4.4 In-Flight Advisories: AIRMETs, SIGMETs & Convective SIGMETs

Inflight Advisories: Products, Timing, and Dispatch Decisions

An inflight weather advisory identifies hazardous weather affecting a region, altitude layer, and period. Its polygon is a useful overview, but the text, movement, vertical limits, and validity times matter. Dispatchers compare the advisory with the proposed route and each active flight, then combine observations, forecasts, imagery, and PIREPs. Neither absence of an advisory nor location just outside its drawn boundary proves that conditions are safe.

Current AIRMET formats

Traditional alphanumeric CONUS text AIRMETs were retired January 27, 2025. Current contiguous-U.S. planning uses G-AIRMETs. These present hazards at discrete forecast times, ordinarily no more than three hours apart, out to twelve hours. A snapshot is not a promise that a hazard exists only at that instant: evaluate changes between snapshots and defined onset/end times. Text AIRMETs remain applicable in Alaska and Hawaii under their regional formats and issuance rules.

The familiar hazard families remain useful:

The 2026 Aviation Weather Handbook identifies CONUS IFR advisory thresholds as ceilings below 1,000 feet and/or visibility below 3 SM. Non-convective LLWS concerns the layer below 2,000 feet AGL. The handbook's CONUS strong-surface-wind criterion is greater than 30 knots; regional text descriptions should be read in their own scope. Do not present every AIRMET everywhere as a six-hour text bulletin. AIRMET-level weather is relevant to transport airplanes, especially during diversion, descent, approach, and ground operations.

CONUS non-convective SIGMETs

A SIGMET addresses significant hazards to aircraft, not just light aircraft. In the CONUS, non-convective criteria include severe or greater turbulence, severe icing, widespread dust or sand storms, and volcanic ash. These are unscheduled products with validity up to four hours. A continuing phenomenon is reissued as necessary. Read the top and bottom altitudes, movement, and expected continuation beyond the current bulletin.

International SIGMETs and tropical-cyclone/volcanic-ash products outside the CONUS have their own formats and validity provisions, including some periods up to six hours. Applying that six-hour value to every CONUS ash bulletin is incorrect. Identify the issuing meteorological watch office and flight information region before importing a domestic mnemonic into an oceanic briefing.

Convective SIGMETs

CONUS Convective SIGMET bulletins are issued hourly at 55 minutes past the hour for eastern, central, and western regions. They are valid for two hours or until superseded. A “NONE” bulletin is issued if the criteria are not met. Any Convective SIGMET implies severe or greater turbulence, severe icing, and LLWS associated with the convection.

Routine criteria include a line of thunderstorms at least 60 miles long with thunderstorms affecting at least 40 percent of its length; an area of active thunderstorms with significant impact, at least 40 percent coverage and a strong radar/satellite/lightning signature; and embedded or severe thunderstorms expected for more than 30 minutes. Special criteria include tornadoes, surface hail at least 3/4 inch, or surface wind gusts at least 50 knots, with the time and forecaster-judgment provisions described in the handbook. Do not invent a universal 3,000-square-mile requirement for every current Convective SIGMET.

That 3/4-inch aviation advisory criterion is distinct from the National Weather Service's general severe-thunderstorm warning hail criterion and from urgent PIREP categorization, where reported hail is urgent without that size cutoff. Questions mixing these products require attention to the actual product named.

Center Weather Advisories and planning outlooks

A Center Weather Advisory (CWA) is a short-term, unscheduled advisory from a Center Weather Service Unit supporting its ARTCC. It can address conditions meeting or approaching national advisory criteria, conditions not adequately covered by another product, or hazards needing a prompt local update. Its validity is up to two hours. A Meteorological Impact Statement (MIS) supports longer planning, with validity up to 48 hours; it is not a tactical penetration clearance.

During flight watch, a CWA can reveal developing weather before a broader bulletin is updated. Brief the crew promptly, assess the affected altitude and time, and check whether a reroute, delay, or diversion is needed. A new CWA does not automatically cancel every release in the center, and a dispatcher's duty does not depend on waiting for a particular product to be issued.

Icing and the correct legal rule

Section 121.629(a) prohibits dispatch, release, continued enroute operation, or landing when, in the applicable PIC/dispatcher opinion, icing expected or encountered might adversely affect safety. Section 121.341 supplies icing-equipment requirements and particular restrictions for the non-transport airplanes described in paragraph (c). It is not a universal wording that bans every transport-category flight inside every forecast severe-icing polygon.

A severe-icing SIGMET along the planned altitude is a serious planning issue. Verify the aircraft's certified envelope, AFM limitations, actual weather information, and carrier procedures. If the plan exposes the flight to unsafe icing, change altitude or route, delay, or cancel. A generic belief that thermal anti-ice makes any forecast harmless is equally incorrect. The required decision connects the hazard to the airplane and proposed operation.

Worked flight-watch exercise

A hypothetical flight is planned at FL320. A SIGMET describes severe icing between FL280 and FL360 across a segment it will reach at 0210Z. The dispatcher first checks the bulletin's validity and movement, then compares current imagery and PIREPs with the route. A southern reroute adds 20 minutes; a lower route introduces terrain and additional fuel considerations. Evaluate both options using approved performance and fuel data, brief the PIC, and record the chosen amendment. Send timely safety information under § 121.601 rather than merely archiving the bulletin.

For thunderstorms, FAA guidance recommends at least 20 NM avoidance for severe or intense cells, with additional care downwind and around anvils. This is weather-avoidance guidance, not a universal legal dBZ/flight-level formula. Current radar polygons move and data links have latency. Plan a viable strategic corridor while the crew uses appropriate real-time information and ATC clearance for tactical deviations.

Official References

Source check: 2026-10-12. 2026 FAA Aviation Weather Handbook; Current NWS TAF policy (October 2024); AWC current products; Dispatcher weather information § 121.601. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.