After a breakneck start to the year for tornadoes, May came in like a lamb across the Lower 48. Severe weather was scarce until May 10, when a day-long outbreak of dangerous storms tore through the Plains. Since then, however, tornado risk has flatlined again, with no signs of a resurgence in the near future.
From the start of the year to the end of April, some 595 tornadoes were recorded nationwide. That means 2023 bore witness to the third-most-active start to the year on record. At least 60 people have died in the storms.
But the remainder of May — a month feared for its inveterate meteorological malice and calamitous storms — looks to be quiet. Now, meteorologists are racking their brains to understand when the next flurry of activity could be.
Why May is a month for concern
Historically, May is the most active month for severe weather across the United States. An average of 275 tornadoes spin up nationwide, with the greatest concentration in the Plains. Oklahoma averages 34 tornadoes during the month; Texas tends toward 38; and Kansas sees 37, give or take. There’s enormous year-to-year variability, however: Some years may have a dearth of tornadoes, while others may be punctuated by near-incessant tornado swarms.
May is part of a “fringe” season, which is more apt to have severe weather. In the fringe, winter has mostly retreated into Canada, but high-altitude temperatures, and readings in the Rockies, are often still frigid. At the surface, meanwhile, warm, moist air begins wafting north from the Gulf of Mexico. The resulting clash of opposing air masses can brew violent thunderstorms that tower more than 10 miles high.
The secret behind tornado formation, however, stems from the jet stream. Tornadoes require rotating thunderstorms known as supercells, which exist in an environment full of shear — or a change of wind speed and/or direction with height.
That shear can be found near the jet stream, or a river of winds in the upper atmosphere. The jet stream oscillates north and south with the changing of the seasons. During the winter, it dives south over the contiguous United States, but it retreats into Canada in the summer.
By late spring, the jet stream is somewhere in between, having begun its northward migration. That additional wind energy in the upper atmosphere helps sculpt thunderstorms into rotating supercells, and tornadoes can quickly follow.
May so far
After a virtually silent start to the month, May 10-13 was busy with storms.
A lobe of high-altitude cold air, low pressure and spin — known as a shortwave — dived out of Colorado, causing pockets of surface air below to rise. In Colorado, northwest Kansas, southwest Nebraska and Iowa, the extreme availability of twist caused even low-topped, shallow storms to rotate profusely.
That produced dozens of tornadoes, including a highly photogenic wedge tornado Friday near Spaulding, Neb.:
Large Wedge Tornado Crossing Highway 281 near Spaulding, NE - Contact @koryhartman for licensing at @SevereStudios pic.twitter.com/GxLmEvkiKl— Norman Smith (@Norman_Wx) May 13, 2023
Renowned storm chaser and meteorologist Reed Timmer deployed his “tornado intercept vehicle” in the path of the twister, braving wind gusts of 150 mph and collecting air-pressure data:
The most intense Dominator #tornado intercept yet https://t.co/dStnxD9B5P pic.twitter.com/68jrJWLEyx— Reed Timmer, PhD (@ReedTimmerAccu) May 12, 2023
A string of tornadoes touched down in Iowa on Saturday beneath the same cold-core-low setup:
Timelapse of the Knoxville, Iowa tornado producing supercell as seen from Harvey, Iowa 5/13/2023 #iawx pic.twitter.com/2IHC6ZrsAf— Murphy Roberts (@MurphyRobertsWx) May 14, 2023
Several of the tornadoes featured several vortices dancing around a common center:
A multi-vortex tornado yesterday in NW Iowa pic.twitter.com/WAIUikS38A— George Ilstrup (@georgeilstrup) May 14, 2023
Removed from the cold-core setup, the same sprawling parent low-pressure system also dragged a “dryline” into central Oklahoma on Thursday night, forming a dozen or more tornadoes near and south of the Oklahoma City metro area. This dryline is the leading edge of bone-dry air from the Desert Southwest encroaching on moisture-rich Gulf of Mexico air to the east. The colliding air masses again brought storms, several of which tapped into jet-stream energy around sunset and began to spin:
One of the tornadoes hit Cole, Okla., and was rated an EF1 on a 0-to-5 scale, meaning it had winds of 105 mph. Cole was also hit on April 19 by a killer EF3 tornado.
Thus far, eight tornadoes have been confirmed by the National Weather Service in Norman, Okla., but more may be tallied as investigations and surveys continue.
All told, the four-day outbreak over the central United States resulted in 94 preliminary reports of tornadoes, though many are likely duplicates.
So far, in the medium-to-long range — i.e., the next week to 10 days — no large-scale severe weather setups are anticipated.
In fact, the National Weather Service Storm Prediction Center, which forecasts severe thunderstorms up to eight days in advance, notes that broad-scale severe weather is unlikely.
“Surface high pressure would spread southeastward out of Canada, and appears likely to encompass most of the central and eastern [contiguous United States],” the center wrote. “This [cool high pressure] would then limit severe potential through the latter stages of the medium range.”
A dip in the jet stream, with persistent northwesterly flow over the Plains, should also scour away most of the warm, humid Gulf of Mexico air. That would limit how much instability, or fuel, would be available for severe thunderstorms.
The overarching pattern looks to predominate into late May, with uncertainty abundant into the start of June.