3. Synoptic Overview of the Event
3a. General
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, the strong meridional temperature gradient that
characterized a deep layer of the troposphere (Fig. XX) was associated with an
unusually strong polar jet stream, characterized by winds exceeding 100 knots at
500 mb (Fig. XY) and 200 knots at 250 mb (Fig. XZ). The interaction of features
associated with 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 Palmén 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)