A surface observation based climatology of Diablo-like winds in California's wine county and western Sierra Nevada

Sundowners are a downslope windstorm caused by internal gravity wave (IGW) breaking over the Southern slopes of the Santa Ynez range along the Central California coast that have resulted in 23^oF early evening positive temperature ramps at a surface met station less than one mile away from the Coast. Most catastrophic fire events in the area have occurred contemporaneously with extreme Sundowner meteorological conditions. Using an 11 year dynamically downscaled climatology create using the Weather Research and Forecasting (WRF) model which was validated against observations taken during the Sherpa Fire, we have elucidated the following general mesoscale dynamical characteristics of Sundowners:

• IGW breaking over the southern slopes of the Santa Ynez range driven by a synergy between the
• diurnally forced onshore sea breeze and a
• synoptically driven strong North-northwesterly offshore jet in the marine boundary layer (MBL) and a
• transcritical transition of the MBL barotropic shallow water model around the Southern California Bight

Here, we consider modeled case studies, long term observations and reanalysis to inform on the following remaining unresolved science questions:

• The extent to which the lee slope jet dynamics are controlled by upstream profiles of wind speed and static stability versus mixing or impingement of the lee slope jet into the MBL south of the mountain range
• The influence of the offshore shallow water mode in reinforcing the forcing and nonlinearity of mountain waves over the Santa Ynez Ridge
• Atmospheric controls on upstream constructive and destructive interference of IGW and the net contribution of air parcels from the California Central Valley