The Southern Great Plains are subject to fluctuating precipitation extremes that pose significant challenges to agriculture and water management. Despite advancements in forecasting, the mechanisms driving these climatic variations remain incompletely understood. This study investigates the relative contributions of the tropical Pacific and Atlantic Oceans to April-May-June precipitation variability in this region. Using partial ocean assimilation experiments within the Community Earth System Model version 1.0, we identify a substantial influence of inter-basin interactions, with the Pacific and Atlantic contributing approximately 70% and 30%, respectively, to these variations. Our statistical analysis suggests that these tropical inter-basin contrasts offer a more reliable indicator for late-spring precipitation anomalies than the El Niño-Southern Oscillation. This finding is corroborated by analyses from seven climate forecasting systems in the North American Multi-Model Ensemble, providing a promising outlook for improving real-time forecasting in the Southern Plains. However, the predictive skill of these inter-basin contrasts is currently limited by the lower predictability of the tropical Atlantic, underscoring the need for future research to enhance climate prediction models.