so powerful that it forced the surface air upward through the cap.
No rest for the weary forecaster: Another severe weather outbreak looms for late week
The bundle of high winds and frigid air aloft that swept across the Southern Plains on Sunday will continue to pose a severe weather threat on Monday, this time from Illinois to Ohio. As the surface cold front sweeps through the area, another fast-moving band of thunderstorms is expected. Embedded tornadoes and high winds will again be possible; several tornadoes were reported Monday morning in Illinois. Overall, surface warmth and moisture should be more limited and thus the threat less than on Sunday, with intense tornadoes unlikely. As of late morning Monday, the Storm Prediction Center had a slight risk of severe weather (level 2 out of 5) across the region.
Meanwhile, the rest of the cold upper-level system that’s been loitering over the West for days remains in place. This parent system will soon “reload,” as another strong packet of cold upper-level air and high winds rotates around the upper low and swings through the south-central U.S. later this week. Plenty of warm, moist surface air is expected to surge from the Gulf of Mexico back into the Southern Plains and Southeast, setting the stage for a potentially expansive, serious outbreak of tornadoes and other severe weather, perhaps emerging over the Southern Plains on Thursday and sweeping into and through the Mississippi Valley that night.
The Storm Prediction Center’s Day 4 outlook, issued Monday and valid Thursday, warned: “A regional outbreak of severe weather appears increasingly likely Thursday afternoon and Thursday night including the potential for large hail, damaging winds, and tornadoes, some of which may be strong.” It’s quite possible this outlook will be ratcheted upward as the event draws closer. Climatology should only heighten our concern and awareness, as early March is close to peak tornado season for the lower Mississippi Valley.
Jeff Masters contributed to this post.
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