Modification of fronts and precipitation by coastal blocking during an intense landfalling winter storm in southern California: Observations during CALJET

The California Land-falling Jets Experiment (CALJET) was carried out along the California coast, and up to 1000-km offshore, during the winter of 1997/98 to study the underlying physical processes that cause flooding rains and high winds in the orographically complex coastal zone and to explore the impact of potential future observing systems on short-term (<24 h) quantitative precipitation and wind forecasts during the landfall of winter storms from the data- sparse eastern Pacific Ocean. Using the suite of experimental and operational observing systems that were available during CALJET, this study documented the mesoscale modification of an intense landfalling cyclone by the steep coastal orography on 3 February 1998. This storm heavily impacted the populous and highly vulnerable coastal zone of southern California with flooding rains, strong winds, and major beach erosion. A pair of landfalling cold-frontal zones produced most of the damaging weather, while the primary cyclone circulation remained offshore. Special attention is given to the development of blocking of the low-level flow by the steep coastal mountains of southern California and to the influence of this blocked flow on the observed nearshore frontal evolution. In particular, unique observations are presented of blocking-induced frontal splitting and frontal merging, as well as unparalleled documentation of terrain-forced frontal waves. The impact of these frontal modifications on rainfall distributions is explored. This study also provides clear observational evidence of the orographic modulation of a landfalling prefrontal low-level jet (LLJ) near the coast of southern California. This is especially important, given that LLJs efficiently transport moisture into the coastal mountains, often resulting in orographically enhanced flooding. The results described in this study have important generalized implications for understanding the complex interactions that occur between shallow blocked flows and landfalling winter storms along the mountainous west coast of North America and for understanding the impact of these interactions on rainfall, winds, and erosion in the coastal zone.