Theme for 1994:
Predicting Heavy Rainfall Events in California
Sierra College, Rocklin, CA
Historical Anecdotes of the 1861-62 Storms
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Quotes from Up and Down California in 1860-1864. The Journal of William H. Brewer, University of California Press, 1966. Reproduced with permission of the publisher.
A Study of 1,000 Year Storms in California
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Record high rainfalls have occurred in California several times in the memory of most California residents. This study compares large storms which have occurred since 1862, by mapping lines of equal return period of rainfalls where the return period exceeds 1,000 years. A few other notable storms are also described. The largest storms in terms of return period were found to cover only limited portions of the State.
The big questions for this study are:
- What is a once in a thousand year storm?
- What were the biggest rainfalls of record?
- Is there any evidence of climatic variation which is effecting the occurrence of flood producing rainfalls?
A 1,000 year rainfall is in general the mean annual maximum rainfall plus about 5 standard deviations as will be shown. The maximum daily rainfall recorded in California occurred on January 21, 1943 at Hoegees located Northwest of Mount Wilson in the San Gabrial Mountains. The long term trends in rainfall extremes were found, as well as trends in total yearly rainfall.
Analysis of Major Storms in Southern California
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The National Weather Service (NWS) is involved in an intensive study of major California storms. These storms will form the basis for developing depth-area-duration curves in this region to determine Probable Maximum Precipitation (PMP) estimates. PMP has been determined for all of the United States and is detailed in the Hydrometeorological Report series. The current study will result in Hydrometeorological Report 58 (HMR-58) that will supersede HMR-36. The United States Army Corps of Engineers (COE) and other agencies responsible for large dams need PMP to set dam construction and spillway criteria.
Using the TEREC Weather Pattern to Predict Heavy Rainfall in California
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In California, the greatest daily rainfalls occurring during the winter months have come from the same weather pattern. The term used to refer to this weather pattern is "TEREC" (pronounced ter'ek) standing for "Truly Extraordinary Rainfall Event in California."
TEREC is a weather pattern which has been identified as bringing heavy rainfall to California. Greater awareness of and further development of this knowledge can provide advance warning of these events to operators of flood control dams and reservoirs, to office of emergency services, to disaster relief organizations, and to the citizens of California.
The purpose of this paper is to
- Explain the basic stages of the TEREC weather pattern
- Show how using the pattern can help predict heavy rainfall
- Identify areas where additional study and research is needed
The April 1982 "Big Sur" Storm: An Example of Extreme Rainfall in California
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During the winter of 1982 there were two periods of unusually heavy rainfall in California. In January a major storm dumped a lot of snow in the central Sierra Nevada mountains. This storm closed interstate highway 80 for nearly 3 days! In April, the second stormy period was punctuated by an intense low of subtropical origin which brought heavy rain along a diagonal band from the central California coast to the northern Sierras. This talk focuses upon this second system.
Mesoscale Numerical Simulation of the 1986 Sacramento Valley Flood Event
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The University of California, Davis and Lawrence Livermore National Laboratory have recently developed a Mesoscale Atmospheric Simulation (MAS) model to study the airflow and precipitation over areas of complex terrain like those in California. The model includes parameterized physical processes for the explicit cloud microphysics (cloud, ice, rain, snow, and graupel), radiation, soil, surface, and boundary layer. This model employs a 3rd order accurate advection scheme designed by Takacks (1987) and Hsu and Arakawa (1990). This scheme preserves the peak value well and produces no phase error with little computational oscillations. The steep and complex topography over our area of interest will generate spurious numerical oscillations that need proper treatment.
The model has been used to simulate the case of the 1986 Sacramento Valley flood between February 11 to 22. The simulated maximum precipitation over northern Sierra is 52 inches, which differs from the observation by less than 2 inches. The simulation also demonstrated the capability of the model to predict surface hydrology such as the accumulated snow depth during the storm, which are crucial to the water supply in California.
Analyses of Flooding Caused by the February 18,1986 Cloudburst in Placer County, California
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An unprecedented period of precipitation during the February 10-17, 1986 period resulted in saturated soils over most of northern California. A series of cloudbursts, embedded in a convergence zone moving with moist unstable southwest flow, formed over the Sacramento Valley during the afternoon and evening of February 18, 1986. An unusually large cloudburst storm generated just south of Rancho Cordova at approximately 2100 PST, intensified, moved north and eventually dissipated northwest of Auburn at 2230 PST. Fifteen-minute precipitation data for this event were available to allow an adequate description of the areal precipitation pattern. Gage records and isohyets for the most intense precipitation period are shown. Flooding, with peak flow recurrence intervals from 10 to 100 years, was observed in stream basins influenced by this cloudburst. The most severe flooding was observed in Placer County on Linda Creek and Miners Ravine east of Roseville and in streams immediately west of Auburn. Precipitation and flow data collected for this storm provided an opportunity to test the hydrologic design methodology recommended by Placer and Sacramento counties. Comparisons were made of observed time distribution patterns and areal reduction factors to those described in the literature.
California Precipitation Simulation in the NMC Nested Spectral Model: 1993 January Event
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Over California there is very noticeable low frequency variability in the precipitation and surface temperature extreme events. The winter-mean time series of precipitation along the west slope of the Sierra Nevadas in Northern California from 1895 through 1991 is shown in Fig. 1. Several multi-winter wet and dry spells have occurred with anomalous amplitudes comparable to climatological mean. For example, the state of California has had a severe drought since 1987 (State of California 1992). In fact, during January and February of 1993, record breaking precipitation fell over Southern California and effectively ended a near-record, long-term drought. However, the drought reappeared again in 1994.
Hydrology Manual Verification Using the March 23, 1993 Storm at Redding, California
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During development of a new hydrology manual for the City of Redding, California, an unusual storm occurred on March 23, 1993 which provided a unique opportunity for verification of the hydrology manual methodology prior to adoption. From 1700 to 2300 PST on March 23, 1993, an intense storm passed over the western and northern portions of the City of Redding causing unprecedented flooding in several streams. Storm total six-hour precipitation depths of up to 7.0 inches were recorded. Street and subdivision flooding occurred at many locations. A data collection effort initiated immediately after the storm obtained precipitation data at 68 locations and high water marks at 94 locations. An isohyetal map of storm total precipitation was prepared. Storm event flood flows estimated by HEC-1 models were compared to flood flows determined from hydraulic analyses of the high water marks. A comparison was made of recurrence periods for design events, various durations for precipitation in the March 23, 1993 storm, and the stream flood peaks. At nearly all locations, peak discharges estimated from hydraulic analysis of high water marks matched the HEC-1 peak discharges from the March 23, 1993 storm precipitation within 20 percent.
Development of Design Storm Procedures for San Joaquin County
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A design storm procedure for the San Joaquin County Hydrology Manual was developed to provide the data for storm duration, point rainfall depth, areal depth adjustment, storm intensity, and time distribution pattern. These data were developed from historical records of rainfall in the San Joaquin County region. Records for twenty-one rainfall gages with lengths of record from 5 to 101 years within the county and seven gages located adjacent to the county were were used. Five of the gages had short duration rainfall data.
Rainfall for specified frequency and duration is calculated from the average maximum daily rainfall, which in turn is determined from the mean annual precipitation. The data are arranged into a pattern to form a design storm which is used with a computer model of the rainfall-runoff process.
- Floodplain Management Association
- Sierra College Science Center
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