Calibrated probabilistic sub-seasonal forecasting for Pakistan’s monsoon rainfall in 2022

In 2022, a record-breaking monsoon caused flooding throughout Pakistan, particularly in the southern regions, resulting in deaths, property losses, and severe crop damage, affecting the food supply chain that could last for years. This study assesses the accuracy of sub-seasonal calibrated probabilistic rainfall forecasts for Pakistan. The evaluation focuses on forecasts initialized throughout the summer monsoon season (June–September) and utilizes the European Center for Medium Range Forecast (ECMWF) ensemble prediction system. Forecasts are calibrated using a canonical correlation analysis (CCA) and evaluated using cross-validated hindcasts from 2002 to 2021. The calibrated hindcasts exhibit positive ranked probability skill score and are reliable for weeks 1 (days 1–7), 2 (days 8–14), and 3 – 4 (days 15–28), lead times. In the extraordinary monsoon season of 2022, tercile-category probabilistic forecasts provided useful information up to 4 weeks ahead. Furthermore, the occurrences of intense monsoon rainfall in the highly affected southern region of Pakistan were forecasted reasonably well up to 2 weeks in advance. The ECMWF model's ability to predict sub-seasonal monsoon rainfall in Pakistan during 2022 is attributed to the model’s successful prediction of monsoonal intra-seasonal oscillations.