A Severe Weather Warning is a short-fuse National Weather Service product issued when a specific thunderstorm is producing or is imminently expected to produce a defined severe hazard within a defined polygon. Where a watch describes potential, a warning describes an actual or imminent threat.
The main warning types
A Severe Thunderstorm Warning is issued when a storm is producing or expected to produce hail 1 inch or larger, wind gusts of 58 mph or greater, or both. A Tornado Warning is issued when a tornado is indicated by radar or reported by a trained spotter, or when a storm shows radar signatures consistent with imminent tornado development.
Severe Thunderstorm Warnings also carry an impact-based tiering. A baseline warning uses the standard 1-inch, 58-mph criteria. A Considerable tag flags storms expected to produce hail of 1.75 inches or wind of 70 mph. A Destructive tag flags storms expected to produce hail of at least 2.75 inches or wind of at least 80 mph, and it triggers Wireless Emergency Alerts on mobile phones inside the polygon.
How warnings are issued
The local NWS Weather Forecast Office issues warnings for its county warning area. Warnings are polygon-based, drawn to include only the ground a specific storm is expected to affect, and they typically last on the order of 30 to 60 minutes. Warnings can be reissued, extended, or upgraded as the storm evolves.
Warnings are triggered by a combination of radar signatures, satellite trends, environmental parameters, and ground-truth reports from trained spotters, chasers, and law enforcement. Ground truth remains valuable because it confirms what radar can only infer.
How to respond
A Tornado Warning is an instruction to move to a safe shelter immediately. Interior room, lowest floor, away from windows. Do not wait to see the storm.
A Severe Thunderstorm Warning is not just a rain warning. Hail of 1 inch and winds of 58 mph will damage vehicles and can injure people caught outside. Considerable and Destructive tags escalate that risk substantially. If you are in the threatened region and a warned storm is approaching, have a plan to take shelter and act on it before the storm arrives.