A Convective SIGMET would be issued for which of the following conditions?
- AA line of thunderstorms affecting at least 3,000 square miles, or thunderstorms producing surface winds of 50 knots or hail 3/4 inch or greater in diameter
- BModerate turbulence forecast below 15,000 feet MSL due to mountain wave activity
- CWidespread stratus clouds producing continuous light rain over several states
- DA single isolated cumulus cloud with no precipitation over a 20-mile area
Show answer & explanationAnswer & explanation
Correct answer: A. A line of thunderstorms affecting at least 3,000 square miles, or thunderstorms producing surface winds of 50 knots or hail 3/4 inch or greater in diameter
Convective SIGMETs are issued for severe or embedded thunderstorms, lines of thunderstorms affecting an area of 3,000 square miles or more, thunderstorms associated with hail 3/4 inch or greater in diameter, surface wind gusts of 50 knots or greater, or tornadoes. Isolated non-precipitating clouds, stratus rain, and mountain wave turbulence do not meet these criteria.
Why the other options are wrong
- B. Mountain wave turbulence is addressed by AIRMET Tango, not Convective SIGMET, which is thunderstorm-specific.
- C. Stratus and continuous light rain are non-convective and do not trigger a Convective SIGMET.
- D. A small non-precipitating cumulus cloud does not meet any convective SIGMET criteria.
Convective SIGMET Criteria
A Convective SIGMET is issued for severe thunderstorms, embedded thunderstorms, lines of thunderstorms affecting 3,000+ square miles, or thunderstorms with surface winds ≥50 knots, hail ≥3/4 inch, or tornadoes.
- Valid for 2 hours, issued for the Eastern, Central, and Western U.S. regions plus outlooks
- Implies severe or greater turbulence, severe icing, and low-level wind shear
- Tornado, hail ≥3/4 inch, and wind gusts ≥50 knots are specific severe thresholds
- Issued hourly at 55 minutes past the hour, with unscheduled updates as needed
Memory trick: 3000 square miles, 50 knots, 3/4 inch hail — remember the 'severe three'