R1: the highest factor wins (after caps). R2: news never triggers alone. R3: two physical hazards at Prepare = Act. R4: Leave only for a close orange/red cyclone or extreme heat plus a water crisis.
Koh Samui watch
The engine combines the factors with four documented rules:
R1, the highest factor wins. Overall level = the highest factor level, after applying each factor's cap. Caps: El Nino 2; water, heat, flood, cyclone/wind, sea state and air 3; marine heat 1; news 1.
R2, news never decides alone. The news factor only counts if another factor is at 1 or more.
R3, combined hazards. If two physical hazards (among water, heat, flood, cyclone/wind, sea state, air) are both at 2 Prepare or more, the overall level is at least 3 Act. Example: heavy rain while 2 m seas stop the ferries.
R4, Leave (4) only when safety or basic supplies are directly threatened:
R4a: an orange or red GDACS tropical cyclone within 300 km of Samui (cyclone/wind factor = 4).
R4b: extreme heat (heat >= 3) together with a water supply crisis (water >= 3).
The Samui page lists which rules fired, and "what would raise the level" (the next trigger for each factor).
Example
ONI at +1.8 °C gives the El Nino factor 2 (Prepare). All other factors at 0 or 1. Overall = 2 Prepare by R1, even if the weather today is fine.