When weather conditions along an airway deteriorate to the point where aircraft safety is genuinely threatened, forecasters at Meteorological Watch Offices (MWOs) issue a Significant Meteorological Information advisory — a SIGMET. Unlike routine forecasts, a SIGMET is an unscheduled, concise, plain-language alert transmitted to all pilots in flight as rapidly as practical. Understanding what triggers a SIGMET, who issues it, how to decode it, and what its limitations are is essential knowledge for any pilot operating in the National Airspace System.
This article focuses on non-convective SIGMETs for the contiguous United States (CONUS) — the advisories issued for severe weather phenomena that are not thunderstorm-related. Convective SIGMETs, which cover thunderstorm hazards, follow a separate issuance process and are treated as a distinct product, though they are briefly contrasted here for clarity.
What a SIGMET Is and Who Issues It
A SIGMET is formally defined as a concise description of the occurrence or expected occurrence of specified en route weather phenomena that may affect the safety of aircraft operations. The emphasis on safety distinguishes a SIGMET from lower-level advisories like AIRMETs, which address conditions hazardous primarily to lighter aircraft or less-experienced pilots. SIGMETs apply to all aircraft categories and demand immediate pilot attention.
The United States operates three MWOs authorized to issue SIGMETs:
- Aviation Weather Center (AWC), located in Kansas City, MO (identified by the station identifier KKCI) — responsible for the 20 domestic Air Route Traffic Control Center (ARTCC) Flight Information Regions (FIRs) covering the CONUS and adjacent coastal waters, grouped into six SIGMET areas. The AWC also covers the New York, Houston, Miami, and San Juan Oceanic FIRs, plus the Oakland Oceanic FIR north of 30° N latitude and the portion east of 140° W longitude between the Equator and 30° N.
- Alaska Aviation Weather Unit (AAWU) — responsible for the Anchorage Continental FIR and the Anchorage Oceanic FIR.
- WFO Honolulu — responsible for the Oakland Oceanic FIR south of 30° N latitude and between 140° W and 130° E longitude.
Each issuing office covers a defined geographic domain, ensuring there are no gaps in SIGMET coverage across U.S.-controlled airspace and oceanic regions.
Non-Convective SIGMET Issuance Criteria
For the CONUS, a non-convective SIGMET is issued when any of the following phenomena are occurring or expected to occur and are judged by the forecaster to have a significant impact on the safety of aircraft operations:
- Severe or greater turbulence (SEV TURB) — turbulence intense enough to cause momentary loss of aircraft control, large attitude variations, or passenger injuries.
- Severe icing (SEV ICE) — ice accretion at a rate that exceeds the capacity of deicing/anti-icing equipment or creates a critical handling hazard.
- Widespread dust storm (WDSPR DS) — a large-scale dust event reducing visibility to hazardous levels across a broad area.
- Widespread sandstorm (WDSPR SS) — similar in scope and hazard to a widespread dust storm but composed of sand particles.
- Volcanic ash (VA) — the presence or expected presence of ash clouds from volcanic eruption, which can cause engine flameout, abrasive damage to flight surfaces, and instrument failure.
Notice what is not on this list: thunderstorms. In the CONUS, convective hazards are handled exclusively by Convective SIGMETs, which carry an implied warning of severe or greater turbulence, severe icing, and low-level wind shear. A Convective SIGMET is issued instead of a non-convective SIGMET whenever thunderstorms are the triggering phenomenon.
Valid Period, Timing, and Continuity
The non-convective SIGMET is an unscheduled product — it is issued any time conditions meeting SIGMET criteria are occurring or are expected to occur within the next four hours. The maximum valid period for a single issuance is four hours. If hazardous conditions persist beyond that window, the forecaster must reissue the SIGMET at least every four hours for as long as SIGMET-level conditions continue in the area of responsibility.
This reissuance requirement is operationally important: a pilot checking a SIGMET near its expiration time should understand that a new issuance may already be in preparation, or that conditions may have improved enough to no longer warrant one. Pilots should always check for the most current version before departure and monitor for updates during flight.
SIGMET Identification and Series Naming
Each non-convective SIGMET is assigned a phonetic alphabet series name and a sequential number. For the CONUS, the AWC uses the identifiers NOVEMBER, OSCAR, PAPA, QUEBEC, ROMEO, UNIFORM, VICTOR, WHISKEY, XRAY, and YANKEE. Two letters are deliberately excluded from the CONUS series: SIERRA and TANGO, because those identifiers are reserved to avoid confusion with other products. The Honolulu MWO, by contrast, does use SIERRA and TANGO in its series for the Oakland Oceanic FIR, and the AAWU uses INDIA through MIKE for Anchorage FIR SIGMETs.
The sequential number increments with each new issuance for a continuing phenomenon — for example, ROMEO 1, ROMEO 2, ROMEO 3. At 0000 UTC each day, all active SIGMETs are renumbered back to 1, regardless of whether the underlying phenomenon is continuing. This daily reset means the series number alone does not tell you how many times a phenomenon has been reissued; always note the issuance date/time (UTC) for situational awareness.
