PUB00131

• Year: 1998
• Title: Layperson's Guide to Flood Management
• CWCB Section: Watershed & Flood Protection
• Author: California Water Education Foundation
• Description: Layperson's Guide to Flood Management
• Publications - Doc Type: Other

Flood Forecasting

Predicting the weather is an unsure science but
enhanced computers that analyze data have
improved the accuracy of short-term weather
forecasts, Long.term forecasts (30 to 90 days in
advance) usually are limited to general terms such
as "wetter than normal," as opposed to delineating
specific weather events. Long-term forecasts are
about 55 percent accurate.

Determining when to issue flood warnings is fraught
with uncertainty because of the unknowns inherent
in weather forecasting. As one DWR official noted:
"We have to be careful not to cry wolf too often or

At the .S'tate/Federal
Flood COf/trol Operations
Ce11fer in Sacramento,
hydrologists, Ivcather
forecasters llnd olhers
)vork together to
coordinate floodlights
lllld reservoir releases.

16

we lose credibility, But at the same time, we don't
want to lose any lead time that could be used to
safely evacuate people," Part of the uncertainty
stems from the fact that there are only 100 years
of climate data upon which to base weather
predications.

Flood control managers, however, often are able to
predict with a high degree of accuracy when local
flooding is likely to take place, Their forecasts
combine storm runoff from unregulated tributaries
with reservoir releases to predict river levels. In
northern California, the joint state-federal forecast.
ing is done by the National Weather Service (NWS)
in cooperation with DWR's Division of Flood
Management at the California-Nevada River
Forecast Center in Sacramento. Federal and state
hydrologists can estimate high river stages within
12 to 24 hours before the event because of updates
on weather, precipitation amounts, and reservoir
and river levels. The information is distributed by
the State/Federal Flood Operations Center in
Sacramento, which cooperates with local flood
control and emergency services agencies. Two days

before the full force of the 1997 subtropical series
of storms was unleashed, forecasters predicted that
40 inches of rain would fall in the upper Feather River
watershed, which was within 1 inch of the actual
total downpour.

On large, slower-moving rivers such as the Sacra-
mento and San Joaquin, forecasters can predict high
river stages more than 48 hours in advance. This is
because releases from upstream dams can take
days to flow down the river to the Delta. In contrast,
on smaller, faster-moving rivers, such as the Eel and
Smith on the north coast of California, and most
southern California waterways, officials can produce
accurate flood warnings no more than 12 hours
ahead of time,

Weather patterns that worry climatologists and
flood managers are those arising from EI Nino and
La Nina weather conditions. EI Nino, known formally
as EI Nino-Southern Oscillation (ENSO), refers to
unusually warm currents along the coasts of Peru
and Ecuador, which heats other parts of the ocean
and atmosphere. This weather phenomenon can
increase tropical water temperatures in the eastern
Pacific by as much as 5 degrees Celsius and cause
heavier and more frequent storms, particularly in the
south coast regions and areas south of San
Francisco, In 1982-1983, which was an EI Nino
event, more than 26 inches of rain lei I and a record
37.7 million acre.feet flowed through the Sacramento
River system. However, most of the impact was along
the coastal regions because of high tides and waves.
There was $100 million in damage to coastal homes,
businesses and public recreation facilities.

La Nina, which is the inverse of EI Nino, causes
intense, wet warm storms. The January 1997 deluge,
along with the floods of 1955 and 1964 were a result
of La Nina, which is caused by the interaction of cold
surface ocean water near the equator and air.

Although weather patterns are discernible, the nature
of a given storm and the regional impacts are nearly
impossible to predict. As a precautionary measure,
legislation was enacted in 1997 that allocated $7,5
million to better prepare California to cope with
potentially severe storms associated with the EI Nino
that battered parts of California in 1998, The funds
were for repairing and upgrading levees battered by
the New Year's floods, increasing staff to monitor
flood forecasts and reservoir operations and for
advanced deployment of personnel in the event of
flooding, During both the 1982-1983 and late 1997
EI Niiio events, tropical fish were found swimming in
the waters off the Golden Gate,