Tornado watch vs. warning: what each one means

A tornado watch means conditions favor tornadoes in and near a large area over the next several hours; a tornado warning means a tornado has been sighted or indicated by weather radar and the danger is imminent for a much smaller area. The watch is a forecast of the environment. The warning is a statement about a specific storm.

The National Weather Service compresses the difference into two headlines on its tornado safety page: a Tornado Watch is “Be Prepared!” and a Tornado Warning is “Take Action!” Its definitions are just as short. A watch: “Tornadoes are possible in and near the watch area.” A warning: “A tornado has been sighted or indicated by weather radar. There is imminent danger to life and property.” The rest of this page is about what sits behind those two sentences.

Two different offices

The Storm Prediction Center in Norman, Oklahoma, issues every tornado watch and every severe thunderstorm watch in the contiguous United States. Warnings come from the local NWS Weather Forecast Office responsible for the area. The SPC’s FAQ states it directly: “The SPC issues all Severe Thunderstorm Watches and Tornado Watches. All warnings are issued by local NWS offices.”

The split follows the scale of the problem. A watch is the last step in the SPC’s chain of forecast products, after the convective outlook and, often, a mesoscale discussion. A warning is issued by forecasters watching individual storms on their own radars, with spotter and public reports coming in.

The watch: an area and a window

The SPC describes a typical watch as 20,000 to 40,000 square miles, “though some are smaller and others larger,” with a typical duration of 6 to 8 hours, and notes it “may be canceled, extended in time or space, or replaced as required.” Its FAQ gives a slightly different typical figure of about 25,000 square miles, so read these as orders of magnitude rather than rules. For scale, the SPC compares a typical watch to about half the size of Iowa.

The choice between a tornado watch and a severe thunderstorm watch depends on what the environment supports. A severe thunderstorm watch covers hail 1 inch or larger and damaging thunderstorm winds; a tornado watch covers those threats “as well as the possibility of multiple tornadoes or a single intense tornado.” The SPC also notes that tornadoes can occur in either type of watch. A severe thunderstorm watch is not a statement that tornadoes are impossible.

A watch is also not a guarantee. In the SPC’s words: “A watch is not a warning, and should not be interpreted as a guarantee that there will be severe weather!”

Particularly Dangerous Situation watches

When forecasters have high confidence in multiple intense tornadoes, the SPC adds the line “THIS IS A PARTICULARLY DANGEROUS SITUATION” to a tornado watch. The SPC says PDS tornado watches are for “rare situations when long-lived intense tornadoes are likely,” and that it aims for 3 of every 4 to verify with multiple intense tornadoes.

They are uncommon. In a study of 1996–2005 watches, Dean and Schaefer of the SPC and CIMMS found that 216 of 3,058 tornado watches, 7%, carried PDS wording. PDS severe thunderstorm watches exist too, reserved for the threat of a high-end derecho with widespread wind damage; only 22 were issued in that ten-year period.

The warning: a polygon, not a county

Since October 1, 2007, NWS offices have issued tornado and severe thunderstorm warnings as storm-based warnings. The NWS notice announcing the change defined the warned area as “the area defined by the latitude and longitude pairs listed at the bottom of the warning product.” Before that, warnings were issued for whole counties. The county list is still included for users without a map, including NOAA Weather Radio listeners, but the polygon is the warning. A county named in the text may be only partly inside it.

Warnings are short. Brooks and Correia (2018, Weather and Forecasting) found that tornado warning durations cluster near 30, 45 and 60 minutes, and that from 2012 the default length dropped from 45 to 30 minutes, after which warnings near 45 and 60 minutes dropped sharply. Each warning also carries a time, motion and location line giving the storm’s position and movement.

What goes into a tornado warning

Tornado warnings use the NWS impact-based warning format, which separates hazard, source and impact and ends with a set of tags. The source tag says what the warning rests on. According to the NWS tornado impact-based warning fact sheet:

• TORNADO...RADAR INDICATED: “Radar indication of a tornado or developing tornado, but the tornado is not confirmed.” The rotation behind this tag is usually a mesocyclone with a tight velocity couplet.
• TORNADO...OBSERVED: “Evidence of an ongoing tornado via radar confirmation (e.g., debris indicated) or reliable reports.” A tornado debris signature is one route to this tag.

A radar-indicated warning is issued before anyone can confirm a tornado, which is the point of it. The damage threat tag then says how bad the tornado is expected to be:

The NWS glossary defines a Tornado Emergency as “an exceedingly rare tornado warning issued when there is a severe threat to human life and catastrophic damage from an imminent or ongoing tornado.” It is not a separate product from the warning; it is the top tier of one. The NWS office in Norman describes the Tornado Emergency it issued for the Oklahoma City metro area on the evening of May 3, 1999, as “the first warning of its kind.” Note that “particularly dangerous situation” therefore appears in two places: in some SPC watches, and in CONSIDERABLE tornado warnings from local offices.

The NWS fact sheet also states that all tornado warnings trigger Wireless Emergency Alerts on mobile phones, regardless of tag.

Severe thunderstorm watch vs. warning

The same structure applies one level down. The NWS headlines are identical: a severe thunderstorm watch is “Be Prepared!” and a warning is “Take Action!”, issued when “severe weather has been reported by spotters or indicated by radar.”

Since August 2, 2021, severe thunderstorm warnings have carried their own damage threat tags. The NWS thresholds:

The NWS says that on average only about 10% of severe thunderstorms reach the destructive category each year nationwide.

Lead time, as measured

Brooks and Correia measured the mean lead time for tornadoes that were warned in advance: 18.8 minutes averaged over 1986–2011, and 15.6 minutes over 2012–2016, when the NWS placed more emphasis on reducing false alarms and fewer warnings were issued. That figure excludes tornadoes that received no warning before they formed. The same study found that the probability a tornado was warned in advance rises with its EF rating, and fell below 0.60 for EF0 tornadoes in the storm-based era.

WeatherStory Radar shows NWS warnings alongside full-resolution Level II radar for analysis; it has no alerting of its own.