On 25 February 2004, torrential rain and extensive flash flooding occurred in portions of San Francisco and San Mateo counties, in association with a convective cell embedded in a landfalling frontal band. Rainfall surveys indicate that between 1 and 1.5 in (2.5 to 3.8 cm) of rain occurred between 8:30 and 9:00 am PST from northwestern San Mateo to southwestern San Francisco counties. Maximum rainfalls occurred in the immediate vicinity of San Francisco State University (SFSU, see Fig. 1 for location); a rain gauge on the roof of Thornton Hall at SFSU recorded 1.9 in (4.6 cm) in the 45-min period ending at 9:00 am. As a result, over 40 homes were seriously damaged, along with significant additional infrastructure damage (street washouts, damage to building contents, etc.). The final flood-related damage total has been estimated to be in excess of $20 million.

Analysis of mesonet rainfall data across the city of San Francisco in conjunction with California Department of Water Resources calculated rainfall-depth-duration frequency for peak 30-min amounts indicates that this was at least a 1000-yr rainfall event (for 30-min) across a portion of southwestern San Francisco, including the SFSU campus (Fig. 2). This is consistent with the reflectivity data available from the KMUX WSR-88D National Weather Service radar in the Santa Cruz Mountains (located approximately 80 km southwest of SFSU). The radar plot for 8:27 am 25 February (Fig. 3) shows a small but intense, elongated area of convection oriented from southwest to northeast along the front which was then moving through southwestern San Francisco.

In this paper we will further examine both the meteorological conditions associated with this significant coastal central California rainfall event and the resulting flooding that was produced. Impacts on operational forecast/nowcast activities will also be discussed.

FIG. 1. Topographic map of central San Francisco Bay Region.

FIG. 2. Map of the city and county of San Francisco. Return periods (years) are indicated for peak 30-min rainfall amounts observed during the morning of 25 February 2004. Analysis is based on data from 16 sites in the San Francisco public utilities commission rain gauge network, located where return periods are marked (6 additional network gauges did not report ).

FIG. 3. KMUX base reflectivity (0.5o tilt, in dbZ) for 1627 UTC (8:27 am PST).