In the U.S. there are roughly 2,300 tornado warnings issued each year. Some years there are more, and some less. However, there are 1,200 tornadoes confirmed on average each year. Some tornadoes are brief spin-ups that cause very little damage, and others cause catastrophic damage, wiping homes completely off their foundations.
When a tornado warning is issued you should always take it seriously and seek shelter, but not all tornado warnings are created equally. Knowing the multiple types of warning can help you understand more about this urgent situation.
What Prompts A Tornado Warning?
A tornado warning is issued by the National Weather Service (NWS) when a tornado has been sighted and/or indicated by radar. Each tornado warning that is issued will indicate if the warning was "radar indicated rotation" or "observed," which means it was spotted by a trained storm spotter or law enforcement.
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A "Radar Indicated" Tornado Warning
Most tornado warnings cite "radar indicated rotation" as the reason the warning was issued. This does not mean that you should wait until the tornado is "observed" to take action. This type of warning is just as serious as any, so you should seek shelter immediately.
A radar indicated tornado means a severe thunderstorm is exhibiting signs of strong rotation on Doppler radar, which indicates that a tornado could be in progress or that one could form at any time.
When "radar indicated rotation" is cited in the warning, it's not yet known if an actual tornado has formed, but the NWS forecaster wants to provide as much lead time as possible before a tornado occurs to keep you safe.
(MORE:Tornado Watch Vs. Warning)
Types Of Warnings For Confirmed Tornadoes
A tornado can be confirmed as in progress in two different ways: radar confirmed and spotter confirmed.
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Radar Confirmed Tornado: This will appear in the text of some warnings that are issued based on radar. It means Doppler radar is showing signs that debris has been lofted into the air near the area of low-level rotation in a storm, giving the NWS high confidence a tornado is in progress.
Spotter Confirmed Tornado: Sometimes, you will see "confirmed tornado from trained spotters" as the source cited in a tornado warning. That means a tornado has been spotted by a trained spotter and/or law enforcement.
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The Highest Level Tornado Warnings
The following types of tornado warnings are extremely rare and indicate the highest level of tornado warning issues. These are reserved for the most dire situations, when a tornado is either confirmed and showing major damage, and/or a tornado has the potential to be very strong and life-threatening.
Particularly Dangerous Situation (PDS) Confirmed Tornadoes: You might see this line in warnings for radar confirmed or spotter confirmed tornadoes that are likely to be strong. These particular warnings are rare and an elevated version of a tornado warning that is meant to suggest an unusually high threat of damage and loss of life exists for a particular storm.
A Tornado Emergency: If a tornado warning indicates a "tornado emergency," know that this is the rarest type of warning and is reserved for the most dire situations. This is only issued "when there is a severe threat to human life and catastrophic damage from an imminent or ongoing tornado", the NWS says.
A tornado emergency can be issued based on a confirmed sighting of a tornado or one that has been confirmed by radar.
The first tornado emergency ever issued was by the NWS office in Norman, Oklahoma, during theMay 3, 1999 outbreak.The warning was issued for a tornado that caused F5 damage just south of Oklahoma City.
Warnings Can Change Types
Tornado warnings can change throughout the life of a tornado.
For example, a tornado warning might first be issued as a "radar indicated" tornado, based on radar rotation alone.
Then, the NWS might receive credible information that a tornado has been sighted or has been confirmed by debris on radar. They will then update the messaging in that warning and refer to the tornado as a "confirmed tornado."
If the NWS sees that the torando is indeed causing extreme damage and there is a higher potential for loss of life, the office might upgrade the warning to a PDS warning or a tornado emergency.
This is why you should always stay up to date with the latest information about the warning, and know what the warning means for you.
MORE: (15 Severe Weather Tips That Could Save Your Life)
Jennifer Grayis a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.