Tornado and Severe Thunderstorm Warnings vs. Watches

A Severe Thunderstorm Warning (SVR) or Tornado Warning (TOR) is not the same product as a Severe Thunderstorm Watch or Tornado Watch — the two pairs come from different directives, different issuing offices, and different criteria. SVRs and TORs are issued by a local National Weather Service Weather Forecast Office (WFO) under stated radar/report criteria and stated valid-time windows, governed by National Weather Service Instruction (NWSI) 10-511. Severe Thunderstorm Watches and Tornado Watches are issued by the Storm Prediction Center (SPC) under stated areal-size, duration, and hazard-count thresholds, governed by NWSI 10-512. The National Weather Service’s own directives index lists the two instructions by title: “10-511: WFO Severe Weather Products Specification … 10-512: National Severe Weather Products Specification” (NWS Directives index, series 010).

This original record-reading guide is a general reading aid, not cited evidence. It does not depict a source record; the article text and source links provide the facts.

Key points

• SVRs are issued by local WFOs “when there is radar or satellite indication and/or reliable reports of wind gusts equal to or in excess of 50 knots (58 miles per hour (mph)) and/or hail size of one inch (U.S. quarter-size) in diameter or larger” (NWSI 10-511, §2.2.2).
• TORs are issued by local WFOs “when there is radar indication and/or reliable reports of a tornado or developing tornado” (NWSI 10-511, §3.2.2).
• SVR valid times run “within 30 to 60 minutes of issuance,” and TOR valid times run “15 to 45 minutes from issuance” (NWSI 10-511, §§2.2.4, 3.2.4).
• SPC issues Severe Thunderstorm Watches for forecast hail/wind thresholds and Tornado Watches for forecast tornado thresholds, both requiring “at least 2 hours over an area at least 8,000 square miles” (NWSI 10-512, §§12.2.2, 13.2.2).
• The familiar watch-area graphic (the SEL product) “is an approximation of the area in a watch”; the official area is the separate WOU product (NWSI 10-512, §§12.1, 13.1).

Warnings: WFO products under NWSI 10-511

Warnings are governed by National Weather Service Instruction 10-511. The current PDF text identifies itself as an administrative-only update: “NATIONAL WEATHER SERVICE INSTRUCTION 10-511 JANUARY 26, 2023 … OPR: W/AFS21 (K. Laws) Certified by: W/AFS21 (S. Bieda) Type of Issuance: Administrative … SUMMARY OF REVISIONS: This directive supersedes NWSI 10-511, WFO Severe Weather Products Specification, dated January 26, 2023. This is an administrative update only to fix broken links NWSI 10-313 and NWS-1701 within the directive. No content changes were made with this update, and the effective date was not affected.” The current PDF is digitally signed: “Digitally signed by ALLEN.ALLISON.LEIGH.1365831072 Date: 2024.09.16 04:52:04 -04’00’” (both from NWSI 10-511).

Severe Thunderstorm Warning (SVR)

The issuance criterion: “2.2.2 Issuance Criteria. WFOs should issue SVRs when there is radar or satellite indication and/or reliable reports of wind gusts equal to or in excess of 50 knots (58 miles per hour (mph)) and/or hail size of one inch (U.S. quarter-size) in diameter or larger” (NWSI 10-511).

The valid-time window: “2.2.4 Valid Time. Valid times should be within 30 to 60 minutes of issuance. For thunderstorms that are expected to remain severe beyond the valid time of the original warning, WFOs should issue a new warning” (NWSI 10-511).

Tornado Warning (TOR)

The issuance criterion: “3.2.2 Issuance Criteria. WFOs should issue TORs when there is radar indication and/or reliable reports of a tornado or developing tornado” (NWSI 10-511).

The valid-time window: “3.2.4 Valid Time. Valid times should be 15 to 45 minutes from issuance. For a tornado expected to continue beyond the valid time of the original warning, WFOs should issue a new warning” (NWSI 10-511).

Watches: SPC products under NWSI 10-512

Watches are governed by a separate directive, National Weather Service Instruction 10-512. Its current PDF text likewise identifies itself as an administrative-only update: “NATIONAL WEATHER SERVICE INSTRUCTION 10-512 APRIL 9, 2021 … OPR: W/AFS21 (R. Bowers) Certified by: W/AFS21 (S. Bieda) Type of Issuance: Routine – Admin Update … SUMMARY OF REVISIONS: This directive supersedes NWSI 10-512, “National Severe Weather Products Specification”, dated April 9, 2021. This is an administrative update made only to change all references of “Gulf of Mexico” to “Gulf of America”. No content changes were made with this update, and the effective date was not affected“ (NWSI 10-512).

Severe Thunderstorm Watch

The mission and the graphic caveat: “12.1 Mission Connection. SPC issues Public Severe Thunderstorm Watch Notification Messages to alert CONUS WFOs, the public, media and emergency managers to organized thunderstorms forecast to produce six or more hail events of one inch (quarter-size) diameter and/or greater or convective damaging winds of 50 knots (58 mph) or greater. The SEL product is an approximation of the area in a watch, for the official area covered by a watch see the corresponding WOU product” (NWSI 10-512).

The thresholds: “12.2.2 Issuance Criteria. SPC should issue a Public Severe Thunderstorm Watch Notification Message when there is a forecast of six or more hail events of one inch (quarter-size) diameter or greater or convective damaging winds of 50 knots (58 mph) or greater. The forecast event minimum thresholds should be at least 2 hours over an area at least 8,000 square miles” (NWSI 10-512).

Tornado Watch

The mission and the graphic caveat: “13.1 Mission Connection. SPC issues Public Tornado Watch Notification Messages to alert CONUS WFOs, the public, media and emergency managers to organized thunderstorms forecast to produce two or more tornadoes or any tornado which could produce EF2 or greater damage. The SEL product is an approximation of the area in a watch, for the official area covered by a watch see the corresponding WOU product” (NWSI 10-512).

The thresholds: “13.2.2 Issuance Criteria. SPC should issue a Public Tornado Watch Notification Message when there is a forecast of multiple weak tornadoes or any tornado which could produce EF2 or greater damage. The forecast event minimum thresholds should be at least 2 hours over an area at least 8,000 square miles” (NWSI 10-512). Note that the directive itself uses two different phrasings for the tornado-count trigger — “two or more tornadoes” in the Mission Connection quoted above and “multiple weak tornadoes” here in the Issuance Criteria — both quoted exactly as written in NWSI 10-512.

Watch graphic vs. official area

Both watch sections repeat the same caveat: the SEL product — the shaded polygon most people see in a watch graphic — “is an approximation of the area in a watch,” and the record points to a separate product, the WOU, as the one that carries the official watch area (NWSI 10-512, §§12.1, 13.1). This article does not verify or state what that official WOU boundary is for any specific watch; it only reports that the record distinguishes an approximate public graphic from an official area product.

What this article does not cover

This article does not tell you what to personally do if a watch or warning is issued for your area — that is protective-action guidance, not a definition of the products. It does not assess whether any specific reader, location, or property is currently under a watch or warning. It does not cover non-convective hazard criteria, which are governed by separate NWS directives outside the scope of the two instructions quoted here. It also does not verify or state the official WOU boundary or any county list for any specific watch.

Source links

• NWS Instruction 10-511 — WFO Severe Weather Products Specification
• NWS Instruction 10-512 — National Severe Weather Products Specification
• NWS Directives Series 010 Index