NWS VTEC Significance Codes: What W, A, Y Mean

When a National Weather Service alert identifier includes a code like TO.W or FF.A, the letter after the period is called the “significance” code. The governing federal record is National Weather Service Instruction 10-1703, “Valid Time Event Code (VTEC),” current edition dated April 6, 2021, which supersedes an earlier edition effective May 5, 2017. Section 2.1.5 of that instruction states the significance character “Identifies the level of importance (e.g., watch, warning, advisory, etc.) of the weather or non-weather occurrence.” The instruction’s Appendix A lists the significance codes in use, with a caveat that not every listed code currently appears in NWS products.

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.

The governing record

The document is titled “NATIONAL WEATHER SERVICE INSTRUCTION 10-1703” (https://www.weather.gov/media/directives/010_pdfs/pd01017003curr.pdf). Its cover page dates the current edition “APRIL 6, 2021” and records “Type of Issuance: Routine.” It is attributed to “OPR: W/DIS (A. Hardy)” and “Certified by: W/DIS (M. Mainelli).” The same page states: “This directive supersedes NWSI 10-1703, “Valid Time Event Code,” effective May 5, 2017.“

The public landing page at https://www.weather.gov/vtec/ carries no date of its own. It functions as a pointer to the numbered, dated instruction rather than a standalone authority — the dated PDF above is the actual governing record for the codes discussed here.

What “significance” means

Section 2.1.5 of the instruction defines the significance character this way: “Identifies the level of importance (e.g., watch, warning, advisory, etc.) of the weather or non-weather occurrence.” The same section directs readers onward: “See Appendix A for the complete list of Significance codes in use in NWS products.”

The seven significance codes

Appendix A lists the significance codes as: “W Warning, A Watch, Y Advisory, S Statement, F Forecast, O Outlook, N Synopsis.” The instruction adds an explicit caveat about that list: “Note that not all significance codes are being used in NWS products.” In other words, the table names seven possible significance letters, but not all seven are actively assigned to current NWS product types.

Alert code ETN

A VTEC string also carries a four-digit Event Tracking Number (ETN), which the instruction defines separately from significance. Section language describes it this way: “The ETN is a four-digit number assigned to keep track of how an event (as defined in Section 1.4) is addressed by various VTEC actions and/or products issued over the lifetime of the event.” The instruction also specifies how that number is generated: “The ETN is assigned automatically by NWS applications software for each type of event issued by each office, sequentially starting with 0001 for the first new event of its type for the calendar year starting at 0000 UTC on January 1.” The ETN tracks a specific event’s lifecycle; the significance code (W, A, Y, etc.) separately marks that event’s level of importance.

Key points

• The significance code in a VTEC identifier is defined by NWS Instruction 10-1703, current edition dated April 6, 2021, which superseded the May 5, 2017 edition.
• Section 2.1.5 defines significance as identifying “the level of importance (e.g., watch, warning, advisory, etc.)” of an occurrence.
• Appendix A lists seven significance codes — W (Warning), A (Watch), Y (Advisory), S (Statement), F (Forecast), O (Outlook), N (Synopsis) — but the instruction itself notes not all are currently used in NWS products.
• The public weather.gov/vtec/ page is undated and directs readers to the numbered, dated instruction as the actual governing record.
• A separate element, the four-digit ETN, tracks an individual event’s lifecycle across VTEC actions and products; it is distinct from the significance letter.

What this article does not answer

This article explains only what the significance letter itself means and which document defines it. It does not explain any individual hazard’s issuance threshold — those thresholds live in separate product-specification instructions (such as NWSI 10-511, 10-512, 10-513, or 10-922), not in the VTEC instruction covered here. It also does not establish that an “advisory” is always less serious than a “watch” across every hazard family; each hazard’s severity ordering is set by that hazard’s own product-specification instruction, not by the VTEC instruction covered here. Finally, this article does not restate a specific product-name upgrade or downgrade example sometimes attributed to Section 3.1 of this instruction, because that exact wording could not be confirmed verbatim in the current April 6, 2021 edition on direct re-read; the section’s actual worked examples differ from that quoted version, so it is omitted here rather than repeated inaccurately.

Source links

• https://www.weather.gov/media/directives/010_pdfs/pd01017003curr.pdf
• https://www.weather.gov/vtec/