Trends in Critical Fire Weather Conditions in the Santa Ynez Mountains in Santa Barbara, CA, Based on 30 Years of High Resolution Downscaling

Coastal Santa Barbara County (SB), with a population exceeding 130,000 inhabitants, is among the most exposed communities to wildfire hazards in southern California. The Santa Ynez Mountains (SYM) rise abruptly from coastal SB separating the Pacific Ocean on its south face from the Santa Ynez Valley (SYV) on its north face. The SYM is influenced by the semi-stationary North Pacific anticyclone, and receives most of the rainfall from frontal systems and atmospheric rivers during the winter months. Downslope, dry and gusty windstorms are frequently observed on the southern-facing slopes of the SYM. These winds typically intensify from early evening to morning hours and are known as “Sundowner winds” or “Sundowners”. These winds have been responsible for the fast spread of wildfires toward populated areas in Santa Barbara. While wildfires are not uncommon in the slopes of the SYM, the number of these events has dramatically increased in the first 20 years of the 21^th century compared to the last 50 years of the 20^th century. In this study we discuss trends in several variables associated with the increase in critical fire weather conditions with focus on the SYM using 30yr of downscaling with the Weather Research & Forecast (WRF) model at 1km grid resolution and 1 hour temporal resolution. We show that temperatures have increased in the Southern Slopes of the SYM at higher rates (~2.5^oC in 30 yrs.) compared to coastal areas, with impacts on relative humidity. Trends are greater in the summer and fall than in other seasons, which is critical for the buildup of dry fuels and the extent of the fire season in the fall and winter. While wind speeds have not increased in most of the domain, the Forsberg fire index (calculated based on modeled temperatures, relative humidity, precipitation and winds) has increased in most of the slopes of the SYM consistent with the growing risk of major wildfires in the region. Additionally, this study discusses climatic drivers of temperature and winds during the 30yr period using reanalysis. We discuss the influenced of El Nino/southern Oscillation (ENSO) on the variability of temperatures in the slopes of the SYM on interannual time scales and the Pacific Decadal Oscillation on decadal time-scales. We also investigate relationships between temperature variability and sea surface temperature (SST) anomalies in the Santa Barbara Channel.