Observed: Because IVT is not easily directly measured, a modeled 0-hour forecast, often referred to as model analysis, is used for observations of IVT.

Global Ensemble Forecast System (GEFS): The US national weather model developed and operated by the National Centers for Environmental Prediction (NCEP), which is made up of 21 separate forecasts, or ensemble members.

European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS): The European weather model developed and operated by ECMWF, which is made up of 51 separate forecasts, or ensemble members.

Ensemble Mean: The average of the set of forecasts created by the different forecasts members within each ensemble model.

Control Forecast: The model run that is not perturbed, or altered from the current conditions. This is thought to be the best guess for a single model, also known as the deterministic model.