VFR departure with thunderstorms on the Rodrigues route
You plan a VFR flight from FIMP to FIMR, 350 NM, estimated 2 h 10 min. Fuel endurance 3 h 00 min. At 1200Z the FIMP METAR is 12015G25KT 9999 SCT018 BKN030 28/24 Q1011, and the TAF TEMPO 1200/1400 3000 +TSRA BKN015CB. The route forecast shows isolated cumulonimbus tops FL280, widespread showers, and a low-level jet 30 kt at 2,000 ft. FIMR METAR is 11012KT 9999 SCT020 29/24 Q1010, no TAF. Your aircraft has no weather radar, no storm scope, and no oxygen. You are VFR only. It is 1215Z. You have a handheld GPS and a serviceable VHF radio. There is no alternate with fuel available on Rodrigues; if you go, you must return to FIMP or land at FIMR.
1. Assess the weather hazards and decide whether to depart now.
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Do not depart now. The TEMPO period 1200/1400 overlaps your planned departure and the first part of the flight. It forecasts 3,000 m visibility in thunderstorms and heavy rain with a broken cloud base at 1,500 ft and cumulonimbus. Your aircraft has no weather radar or storm scope, so you cannot see the cells at night or in haze, and you have no oxygen to go over high tops. The route forecast gives CB tops to FL280, which is far above a VFR light aircraft.
The low-level jet adds mechanical turbulence and low-level wind shear, especially near the island terrain. A 350 NM over-water leg with no fuel alternate at Rodrigues means a diversion decision must be made early. The destination weather looks better, but that does not make the route safe. The correct decision is to delay until after 1400Z, recheck the TAF and actual radar, or cancel. Do not launch and hope to weave around the cells.
2. If you delay, what specific conditions must be met before departure?
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Wait until the TEMPO period has passed and the actual weather shows no thunderstorms or cumulonimbus within at least 20 NM of the route. The TAF should no longer forecast TS, and the visible satellite or radar picture should confirm that the cells are not building along the track. Cloud base should allow VFR at a safe cruising level with standard cloud clearance, and visibility should be at least 10 km.
Wind must be within your personal and aircraft limits for the whole route, including the low-level jet and gust spread. Fuel must cover the trip, a 30-minute VFR reserve, and a realistic diversion. Because Rodrigues has no fuel alternate, you need a clear return-to-FIMP plan if the destination weather deteriorates. If any of these conditions are not met, do not go; a delayed meeting is cheaper than an encounter with a CB.
3. What would you do if a thunderstorm develops ahead when you are 150 NM from FIMP with 1 h 40 min fuel?
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Turn back immediately or divert around the storm by a wide margin. At 150 NM from FIMP, you need about 75 minutes at 120 kt to return, leaving only 25 minutes of fuel. That is below a comfortable reserve, so you should declare PAN PAN, state your position and fuel, and request any available weather information. Do not continue toward the CB to save time.
Avoid the storm by at least 20 NM, especially on the upwind side, and do not attempt to fly under the anvil or through heavy rain. If you are caught, reduce power, maintain a safe attitude, and turn away from the worst returns. Expect turbulence, hail, and possible microbursts. Landing with less than 30 minutes reserve is undesirable, but penetrating a cumulonimbus is far worse. Plan to land at FIMP and accept the low-fuel state as an emergency consequence of the earlier decision.