Daniel Swain PhD at Weather West:
Unlike most winter storms that affect California, which more typically develop well to the west out over the open Pacific and often make landfall as they either maintain their strength or are already beginning to weaken, the storm slated to slam California starting later today will do just the opposite. This system did not really exist in coherent form out over the remote North Pacific, but is instead now rapidly developing “in situ” only 400-500 miles off the coast of central and southern California. This system will take the form of an explosively deepening surface low pressure center (or possibly a pair of similarly deep centers orbiting around a common center) just west of the Central California coast by late tonight. Several models are suggesting that this storm will meet the formal (latitude-adjusted) meteorological criteria for a “bomb cyclone” (i.e., a low pressure system that undergoes a rapid rate of deepening (at California’s latitude, around 17 mBb in 24 hours). Regardless of whether it meets the technical threshold, the main point is that this storm will be become rapidly stronger precisely at the moment it makes its closest approach to the Central Coast; this temporal evolution, plus its proximity to coast as it does so, will greatly elevate impacts relative to more ordinary California winter storms.