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Large. long-lived thunderstorm complexes forecast for Central U.S.

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With summer heat building, it’s the season of “mesoscale convective systems,” or large, often intense, long-lived thunderstorm complexes. For residents of the central United States, this means a daily chance of torrential rain, strong to locally damaging winds and occasional hail and tornadoes.

These systems, which can cover an entire state, also have the potential to incite flash flooding. It is often challenging to predict exactly where these systems will develop, decay and regenerate and how strong they’ll become.

The National Weather Service Storm Prediction Center has declared an elevated risk of severe thunderstorms in the central states daily through Friday, largely because of the potential for these systems. The Weather Service has also placed large parts of the central United States in an elevated risk zone for excessive rainfall through at least Wednesday.

In addition to damaging winds and heavy rainfall, these systems can generate tremendous amounts of lightning and have set records for “megaflashes” — or bolts spanning hundreds of miles.

World record 477-mile-long lightning ‘megaflash’ confirmed over U.S.

A mesoscale convective system starts as a single thunderstorm cell or a broad cluster that becomes organized and bows out into a curved line. Thunderstorm downdrafts all interact to produce a single outflow boundary — or the leading edge of cool-air exhaust exiting the storms — as a squall line evolves. The squall taps into jet stream energy from aloft, usually moving parallel to or to the right of the upper-level winds, and transfers that momentum to the surface in the form of damaging wind gusts.

The surge of winds pushes the middle of the thunderstorm complex outward, causing the entire line to take on its signature backward-C shape.

It’s not unusual for widespread winds of 60 to 80 mph to accompany an MCS, along with prolific lightning. Most MCSs rage all night long before dying around sunrise as the low-level jet stream, which helps fuel the thunderstorms, weakens.

Most days this week there’s the potential for mesoscale convective systems to develop or regenerate. Here’s the day-by-day breakdown:

  • Setup and hazards: A decaying mesoscale convective system was rolling through central Missouri with downpours and a bit of thunder Monday morning, but daytime heating is expected to rejuvenate it over the Tennessee Valley. In the wake of the MCS, increasing instability, or fuel for storms, could favor a few supercell or rotating thunderstorms with a low-end tornado, hail and wind risk.
  • Areas affected: A Level 2 out of 5 slight risk of severe thunderstorms is in effect for parts of the Tennessee Valley, including Evansville, Ind., and Nashville and Jackson, Tenn. A lesser marginal risk extends from Detroit to near Little Rock. Much of the High Plains are also included in a slight risk zone.
  • Additional risk: There will also be isolated to widely scattered supercell thunderstorms over the High Plains, including in western Kansas and Nebraska, the Black Hills of South Dakota and extreme northeast Wyoming. Those could produce strong winds and hail as large as hen eggs, along with an isolated tornado. Thunderstorms could eventually merge into multiple windy MCSs — one in southern Nebraska that should head toward Kansas City, and perhaps another in the Texas and Oklahoma panhandles that heads toward the Red River.
  • Setup and hazards: A contrast between dry air banked over the mountains to the west and gulf moisture streaming north over the Plains will brew strong to severe thunderstorms. Wind shear, or a change of wind speed or direction with height, favors supercells. Those supercells could produce damaging straight-line winds, hail as large as pool balls and a few tornadoes. Eventually, storms may merge into another MCS that will shift east with time overnight and bring strong winds. A few strong thunderstorms could fire as far east as Memphis along Interstate 40.
  • Areas affected: The central High Plains will once again be affected, with a Level 2 out of 5 severe storm risk drawn for parts of the Texas and Oklahoma panhandles, western Kansas and western Nebraska. Liberal, Garden City and Colby, Kan.; and McCook, Kearney and North Platte, Neb.; and much of Interstate 80 are within that risk zone.
  • Hail-producing supercells developing over the southern Plains could combine to form another MCS during the evening. A Level 1 out of 5 risk of severe weather covers the zone from roughly Pueblo, Colo., into northeast New Mexico and adjacent parts of the Texas Panhandle.
  • A second Level 1 risk area has been drawn from northeast Oklahoma east through Arkansas and southern Missouri into western Tennessee and northern Mississippi and Alabama. Here, an MCS could be ongoing in the morning and strengthen with the rising sun.
  • The remnants of this MCS could reach the Mid-Atlantic on Wednesday night, but confidence is low.
  • For Thursday, the Storm Prediction Center wrote that “a severe MCS appears likely to develop somewhere across NE/KS and shift southeast into OK overnight.”
  • An elevated risk of severe weather covers most of Nebraska, Kansas and Oklahoma, including much of the heavily populated Interstate 35 stretch. Oklahoma City, Tulsa and Woodward, as well as Wichita, Colby, Hays and Garden City, Kan., and McCook and Hyannis, Neb., are included in the risk.
  • On Friday, re-intensification of Thursday night’s MCS, or redevelopment near a weak low pressure swirl left behind by the storms, is expected. The environment would support severe weather.
  • An elevated severe weather risk Friday covers the zone between Little Rock and northwestern Alabama, including Huntsville, Birmingham and Hoover, as well as northern Mississippi.

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