Severe Thunderstorm Warnings: Global Criteria and Safety Protocols
A severe thunderstorm warning is a critical alert indicating that a specific area is in immediate danger from hazardous weather. While these storms are primarily characterized by damaging winds and large hail, they also bring risks from lightning and hydrological impacts, such as flash flooding. It is important to note that severe thunderstorms can occasionally produce tornadoes without prior warning.
Although not every severe storm spawns a tornado, they can generate powerful straight-line winds. These winds can cause damage comparable to a hurricane or a low-category tornado, often impacting a much wider geographic area than the narrow path of a typical tornado.
When meteorologists detect a tornado via radar or visual sighting, a tornado warning is issued, either replacing or running alongside the severe thunderstorm warning. In most cases, a "watch" (a notification that conditions are favorable for severe weather) precedes the actual "warning" (a notification that the weather is occurring or imminent).
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Key Facts
- Severe Thunderstorm Warning: Issued when hail or wind speeds meet specific danger criteria.
- Tornado Relationship: Severe thunderstorms can produce tornadoes, but a specific Tornado Warning is issued when strong rotation is detected via Doppler radar.
- Straight-Line Winds: These can be as destructive as lower-category tornadoes but cover larger areas.
- US Criteria: A storm is "severe" if it has hail ≥ 1 inch or wind speeds ≥ 58 mph.
- Impact-Based Warnings: Modern systems now use tags like "Considerable" or "Destructive" to communicate the level of threat to the public.
Severe Weather Systems in the United States
In the U.S., the National Weather Service (NWS) manages these alerts. Since January 2010, the hail threshold for a severe warning has been one inch (2.5 cm) in diameter. This was increased from 0.75 inches to reduce public complacency and better align with actual ground damage data.
Warning Dissemination and Technology
Local NWS offices use WarnGen software within the Advance Weather Interactive Processing System (AWIPS) to create warnings. These are distributed via NOAA satellites, NOAA Weather Radio, the internet, and the Emergency Alert System (EAS), including Wireless Emergency Alerts (WEA) for mobile phones.
Warnings are mapped as polygonal shapes based on the storm's projected path from Doppler radar. In text reports, these are often listed by county or parish. On visual maps, these areas are typically highlighted in yellow or orange.
High-Intensity Alerts and Impact-Based Warnings (IBW)
For extremely dangerous storms, the NWS may use enhanced wording such as "particularly dangerous situation" (PDS) or "severe thunderstorm emergency." These are reserved for storms likely to cause major structural damage or death to those outdoors.
The Impact Based Warning (IBW) system, fully implemented by 2016, adds specific tags to the bottom of warnings to clarify the threat:
- Tornado: Indicates a mesocyclone (a rotating updraft) is producing circulation near the cloud base.
- Hail: Lists the estimated size (e.g., "HAIL...1.25IN") and compares it to common objects, such as baseballs.
- Wind: Lists estimated gust speeds (e.g., "WIND...70MPH") and describes potential damage to trees and mobile homes.
As of July 2021, two categorical damage indicators were added for extreme events:
- Considerable: Hail ≥ 1.75 inches or winds ≥ 70 mph.
- Destructive: Hail ≥ 2.75 inches or winds ≥ 80 mph. These warnings include the phrase "THIS IS A DESTRUCTIVE STORM FOR[...]" and may trigger Wireless Emergency Alerts.
International Severe Thunderstorm Criteria
Different countries employ varying thresholds to define a "severe" thunderstorm based on regional climate and risk factors.
Canada, UK, and Australia
In Canada, severe storms are defined by wind gusts over 90 km/h, hail over 2 cm, or heavy rainfall (50mm/hr or 75mm/3hrs). Environment and Climate Change Canada disseminates these via the National Public Alerting System (Alert Ready). In the United Kingdom, the Met Office defines severity as hail over 2 cm or winds over 89 km/h, using a color-coded system (yellow, amber, red).
Australia's Bureau of Meteorology (BOM) sets the threshold at winds over 90 km/h, hail ≥ 2 cm, or conditions conducive to flash flooding. They provide both broad-based and detailed warnings with 60-minute forecast movement maps.
New Zealand and the Philippines
New Zealand's MetService defines severe storms as having hail of 20 mm, winds of 110 km/h, or rainfall of 25 mm/hr. They also include tornadoes rated F1 or stronger on the Fujita Scale. In the Philippines, PAGASA issues advisories for strong winds, large hail, torrential rain, and landslides, utilizing the Emergency Cell Broadcast System.
| Country | Tornado Criteria | Min. Hail Size | Min. Wind Speed | Min. Rainfall Rate/Hr |
|---|---|---|---|---|
| United States | See Tornado Warning | 1 inch (25 mm) | 58 mph (93 km/h) | See Flash Flood Warning |
| Canada | See Tornado Warning | 20 mm (0.79 in) | 90 km/h (56 mph) | 50 mm (2.0 in) |
| Australia | All | 20 mm (0.79 in) | 90 km/h (56 mph) | Conducive to flash flooding |
| New Zealand | F1 or stronger | 20 mm (0.79 in) | 110 km/h (68 mph) | 25 mm (0.98 in) |
| Germany | All | 15 mm (0.59 in) | 105 km/h (65 mph) | 20 mm (0.79 in) |
| Mexico | All | 5 mm (0.20 in) | 89 km/h (55 mph) | 20 mm (0.79 in) |
| United Kingdom | All | 20 mm (0.79 in) | 55 mph (89 km/h) | Conducive to flash flooding |
Frequently Asked Questions
What is the difference between a severe thunderstorm watch and a warning?
A watch means that atmospheric conditions are favorable for severe thunderstorms to develop. A warning means a severe thunderstorm has actually been detected or is imminent, and immediate action should be taken.
Can a severe thunderstorm produce a tornado without a warning?
Yes, severe thunderstorms can occasionally produce tornadoes without advance warning, which is why safety precautions are advised even during non-tornadic severe thunderstorm warnings.
What does a "Destructive" tag mean in a US weather warning?
A "Destructive" tag indicates an extreme event where radar or observations show hail of 2.75 inches or larger, or wind speeds of 80 mph or greater.
How is hail size measured and communicated?
Hail is measured by diameter. To help the public visualize the risk, the NWS compares hail sizes to common objects, such as pennies (0.75 inches) or baseballs (2.75 inches).
What is a mesocyclone?
A mesocyclone is a rotating updraft within a thunderstorm. If this rotation strengthens and reaches the cloud base, it may lead to the issuance of a tornado warning.