The development of statistical relationships between local hydroclimates and large-scale atmospheric
variables enhances the understanding of hydroclimate variability. The rainfall in the study basin (the Upper Chao
Phraya River Basin, Thailand) is influenced by the Indian Ocean and tropical Pacific Ocean atmospheric circulation.
Using correlation analysis and cross-validated multiple regression, the large-scale atmospheric variables,
such as temperature, pressure and wind, over given regions are identified. The forecasting models using atmospheric
predictors show the capability of long-lead forecasting. The modified k-nearest neighbour (k-nn) model,
which is developed using the identified predictors to forecast rainfall, and evaluated by likelihood function, shows
a long-lead forecast of monsoon rainfall at 7–9 months. The decreasing performance in forecasting dry-season
rainfall is found for both short and long lead times. The developed model also presents better performance in
forecasting pre-monsoon season rainfall in dry years compared to wet years, and vice versa for monsoon season
rainfall.