The term "severe thunderstorm" is often used by meteorologists to describe intense thunderstorms that are capable of producing damage. 

What makes a thunderstorm "severe" exactly? 

This article explains the criteria that qualify a thunderstorm as being "severe" in the United States, as opposed to a "strong" or "ordinary" thunderstorm. We also take a look at severe thunderstorm criteria in other parts of the world.

Severe Thunderstorms – U.S. National Weather Service Definition

For a thunderstorm to be considered severe in the United States, it must meet at least one of these three criteria...

1. Straight-line wind gusts of 58 mph (50 knots) or higher
   
   
2. Hail of 1-inch (quarter size) or larger in diameter
   
   
3. Tornado

The following thunderstorm hazards can also occur with severe thunderstorms but do not alone qualify a storm as being severe in the U.S...

• Frequent cloud-to-ground lightning
  
  
• Heavy rain
  
  
• Flash flooding

However, outside of the U.S., some countries consider these criteria as "severe" (we cover this near the end of the article).

Let's examine the three severe weather criteria used by the U.S. National Weather Service...

Straight-line wind gusts of 58 mph (50 knots) or higher

This is the most common type of severe weather from thunderstorms. These are typically outflow winds that form from the downdrafts of thunderstorms, which spread out horizontally across the surface, often in the form of "gust" fronts that are similar to cold fronts.

Thunderstorm squall lines, which take the shape of long thunderstorm lines on radar, often making a "bow" signature, are common sources of powerful straight-line winds. Strong straight-line winds typically occur at the leading edge of thunderstorms, often before rain begins.

Wind gusts of 50 knots or 58 mph are considered the minimum criteria as winds of these magnitudes can knock down trees and power lines, and damage buildings. During particularly intense storms, wind gusts can exceed 80 mph and cause widespread damage. 

Powerful straight-line winds are often accompanied by hail, heavy rain, and frequent lightning. However, damaging straight-line wind gusts in excess of 58 mph can also occur beneath seemingly "weak" dry thunderstorms due to the evaporation of moisture in thunderstorm downdrafts, which causes the falling air to become cooler and more dense, accelerating as a result.

National Weather Service meteorologists issue severe thunderstorm warnings based on wind (meaning it is imminent or occurring) using Doppler radar, which can detect wind velocities. Also, observed wind gust measurements of 58 mph or greater from weather stations in the path of the storm factor can trigger warnings and verify a storm as being severe. 

Shelf clouds (pictured above) are often precursors to powerful straight-line wind events, typically occurring at the leading edge of squall lines. Shelf clouds can be identified as elongated cloud base lowerings that extend a great distance horizontally, as opposed to wall clouds which occur over a smaller area. Source: Image Source NWS Milwaukee

Hail of 1 inch (quarter size) or larger in diameter

Hail forms when raindrops are carried upwards by a thunderstorm's updraft into cold air aloft, which causes the rain droplets to freeze. Hailstones grow by colliding with water droplets that freeze upon contact with the hailstone.

The hailstone will eventually fall when the thunderstorm updraft can no longer support its weight. Severe thunderstorms often have very strong updrafts that can lead to hailstones growing quite large before falling to earth.

Hailstones reaching or exceeding 1 inch in diameter can cause significant damage to vehicles, property, gardens, and crops and can injure humans and animals. 

National Weather Service meteorologists issue severe thunderstorm warnings due to hail based on Doppler radar which can detect hail "signatures" and by the Multi-Radar/Multi-Sensor (MRMS) system that is used to estimate hail size. Also, first-hand reports of severe hail by trained spotters and the public factor into warnings and verification.

Also, in advance of thunderstorms, atmospheric criteria can be identified that is supportive of large hail, such as instability in the atmosphere (high instability supports strong updrafts), wind shear (increasing winds with height can sustain a thunderstorm's updraft), and the height of the freezing level above the surface (lower freezing levels are more supportive of hail).

Quarter-size or larger hail in diameter is sufficient for a thunderstorm to be considered "severe" due to its ability to produce damage. Sometimes, hail can be much larger than quarter-size, occasionally even reaching golf ball (2-inch diameter) or baseball (4-inch diameter) size, which can cause extensive damage. Source: Image Source NWS Cheyenne

Tornadoes

Tornadoes are the rarest but most destructive form of severe weather. Even though they are less common than other forms of severe weather, the United States still averages a total of 1,200 tornadoes per year which is significant.

A tornado is a violently rotating column of air extending vertically from the ground to the base of a thunderstorm. Tornadoes often have a visible funnel with a rotating cloud of debris from the ground, but in some instances may be completely obscured by rain. 

A tornado has its own category of watches and warnings. While large hail and/or strong straight-line winds will trigger severe thunderstorm watches/warnings, tornadoes will trigger tornado watches/warnings.

National Weather Service meteorologists issue tornado warnings based on Dopper radar signatures, which include "hook" echoes" and wind velocity couplet signatures (a couplet of strong winds blowing in opposite directions, which indicates rotation). Also, first-hand reports of ongoing tornadoes from trained spotters are relied upon for warnings and verification.

