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

With the increase in summer heat, this is the season for “medium-scale convective systems”, or large, often intense and long-lasting storm complexes. For residents of the central United States, that means a daily risk 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 cause flash flooding. It is often difficult to predict exactly where these systems will grow, decay and regenerate and how strong they will become.

The National Weather Service Storm Prediction Center said a high risk of severe thunderstorms in central states every day through Friday, largely due to the potential of these systems. The weather service also placed large parts of the central United States in a high risk zone for excessive rainfall until at least Wednesday.

In addition to damaging winds and heavy rain, these systems can generate huge amounts of lightning and have set records for “megaflashes” – or lightning spanning hundreds of miles.

World record 477 mile ‘megaflash’ lightning confirmed over US

A mesoscale convective system begins as a single thunderstorm cell or large cluster that organizes and tilts in a curved line. The downdrafts of thunderstorms all interact to produce a single outflow boundary – or the leading edge of the exhaust of cool air exiting the storms – as a squall line evolves. The gust taps into the energy of the jet stream aloft, typically moving parallel or to the right of upper-level winds, and transfers that momentum to the surface in the form of damaging gusts of wind.

The thrust of the winds pushes the middle of the storm complex outward, causing the entire line to take its characteristic C-shape backwards.

It is not uncommon for widespread winds of 60-80 mph to accompany an MCS, as well as prolific lightning. Most MCS rage through the night before dying off at sunrise as the low-level jet stream, which helps fuel thunderstorms, weakens.

On most days this week, mesoscale convective systems may develop or regenerate. Here is the day-to-day breakdown:

  • Setup and Hazards: A decaying medium-scale convective system moved through central Missouri with showers and some thunder Monday morning, but daytime heating should rejuvenate it in the Tennessee Valley. In the wake of the MCS, growing instability, or fuel for storms, could favor a few rotating supercells or thunderstorms with the risk of tornado, hail and low-end winds.
  • Areas concerned: A slight risk of a Level 2 of 5 severe thunderstorm is in effect for parts of the Tennessee Valley, including Evansville, Ind., and Nashville and Jackson, Tenn. Less marginal risk extends from Detroit to near Little Rock. Much of the high plains are also included in a low risk zone.
  • Additional risk: There will also be isolated to widely scattered supercells over the high plains, including western Kansas and Nebraska, the Black Hills of South Dakota, and extreme northeast Wyoming. These could produce strong winds and hail as big as chicken eggs, as well as a lone tornado. The thunderstorms could eventually merge into multiple blustery MCSs – one in southern Nebraska that is expected to track toward Kansas City, and perhaps another in the Texas and Oklahoma panhandles that tracks toward the Red River.
  • Setup and Hazards: A contrast between dry air pooling over the mountains to the west and moisture from the gulf flowing north over the plains will produce strong to severe thunderstorms. Wind shear, or a change in wind speed or direction with height, favors supercells. These supercells could produce damaging straight-line winds, hail as big as billiard balls and a few tornadoes. Eventually the storms could merge into another MCS that will move eastward over time overnight and bring strong winds. A few severe thunderstorms could kick in as far east as Memphis along Interstate 40.
  • Areas concerned: The central high plains will again be impacted, with a level 2 out of 5 severe storm risk established for parts of the Texas and Oklahoma panhandles, western Kansas and western Nebraska. Liberal, Garden City and Colby, Kansas; and McCook, Kearney and North Platte, Neb. ; and much of Interstate 80 is in this risk area.
  • Hail-producing supercells developing over the southern plains may combine to form another MCS during the evening. A level 1 in 5 severe weather hazard covers the area from approximately Pueblo, Colorado to northeastern New Mexico and adjacent portions of the Texas Panhandle.
  • A second Level 1 risk zone has been drawn from northeast Oklahoma to eastern Arkansas and from southern Missouri to western Tennessee and northern Mississippi and Alabama. . Here, an MCS could be underway in the morning and strengthen with the rising sun.
  • Remnants of this MCS could reach the mid-Atlantic Wednesday evening, but confidence is low.
  • For Thursday, the Storm Prediction Center wrote that “a severe MCS appears likely to develop somewhere across NE/KS and move southeast towards OK overnight.”
  • A high risk of severe weather covers most of Nebraska, Kansas, and Oklahoma, including much of the heavily populated stretch of Interstate 35. 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, a reintensification of Thursday evening’s MCS, or redevelopment near a weak low pressure eddy left behind by the storms, is expected. The environment would withstand severe weather events.
  • A high risk of severe weather on Friday blankets the area between Little Rock and northwestern Alabama, including Huntsville, Birmingham, and Hoover, as well as northern Mississippi.

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