A Severe Thunderstorm Warning is the National Weather Service’s most urgent alert for non-tornadic severe weather, signifying an immediate threat to life and property. This warning means a dangerous storm is either already occurring or is imminent in the warned area. This alert demands immediate protective action from anyone in its path. Understanding the specific criteria for severity is the first step in assessing the true danger and knowing how to respond quickly.
What Makes a Thunderstorm Severe?
A thunderstorm is officially classified as severe by the National Weather Service (NWS) when it meets or exceeds specific minimum intensity thresholds for wind or hail. The minimum requirement for wind is a measured gust of 58 miles per hour or greater, which is powerful enough to cause substantial damage. This speed is equivalent to a weak Category 1 hurricane.
The minimum size for hail to qualify a storm as severe is one inch in diameter, roughly the size of a quarter. While these are the baseline criteria, many severe thunderstorms can far exceed these minimums, producing larger hail and straight-line winds exceeding 70 miles per hour. The presence of a tornado also automatically classifies any thunderstorm as severe. These criteria are set because damage and injury become substantially more likely once these thresholds are reached.
Understanding the Difference Between a Watch and a Warning
Distinguishing between a Severe Thunderstorm Watch and a Severe Thunderstorm Warning is important because each requires a different response. A Severe Thunderstorm Watch is issued when conditions are favorable for severe thunderstorms across a large area, often encompassing multiple counties or states. This alert means people should be prepared for the possibility of severe weather later, but the danger is not yet immediate.
A Severe Thunderstorm Warning is a much more urgent directive, indicating that severe weather is either occurring now or is expected to begin very soon. Warnings are typically issued for much smaller, specific areas that are directly in the storm’s path. When a warning is issued, it is no longer a time for preparation but for immediate action to secure safety. The shift from a watch to a warning represents the change from a potential threat to an imminent danger.
The Primary Hazards of a Severe Thunderstorm
The hazards associated with a Severe Thunderstorm Warning are tangible threats capable of causing widespread destruction and physical harm. Wind gusts of 58 miles per hour or greater, often occurring as straight-line winds, are strong enough to snap large tree branches, uproot smaller trees, and cause structural damage to sheds and mobile homes. These high winds can also turn loose outdoor objects into dangerous, high-speed projectiles.
Hail that reaches the one-inch diameter threshold causes significant property damage, particularly to vehicles and home roofs. Larger hailstones pose a serious risk of injury to anyone caught outside, as they can fall at speeds exceeding 100 miles per hour. Additionally, all thunderstorms carry the risk of dangerous cloud-to-ground lightning, which can strike up to ten miles away from the rain area.
Heavy rainfall accompanying severe storms can lead to localized flash flooding, which is the number one cause of weather-related fatalities in the United States. The intense downpour can overwhelm drainage systems and low-lying areas quickly. This combination of damaging wind, large hail, and flash flooding potential signals a multifaceted, immediate threat.
Essential Safety Steps During a Warning
When a Severe Thunderstorm Warning is issued, the immediate action should be to seek shelter inside a sturdy building. The safest place is an interior room on the lowest level, such as a basement or interior hallway, keeping as many walls as possible between you and the outside. Stay away from all windows and glass doors, which are vulnerable to breaking from wind-borne debris or large hail impacts.
If time allows before the storm arrives, quickly secure or bring in loose outdoor items like patio furniture or trash cans to prevent them from becoming dangerous projectiles. Once inside, avoid contact with electrical equipment and plumbing, as lightning strikes can travel through these conductive pathways. This means delaying activities like showering or washing dishes until the storm has safely passed. Never attempt to drive through a flooded roadway; remember the phrase, “Turn Around, Don’t Drown.”