Thunderstorms

Three required ingredients

Every thunderstorm — single-cell pop-up to organized supercell — needs all three:

1. Sufficient water vapor — moisture for condensation and latent-heat release
2. Unstable air — air that continues rising once disturbed (positive buoyancy through the lower atmosphere)
3. Lifting mechanism — something to start the unstable parcel rising

Common lifting mechanisms: converging winds around surface lows and troughs, frontal lift (cold or warm), upslope flow over terrain, drylines, outflow boundaries from prior storms, sea/lake/land breezes, and valley breezes. See AWH Chapter 22 for the full taxonomy.

Life cycle — three stages

• Towering cumulus stage: Continuous updraft, rapid vertical growth (3,000+ fpm). Cloud is building. Precipitation has not yet started. Lasts ~10-20 minutes.
• Mature stage: Both updraft and downdraft present. Precipitation begins reaching the ground. Most dangerous stage — strongest turbulence, lightning, hail, microbursts. Lasts ~10-20 minutes.
• Dissipating stage: Downdrafts dominate. Updraft cuts off because precipitation cools and stabilizes the column. Anvil top spreads downwind on upper-level winds. Storm is dying — but downbursts and hail can still reach the surface from the collapsing cell.

Operational implication: "Dissipating" does not mean "safe to fly under." Strong downbursts often occur right as the cell collapses.

Three thunderstorm types

• Single-cell: Isolated, one updraft / downdraft cycle. Lifespan ~30 minutes. Common over heated terrain on summer afternoons. Easy to circumnavigate visually.
• Multicell (cluster or line): Multiple cells in different life-cycle stages. New cells form on the gust front of the dissipating cell. Can persist for hours. Squall lines are linear multicells — intense, fast-moving, often forming 50–300 NM ahead of strong cold fronts. Penetration is not survivable; the only legal answer is "don't."
• Supercell: One persistent rotating updraft (mesocyclone). The most dangerous form. Persists for hours; produces large hail, damaging straight-line winds, and the majority of strong tornadoes. Recognizable on radar by its hook echo. See SPC convective outlooks in your morning brief — supercell days are flagged in advance.

Hazards

Per AC 00-24C, a single thunderstorm cell carries the following:

• Low ceilings and visibility (precipitation, dust)
• Lightning (can damage avionics and pilot eyesight)
• Adverse winds and rapid wind shifts at the surface
• Downbursts and microbursts — see the wind shear & microbursts page
• Severe to extreme turbulence extending up to 20 NM laterally and 25 NM downwind in some cases
• Icing (especially in the upper portions, mixed and clear)
• Hail — can be encountered in clear air outside the visible cloud
• Rapid altimeter changes from pressure surges along gust fronts
• Static electricity — radio interference and P-static
• Tornadoes
• Engine water ingestion in heavy rain — flame-out risk on turbines

Avoidance — the rules pilots actually follow: circumnavigate by 20 NM minimum; never fly under an overhanging anvil; never try to outclimb a cell. Use ADS-B FIS-B weather as a strategic aid only — its NEXRAD age stamps lag the real cell by 5–20 minutes, which is plenty of time to fly into a building cell that the screen still shows clear.

METAR & TAF coding

From AC 00-45H — Aviation Weather Services:

• TS — thunderstorm without precipitation (dry thunderstorm)
• TSRA — thunderstorm with rain
• VCTS — thunderstorms in the vicinity (5–10 SM of the airport)
• TSB[time]E[time] — thunderstorm began HHmm, ended HHmm. Example: TSB0159E30 = began 0159, ended at :30 of the same hour
• +TSRAGR — heavy thunderstorm, rain, hail (GR ≥ 1/4 inch); GS = small hail/snow pellets

Convective SIGMETs are issued for the CONUS in lieu of regular SIGMETs for thunderstorms — issued every 55 minutes (and as needed) for organized severe convection. Read the current product at aviationweather.gov/sigmet. They cover thunderstorms producing severe surface wind, hail ≥ 3/4", embedded TSs, lines of TSs, or any tornado.

The pre-flight question

Before launching on a day with any chance of convection:

• Have you read the latest SPC convective outlook? "Slight risk" or worse means cells are likely.
• Are there active Convective SIGMETs on your route?
• Where will building cells most likely form along your route — over heated terrain, along a frontal line, on the upslope?
• What is your turn-around point? Brief it out loud.
• Is there a fuel-stop alternate that keeps you 20+ NM from any forecast cell?

Pilots who answer those five questions before launching almost never get caught.