Warm Front Arrival: Reading the Pressure Fall and the Wind Veer
You are the commander of a scheduled flight to a coastal aerodrome in northwest Europe. The 0600 analysis places a warm front 60 nm west of the aerodrome, aligned NE–SW and moving east at 20 kt; the associated cold front lies 380 nm further west. The aerodrome QNH has fallen from 1012 hPa at 0300 to 1004 hPa at 0600. The 0600 METAR reads 14012KT 6000 -RA BKN008 11/10 Q1004. The TAF issued at 0500 reads: BECMG 0800/0900 22015G28KT 9999 SCT018 BKN030, TEMPO 0900/1500 5000 -RA BKN010. Your ETA is 0930 and the flight is 1 hour 20 minutes. The destination has one runway, 27, with a 25 kt tailwind limit and a 20 kt crosswind limit. The two alternates are 40 nm inland (elevation 300 ft, no significant weather, QNH 1008) and 90 nm south. Fuel gives 30 minutes of holding.
1. Estimate the time of warm-front passage and describe the conditions you should expect at your 0930 ETA compared with the 0600 METAR.
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Timing. At 20 kt over 60 nm the front reaches the aerodrome about 3 hours after 0600, i.e. around 0900 - almost exactly the middle of the TAF's BECMG 0800/0900 window. Your ETA of 0930 therefore places you just behind the warm front, in the warm sector.
Cross-check with the pressure tendency. An 8 hPa fall in 3 hours is nearly 3 hPa/h, which is fast. A fall of that rate is more typical of being within a few tens of miles of the frontal discontinuity than 60 nm ahead of it, so treat the frontal position as the optimistic case: the front could be through by 0830–0900 and the wind change could come earlier than the TAF suggests. That matters for your approach wind.
Conditions at 0930.
- Wind veers from SE 140/12 to SW 220/15G28, exactly as the BECMG group says; the gust is mechanical, in the warm sector.
- Temperature and dewpoint rise slightly (warm advection aloft and at the surface) rather than fall: expect T≈12–13 °C, Td≈11 °C, so the dewpoint spread stays small.
- Visibility improves out of the rain (9999 m) but the low stratus persists: the BKN008 of the METAR gives way to SCT018/BKN030, with a real risk of ragged scud below that in the TEMPO group and further drizzle. The small spread means fog is unlikely at 15 kt of wind but mist and low cloud on the approach are likely.
The transition you should brief is: continuous rain and low cloud before the front, then drizzle, a veered gusty wind and a slow improvement in cloud base but a very moist boundary layer.
2. Assess the wind for runway 27 and say whether you anticipate any wind shear on the approach.
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Wind components on runway 27. With the surface wind 220/15G28, the angle between wind direction and runway heading (270) is 50°.
- Headwind component: 15 × cos 50 ≈ 10 kt steady, 28 × cos 50 ≈ 18 kt in the gusts. In the gust the wind is still functionally a headwind, so there is no tailwind problem.
- Crosswind component: 15 × sin 50 ≈ 11 kt steady, 28 × sin 50 ≈ 21 kt in the gusts - right on, and briefly over, the 20 kt limit.
The correct handling is not to look at the mean wind but at the highest gust: if a gust of 28 kt coincides with the crosswind limit, the approach is outside limits and a diversion or a hold for the gust to ease is required. Many operators also apply a gust correction to the landing distance and to Vref.
Wind shear. Two sources are present. First, the frontal shear: the wind veers 80° and increases by 10–16 kt in a shallow layer across the front, and you are arriving just after passage, so the shear is in the lowest few thousand feet. Second, mechanical gustiness in a 15 kt mean wind with 28 kt gusts implies a gust factor of nearly 2, typical of a well-mixed warm-sector boundary layer. Expect an airspeed loss on short final when the gust drops away.
Practical mitigations: add the full gust increment to Vref, fly a stabilised approach with a prompt go-around decision, and brief that a windshear escape may be needed if the airspeed trend is adverse below 500 ft. Consider using the inland alternate if the crosswind at the destination exceeds the limit on arrival rather than after the approach.
3. The TAF was issued at 0500. What is your fuel and alternate decision, and what would make you change it in flight?
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Fuel and alternate. The inland alternate 40 nm away (elevation 300 ft, no significant weather, QNH 1008) is the correct first choice: it lies behind the warm front in the cooler, drier air, it is close, and it has no significant weather. The 90 nm southern alternate is the second choice but doubles the diversion fuel.
With 30 minutes of holding, the arithmetic is tight but workable for an 0930 arrival: the front is forecast to be through by 0900, the wind veer is a post-frontal change, and the TEMPO drizzle/cloud group does not suggest the field will go below limits. The weak point is the timing - the 8 hPa fall in 3 hours tells you the system is more advanced than the chart position implies. Brief a decision point at about 60 nm out: if the ATIS still shows 140/12 with -RA and BKN008 at that point, the front is late, the crosswind and low-cloud risk at the destination grow, and you should divert early rather than hold.
In-flight triggers to change the plan:
- The destination METAR still showing SE wind and rain at the decision point (front late).
- Reported crosswind/gust above limits, or a windshear alert.
- Any holding - 30 minutes is one hold plus an approach and a diversion; there is no margin for a second hold.
- The cold front is 380 nm behind the warm front and will not affect you on this sector, so the only weather risk is the warm front itself. Do not carry fuel for a system that will not arrive.
The judgement the examiner wants: the pressure tendency is a forecast in its own right, and it argues for less faith in a 4½-hour-old TAF position than in the actual wind and cloud at the decision point.