When convective weather erupts across the National Airspace System, two weather products rise above all others in urgency for airline dispatchers and Airline Transport Pilots: the Convective SIGMET and the Center Weather Advisory (CWA). Unlike routine METARs or area forecasts, these products signal hazards severe enough to destroy or incapacitate an aircraft, and understanding how they are generated, what criteria trigger them, and how to integrate them into dispatch and flight release decisions is a foundational ATP-level competency. This article provides a complete operational treatment grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28B).
At the dispatch desk or in the cockpit, time is compressed. A Convective SIGMET issued 15 minutes ago may already be outdated as a fast-moving squall line advances. Knowing the product's lifecycle, geographic scope, and the exact meteorological thresholds that triggered it allows a dispatcher or ATP to assess whether a planned route, alternate, or release is still valid—and whether 14 CFR Part 121 flight release obligations are met.
Convective SIGMETs: What They Are and How They Work
A Convective SIGMET (WST) is a special category of SIGMET issued by the Aviation Weather Center (AWC) specifically for hazardous convective phenomena. Unlike standard SIGMETs that cover a broad range of non-convective hazards (turbulence, icing, volcanic ash, etc.), Convective SIGMETs are dedicated entirely to convection because convective hazards—embedded thunderstorms, severe turbulence, large hail, and microbursts—represent the most acute structural and control threat to transport-category aircraft.
The AWC divides the conterminous United States into three regions for Convective SIGMET issuance: the Eastern (E), Central (C), and Western (W) regions. Bulletins are issued at 55 minutes past each hour (H+55), with special Convective SIGMETs (unscheduled) issued as needed between scheduled times when qualifying conditions develop or intensify suddenly. Each bulletin is numbered sequentially within the region for that day (e.g., 45E, 46E). A Convective SIGMET, including one covering tornadoes, is valid for up to two hours.
Issuance Criteria
According to FAA-H-8083-28B, a Convective SIGMET is issued when any of the following conditions exist or are expected within the next two hours:
- Severe thunderstorm — surface winds of 50 knots (58 mph) or greater, hail at the surface of 3/4 inch diameter or greater, or tornadoes.
- Embedded thunderstorms — thunderstorms hidden within other cloud layers and not easily visible to flight crews, creating a special hazard for IMC operations.
- A line of thunderstorms — at least 60 nautical miles long, with thunderstorms affecting at least 40% of its length.
- An area of thunderstorms — covering at least 3,000 square miles with thunderstorms affecting at least 40% of the area.
The area itself is described by a polygon of lat/long coordinates or relative fixes, and the text includes storm motion, maximum tops, intensity trend (INTSF, DVLPG, DMSHG), and a forecast section describing the expected movement and intensity change during the valid period. Pilots and dispatchers must parse both the observed and forecast portions to make valid routing decisions, since aircraft will enter the area not at the moment of issuance but minutes or hours later.
Center Weather Advisories: A Complementary Product
A Center Weather Advisory (CWA) is an unscheduled, short-fuse weather product issued by a CWSU (Center Weather Service Unit) meteorologist embedded within an Air Route Traffic Control Center (ARTCC). Where a Convective SIGMET is a nationwide, AWC-generated strategic product, a CWA is a tactical, real-time product tailored to the specific ARTCC's airspace—reflecting what controllers and their assigned meteorologist are observing on radar right now.
CWAs are valid for up to two hours and may be reissued if conditions persist. They are not forecasts in the traditional sense; they reflect current or anticipated conditions within approximately two hours of issuance. CWAs are particularly valuable because they capture rapidly developing phenomena that may not yet have met the Convective SIGMET issuance threshold, or that have intensified between scheduled Convective SIGMET bulletins.
What CWAs Cover
CWAs can be issued for any hazardous condition that affects or is forecast to affect the ARTCC's airspace, including but not limited to:
- Convective activity (most common use case in summer months)
- Moderate to severe turbulence
- Icing
- IFR conditions (low ceilings and visibility)
- Volcanic ash or other localized hazards
This broader scope means a dispatcher monitoring weather for a cross-country release must check both products: Convective SIGMETs for the macro-scale strategic picture, and CWAs for the tactical, center-specific details that may show a fast-developing cell or corridor closure not yet captured in the national product.
Why These Products Matter for Dispatch and Flight Release
Under 14 CFR Part 121, a dispatcher and the pilot-in-command share joint responsibility for the safety of a flight. The dispatcher's flight release, or in some operations an operational control authorization, is predicated on the dispatcher's assessment that the flight can be conducted safely given current and forecast weather. Convective SIGMETs and CWAs are primary inputs to that assessment.
