Convective Forecast on the Arrival Window

Decode, do not skim.

• The base forecast (18 kt gusting 30) is already a moderate-to-fresh crosswind with a gust spread of 12 kt - a windshear signature at low level and a handling problem on a light aircraft.
• TEMPO 1500/1900 means the conditions are expected for periods of less than an hour within that window, not 'maybe'. Visibility 3,000 m in heavy rain and thunderstorm, with cumulonimbus bases at 2,000 ft.
• PROB30 1500/2000 is a 30% probability - a real possibility, not an outside chance. Of ten such days, three produce 1,500 m visibility in thunderstorms with CB bases at 1,200 ft.

Timing. Arrival at about 15:50 lands squarely inside both windows and near peak surface heating.

Physics. Temperature 33°C with dew point 24°C gives a spread of 9°C - convective cloud will develop readily. The aerodrome is 8 km inland, so a sea-breeze convergence line is a plausible trigger. The gusty surface wind suggests outflow or gust fronts from cells upwind. For a VFR light aircraft the threats are: loss of visual reference in heavy rain, low-level windshear, hail, severe turbulence under the CB, and an unexpected 180° wind shift on final approach.

Departure: go, conditionally. Nothing in the forecast makes the departure unsafe, and the ridge crossing occurs around 14:40, before the convective window opens. But conditions are attached.

Explicit gates:

1. Before the ridge (14:40): if build-ups are already topping the ridge or the aircraft cannot cross at least 2,000 ft above the ridgeline in clear air, turn back. Do not scud-run a mountain pass with CB in the area.
2. En route, 30 minutes out: obtain an actual at destination and the alternate. If destination is reporting a thunderstorm overhead, a spread-out gust front, or visibility below 5 km, the destination is out; proceed to the alternate.
3. Arrival: do not launch an approach into a cell. Plan to hold clear, ideally over the alternate, and let it pass. If a TS is over the field, the correct aerodrome is the one without one.
4. Hard limit: no arrival into a reported TS or gust front, no landing in a 30 kt gust with a 12 kt spread in a light aircraft, and no VFR descent below the cloud base to 'have a look'.

Why this shape. A go/no-go answered at the departure aerodrome is answered with the least information. The decision belongs at the point where the evidence exists, which is why the gates are defined in advance - after an hour in the air, a tired pilot will rationalise a bad arrival.

Fuel arithmetic:

• Trip: 1.83 h × 32 L/h ≈ 59 L
• Contingency 5% ≈ 3 L
• Alternate 25 min ≈ 13 L
• Final reserve 45 min = 24 L
• Taxi 5 L
• Total ≈ 104 L against 150 L on board → surplus ≈ 46 L, about 1 h 26 min of cruise fuel.

Does it hold? Yes, with real margin. The surplus buys roughly 1 h 25 min of holding or one extra approach - important because 'TEMPO' means a temporary spell, so waiting is a legitimate strategy rather than an automatic diversion. It also covers a second diversion if the first alternate fills in.

Cautions.

• The alternate is the only planned escape; identify a second, en-route diversion field before departure, ideally on the near side of the ridge.
• If a diversion is made, remember the aircraft may then be low on fuel at an unfamiliar field - reserve must survive the diversion, not merely the plan.
• Hot day, high DA: the go-around and climb performance at destination will be degraded exactly when a gust front may force one.

Conclusion. Fuel is the strongest part of this plan; the weak point is decision-making under convective pressure. Plan to arrive able to divert, not committed to land.