Flash Flood Forecasting Over Complex Terrain With an Assessment of the Sulphur Mountain NEXRAD in Southern California (2005) / Chapter Skim
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6 Flash Flood Warning Process in Los Angeles and Ventura Counties
6 Flash Flood Warning Process in Los Angeles and Ventura Counties
Pages 74-90
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From page 74... ...
Figure 6.2 includes a view of the Los Angeles basin and metropolitan area; Los Angeles International Airport is in the direction of 125° from the north. Of the major rivers in Los Angeles County, the Santa Clara River originates in Los Angeles County and then flows into Ventura County, and the Los Angeles and San 74
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From page 75... ...
The forecast process requires a forecaster to be cognizant not only of the antecedent hydrological and current meteorological conditions that may trigger a flash flood, but also of modifications of approaching weather systems as they impinge on the complex, mountainous terrain surrounding the highly populous urban areas in southern California. The impact of urbanization on both weather patterns and water runoff and the potential for recent fire-burned regions to modify watershed response and runoff must
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From page 76... ...
also be factored into the flash flood warning process. When it becomes clear that the conditions are ripe for the potential for flash flooding, the NWS Los Angeles-Oxnard Weather Forecast Office (WFO)
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From page 77... ...
. These tables are based on data from the National Climatic Data Center and the Southwest Regional Climate Center for a sampling of stations.
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From page 78... ...
Highest measured precipitation events in Southern California include more than 24 inches in 24 hours (at Haynes Canyon) in mountainous regions.
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From page 79... ...
. TABLE 6.2 Other Stations, Variable Lengths of Record Rancho Riverside Cucamonga Orange Oxnard January 2.1 2.4 2.7 2.9 February 2.0 2.2 2.4 2.4 March 1.7 2.1 2.5 2.0 April 0.8 1.1 1.0 0.8 May 0.2 0.4 0.2 0.1 June 0.1 0.1 0.1 <0.1 July 0.1 0.1 <0.1 <0.1 August 0.2 0.2 0.1 0.1 September 0.3 0.4 0.2 0.3 October 0.4 0.5 0.3 0.2 November 0.9 1.5 1.3 1.5 December 1.5 1.9 1.8 1.7 Annual 10.1 12.7 12.6 12.0 SOURCE: Southwest Regional Climate Center.
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From page 80... ...
80 FLASH FLOOD FORECASTING OVER COMPLEX TERRAIN TABLE 6.3 Los Angeles Area Average Number of Days with Precipitation, 5867 Years of Data Los Angeles Los Angeles Santa Rancho Long Beach Airport Civic Center Barbara Riverside Cucamonga January 6 6 6 6 6 6 February 5 6 6 6 6 5 March 5 6 6 6 6 7 April 3 3 3 2 4 4 May 1 1 1 1 1 2 June <1 1 1 1 1 1 July <1 1 <1 1 1 1 August <1 <1 1 <1 1 1 September 1 1 1 1 1 1 October 2 2 2 2 2 2 November 3 3 3 3 3 4 December 5 5 5 5 5 5 Annual 31 35 35 33 37 39 SOURCE: Southwest Regional Climate Center. TABLE 6.4 Monthly Record Precipitation Amounts Los Angeles Santa Rancho Long Beach Airport Barbara Oxnard Riverside Orange Cucamonga January 12.8 10.0 13.0 13.2 9.0 12.6 11.2 February 9.4 12.4 13.6 13.8 8.9 11.7 10.5 March 8.8 8.1 11.4 7.6 6.1 10.2 8.1 April 4.4 9.9 5.8 4.2 4.6 5.4 4.8 May 2.3 3.6 1.8 1.0 2.1 1.2 2.5 June 0.9 1.1 0.5 0.6 0.9 1.0 0.7 July 0.2 <0.1 0.9 0.2 1.5 0.3 0.4 August 2.0 0.4 1.9 1.2 2.4 0.8 2.2 September 1.4 5.7 4.1 5.0 3.0 2.2 3.9 October 1.6 2.3 2.4 2.2 2.9 1.7 3.0 November 6.0 6.0 6.9 6.4 5.6 6.7 8.0 December 5.3 8.5 5.9 5.3 6.1 5.5 7.8 Annual 27.7 31.3 40.7 29.9 24.0 31.4 28.8 SOURCE: Southwest Regional Climate Center.
