VFR Into a Cumulonimbus Forecast
You are planning a VFR flight from an airport in central Iran to a destination 180 nm east. Planned time en route is 1 h 30 min. The aircraft has fuel endurance of 3 h 15 min at planned power, including 45 min reserve. The destination TAF reads: wind 240/18G35 kt, visibility 6 km, thunderstorms with rain, scattered cumulonimbus at 2,500 ft, broken at 6,000 ft; TEMPO 2,000 m in heavy thunderstorm rain and hail. The alternate is 35 nm west of destination, forecast broken at 3,000 ft, visibility 10 km, wind 250/12 kt. A line of cumulonimbus is visible on the weather radar 20 nm ahead, covering the direct track; tops are above 35,000 ft. You are 45 min into the flight, in VMC, with 2 h 30 min of fuel remaining. ATC offers a 40 nm deviation to the north or a return to departure. The destination is not reporting any PIREP.
1. Assess whether continuing VFR to the destination is acceptable. Include the weather and fuel factors.
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Continuing VFR to the destination is not acceptable. The TAF includes thunderstorms, cumulonimbus, gusts to 35 kt, and a TEMPO of 2,000 m visibility with heavy rain and hail. Those conditions imply severe turbulence, icing, hail, wind shear and embedded IMC; VFR cloud clearance cannot be assured. The visible CB line 20 nm ahead covers the direct track, and tops above 35,000 ft mean you cannot overfly it.
Fuel is not yet the limiting factor: 2 h 30 min remaining covers the 45 min to destination, 25 min to alternate, and 45 min reserve. But fuel margin does not make a thunderstorm penetration safe. No PIREP is not evidence of safe conditions; it may simply mean no one has reported recently.
2. Compare returning, taking the 40 nm northern deviation, or continuing to destination then diverting to the alternate. What is the safest decision?
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Return to departure: about 45 min, leaving 45 min reserve plus margin, and it keeps you in known VMC. Northern deviation: 40 nm extra is roughly 15–20 min, but it only avoids the line temporarily; the destination still has TS/CB, so it is useful only if it leads to an airport with confirmed VMC.
Destination then alternate: about 45 min to destination plus 25 min to alternate plus 45 min reserve = 115 min, within the 150 min remaining. However, it relies on reaching a destination with thunderstorms and then flying an approach; a go-around could trap you near the CB line with the alternate to the west. The safest decision is to divert now to an airport with confirmed VMC, or return. Do not press toward the CB line.
3. What specific cues would tell you the thunderstorm threat is affecting the destination earlier than the TAF suggests?
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Cues include a darkening cloud base, frequent lightning, a strong green or purple radar return, abrupt wind shifts or gusts, a visible rain shaft or hail, and blowing dust near the airport. On radio, request current weather and PIREPs; if the destination reports a thunderstorm overhead, the TEMPO is verifying.
If the storm is moving away and there is a clear VMC window with safe cloud clearance and fuel to the alternate, a cautious arrival could be considered. But do not attempt a VFR arrival through the CB line or beneath a storm base, and keep the diversion fuel intact.