Climate and Phenology in Napa Valley:A Compilation and Analysis of Historical Data

Daniel R. Cayan, Dr. Kimberly Nicholas, Mary Tyree, Dr. Michael Dettinger

Executive Summary

In response to, and sponsored by the Napa Valley Vintners (NVV), this study was designed to evaluate the climate of the Napa Valley (NV) and explore links between climate and wine grape phenology and composition. Specific objectives of this study are to a) describe the spatial and temporal structure of climate in NV; b) explore linkages between climate and phenology/harvest; and c) evaluate historical trends in temperature.

NV shares traits of both coastal and interior climates. The southern portion of NV is more coast-like while the central northeastern side of NV is more interior-like. Elevation also plays a strong role, e.g., heavier precipitation and lower summer daytime temperatures generally occur in higher elevations relative to the valley floor.

Approximately 30 private weather stations distributed throughout NV allow inspection of fine-scale temperature variability from north to south, and along a transect of changing elevations across the valley from east to west. These records vary in length, but mostly cover two to 15 years from the last two decades. Generally, all stations display similar variations (i.e., warm periods are relatively warm at all stations). There is marked variability by location (i.e., southerly and higher-elevation sites are cooler in summer months, containing lower daytime maximum temperatures and higher nighttime minimum temperatures than more inland and valley floor sites. The daily and seasonal temperature ranges and temperature extremes observed in NV are greater than cool coastal climates, but smaller than warmer inland climates. During summer, cloud cover in NV shares patterns more similar to the coast than to the interior Central Valley. Cloud cover in the southern part of the NV is greater in July and August than it is in April, May, and June. Days with higher morning cloud cover in NV generally do not attain afternoon temperatures as high as those with lower morning cloud cover, even though cloudiness may dissipate by early afternoon.

In addition to local influences, climate in NV has a very clear, significant association with large-scale variations and trends. Large-scale atmospheric circulation plays a strong role in setting up anomalously warm and anomalously cool days in NV. Anomalous Pacific Ocean temperature patterns are linked quite strongly to NV temperatures during winter and early spring months, but not so much during summer months.