BMKG warns cumulonimbus clouds can trigger sudden winds off Labuan Bajo

Labuan Bajo — Indonesia’s meteorological agency has warned that the rapid growth of cumulonimbus clouds over waters around Nusa Tenggara Timur can trigger sudden strong winds, heavier rain, lightning and short-lived increases in wave height, and is urging small boat operators and marine tourism operators to delay departures when dark, towering clouds appear.

The advisory is based on a marine warning issued by the Meteorological Marine Station Tenau Kupang and national guidance from the Meteorology, Climatology and Geophysics Agency (BMKG) covering the period 10–13 November 2025, which forecasts wave heights of 1.25 to 2.5 metres in several Nusa Tenggara waters and highlights the role of thunderstorms in rapidly changing sea conditions, according to Metro TV and BMKG information releases.

According to BMKG, cumulonimbus — the dark, vertically developing clouds commonly associated with thunderstorms — can cause abrupt changes in wind direction and speed as well as sudden increases in wave height, rather than a gradual deterioration that crews can easily anticipate.

What crews should watch for

In guidance reported by Metro TV and other Indonesian media, BMKG urges fishers, marine-tourism businesses and boat operators in Nusa Tenggara Timur to postpone departures if they observe dark clouds rising vertically along the intended route or clusters of cumulonimbus accompanied by thunder.

BMKG describes cumulonimbus as dark, cauliflower-like clouds that can significantly and suddenly alter both wind speed and direction, and increase wave height, creating hazardous conditions for small vessels.

A line or cluster of thunderstorms can form a squall line, a band of storms capable of producing concentrated wind gusts and locally higher waves. BMKG notes that such systems can briefly push waves into the 1.25–2.5 metre range in affected areas of Nusa Tenggara waters, with the exact height depending on the intensity and structure of the storm.

BMKG does not prescribe a universal “safe distance” from a storm cloud or a single threshold at which all boats must return to port. Instead, its guidance indicates that vessel size, route, shelter options and the position and movement of storm systems should all be taken into account by crews using official forecasts together with real-time observations at sea.

Operators are therefore advised to combine visual cues — such as rapidly growing, dark clouds along their track — with BMKG marine forecasts and early warnings before deciding whether to depart, to continue or to seek shelter.

Forecast ranges and operating guidance

BMKG’s early warning for Nusa Tenggara Timur valid from 10 to 13 November 2025 identifies a 1.25–2.5 metre wave-height range for several sea areas and straits, including the southern Sape Strait, the Flores–Lamakera Strait, southern Flores, southern Alor–Pantar, the western Sumba Strait, the Savu Sea, southern Sumba, northern Sabu–Raijua, northern Kupang–Rote, southern Sabu–Raijua and Timor–Rote waters.

The same guidance states that prevailing winds in Nusa Tenggara Timur are generally blowing from the southeast to the southwest at 8 to 30 knots over the warning period.

Operating thresholds are also outlined. For users of small fishing boats, BMKG advises particular caution once wind speeds reach around 15 knots and significant wave heights of about 1.25 metres. For barge operators, the agency recommends heightened vigilance at wind speeds of about 16 knots with waves of roughly 1.5 metres, according to BMKG-based reports.

BMKG emphasises that these figures are general guidance for the wider warning area and cannot be taken as a guarantee of actual conditions at a specific reef, island or harbour, where local geography and weather patterns can produce different seas.

If a boat is already offshore

For vessels already at sea during periods of active cumulonimbus development and elevated waves, BMKG urges mariners to prioritise safety over schedules and to seek more protected waters or shelter behind nearby islands or within bays until conditions improve, while continuing to monitor official weather information.

Short-lived but intense wind bursts associated with thunderstorm cells can quickly steepen waves, making seas uncomfortable or hazardous for smaller craft, especially in exposed areas of Komodo National Park and adjacent straits. BMKG advises that captains remain ready to alter course or delay operations as thunderstorms approach or develop along their route.

For visitors planning boat excursions from Labuan Bajo and other ports in western Flores, the practical implication is that itineraries should remain conditional on the latest BMKG marine forecast and the skipper’s assessment. A delayed departure may be safer than attempting to sail through a developing thunderstorm, and boats already offshore are advised to move toward suitable shelter rather than wait for the strongest winds and highest waves to arrive.

Sources

• Metro TV News – BMKG Imbau Warga Waspadai Gelombang Laut 2,5 Meter di Perairan NTT pada 10–13 November 2025
• Kompas TV – BMKG: Siklon Tropis Picu Gelombang Tinggi 1,25–2,5 Meter di Sejumlah Perairan 10–13 November 2025
• BMKG – Strengthening Preparedness for 2025–2026 Year-End Holidays, Public Urged to Monitor Official BMKG Information
• Harian Jogja – BMKG Peringatkan Gelombang Tinggi Labuan Bajo hingga Akhir 2025
• MerahPutih – BMKG Beri Peringatan Dampak Pertumbuhan Awan Cumulonimbus di Labuan Bajo