How a 15-30 day forecast works and how to read it
The long-range forecast is one of the most common questions in weather services: "what will the weather be like the week of my trip to Lisbon", "will it rain on my wedding day three weeks from now", "will August be hot". The honest answer is that the atmosphere is chaotic and beyond 7-10 days a deterministic forecast quickly loses skill. However, modern numerical models still provide useful information well beyond that, as long as we understand the type of information shifts: it is no longer "12 mm of rain on Tuesday", but "week 3 shows a warm and dry signal compared to climatology".
WhatAWeather combines two complementary Open-Meteo sources:
- Forecast API (16 days): uses high-resolution deterministic models (ICON-EU, GFS, ECMWF-IFS). Days 1-7 are highly reliable; days 8-15 give a trend.
- Ensemble API (30 days): uses the GFS-Ensemble (GEFS) model that runs the model dozens of times with slight perturbations. Averaging the members yields the most likely signal beyond two weeks, with growing uncertainty.
It's essential to interpret reliability correctly. Forecast skill drops with time: at 1 day it is virtually perfect, at 7 days temperature is captured with 1-2°C mean error, at 14 days the error rises to 3-4°C, and at 21 days the model only distinguishes between warmer or cooler than average. Rain is even harder: the exact location of a low-pressure system 10 days out is almost impossible to nail, but the ensemble can detect whether the week will be statistically wet or dry.
Recommended uses for this tool: planning trips with a margin, preparing agricultural seasons, anticipating heatwaves or cold spells on weekly scales, and watching the consistency of the forecast day after day (if three consecutive runs flag rain on day 12, the signal gains confidence). Do not use it for critical hour-by-hour decisions beyond 7 days.