Radar-Rainfall Analysis for Extreme August Storm in Sacramento

The climate in California's central valley is distinctly divided into two seasons: wet and dry. Sacramento's six-month wet season runs roughly from mid-October through mid-April, while the dry season runs through the remaining six months from mid-April to mid-October. While rainfall during Sacramento's "dry" season is not unheard of, it is relatively rare, especially in August. At one long-term climatological observing station in Sacramento from 1877 to 2006, there were 4024 non-missing daily August rainfall observations. Of those 4024 rainfall observation days, only 41 days (1.02%) recorded more than a "trace" and daily observations of more than 0.30 inches were reported just two times. The maximum August daily rainfall in Sacramento from 1877–2006 was 0.66 inches. That is why the events of August 21–22, 2003 were remarkable. A series of unusually strong thunderstorms rolled through region during the late afternoon on August 21^st and continued sporadically through the early morning hours of August 22^nd. Rainfall totals in excess of 1.4 inches were recorded in 24 hours, more than double the maximum daily August rainfall observed in the 130 year record. High resolution radar rainfall estimates were calibrated using gage observations from the local flood ALERT system. The gage-adjusted radar rainfall estimates were used to track the intense storm cells through the city of Sacramento and to the localized flooding that occurred.