December 5, 2025 | 03:03 pm
TEMPO.CO, Jakarta - Indonesia’s Meteorology, Climatology, and Geophysics Agency (BMKG) has issued a warning of possible volcanic mudflows and flash floods on the slopes of Mount Semeru, one of the country’s most active volcanoes in East Java.
The alert was issued after light to moderate rainfall was detected around the volcano on Friday morning, December 5, 2025, and further rain was expected into the early afternoon.
BMKG forecaster Oki Sukma Hakim reported that rainfall was concentrated across several districts in Lumajang Regency, including Pronojiwo, Candipuro, and Pasrujambe.
He explained that rain clouds were moving slowly toward the northwest and north, and were likely to persist for at least two more hours.
The weather conditions, BMKG said, could trigger volcanic mudflows, flash floods, regular flooding, and landslides in both Malang and Lumajang Regencies, particularly in Ampelgading, Pasrujambe, Pronojiwo, and Candipuro.
Authorities urged the public to remain alert by staying away from riverbanks connected to Mount Semeru and avoiding landslide-prone slopes. Residents were also advised to monitor official updates from BMKG and follow directions from local officials.
Local governments were asked to intensify early warning dissemination and, if necessary, temporarily restrict access to high-risk locations to reduce disaster impacts.
The Geological Agency also reported that Mount Semeru’s summit was obscured due to low visibility caused by cloud cover and rainfall.
Observers at the Mount Semeru Volcano Observation Post in Lumajang warned that people near river channels and surrounding slopes should heighten vigilance, as volcanic mudflows and pyroclastic flows could occur without warning.
Meanwhile, seismic monitoring from 06:00 AM to 12:00 PM WIB on Friday recorded 31 eruption or explosion earthquakes with amplitudes ranging between 10 and 22 millimeters and durations of 77 to 160 seconds.
Three gust-type earthquakes were also detected with amplitudes of 5 to 8 millimeters and durations of 30 to 67 seconds.
In addition, one harmonic tremor with a maximum amplitude of 20 millimeters and a duration of 149 seconds was observed, along with one flood-related vibration reaching 35 millimeters in amplitude and lasting 6,360 seconds.
BMKG and the Geological Agency emphasized that the situation remains dynamic, urging continued caution as Semeru’s volcanic activity and seasonal rainfall continue to overlap.
Read: North Sumatra's Batangtoru River Still Clogged with Logs After Flash Floods
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