A valid Convective SIGMET overlapping a planned route does not automatically prohibit flight, but it creates a mandatory evaluation burden. The dispatcher must consider whether the aircraft can legally and practically deviate around the hazard area, whether adequate alternates exist, whether fuel reserves accommodate routing deviations, and whether the flight crew has been briefed on the specific hazard. For Part 121 operations, this assessment is documented in the flight release.
Embedded thunderstorms are especially critical at the dispatch level. Because embedded cells are invisible to the flight crew without airborne weather radar, a Convective SIGMET calling out embedded thunderstorms in an overcast layer along a planned route demands either route modification or clear crew coordination about the hazard and their on-board radar interpretation responsibilities. FAA-H-8083-28B emphasizes that embedded thunderstorms are among the most dangerous hazards for air carrier operations precisely because the crew may have no visual warning before encountering extreme turbulence, hail, or wind shear.
Key Numbers and Rules
- Convective SIGMET valid time: up to 2 hours (including tornado-related Convective SIGMETs).
- Issuance schedule: H+55 each hour; specials issued as needed between bulletins.
- Severe thunderstorm criteria: surface winds ≥50 kt, surface hail ≥3/4 inch, or tornado.
- Line of thunderstorms threshold: ≥60 NM long, ≥40% coverage.
- Area threshold: ≥3,000 sq mi, ≥40% coverage.
- CWA valid time: up to 2 hours; may be reissued.
- CWA issuing authority: CWSU meteorologist within the ARTCC facility.
- Geographic coverage: Convective SIGMETs cover the conterminous U.S. (CONUS); CWAs cover a specific ARTCC's airspace.
- Regions: Eastern (E), Central (C), Western (W) for Convective SIGMET bulletins.
Integrating Both Products in a Dispatch Scenario
Consider a scenario: a Part 121 flight is released from Dallas/Fort Worth to Chicago O'Hare. At flight plan time, Convective SIGMET 38C covers a large area of southern Missouri and Illinois with tops to FL450 and a line of thunderstorms moving at 250/35 kt. The ZAU (Chicago ARTCC) CWSU has also issued a CWA valid for the next 90 minutes covering the approach corridor into O'Hare with moderate to severe turbulence below FL200 and embedded convection.
The dispatcher's response is layered: first, evaluate whether the en-route segment can be rerouted north or south of the Convective SIGMET polygon using the forecast movement vector—if the line is moving at 35 knots eastward, the dispatcher must project the polygon forward to the estimated aircraft arrival time in the area, not just its current position. Second, the CWA near O'Hare means the destination itself may be temporarily unusable; the dispatcher re-evaluates the alternate requirement and fuel load. Third, the flight crew is briefed on both products, including the embedded nature of the convection, so the crew can configure their on-board weather radar appropriately and coordinate early with ZAU Center for radar advisories.
This integrated approach—using the strategic Convective SIGMET for route planning and the tactical CWA for real-time terminal and en-route adjustments—represents the intended design of the two-product system and is what examiners expect ATP candidates to articulate clearly.
Common Test Traps
- Confusing issuance authority: Convective SIGMETs come from the AWC (national); CWAs come from CWSU meteorologists inside individual ARTCCs. These are different issuing authorities with different scopes.
- Forgetting the 40% coverage rule: An area of thunderstorms only generates a Convective SIGMET if thunderstorms affect at least 40% of the 3,000-square-mile area. A sparse cluster that doesn't meet area or coverage thresholds won't trigger one—but conditions may still be hazardous.
- Treating a Convective SIGMET as static: The product reflects observed conditions at issuance plus a forecast. Dispatchers must apply the storm motion vector to project where the hazard polygon will be when the aircraft arrives—not where it was when the bulletin was written.
- Overlooking embedded thunderstorm SIGMETs: Unlike a discrete severe storm that a crew might visually avoid, embedded thunderstorms have no threshold based on storm size or severity criteria other than their hidden nature. Students sometimes focus only on the severe/tornado criteria and miss embedded convection as a standalone trigger.
- Assuming a CWA equals a Convective SIGMET for regulatory purposes: While CWAs are critical operationally, they are advisories, not SIGMETs. Check the specific regulatory language in your operation's Operations Specifications regarding which products trigger specific dispatch or deviation requirements.