12.convective-sigmets-criteria. Convective SIGMETs
A Convective SIGMET (WST) is an inflight weather advisory issued by the Aviation Weather Center (AWC) for hazardous convective weather that is significant to the safety of all aircraft. Unlike non-convective SIGMETs, Convective SIGMETs are issued for thunderstorm-related phenomena only, and they implicitly imply the existence of severe or greater turbulence, severe icing, and low-level wind shear — so these hazards are not listed separately in the bulletin.
Issuance Schedule
Convective SIGMETs are issued for the contiguous United States in three regional bulletins:
- Eastern (WSTE) — east of 87°W
- Central (WSTC) — between 87°W and 107°W
- Western (WSTW) — west of 107°W
They are issued routinely at H+55 (55 minutes past each hour) and are valid for 2 hours. Special (unscheduled) Convective SIGMETs may be issued at any time as conditions warrant. If no hazardous convective conditions exist, the bulletin reads "CONVECTIVE SIGMET...NONE."
An Outlook is appended to each bulletin forecasting possible Convective SIGMET issuance for the period 2 to 6 hours beyond the current valid time.
Criteria for Issuance
A Convective SIGMET is issued for any of the following hazardous convective conditions:
- Severe thunderstorms due to:
- Surface winds greater than or equal to 50 knots
- Hail at the surface greater than or equal to 3/4 inch in diameter
- Tornadoes
- Embedded thunderstorms (obscured by massed clouds or haze and not readily seen or circumnavigated)
- A line of thunderstorms (a line at least 60 miles long with thunderstorms affecting at least 40 percent of its length)
- Thunderstorms producing precipitation greater than or equal to heavy intensity affecting 40 percent or more of an area at least 3,000 square miles
Any one of these criteria triggers a Convective SIGMET. The threshold is a true safety-of-flight level: these are storms no aircraft, regardless of size or capability, should penetrate.
Format and Content
A Convective SIGMET bulletin contains:
- A header identifying region and sequence (e.g., MKCC WST 221855 / CONVECTIVE SIGMET 21C)
- Valid time (a 2-hour window)
- States or flight information regions affected
- A description of the hazard: location (referenced to VORs), movement, intensity, tops (in hundreds of feet MSL), and the specific phenomenon (line, area, isolated cells, embedded)
- An outlook section
Sequence numbering restarts daily at 00Z, with letters denoting region (E, C, W). For example, Convective SIGMET 15E is the 15th Eastern Convective SIGMET issued that day.
Sample Bulletin
MKCC WST 221855 CONVECTIVE SIGMET 21C VALID UNTIL 2055Z KS OK TX FROM 40W GLD-30N AMA-50SW ABI LINE TS 30 NM WIDE MOV FROM 26035KT. TOPS ABV FL450. HAIL TO 1 1/2 IN...WIND GUSTS TO 65 KT POSS.
This tells the pilot: a 30-NM-wide line of thunderstorms across Kansas, Oklahoma, and Texas, moving from 260° at 35 knots, with tops above 45,000 feet, 1.5-inch hail and 65-knot gusts possible.
Operational Use
For preflight, Convective SIGMETs should be reviewed as part of a standard weather briefing under 14 CFR 91.103 (preflight action). In flight, they are broadcast by Flight Service via HIWAS (where available, though largely decommissioned), EFAS-equivalent services, and ATC controllers will alert pilots to Convective SIGMETs pertinent to their route.
Key points for students:
- Because thunderstorms inherently produce severe turbulence, severe icing, and low-level wind shear, a Convective SIGMET implies all three — there is no separate SIGMET issued for those when a Convective SIGMET already covers the area.
- A Convective SIGMET applies to all aircraft, not just light aircraft (unlike AIRMETs, which target light aircraft).
- Distance from a thunderstorm matters: the FAA recommends avoiding severe storms by at least 20 NM, and Convective SIGMET areas should be treated similarly.
Comparison Quick Reference
- AIRMET — moderate icing/turbulence, IFR, mountain obscuration; primarily for light aircraft
- SIGMET (non-convective) — severe icing, severe/extreme turbulence, dust/sandstorms, volcanic ash; valid 4 hours (6 for volcanic ash)
- Convective SIGMET — thunderstorm hazards; valid 2 hours; implies severe turbulence, icing, and wind shear
Understanding the issuance criteria allows a pilot to translate a brief alphanumeric bulletin into a vivid mental picture of the weather and make a sound go/no-go or reroute decision.