ENSO forecasts made in the northern spring (March-May) are much less reliable. From mid-year, forecasts for the coming winter peak become quite skilful.
Forecasts and bulletins
ENSO forecasts have a weak spot every year, called the spring predictability barrier. Forecasts that cross the northern spring (March to May) are much less accurate.
Why (NOAA Climate.gov):
Spring is a transition time: events decay after their winter peak, pass through neutral, or start to form.
The ocean and atmosphere are only weakly coupled then, because the temperature contrasts across the Pacific are smallest.
In short: the signal is small and the noise is large.
After spring, skill improves. Using July-August data, models predict about three-quarters of the winter ENSO ups and downs.
Rules of thumb:
A forecast made in September for December is fairly reliable.
A forecast for next May to July is much less reliable, whoever makes it.
Forecasts of local impacts (rain on Samui) are less certain than forecasts of Nino 3.4.
Why it matters for Koh Samui
Forecasts for how the 2026 El Nino peaks around the end of 2026 are fairly solid now. Forecasts for when it ends in mid-2027 will stay uncertain until spring 2027.