Severe Thunderstorm Warning vs Watch: what each one means

A Severe Thunderstorm Warning means a storm that is already producing, or is about to produce, wind gusts of 58 mph or more and/or hail 1 inch in diameter or larger is moving through a specific area, usually within the next 30 to 60 minutes. A Severe Thunderstorm Watch means the atmosphere over a much larger region can produce such storms over the coming hours, but none may have formed yet. The National Weather Service issues tens of thousands of these warnings a year, more than any other warning type, which is exactly why people tune them out. About 1.4 million Americans a month search the phrase, most of them while the phone is still buzzing. Every US city page on this site shows the live NWS alerts for that location, so you can see the warning text and the expiry time without opening another app.

The two thresholds, and why lightning is not one of them

"Severe" has a precise meaning to the NWS. A thunderstorm is severe when it produces one or more of:

• Wind gusts of 58 mph (50 knots) or greater. That is the speed at which large branches break, shallow-rooted trees come down and power lines start failing.
• Hail of 1 inch or larger, the size of a quarter. Below that, hail is a nuisance; at and above it, it dents cars and breaks skylights.
• A tornado. Any tornado makes the storm severe, but a tornado threat gets its own product, the Tornado Warning, covered in Tornado Watch vs Tornado Warning.

Lightning, heavy rain and frequent thunder do not make a storm severe, no matter how loud it is. A storm that kills someone with lightning and floods a street can be entirely non-severe by definition. That is not the NWS being pedantic: the severe criteria exist to flag storms that damage structures, and lightning safety is handled by a different rule, "when thunder roars, go indoors". Our guide to how far away lightning is covers the flash-to-bang count.

Watch vs warning

• Severe Thunderstorm Watch: issued by the NOAA Storm Prediction Center in Norman, Oklahoma, for an area that typically spans several counties or parts of several states, valid for roughly 4 to 8 hours. It means conditions are favorable. Use the time to move cars under cover, secure patio furniture and decide where you will be when storms arrive.
• Severe Thunderstorm Warning: issued by your local NWS forecast office for a polygon drawn around the storm's path, usually valid 30 to 60 minutes. It means the storm is severe now, based on radar, spotter reports or both. Go inside, away from windows, and stay there until it expires.

The same grammar applies across every hazard the NWS covers; if the two words still feel interchangeable, start with watch vs warning, and what the difference means.

The damage threat tags: base, considerable, destructive

Since 2021 every Severe Thunderstorm Warning carries a damage threat tag, and the tag is the part worth reading:

• Base (no tag shown): 58 mph winds and/or 1 inch hail. Damage to trees and power lines possible.
• Considerable: 70 mph winds and/or 1.75 inch (golf ball) hail. Expect downed trees, roof damage and outages.
• Destructive: 80 mph winds and/or 2.75 inch (baseball) hail. This is hurricane-force wind from a thunderstorm, the tag applied to derechos and the worst supercells. A destructive-tagged warning is the only severe thunderstorm product that triggers a Wireless Emergency Alert on your phone.

That last point matters. Roughly nine in ten Severe Thunderstorm Warnings never reach your phone as a push alert. If you want them, you need an app that delivers NWS warnings or a NOAA Weather Radio; how to get weather alerts on your phone explains what arrives by default and what does not.

What to do during a warning

• Get inside a sturdy building. Severe wind kills mainly through falling trees and flying debris, and hail injures people who are outdoors.
• Move away from windows. A 1 inch hailstone at 60 mph breaks glass; an 80 mph gust throws it.
• If you are driving, pull over somewhere without trees overhead and stay in the car. Do not park under an overpass on a highway; it blocks traffic and does not protect from wind.
• Assume the storm can produce a tornado. A meaningful share of tornadoes begin inside a Severe Thunderstorm Warning before the office has time to upgrade.
• Wait for the expiry time. Warnings are polygon-based and often end before the rain does; the polygon is the dangerous part.

Seeing it coming

Severe days are usually flagged one to three days ahead by the Storm Prediction Center's convective outlook. The colors, from Marginal to High, are explained in the SPC severe weather outlook categories. Timing is where the forecast models earn or lose their keep: a line of storms at 3 PM and the same line at 9 PM are different problems for a school pickup or a ballgame. On our city pages, Dallas or Chicago for instance, the NWS blend, the American GFS and the European model sit side by side hour by hour, and our accuracy scoreboard shows which one has been right most often in your city. Once the storms are up, the radar and how to read it take over.