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

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