Model Spread, Ensemble Probability, and a Low-Ceiling Forecast for a Coastal Aerodrome
A forecaster must issue a TAF for SBFL (Florianópolis) valid 00:00–06:00 UTC. The 12:00 UTC global ensemble shows a bimodal solution: 60 % of members produce a marine stratocumulus deck with a ceiling of 300–500 ft and visibility of 2,000–4,000 m, while 40 % produce a clear night with a ceiling above 3,000 ft. The deterministic run follows the clear solution. The airport has a 200 ft decision height and no ILS on the secondary runway. Surface observations at 23:00 UTC show a dewpoint depression of 1 °C, a wind from the southeast at 6 kt, and a satellite image showing a low cloud bank 20 nm offshore moving toward the coast. The forecaster has 30 minutes to issue the TAF.
1. How should the forecaster weigh the ensemble spread against the deterministic run, and what TAF should be issued?
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The deterministic run is a single member of the same distribution and should not be given more weight than the ensemble simply because it is the operational product. The bimodal structure means the forecast is genuinely uncertain, and the correct approach is to use the ensemble probability as the primary guidance. The 60 % probability of a 300–500 ft ceiling, combined with a dewpoint depression of 1 °C, a light onshore southeast wind, and an offshore cloud bank moving toward the coast, tilts the balance strongly toward the low-ceiling solution. A TAF that carries only the clear solution would be inconsistent with the evidence. The appropriate TAF is to include a TEMPO or BECMG group for BKN004 and visibility 3,000 m, with a PROB30 for the lower end, and to state the ceiling and visibility in the remarks. The forecaster should also note that the 200 ft decision height makes the difference between 300 ft and 500 ft operationally significant, so the more conservative ceiling should be used in the TAF.
2. What observational and nowcasting actions should be taken in the 30 minutes available, and how should the TAF be updated after 00:00 UTC?
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In the 30 minutes available, the forecaster should check the latest satellite image for the cloud bank's leading edge, request a pilot report from any arrival, and examine the 23:00 UTC surface observations for the dewpoint depression trend. The ceilometer at the aerodrome should be monitored for the first sign of a lowering ceiling, and the forecaster should coordinate with the adjacent MET office for a cross-check on the offshore cloud. If the cloud bank is 20 nm offshore and moving at 6 kt, the earliest arrival is about 3.3 hours, so the low ceiling is more likely in the second half of the TAF validity period. The TAF should therefore place the BECMG group around 03:00 UTC rather than at the start of the period. After 00:00 UTC the forecaster should issue amendments as soon as the ceilometer shows a ceiling below 1,000 ft, and should convert the TEMPO to a prevailing group if the low ceiling becomes established.