When a tropical cyclone or a space weather event threatens aviation, the inflight advisory system communicates that hazard through specific products that follow a precise format. Knowing how to find, read, and act on these advisories separates a prepared crew from one caught off guard. The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 26, details exactly how these products work, when they are issued, and what every line of text means. This article walks through both the Convective SIGMET system — which folds tropical cyclone remarks into its structure — and the broader SIGMET framework used outside the contiguous United States (CONUS), where tropical cyclone and space weather events receive their own dedicated treatment and extended valid periods.
Understanding these products requires recognizing that inflight advisories are layered. The most intense convective hazards in the CONUS are covered by Convective SIGMETs, while operations over oceanic and other non-CONUS airspace rely on SIGMETs that follow International Civil Aviation Organization (ICAO) conventions. Tropical cyclones and space weather bridge both worlds and demand special attention because of their scale, duration, and the long valid periods they carry.
Convective SIGMETs: Structure and Tropical Cyclone Remarks
Convective SIGMETs are issued by the Aviation Weather Center (AWC) for the eastern (E), central (C), and western (W) regions of the CONUS. They are published on a scheduled basis — hourly at 55 minutes past the hour — and each bulletin is valid for 2 hours or until superseded by the next issuance. When no qualifying conditions exist in a region, the AWC transmits a CONVECTIVE SIGMET...NONE message; a bulletin must go out every hour regardless. Unlike regular SIGMETs, Convective SIGMETs are not cancelled and not amended; they are simply superseded by the next issuance in the series. Corrections, however, are issued when needed and carry a COR indicator at the end of the first line.
Each Convective SIGMET in a bulletin carries a series number and region letter (e.g., 20C = the 20th Convective SIGMET of the day for the central region). Reading down the product, you will find the valid ending time in UTC, the affected states, the geographic boundary drawn using high-altitude VOR coordinates, a phenomena description (such as INTSFYG AREA SEV TS), movement direction and speed (e.g., MOV FROM 24045KT, meaning the storms are moving toward the northeast at 45 kt), cloud top height (e.g., TOPS ABV FL450), and finally a remarks section. Tropical cyclone information is inserted in the remarks section of CONUS Convective SIGMETs whenever a tropical cyclone affects or is expected to affect the region. This means a pilot flying along the Gulf Coast during hurricane season may see a Convective SIGMET whose remarks flag an approaching or overhead tropical system, providing critical context for the broader convective environment being described.
Decoding a Convective SIGMET Line by Line
Consider the example in Chapter 26. The product identifier MKC C WST 221855 tells you the AWC issued a Convective SIGMET for the central region on the 22nd day of the month at 1855 UTC. The next line, CONVECTIVE SIGMET 20C, confirms this is the 20th central-region issuance of the day. VALID UNTIL 2055Z gives you exactly two hours of validity. The affected states (ND SD) are listed next, followed by the polygon drawn with VOR references: FROM 90W MOT-GFK-ABR-90W MOT, meaning 90 NM west of Minot to Grand Forks to Aberdeen and back. The last block of text is the richest: an intensifying area of severe thunderstorms moving from 240° at 45 kt, tops above FL450, wind gusts of 60 kt reported, and tornadoes, 2-inch hail, and 65-kt gusts possible in the North Dakota portion. That remarks section is precisely where a tropical cyclone notation would appear if the system were tropical in nature.
A 2-to-6-hour outlook is appended to every Convective SIGMET bulletin. It uses the same high-altitude VOR coordinate format to depict where new Convective SIGMETs are likely to be required during that window, giving dispatchers and pilots a planning horizon beyond the 2-hour valid period.
SIGMETs Outside the CONUS: Where Tropical Cyclones Live
Outside the CONUS, the U.S. SIGMET framework expands to cover phenomena that simply do not appear in the domestic product. The issuance criteria include several thunderstorm types (obscured, embedded, widespread, squall line, isolated severe), severe turbulence, severe icing, widespread dust and sandstorms, volcanic ash — and tropical cyclones (TC). Tornado, funnel cloud, waterspout, and heavy hail may be added as further descriptors of thunderstorm activity as needed.
Non-CONUS SIGMETs are unscheduled, meaning they are issued any time SIGMET-criteria conditions are occurring or expected within a 4-hour window. The standard valid period is up to 4 hours, but tropical cyclone (TC) and volcanic ash (VA) SIGMETs may be valid for up to 6 hours. That extended window reflects the large geographic footprint and slower evolution of these phenomena compared with mesoscale convective events. As long as conditions continue, the SIGMET is reissued at least every 4 hours (or every 6 hours for TC and VA) to maintain continuous coverage.
