Compares rain over the last 90 days (180 days in the dry season) with the 1991-2020 normal, and adds PWA notices, the seasonal forecast, the regional dam and the El Nino season.
Koh Samui watch
Main measure: rain observed (ERA5) over the last 90 days as a % of the 1991-2020 normal for the same days. In the dry season (January-May) the 180-day window drives the level when it is worse, because it holds the last monsoon refill.
Then, raise the level (never lower it) when:
60-day rain is below 50% of normal, or 180-day rain below 75%: at least 1.
October-December (refill season) + strong El Nino (ENSO factor 2) + 90-day rain below 75%: at least 2.
January-May + strong El Nino: at least 1, or 2 if rain is below 75% of normal.
ECMWF seasonal forecast below 80% of normal for the next 3 months: at least 1.
Ratchaprapa dam (mainland, a regional indicator only) below 40%: at least 1; below 25%: at least 2.
A PWA Samui water-supply notice in force: at least 1; a notice lasting more than 48 h, or "no supply": at least 2.
Safeguards:
A rain-only level 3 needs at least 100 mm missing in absolute terms; otherwise it stays at 2 (40% of a tiny dry-season normal is not an emergency by itself).
Level 4 only when a shortage is confirmed: level 3 AND (at least 2 news reports of water shortage on Samui in 14 days, or a long PWA notice).
Not tracked automatically (no machine-readable source): the island reservoir levels and the PWA rotation schedule.
Why it matters for Koh Samui
October-December is when Samui's reservoirs refill (about half of the year's rain). A deficit now is the strongest warning for the 2027 dry season.
Thresholds
Rain, % of 1991-2020 normal (90 d; 180 d in the dry season)
Factor level
75% or more
0 Normal
60 to 75%
1 Watch
40 to 60%
2 Prepare
below 40% and at least 100 mm short
3 Act
level 3 + shortage confirmed on the island
4 (counts as 3 in the overall level, 4 only through rule R4b)
Cap in the overall level: 3. Critical factor. Stale if ERA5 has not updated for 3 days or the last ERA5 day is more than 10 days old.