GPTCast: a weather language model for precipitation nowcasting

Abstract. This work introduces GPTCast, a generative deep learning method for ensemble nowcasting of radar-based precipitation, inspired by advancements in large language models (LLMs). We employ a generative pre-trained transformer (GPT) model as a forecaster to learn spatiotemporal precipitation dynamics using tokenized radar images. The tokenizer is based on a Variational Quantized Autoencoder (VQGAN) featuring a novel reconstruction loss tailored for the skewed distribution of precipitation that promotes faithful reconstruction of high rainfall rates. This approach produces realistic ensemble forecasts and provides probabilistic outputs with accurate uncertainty estimation. The core architecture operates deterministically during the forward pass; ensemble variability arises from sampling the categorical probability distribution predicted by the forecaster during inference, rather than requiring external random inputs such as noise injection common in other generative models. All forecast variability is thus learned solely from the data distribution. We train and test GPTCast using a 6-year radar dataset over the Emilia-Romagna region in northern Italy, showing superior results compared to state-of-the-art ensemble extrapolation methods.