Intense and out-of-season thunderstorms led to significant delays and diversions at Bali’s International Airport this past Sunday.
The satellite loop below shows thick thunderstorm clouds across the Indonesian islands of Lombok, Bali and Java on Sunday.
Video: Enhanced infrared satellite imagery showing intense thunderstorm activity across Bali and surrounding areas on the evening of Sunday, July 6, 2025.
A number of flights coming in and out of Bali’s Denpasar Ngurah Rai International Airport between about 4:30pm and 10pm local time were delayed or diverted to nearby islands due to the intense thunderstorms. Low visibility in heavy downpours and the risk of lightning strikes hitting planes and operators on the tarmac make it unsafe for landing and taking off during thunderstorms.
Operating costs of a Boeing 737 or an Airbus 320 or 330, which are typically used for international flights in and out of Denpasar, range from $5,000 to $10,000 per hour. In-flight delays and additional fuel requirements needed in cases like Sunday end up costing aviation operators tens of thousands of dollars.
Image: A now retired Cathay Pacific Boeing 747-400 “Jumbo Jet” at Denpasar in 2009, an aircraft type that has double the operating cost of the current twin-engine planes used to service the Island of Gods. Source: iStock / Fabian Gysel
Intense tropical rainfall as seen this weekend across the region was partly due to an equatorial Rossby Wave. July is in the middle of the dry season, which typically sees less frequent rain and thunderstorms under a pattern of more predictable trade wind conditions. Tropical waves can however disrupt this pattern, bringing weather more typical to the lush wet season that runs between November and March.
What is an Equatorial Rossby Wave?
Equatorial Rossby waves travel through the tropical atmosphere from east to west, typically producing an alternating sequence of high and low pressure systems on either side of the equator. This brings mirrored zones of cloudiness and clear weather in the tropics of both hemispheres.
The map below shows the modelled influence of the equatorial Rossby wave, with areas of enhanced and suppressed convection (rain and cloudiness) north and south of the equator near Indonesia.
Image: Forecast outgoing long-wave radiation anomalies for July 6, 2025, associated with the equatorial Rossby wave. The green areas show where cloud cover is expected to be enhanced, while the brown shading represents suppressed cloud cover. Source: BoM.
The westwards moving Equatorial Rossby wave may still bring showers and thunderstorms on Monday across parts of Bali. Heavier falls and thunderstorms are possible this week across parts of Java, including Jakarta, as the wave progresses west.
DTN Total Lightning Network
Image: Lighting strikes reported by DTN’s Total Lightning Network (TLN) within 150km of Denpasar International Airport.
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Image: The DTN TLN gives you maximum lead-time ahead of lightning, allowing you to maximise efficiency and safety around thunderstorms. Source: iStock / Matthew Perkins
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DTN APAC ObsConnect Weather Stations
Strong winds generated by the thunderstorms can be erratic in direction, coming across the runway (as a “crosswind”) making it difficult to keep the aircraft in line with the runway. Denpasar airport operates a single runway direction, which goes in an east-west direction, making it vulnerable to winds from the north or south. A 52 km/h wind gust from the southeast generated by a thunderstorm was recorded yesterday.
Image: the DTN APAC Stormtracker showing the enhanced infrared satellite, Total Lightning Network and observed wind gusts at 4pm CIT on Sunday, July 6, 2025.
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Thumbnail image source: iStock / gionnixxx
