A dispatcher is deciding whether current thunderstorm activity meets the criteria for issuance of a Convective SIGMET. Which of the following conditions would meet that criteria?
- AA line of thunderstorms at least 60 nautical miles long affecting 40 percent of the line's length
- BA single cumulus cloud building vertically with no lightning reported
- CScattered stratocumulus clouds with a ceiling of 2,500 feet
- DAn isolated single-cell thunderstorm producing light rain and no wind gusts
Show answer & explanationAnswer & explanation
Correct answer: A. A line of thunderstorms at least 60 nautical miles long affecting 40 percent of the line's length
Convective SIGMETs are issued for severe or embedded thunderstorms, a line of thunderstorms at least 60 nautical miles long affecting at least 40 percent of its length, or an area of thunderstorms affecting 40 percent or more of a 3,000-square-mile region; isolated weak storms and non-convective cloud layers do not meet this criteria.
Why the other options are wrong
- B. A single building cumulus cloud without severe weather is not sufficient for issuance.
- C. Stratocumulus with a moderate ceiling is not a convective hazard.
- D. An isolated weak storm without severe characteristics does not meet Convective SIGMET criteria.
Convective SIGMET Criteria
A weather advisory issued for hazardous convective weather meeting specific criteria: severe thunderstorms, embedded thunderstorms, a line of storms 60 NM long affecting 40% of its length, or an area of storms affecting 40% of a 3,000 square mile region.
- Valid for 2 hours, with new bulletins issued every hour
- Implies possible severe turbulence, severe icing, low-level wind shear, hail ≥3/4 inch
- Issued for the continental U.S. only
Memory trick: Convective SIGMET = 60 miles of storm, 40% coverage, severe hazard guaranteed.