"California
Flood"
31 December 1996
SUMMARY OF HEAVY PRECIPITATION EVENT ALONG THE WEST
COAST
1-3 January 1997
A major hydrologic event took place from 26 Dec 1996 through 3 January
1997 in northern California. The most impressive rainfall and subsequent river stages and
levee breaches occurred on 1-3 January.
- It was not enough to know this was a Western Type III event (Maddox et
al, 1980). Orographically forced low-level wind flow up the valley into the higher terrain
north of the central lowlands enhanced precipitation production. Precipitation was
enhanced here as much as it was along the more NNW-SSE oriented ranges that were
perpendicular to the geostrophic southwesterlies.
- Moist air from the tropical Pacific brought impressive dewpoints into the
region and raised the snow levels. Flooding in Reno, NV, east of the Sierra was mainly the
result of high elevation rainfall onto snowpack. The connection with the tropical Pacific
is essential. Precipitation efficiency is enhanced.
- Jet dynamics would suggest upper divergence (left exit) to the north of
where some of the heavy precipitation was being produced. It appears that the
precipitation production was very low-level, driven by the strong low level moist upslope
flow into the terrain and northward up the valley. In fact, that low-level air may be
sloping up toward the upper divergence area. The most important precip-producing lift of
the near-saturated, maritime air was the initial lift in the low levels (south of the jet
axis).
- Human-made objects, such as levees, broke and resulted in many of the
"flash" flood episodes.
- Pay attention to:
- connection with tropical Pacific
- orographically induced wind flow (vertical and horizontal)
- the path the inflow air may take as is rises ( initial lift)
Case Study 008