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From page 81... ...
FLASH FLOOD WARNING PROCESS 81 TABLE 6.5 Most Rain Reported in a Day Rancho Long Los Angeles Los Angeles Santa Riverside Cucamonga Beach Airport Civic Center Barbara Oxnard January 3.1 4.0 3.75 4.56 5.71 4.02 5.96 February 2.4 2.3 2.78 3.91 4.25 3.97 4.29 March 2.4 2.2 3.46 3.10 5.87 4.73 4.60 April 1.6 1.6 1.61 1.35 2.74 2.83 2.00 May 1.1 1.5 2.03 1.67 2.02 1.16 1.35 June 0.9 0.5 0.48 0.74 0.76 0.41 0.62 July 1.4 0.3 0.26 0.28 0.13 0.91 0.58 August 1.7 1.9 1.75 2.09 2.06 0.98 0.89 September 2.1 3.3 1.39 1.66 3.95 3.01 3.72 October 1.1 1.6 1.81 1.75 1.71 2.39 1.74 November 2.1 3.8 2.03 5.60 3.85 3.69 3.78 December 2.7 3.9 3.16 2.84 4.85 3.28 5.10 Annual 3.1 4.0 3.75 5.60 5.87 4.73 5.96 SOURCE: Southwest Regional Climate Center. 30 25 20 15 Count 10 5 0 Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Month FIGURE 6.5 Monthly distribution of heavy rainfall in which at least three stations in Southern California had at least 3 inches of rain.
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From page 82... ...
THE NWS LOS ANGELES-OXNARD FLASH FLOOD WARNING PROCESS During the several days prior to the onset of heavy rainfall in the Los Angeles basin, forecasters at the NWS LOX office monitor weather patterns around the hemisphere to detect weather systems that may affect Southern California. They make heavy use of numerical weather prediction model guidance and their own experience to forecast the timing and amount of upcoming precipitation.
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From page 83... ...
The LOX office obtains flash flood guidance values from the NWS California-Nevada River Forecast Center in Sacramento, California, and from the Ventura County Watershed Protection District (Table 6.7)
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From page 84... ...
2. Small area mudslides could occur Santa Monica Mountains (Pacific Coast Highway 23, canyon roads)
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From page 85... ...
, plays a vital role in conveying weather information to the public and other commercial and government clientele. CWI meteorologists, sometimes employed by local television stations or operating remotely from a commercial weather service, generally have access to the same weather data and numerical model guidance as do forecasters at the local NWS WFO.
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From page 86... ...
Private weather firms play important roles in the hydrological services of the Los Angeles area as well. For example, Fox Weather LLC provides detailed precipitation forecasts to the Ventura County Watershed Protection District.
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From page 87... ...
Real-time data also go directly to the county's Emergency Operations Center and county road crew supervisors to prepare them for the potential of flash flooding. FIGURE 6.6 Topographic map of Ventura County and neighboring Los Angeles and Santa Barbara Counties showing the ALERT rain (blue dots)
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From page 88... ...
However, a review of the newspaper coverage of flash floods and warnings for Los Angeles and Ventura Counties reveals that numerous local, state, and federal agencies as well as nonprofits and private interest groups are communicating with the public about the risks and appropriate actions to take. A review of the broadcast media would probably also highlight numerous other agencies and parties providing infor mation to the various "publics" in the greater Los Angeles metropolitan area.
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From page 89... ...
One may also wonder what role these messages play in the public's perception of flash flood threats and its willingness to take, or not to take, timely actions when flash floods are imminent. The warning process is complex and involves the NWS and many others working together to provide timely dissemination of flash flood warnings to the community in a format that both is useful and elicits an appropriate, anticipatory response by the stakeholders and the public at risk.
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From page 90... ...
In addition, in partnership with the media and emergency management officials, the NWS should continue to educate the public about the risks of flash flooding.
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