Decoding a Non-Convective SIGMET: Format and Example
SIGMETs are written in abbreviated plain language using contractions from FAA Order JO 7340.2 for domestic products. All heights and altitudes are referenced to MSL unless otherwise noted, expressed as three-digit hundreds of feet (e.g., 040 = 4,000 ft MSL). At or above 18,000 ft MSL, the flight level prefix FL is used (e.g., FL280). Geographic boundaries are described using high-altitude VOR coordinates and, where necessary, latitude/longitude in whole degrees and minutes.
A typical non-convective SIGMET contains these elements in order: the series name and number; the valid beginning and ending time in UTC; the list of affected states; the geographic boundary of the phenomenon described by high-altitude VOR coordinates; the phenomenon description (e.g., SEV ICE or SEV TURB); the vertical extent (base and top); movement, if applicable; intensity trend (INTSF for intensifying, WKN for weakening, NC for no change); and, when applicable, a statement that conditions are expected to continue beyond the valid time.
Consider this example from FAA-H-8083-28B:
SFOR UWS 100130 / SIGMET ROMEO 1 VALID UNTIL 100530 / OR WA / FROM SEA TO PDT TO EUG TO SEA / SEV TURB BTN FL280 AND FL350. CONDS CONTG BYD 0530Z.
Decoded: This is the first issuance of SIGMET ROMEO in the San Francisco area, issued on the 10th at 0130 UTC and valid until 0530 UTC on the 10th. The affected states are Oregon and Washington. The affected area is bounded by Seattle (SEA) to Pendleton (PDT) to Eugene (EUG) back to Seattle. The hazard is severe turbulence between FL280 and FL350. Conditions are expected to continue beyond 0530Z, alerting the reader that a subsequent issuance is anticipated.
Amendments, Cancellations, and Corrections
Non-convective SIGMET procedures for the CONUS include specific rules for updating or ending an advisory. Amendments are not issued. If the situation changes significantly enough to warrant a revision, the forecaster simply issues a new SIGMET using the next sequential series number. This eliminates confusion about which version supersedes another. Cancellations are issued when the phenomenon is no longer occurring, is no longer expected to occur, or has moved out of the issuing office's area of responsibility. Corrections (for typographical or data errors) are issued when necessary and are identified by the letters COR appearing at the end of the first line, after the issuance UTC date/time.
Graphical Depictions: A Critical Caution
Many preflight and inflight weather products display SIGMET boundaries graphically on a map. The FAA is explicit on one key point: a graphical depiction of a SIGMET area is not the entire SIGMET. Additional information — including intensity trends, vertical extent, movement vectors, and continuation statements — resides only in the text version. Pilots who rely solely on a shaded polygon risk missing operationally critical details. Always read the full text of any active SIGMET, not just its geographic footprint.
Why SIGMETs Matter Operationally
The five triggering phenomena for non-convective SIGMETs share a common characteristic: each is capable of causing rapid, severe degradation of aircraft performance or structural integrity with little or no warning. Severe turbulence can injure unsecured cabin occupants and overstress airframes. Severe icing can overwhelm deicing systems and degrade lift catastrophically. Volcanic ash clouds can simultaneously clog pitot-static systems, sandblast windscreens to opacity, and cause multiple engine flameout — and they are often invisible on radar. Widespread dust and sandstorms can reduce visibility to near zero across vast stretches of desert terrain.
For instrument-rated pilots and airline crews, a SIGMET in the planned route demands a proactive response: re-routing, altitude adjustment, delay, or diversion. Under 14 CFR Part 91, the pilot in command bears ultimate responsibility for the go/no-go decision; a SIGMET does not legally prohibit flight but establishes a clear record that the hazard was known and forecast. In Part 121 and 135 operations, company dispatch and operations specifications add further layers of SIGMET-response requirements.
Key Numbers and Rules
- Non-convective SIGMETs are valid for a maximum of 4 hours.
- They are reissued at least every 4 hours for continuing phenomena.
- Triggering phenomena: severe turbulence, severe icing, widespread dust storm, widespread sandstorm, volcanic ash.
- CONUS series identifiers exclude SIERRA and TANGO (reserved for other uses).
- Sequential numbers reset to 1 at 0000 UTC each day for all continuing SIGMETs.
- Heights below FL180 are expressed in three-digit hundreds of feet MSL; at or above 18,000 ft, the FL prefix is used.
- Amendments are not issued; a new series number is used instead. Corrections are identified by COR on the first line.
Common Test Traps
- Confusing non-convective SIGMETs with Convective SIGMETs. Thunderstorms in the CONUS are covered exclusively by Convective SIGMETs — they are never the trigger for a non-convective SIGMET. Examiners will list thunderstorms among the criteria and ask which requires a non-convective SIGMET; the answer is none of the convective options.
- Assuming the graphical polygon is the complete product. The text version contains elements not shown graphically. Always read the full text.
- Forgetting that SIERRA and TANGO are excluded from the CONUS series. This is a commonly tested identification detail, especially when a question lists phonetic alphabet letters and asks which ones AWC does not use for domestic SIGMETs.
- Misreading the daily renumbering rule. At 0000 UTC, ALL active SIGMETs reset to series number 1. A SIGMET labeled ROMEO 1 could be the first issuance of the day or a continuation — check the issuance date/time to confirm.
- Treating the 4-hour valid period as a hard guarantee of conditions. The SIGMET describes conditions expected within the valid period. Real conditions may change faster than the product cycle; PIREPs and ATC reports should always supplement SIGMET information.