Uses the 3-day rain forecast (biggest day and 72-hour total), island rain gauges, TMD heavy-rain warnings and how wet the ground already is.
Koh Samui watch
Main measure: the largest 24-hour rain and the 72-hour total in the Open-Meteo forecast for the next 3 days. A fresh island rain gauge reading (less than 6 hours old) is also scored with the 24-hour column.
Raised when:
A TMD heavy-rain warning for the area is in force: at least 1.
The past 7 days already brought 150 mm or more and the level is 1 or 2: +1 level (the same rain on saturated ground floods and slides more).
The past 7 days brought 250 mm or more: at least 1, even without more rain forecast (landslides, slow drainage).
Why it matters for Koh Samui
October-December (northeast monsoon) brings Samui's heaviest rain and floods, even in an El Nino year. Coastal roads and the Lamai/Chaweng area can flood; landslides are possible on slopes.
Thresholds
Max 24 h rain
or 72 h total
Factor level
35 mm or less
below 100 mm
0 Normal
more than 35 mm (heavy)
100 mm or more
1 Watch
more than 90 mm (very heavy)
150 mm or more
2 Prepare
150 mm or more
250 mm or more
3 Act
Rain classes from the TMD (see Rain amount classes). Cap: 3. Critical factor.