California is a remarkably dry place during the summer. For example, the average precipitation at the San Francisco airport (KSFO) is 0.11" in June, 0.02" in July, and 0.04" in August.
Nevertheless, San Francisco will get a dousing over the next couple of days thanks to a digging and amplifying upper-level trough over the eastern Pacific.
A long loop of GFS dynamic tropopause analyses and forecasts illustrated the large-scale processes contributing to the upper-level trough evolution. AT 00 UTC 30 May, a trough-ridge system lies over the western and central north Pacific. Both the trough and ridge subsequently amplify, with the ridge closing off and moving over Alaska by 00 UTC 2 June. Subsequently, the trough becomes zonally elongated, with trough fracture occuring over the northern Gulf of Alaska (see cool colors which indicated a depressed tropopause, anomalous cyclonic potential vorticity, and troughing). It is this fractured trough that moves equatorward and sets up off the coast of California. Anticyclonic flow associated with the downstream ridge over Alaska clearly contributes to the trough fracture and subsequent movement to off the coast of California.
With the trough parked off the California coast, the NAM develops a healthy band of precipitation along the central California coast late today, including over 1.5" of rain at KSFO through Sunday morning.
|1200 UTC 3 Jun NAM sea level pressure, 10-m wind, and 12-h |
accumulated precipitation forecasts valid 1200 UTC 4 Jun
(top), 0000 UTC 5 Jun (center), and 1200 UTC 5 Jun (bottom)
An impressive event for an area that averages very little precipitation this time of year. On the other hand, southerly flow ahead of the trough should give Utah a warm weekend.