Understanding Weather Forecasts
How observations, numerical models and probabilities combine to produce a forecast.
Weather is a changing system
Weather describes the state of the atmosphere over a particular place and time. Temperature, pressure, humidity, wind and precipitation interact across many scales, from local breezes to large storm systems.
Because the atmosphere is dynamic, a forecast is an estimate based on observations and scientific models rather than a promise that a particular outcome will occur.
Observations
Weather services collect information from surface stations, weather balloons, ships, aircraft, radar and satellites. Each instrument measures a limited set of variables. Together, the observations help describe the current state of the atmosphere.
The quality and coverage of observations matter. Measurements are checked, compared and processed before they are used in forecasting systems.
Numerical weather prediction
Numerical models represent atmospheric processes using mathematical equations. A computer divides the atmosphere into a three-dimensional grid and calculates how conditions may evolve over time.
Models simplify reality. Their resolution is finite, and some small-scale processes must be represented through approximations. Different models may therefore produce different outcomes, especially several days into the future.
Probability and uncertainty
A forecast may use an ensemble: a collection of model runs that begin with slightly different conditions or use different assumptions. If many runs produce similar results, confidence may be higher. If they diverge, the range of possible outcomes is wider.
A probability of precipitation refers to the chance of measurable precipitation at a specified place and time under the forecast definition. It does not necessarily describe how long it will rain or how much rain will fall.
Using forecasts wisely
Short-range forecasts are generally more reliable than long-range detail, but local conditions can still vary. For important decisions, check updated forecasts and official warnings. Pay attention to timing, confidence and the type of hazard rather than relying on a single number.