3. Synoptic Overview of the Event
The synoptic pattern over the north Pacific during late December 2007 and early January 2008 was very favorable for the development of strong surface cyclones in the Gulf of Alaska .In particular, a strong meridional temperature gradient that characterized a deep layer of the troposphere (Fig. XX) was associated with an unusually strong polar jet stream. 200 mb zonal wind anomalies of 50 knots were found over the northern Gulf of Alaska (Fig. XY) and wind speeds exceeding 150 knots at all levels in the upper troposphere (Fig. XZ) extended zonally just north of the surface polar front. The interaction of features progressing along this strong jet stream pattern with the surface frontal zone is a key in understanding the development of the powerful wave cyclone that advanced across the West Coast on 3-4 January 2008. This cyclone had pressure falls in excess of -1 mb h-1 for 24 hours and thus exceeded the threshold conventionally defined for a "meteorological bomb" (Sanders and Gyakum, 1980).
Several embedded progressive short waves were associated with significant upper tropospheric divergence. These passed along or across the surface frontal zone associated with this jet and initiated rapid cyclogenesis. This is consistent with the observations of Palmˇn and Newton (1969, p. 318) who observed that strong divergence occurring over a preexisting surface frontal zone often results in the development of rapidly deepening surface cyclones in the eastern Pacific during the cold season. They observed that such cyclones often can bring damaging wind and precipitation events to the Pacific coastal states, an observation consistent also with that of Bluestein (1993, p. 119).
The following discussion will center on a diagnosis of the synoptic and mesoscale controls and features associated with the pattern at the time of the greatest rainfall and wind in California on 4 January 2008. While much of the discussion will center on a documentation of the features responsible for important rainfall bursts, and the strong, damaging winds affecting northern and central California, a quasigeostrophic diagnosis of the important contributors to the vertical motion fields associated with the cyclone will be summarized.
3b. Overview of the Pattern in the Middle and Upper Troposphere: 0000 UTC 2 to 12 UTC 4 January 2008
3c. Overview of the Surface Pattern: 0000 UTC 2 to 12 UTC 4 January 2008
3d. Examination of the Synoptic-scale Controls During Explosive Development
3e. Meteorological Controls on Bursts of Heavy Precipitation and Wind on 4 January 2008
Figure XA: Plot of sealevel pressure (mb), sustained wind speed and peak gusts (mph) at San Francisco International Airport from 0000 UTC 3 January to 0000 6 January 2008. The four periods of strong wind (1, 2, 3 and 4), as discussed in the text, are indicated, with Period 3 especially discussed in Section 3e. (Source: METARS archived at National Climatic Data Center)
Figure XB: Plot of hourly rainfall (in) observed at San Francisco International Airport from 0000 UTC 3 January to 0000 6 January 2008 (Source: Hourly Precipitation Data, NCDC, January 2008)