Coastal Fog and a VFR Arrival Deadline
You plan a 90-minute VFR flight from an inland valley airfield to a coastal airfield, departing at 08:00 local; sunrise is 06:30. The coastal TAF says: prevailing SCT012, visibility 10 km; TEMPO BKN006, visibility 4 km in mist, clearing by 11:00. The current coastal METAR is wind 210/06 kt, visibility 5 km, BKN008, temperature 14°C, dew point 13°C, QNH 1018. The coastal airfield has an instrument approach, but you are not instrument rated. Terrain between the two airfields rises to 2,500 ft; en route conditions are clear with 30 km visibility. Your aircraft has 3 h fuel; the flight is 1 h 15 min; the alternate is 25 nm inland with clear weather. You need to arrive by 10:00 for a business meeting. The forecast says the mist will clear by 11:00.
1. Assess the likelihood and trend of fog or stratus at the coastal airfield using the temperature, dew point, wind, and time of day. Is the TEMPO group or the prevailing forecast more relevant to your arrival?
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Temperature 14°C and dew point 13°C give a dew-point depression of 1°C, so the air is close to saturation and mist, fog or low stratus is likely. The wind is light onshore at 6 kt, which can advect moist air over the field but also provides some mixing; with sunrise at 06:30 and arrival at 10:00, daytime heating may lift the stratus, but a shallow marine layer can persist. The current BKN008 already puts the ceiling at 800 ft. The TEMPO BKN006 and 4 km visibility is not a rare outlier; it is consistent with the measured temperature and dew point. The prevailing SCT012 and 10 km is the less relevant part of the forecast for a 10:00 arrival because the TEMPO condition is the one that would make VFR arrival illegal or unsafe. The forecast clearing by 11:00 is after your required arrival time.
2. What VFR planning decision do you make before departure, and what in-flight decision point do you set?
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Do not depart expecting a 10:00 VFR arrival at the coastal field. The probability of ceiling below 1,500 ft and visibility below 5 km is high, and you are not instrument rated, so the instrument approach is not an option. Options are: delay until after 11:00 and obtain a new METAR and TAF before departing; fly to the inland alternate and wait; or cancel the flight. If you do depart, set a hard decision point about 10-15 nm from the coast: obtain the current coastal METAR or ATIS and compare it with the applicable VFR minima. If the ceiling is below 1,500 ft or visibility below 5 km, divert to the inland alternate. Do not scud-run or descend below the cloud to maintain visual contact; that is how VFR-into-IMC accidents happen. Fuel is sufficient to divert, so there is no pressure to press on.
3. If you are already airborne and the coastal METAR updates to BKN006, 4 km, what do you do?
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Treat the update as a stop for the coastal destination. BKN006 means a ceiling of 600 ft, below the likely VFR minimum of 1,500 ft, and 4 km is below 5 km. Turn toward the inland alternate while maintaining terrain clearance and VFR conditions. Do not attempt to pick up a clearance or fly the approach unless you are instrument rated and current; the aircraft may be capable, but you are not. Notify ATC or the appropriate facility if able, and land at the alternate. The fact that the forecast says clearing by 11:00 is not a reason to hold in marginal VFR near rising terrain; the alternate is only 25 nm away and the fuel state allows it. After landing, wait for a reported improvement and a new forecast before attempting the coastal field.