Fog at Palmerston North: TAF versus METAR
At 0610 in May you are airborne in a Cessna 172, 25 nm northeast of Palmerston North at 4,500 ft, on a VFR flight from Napier, in smooth air with a clear sky above and a shallow layer of low cloud visible ahead. The Palmerston North TAF issued at 0400 reads: 0400/0500 04005KT 9999 SCT030 BECMG 0500/0600 04008KT 6000 -RA BKN015 TEMPO 0500/0700 1500 BR BKN006. The 0600 METAR reads: 050/04 0800 FG VV001 06/06 Q1022. Ohakea, 25 nm west, reports 9999 SCT025. You have 1 h 40 min of fuel remaining and Ohakea is 30 minutes away. Sunrise is 0720 and the ground is wet after overnight rain.
1. Decode the TAF and the METAR, and explain why the actual weather is worse than the forecast.
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TAF: wind 040 at 5 kt, visibility 10 km or more, scattered cloud at 3,000 ft initially. Becoming, between 0500 and 0600, wind 040 at 8 kt, 6,000 m visibility, light rain, broken at 1,500 ft. Temporarily, between 0500 and 0700, 1,500 m visibility in mist with broken at 600 ft.
METAR at 0600: wind 050 at 4 kt, visibility 800 m in fog, vertical visibility 100 ft (sky obscured), temperature and dewpoint both 6 C, QNH 1022.
The actual weather is at or beyond the worst case in the TAF. TEMPO describes a temporary fluctuation - in New Zealand practice expected to last less than an hour at a time and for less than half of the period - so 1,500 m and 600 ft were offered as the pessimistic envelope, and the field is below even that. The reasons are visible in the METAR: a temperature-dewpoint spread of zero means the air is saturated, and the wind has gone light at 4 kt, so there is no mixing to break the inversion. After overnight rain, clear skies and light winds, the Manawatu basin cools radiatively and the dewpoint is reached - classic radiation fog. The TAF was issued two hours earlier, before the dewpoint depression closed.
2. Evaluate your options with fuel figures and choose one.
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Options are: continue and hold, divert to Ohakea, or return to Napier.
Holding: Palmerston North is 800 m in fog with vertical visibility 100 ft, far below any VFR criteria and with no approach available to you. Radiation fog in the Manawatu in May typically clears between one and three hours after sunrise, which here means 0830-1030. You have 100 minutes total, so even a best case leaves nothing in reserve, and a continuation would risk a diversion with minimum fuel. Holding to wait for the forecast improvement is not viable.
Diverting to Ohakea: 30 minutes at your groundspeed, landing with about 70 minutes of fuel, which still satisfies a 30-minute VFR reserve plus a comfortable margin. Ohakea is 25 nm west with 10 km visibility and scattered cloud at 2,500 ft. The risk is that the fog bank extends west, so confirm the current report rather than the trend.
Returning to Napier: also viable, but longer and with no advantage over Ohakea.
Decision: divert to Ohakea now, on the current METAR, before the fuel state narrows the options. Do not descend below the minimum safe altitude to have a look at Palmerston North - you have neither the rating nor the equipment to do it, and the field is obscured.
3. What does this case tell you about when the fog will lift and about the limits of a TAF?
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Radiation fog is a local, ground-based effect, so its life cycle is driven by surface heating and by the wind. Here the precursors are all present: overnight rain wetting the ground, light winds, clearing skies, a river basin with cold air drainage, and a temperature-dewpoint spread that has closed to zero. It will burn off when insolation warms the surface and breaks the inversion, which in May is typically one to three hours after sunrise - so between 0830 and 1030. An increase in wind to 10 kt or more would mix it out earlier; a wind shift to a moist onshore direction would turn it into advection fog that does not follow the same diurnal rule.
On the TAF: it is a forecast of the most likely conditions plus specified probabilities, valid for a period, not a now-cast. The TEMPO envelope was exceeded because the fog formed deeper and faster than the model expected after the dewpoint closed. For a short VFR flight the METAR and the visual picture outrank the TAF, and the correct use of a TAF is to identify the trend and the alternate requirement before departure, not to justify pressing on into a field that is reporting below minima.