Impact-based warning tags are short coded lines included in every National Weather Service (NWS) Severe Thunderstorm, Tornado, and Flash Flood Warning. They state the hazard, how it was detected, and the expected level of damage, so people and automated systems can tell how serious a warning is at a glance.
What each set of tags tells you
Each warning's tags cover three things: the hazard type, the source of the information, and the expected impacts. The source reads RADAR INDICATED when the evidence comes from radar, or OBSERVED when the hazard has been confirmed.
The impact level starts at a base level with no tag, rises to CONSIDERABLE, and tops out at DESTRUCTIVE for severe thunderstorms or CATASTROPHIC for tornadoes and flash floods. NWS calls this approach the impact-based warning (IBW).
Severe Thunderstorm Warning tags
These warnings list a maximum hail size in inches and a maximum wind gust in miles per hour, each marked radar indicated or observed. The base level starts at 1 inch hail (quarter size) and 58 mph winds, which appear as 60 mph in the warning.
CONSIDERABLE means at least 1.75 inch (golf ball size) hail or 70 mph winds. DESTRUCTIVE means at least 2.75 inch (baseball size) hail or 80 mph winds, and since 08/02/21 it automatically sends a Wireless Emergency Alert (WEA) to phones in the warned area.
An optional TORNADO…POSSIBLE line flags a storm that may produce a tornado when forecaster confidence does not yet support a Tornado Warning.
Tornado and flash flood tags
TORNADO…RADAR INDICATED means radar shows a tornado or developing tornado that is not confirmed. TORNADO…OBSERVED means reliable reports or radar evidence, such as debris, confirm one.
A CONSIDERABLE tornado tag is for a strong, ongoing tornado and adds Particularly Dangerous Situation wording, and CATASTROPHIC is the Tornado Emergency. Every Tornado Warning sends a WEA regardless of tag.
Flash Flood Warnings use base, CONSIDERABLE and CATASTROPHIC levels, and only the upper two send a WEA. The Flash Flood Warning entry covers those tags in detail.
What the tags do not tell you
Tags describe the worst expected within the warning, not what every spot inside it will see. The base level is the default, and a base warning can still bring damaging wind, large hail, or a deadly tornado.
Raising a warning to the top category requires a new warning, while a move to CONSIDERABLE can come in an update or a new warning.