  We validate the Weather Research and Forecasting (WRF) model for precipitable
water vapour (PWV) forecasting as a fully operational tool for optimizing
astronomical infrared (IR) observations at Roque de los Muchachos Observatory
(ORM). For the model validation we used GNSS-based (Global Navigation Satellite
System) data from the PWV monitor located at the ORM. We have run WRF every 24
h for near two months, with a horizon of 48 hours (hourly forecasts), from 2016
January 11 to 2016 March 4. These runs represent 1296 hourly forecast points.
The validation is carried out using different approaches: performance as a
function of the forecast range, time horizon accuracy, performance as a
function of the PWV value, and performance of the operational WRF time series
with 24- and 48-hour horizons. Excellent agreement was found between the model
forecasts and observations, with R = 0.951 and R = 0.904 for the 24- and 48-h
forecast time series respectively. The 48-h forecast was further improved by
correcting a time lag of 2 h found in the predictions. The final errors, taking
into account all the uncertainties involved, are 1.75 mm for the 24-h forecasts
and 1.99 mm for 48 h. We found linear trends in both the correlation and RMSE
of the residuals (measurements - forecasts) as a function of the forecast range
within the horizons analysed (up to 48 h). In summary, the WRF performance is
excellent and accurate, thus allowing it to be implemented as an operational
tool at the ORM.
