What is the difference between a tornado watch and a warning?

Past disasters should serve as reminder to be prepared in advance for extreme weather events

Tornadoes and severe storms tore through much of the southern US overnight on Tuesday and into Wednesday morning.

At least two people were killed in Montgomery, Alabama, the state’s director of emergency management said, noting the casualties coudl rise as search-and-rescue crews began checking on residents and surveying the damage.

Parts of Mississippi, Alabama, Arkansas, Louisiana, and Tennessee faced at least 17 tornadoes as well as severe flooding and tennis ball-sized hailstones as warnings continued in Alabama into Wednesday morning.

Images and videos of the aftermath of tornadoes showed damaged houses and fallen trees, while injuries began being reported in states like Mississippi and Louisiana.

The incident is just the latest reminder to be prepared for extreme weather events, have a plan in place should an emergency situation arise and to know the difference between a tornado watch and a tornado warning alert when you receive one on your phone.

The former is the preliminary alert issued a few hours in advance of a coming storm, intended to make those likely to be in its path aware of the situation in order to ensure that they keep an eye on developments, remain vigilant and prepare to move should an evacuation prove advisable.

A tornado warning, meanwhile, is more urgent and highly localised and should be regarded as an order to take shelter as soon as possible because the storm’s impact is imminent.

Tornado watch alerts are issued by the National Oceanic and Atmospheric Administration’s Storm Prediction Center (SPC) based in Norman, Oklahoma, like the one below.

Warning alerts, by contrast, come from the regional office of the National Weather Service (NWS) that is closest to the developing storm front.

In the case of Tuesday storm, that happened to be in New Orleans itself.

“A watch is issued when conditions are favourable, for example, either for a severe thunderstorm or tornadoes,” AccuWeather senior meteorologist Dan Kottlowski told Newsweek. “It doesn’t mean severe weather is imminent.”

SPC tornado watch alerts typically cover areas of around 25,000 square miles but the conditions that might prompt them vary.

“There is not just one set of ingredients; every watch may have a different set of [parameters] from one day to the next since it is based on a synoptic situation that may change within several hours,” Mr Kottlowski said.

As tornado warnings are more urgent and require a quick response from recipients, the criteria required by the NWS before they are issued by its meteorologists, watching conditions develop on radar and mapping out potential at-risk locations, are more specific too.

Lightning and heavy rain are insufficient to prompt a warning, for instance. Only hail one inch in diameter and winds of 55mph will move the weather service to act.