Severe storm basics: tornadoes, hail, damaging winds, and when they strike

Severe storms occur during a severe thunderstorm. There are three types of severe thunderstorms.

• Those spawning tornadoes, any confirmed tornado spawned by a storm system.
• Those spawning hailstorms, hail stones one inch or greater and
• Those with damaging winds at least 50 knots (93 km/hr)

How severe storm intensity is measured.

Tornado

Intensity is defined by the Enhanced Fujita Scale (NWS)

EF0 (Light)

65-86 mph (105-137 km/h) - Breaks tree branches, damages gutters, and strips off some roof shingles. Approximately 600+ per year, occurrence odds 1 in 1.9

EF1 (Moderate)

87-110 mph (140-177 km/h) - Uproots small trees, pushes mobile homes off foundations, and peels off roofs. Approximately 400+ per year, occurrence odds 1 in 2.7

EF2 (Considerable)

111-135 mph (179-217 km/h) - Snaps large trees, tears roofs off frame houses, and flips cars. Approximately 120+ per year, occurrence odds 1 in 11

EF3 (Severe)

136-165 mph (219-266 km/h) - Destroys entire stories of well-built houses, overturns trains, and tosses heavy cars. Approximately 30 per year, occurrence odds 1 in 49

EF4 (Devastating)

166-200 mph (267-322 km/h) - Levels well-constructed houses and turns structures into flying debris. Approximately 7 per year, occurrence odds 1 in 222

EF5 (Incredible)

Over 200 mph (322+ km/h) - Sweeps away strong frame houses, carries large steel-reinforced buildings away, and strips bark from trees. Less than 1 per year, occurrence odds 1 in 2393

Hail and windstorm

Hail intensity is measured by the hail size, while windstorm intensity is measured by the wind speed.

How severe thunderstorms form.

Severe thunderstorm formation comes down to four key ingredients coming together (moisture, instability, lift, and wind shear) and the specific combination determines whether you get an ordinary thunderstorm or an organized severe storm capable of large hail, damaging wind, or tornadoes.

• Moisture - Warm, humid air near the surface provides the fuel. This is why severe weather in the US is so concentrated in spring/early summer over the Plains and Midwest
• Instability - This is the atmosphere's willingness to let a rising air parcel keep accelerating upward rather than sinking back down. It develops when warm, moist air sits near the surface underneath cooler, drier air aloft.
• Lift - Something has to actually kick the air parcel upward to get convection started, since instability alone is potential energy, not motion. Common lifting mechanisms:
  • Cold fronts - dense cold air wedges under warm air, forcing it up.
  • Drylines - a boundary between moist and dry air masses (classic in the Southern Plains), where the denser dry air undercuts the moist air.
  • Daytime heating - surface heating alone can be enough to break a weak capping inversion.
  • Outflow boundaries - cold air pools spreading out from existing thunderstorms can trigger new ones nearby.
• Wind shear - This is what separates an ordinary, short-lived "pulse" thunderstorm from an organized severe storm. Shear (change in wind speed and/or direction with height) tilts the storm's updraft.

Storm organization types.

There are three types of storm organization types that vary by severity potential:

• Single-cell (pulse) storms - brief, disorganized, minimal shear can still produce isolated severe hail/wind but short-lived.
• Multicell clusters/lines - a series of updraft/downdraft cells; includes squall lines and bow echoes, the classic producers of widespread damaging straight-line wind (derechos are an extreme version of this)
• Supercells - a single, long-lived rotating updraft (mesocyclone); the most dangerous storm mode, responsible for most strong/violent tornadoes and giant hail, sustained by strong deep-layer wind shear.

Severe storm activity by season.

The centroid of severe thunderstorm activity changes with the season:

Late winter (Dec-Feb): Gulf Coast / "Dixie Alley"

Mississippi, Alabama, and Louisiana see an early-season spike, sometimes starting as early as February, driven by the jet stream still sitting far enough south to interact with Gulf moisture at these latitudes.

Spring (Mar-May): Southern Plains

Research has indicated that peak tornado activity in this area has shifted about 7 days earlier over the past six decades.¹

As the jet stream migrates northward, the activity centroid shifts into Texas, Oklahoma, Kansas, and Arkansas, the classic "Tornado Alley" core. April is typically when the year's most violent outbreaks concentrate, since this is when Gulf moisture, Plains instability, and strong jet-driven shear are all simultaneously at their most favorable overlap.

Late spring/early summer (May-Jun): Central/Northern Plains and Midwest

The centroid continues migrating north into Nebraska, Iowa, South Dakota, and the upper Midwest as the jet stream retreats poleward with the advancing season.

June is climatologically the peak month for the central/northern Plains specifically.

Summer (Jul-Aug): Upper Midwest / Great Lakes

By mid-to-late summer, the jet stream has retreated well north, and the severe weather focus shifts further into the Dakotas, Minnesota, Wisconsin, and the Great Lakes region, often shifting from tornado-dominant to more damaging-wind-dominant (derecho-prone) storm modes.

Fall: reversal back toward the Gulf Coast/Southeast

As the jet stream migrates south again in autumn, a secondary (smaller) severe weather season redevelops across the Southeast and Gulf Coast.

Severe storm records.

The largest hailstone on record is the Vivian, South Dakota hailstone (July 23, 2010) 8 inches (20 cm) in diameter, 18.625-inch circumference, and 1.94 lbs.²

The largest tornado outbreak on record can be split two ways:

• By total tornado count / geographic extent: the 2011 Super Outbreak (April 25-28, 2011), 360+ tornadoes confirmed over 3 days across the Southeast/Midwest, the most extensive tornado outbreak on record. April 27 alone produced 219 tornadoes.³
• By violence/intensity: the 1974 Super Outbreak (April 3-4, 1974), 148 confirmed tornadoes in just ~18 hours but including a record 7 F5s and 23 F4s (30 violent tornadoes total), still the most violent outbreak ever recorded. The record has held since 1974; 2011's outbreak (which had 15 EF4-EF5s) didn't surpass it.⁴⁵⁶

Largest severe windstorm (derecho)

• By path length: the 1999 Boundary Waters (BWCA) derecho, ~1,300 miles, the longest tracked derecho path on record.⁷
• By spatial extent (dual-line event): the April 4-5, 2011 serial derecho, two simultaneous lines that at one point stretched a combined 800+ miles, generating 1,096 wind reports (a record at the time) alongside 46 tornadoes.⁸

Climate change and severe thunderstorms.

Severe thunderstorms are expected to increase and become more intense due to climate change because of more moisture in the atmosphere leading to heavy downpours, and stronger updrafts increasing atmospheric instability and causing storms to be larger and stronger.⁹

For the US models show an increase in environments favorable for severe thunderstorms, especially in eastern and southern regions.¹⁰