Southerly Surge: Abrupt End To Offshore Flow Regime
in Northern and Central California

An Example of a Coastally-trapped Wind Reversal

While the southerly surge (also referred to generally as a coastally-trapped wind reversal) is a mesoscale event, it is triggered by pressure patterns forced at the synoptic scale. These synoptic-scale effects begin to become factors typically in mid to late August, when the polar jet stream begins to make its seasonal progression southward. However, in atypical patterns, such effects can occur in July as well

The diagrams below depict the evolution of a southerly surge schematically.

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Fig. 1: Onset of offshore flow pattern and apparent migration of California Thermal Low to the coast.

Fig. 2: Mature offshore flow pattern as synoptic-scale pressure rises under eastern portion of migratory ridge in the mid and upper troposphere spread southeastward into Pacific Northwest.

Fig. 3: Migratory surface high pressure area and ridge aloft progress southeastward. Subsynoptic low develops along Northern California coastline. Southerly surge develops and stratus advects northward along the coast.

Fig. 4: Mature southerly surge. Mid and upper tropospheric flow returns to climatological normal (unless jet remains in same position). Thermal low pressure returns to Southwest, and soon after, subsynoptic low disspates.