Climate-driven phenological shifts significantly influence ecosystem functioning and agricultural productivity, particularly in temperate regions. Grapevines (Vitis vinifera L.) represent economically and ecologically crucial species whose phenological timing impacts biodiversity and vineyard management. This study investigates how late-winter warming and increased frequency of extreme weather events jointly influence grapevine phenology in Piedmont, Italy. Using a 20-year dataset (2004–2023) of phenological observations for Cortese, Barbera, and Dolcetto cultivars, integrated with local meteorological records and severe weather data from the European Severe Weather Database (ESWD), we quantified climatic effects on budburst timing. February–March temperatures rose significantly (by 2.2 °C and 3.5 °C, respectively), correlating with an advancement of approximately 5 days per 1 °C warming (β = –4.89 ± 0.83 days °C⁻¹, z = –5.91, p < 0.001). Additionally, springs with frequent extreme events (≥2 events in February–March) advanced budburst by 18.13 ± 6.47 days (z = –2.80, p = 0.005). Together, these factors explained 62.5% of inter-annual variation. Our results underline the necessity of integrating extreme weather forecasting with traditional thermal indices to develop robust climate adaptation strategies for sustainable viticulture.