Mesoscale patterns associated with two distinct heatwave events in coastal Santa Barbara, California, and their impact on local fire risk conditions

Abstract

This study investigates mesoscale mechanisms associated with two extreme heatwaves affecting Santa Barbara County (SBC), southern California, and their implication for severe fire weather. We examine these issues using a database consisting of surface stations, radiosoundings, and 1 km grid spacing simulations with the Weather Research & Forecasting (WRF), including a climatology spanning 32 years. During the first heatwave event, synoptic conditions induced downslope winds on the southern-facing slopes of the Santa Ynez Mountains (SYM) on July 6, 2018. One surface station hit an all-time record, and nine surface stations exceeded 99.9th percentiles of surface temperatures. A wildfire (the Holiday Fire) erupted on the slopes of the SYM driven by high temperatures, low relative humidity, and strong winds. The nearby radiosonde registered temperatures at 850 hPa that exceeded the 95th percentile historical records (62 yrs). WRF simulations indicated that mountain wave activity contributed to the excessive surface temperatures on the south-facing slopes of the SYM, and explained the late evening timing for the maximum daily temperatures. The second heatwave broke all-time temperature records at 10 surface stations across SBC on September 6, 2020. Maximum temperatures for most of the SBC occurred during mid-afternoon, the highest observed temperature at the surface was 48.3 °C (118.9∘F), and the 850 hPa temperatures exceeded the 99th percentile. The September 2020 event occurred under weaker synoptic forcing (pressure gradients) than the July 2018 event, resulting in weaker winds in coastal Santa Barbara (including the slopes of the SYM) and Santa Ynez Valley. Nonetheless, the extreme heat and low relative humidity increased the Fosberg Fire Weather Index (FFWI) at critical values for a few hours in some sites when winds were moderate. To evaluate the relative importance of these extreme events in the historical context and to assess the region’s wildfire risk we propose a novel diagram based on the joint behavior of winds, temperature, humidity and FFWI. While no wildfires have broken out during the September 2020 heatwave, our analysis suggests that a combination of extreme heat with stronger winds would lead to unprecedented fire danger. These extreme conditions may become more common in a warming planet.