Decoding a Non-CONUS SIGMET: Format Details
Non-CONUS SIGMETs follow ICAO conventions and carry information in a specific sequence. The WMO product header (e.g., WSPA07) and the issuing Meteorological Watch Office (MWO) identifier appear first, along with the issuance date and time. An NWS AWIPS communication header follows. Then the Area Control Center or Flight Information Region (FIR) is identified, along with the SIGMET series name (an alphabetic identifier such as TANGO), the issuance number, and the full valid period expressed as day/time to day/time in UTC. The body of the product then describes the phenomenon, whether it is observed (OBS) or forecast (FCST), the geographic location using latitude/longitude coordinates, the flight levels affected, movement direction and speed, intensity trend (INTSF, WKN, or NC), and — critically for tropical cyclones and volcanic ash — the forecast position of the center at the end of the valid period.
For a tropical cyclone SIGMET, the product includes the position of the TC center, the radius of associated convection at the start of the valid time, and the projected center position at the end of validity. This gives oceanic flight crews both the current picture and a forward-looking position to evaluate route exposure. An Oceanic FIR like Oakland Oceanic (KZAK), covered by the Honolulu MWO (PHFO), routinely issues these products for Pacific tropical systems. Using the example in Chapter 26, SIGMET TANGO 2 VALID 010410/010800 tells you this is the second issuance of the Tango series, valid from 0410 to 0800 UTC — nearly a 4-hour window — covering a defined area of the Oakland Oceanic FIR.
Space Weather Advisories in the Inflight Context
Space weather — solar radiation storms, geomagnetic storms, and radio blackouts caused by solar activity — can degrade or completely disrupt HF communications, GPS navigation, and onboard avionics. The FAA and NOAA coordinate on space weather products. Within the ICAO framework, Space Weather Advisories (SWX) are issued to alert crews and dispatchers of significant space weather impacts. These advisories are particularly relevant on polar and high-latitude routes, where radiation exposure and communication degradation are most acute. Pilots operating on North Atlantic or transpolar tracks should check Space Weather Advisories as part of preflight planning, just as they would check SIGMETs and AIRMETs. While the FAA-H-8083-28B addresses space weather as a meteorological consideration, the actual SWX advisory product follows an ICAO format and is distributed through the same international SIGMET communication channels, reinforcing the importance of understanding the broader non-CONUS advisory system.
Key Numbers and Rules
- Convective SIGMET issuance time: hourly at 55 minutes past the hour for E, C, and W regions.
- Convective SIGMET valid period: 2 hours or until superseded.
- No cancellations or amendments for Convective SIGMETs; corrections use COR on line 1.
- Non-CONUS SIGMET standard valid period: up to 4 hours.
- Tropical cyclone and volcanic ash SIGMETs: valid up to 6 hours; reissued at least every 6 hours while conditions persist.
- Tropical cyclone remarks in CONUS products: appear in the remarks section of Convective SIGMETs.
- Non-CONUS TC SIGMET content: includes TC center position, radius of convection at start, and forecast center position at end of valid period.
- Outlook appended to each Convective SIGMET bulletin: covers 2–6 hours beyond issuance.
Common Test Traps
- Thinking tropical cyclone SIGMETs are issued inside the CONUS as a stand-alone product: Inside the CONUS, tropical cyclone information is folded into the remarks of a standard Convective SIGMET, not issued separately. A dedicated TC SIGMET exists only outside the CONUS.
- Confusing the valid periods: Standard non-CONUS SIGMETs are valid up to 4 hours. Tropical cyclone and volcanic ash SIGMETs extend to 6 hours. Many students apply one rule to all products.
- Believing Convective SIGMETs can be cancelled: They cannot. The next hourly issuance supersedes the previous one. If the hazard is gone, the region publishes a NONE message at the next scheduled time.
- Misreading the MOV FROM phrase: MOV FROM 24045KT means the weather is moving from 240°, which means it is traveling toward the northeast — not toward 240°. This direction-of-origin convention trips up many students on the written exam.
- Overlooking the outlook: The Convective SIGMET outlook is part of every bulletin and covers 2–6 hours. It is a planning tool, not a guarantee of issuance, but examiners expect pilots to know it exists and what it contains.