If a storm produces a tornado, it is by definition considered to be "severe". However, if a storm is producing or capable of producing a tornado, a Tornado Warning will be issued rather than a Severe Thunderstorm Warning. Image source: NWS Rapid City

Tornadoes produce very strong and damaging winds and strong tornadoes can destroy well-built structures. However, tornadic winds are not the same as straight-line winds. Winds that occur with tornadoes have a rotational element and are usually more intense than straight-line winds, though straight-line winds impact larger areas than tornadoes. 

Thunderstorm Hazards That Do Not Qualify a Storm as Severe in the U.S.

There are several other thunderstorm hazards that can occur with both ordinary and severe thunderstorms, but do not qualify a storm as being "severe" by itself. These hazards should still be taken very seriously, though, especially if you are spending time outdoors.

Lightning

This might be the biggest severe weather misconception in the United States. A thunderstorm can produce frequent cloud-to-ground lightning over a prolonged period, and if you are in its path it will feel like an intense storm.

In the event of a severe thunderstorm, there is a high likelihood that frequent lightning will accompany the storm. However, frequent cloud-to-ground lightning does not make a storm "severe" by U.S. definitions if wind or hail criteria are not met.

Lightning is one of the greatest weather hazards to outdoor recreation, but it is not a qualifier for severe thunderstorms in the U.S. Image source: National Park Service

Heavy Rain and Flash Flooding

Severe thunderstorms can certainly produce heavy rainfall and flash flooding, but these do not always go hand-in-hand.

Sometimes, severe thunderstorms move quickly enough that only brief downpours occur without any risk of flooding. Also, dry thunderstorms may produce damaging winds but little to no rain.

On the other hand, non-severe thunderstorms that develop in high-moisture environments with weak upper level winds (i.e. slow-moving or stalled thunderstorms) can result in prolonged heavy rainfall that can lead to flash flooding. 

The U.S. National Weather Service issues separate watches and warnings for flash flooding, rather than lumping this in with severe thunderstorm watches and warnings. 

Flash flooding is one of the deadliest weather hazards in the U.S. which is why it has its own warning system.

Sub-Severe Hail and Wind

Strong winds and moderate-sized hail can still cause problems even if they don't technically meet severe standards. The National Weather Service will often issue "Special Weather Statements" for thunderstorms that produce hail of less than an inch in diameter and wind gusts of 40-50 mph, along with frequent lightning. These types of thunderstorms can still be very impactful if you are spending time outdoors. 

The Difference Between a Watch and a Warning

Severe weather watches and warnings are often a source of confusion.

A Severe Thunderstorm Watch is issued if atmospheric conditions are favorable for the development of severe thunderstorms. Watches cover larger geographical areas and longer timeframes, often several hours or longer. This is meant to give you lead time to prepare for the possibility of severe weather.

A Severe Thunderstorm Warning is issued if a severe thunderstorm is imminent or occurring. Warnings are issued for more specific areas that are expected to see impacts from the specific thunderstorm very soon. If a warning is ever triggered for your location, it means you should take cover immediately.

Tornado watches and warnings have the same guidelines as severe thunderstorm watches and warnings, except they are issued specifically for tornado potential (watches) and for imminent/occurring tornadoes (warnings).

The same watch and warning system is used for flash flooding as well.

Severe Weather Criteria Outside of the U.S.

Weather services outside of the U.S. have their own criteria for what defines a severe thunderstorm, and these criteria often vary at least a little bit from the U.S.

• Canada – The main difference is the inclusion of heavy rainfall as part of its criteria. Environment Canada (the Canadian Weather Service) classifies thunderstorms as "severe" if they produce any of the following... 1) wind gusts of 90 km/hour (56 mph) or higher, 2) hail of 2 cm (0.8 inches) in diameter or larger, or 3) heavy rainfall.
  
  The criteria for heavy rainfall varies depending on regions and provinces, however. Tornadoes are noted separately in their warning system (similar to the U.S.) but likely are used to classify a storm as "severe" after the fact.
  
  
• Europe – The European Severe Storms Laboratory uses the same wind and hail criteria as Canada, and they also factor in heavy rainfall (a minimum of 25 mm/1" in 30 minutes, or 35 mm/1.4" in 1 hour) and tornadoes. The most notable difference is that they also factor in lightning if it has caused property damage or injuries to humans or animals.
  
  
• Australia – The Australia Bureau of Meteorology uses the same wind and hail criteria as Canada and Europe, though they explicitly state that tornadoes are included in their criteria as well. Also, heavy rainfall is considered, though a specific threshold is not mentioned.
  
  
• New Zealand – The New Zealand Met Service differs slightly from others, and uses the following criteria for severe thunderstorms... 1) wind gusts of 110 km/hour (68 mph) or higher, 2) hail of 2 cm (0.8 inches) in diameter or larger, heavy rain of at least 25mm (1") per hour, or tornadoes with wind speeds of 116 km/hour (72 mph) or higher.

Bottom Line – Severe weather forecasts, watches, and warnings should be taken seriously and factored into plans and decision-making. You should always be prepared to take cover immediately in the event of a severe thunderstorm. However, ANY thunderstorm can be dangerous if you're spending time outside, even if it's not a severe thunderstorm by definition.

Alan Smith