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FLASH FLOODS: DEFINING AND FORECASTING
Matt Kelsch28 & 30 June [email protected]
For more detailed flash flood presentations:www.comet.ucar.edu/class/index.html
for more information on the Fort Collins flash flood and a detailed case summary with links:
www.comet.ucar.edu/resources/cases/c11_28jul97/lab1.htm
for a link to our Venezuela flash flood and landslide page:http://www.comet.ucar.edu/resources/cases/venezuela/index.htm
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FLASH FLOODS: DEFINING AND FORECASTING
What Are They?
What is the correlation with precipitation amount?
What is the correlation with precipitation rate?
How important is the hydrology?
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Flash Floods: Defining and ForecastingPrecipitation
• Intensity
• Duration
Hydrologic Response
• Natural Basin Hydrology
• Altered Basins
Flash Flood forecasting requires more than accurate forecasts of excessive accumulation
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Analyzed Flash Floods
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Defining the Flash Flood
A flash flood is a situation where the causative precipitation and the subsequent runoff are occurring on the same scales.
• Intense Precipitation• Small, Fast-response
basins
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Flash Flood Warnings
Flash Floods lack objective definition and verification
There isn’t a reliable correlation between rainfall amount and flash flood severity.
Severe weather occurs first, if both occur in the same storm.
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Issues With Flash Flood Warnings
Improvements to the Flash Flood Watch stage have occurred since the 70’s
What about Warnings?
1) In an attempt to make sure everyone in the threat area is warned, long-duration warnings are issued for large areas
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Issues With Flash Flood Warnings
2) In a conscientious attempt to provide specific detail in space and time, there is little or no lead time
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Common Features of Flash Floods
Precipitation• Accumulation: 50-500 mm over 1-12 h• Duration of causative precipitation mostly <3 h• Intensity 75-300 mm/h
– Arid/semi-arid climates 75-200 mm/h (avg 118 mm/h)
– Humid climates 125-300 mm/h (avg 192 mm/h)
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Common Features of Flash Floods
Basin Characteristics• Small 10-125 km2 (avg 46 km2)• 70% mountainous watersheds• >50% were altered
– Urbanization– Deforestation, fire scars
• Either thin/impermeable surface layer,– Clay, rocks (low infiltration)– Pavement (no infiltration)
• Or, thick, saturated soil layers
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0600 UTC
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0500 UTC 7 Aug 1999
0700 UTC 7 Aug 1999
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KLOT (Chicago) Radar Storm Total Accumulation as of 1400 UTC 18 July 1996
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Precipitation Intensity
Enhanced Precipitation Efficiency• Moist ambient environment with relatively weak
shear (little entrainment of dry air)• Deep above freezing cloud layer: ~4 km
• Well-defined high-e inflow (low-level jet)
• High Precipitable Water values (30-50 mm)• Low-centroid storms, warm rain process• Enhanced low-level lift (orographic or
meteorological boundary)
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Timetable of Flood Events
5:30
8:00
8:30
9:00
9:40
10:30
11:00
1:20
Rain begins
EOC Activated
Ponds Overflowing, rapid water rescues begin
Most intense rain commences
NWS Warning
Storm begins to dissipate and move northeast
Trailer Park Flooding, Fires, Train Derailment
Declared City Disaster
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Flash Floods: Defining and Forecasting
Precipitation IntensityTropical Maritime Connection
Deep above-freezing cloud layer (4 km)• Atypical precipitable water: 25 (arid)–50 mm
• Low-level jet, high e air
• Extra low-level lift associated with a boundary
** beware of storm-specific enhancement
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Flash Floods: Defining and Forecasting
Fast-Response Basins
Small Size
• 10-125 km2 (avg 46 km2)
• Many <40 km2
Thin, impermeable soil or; deep, saturated soils
Altered Basin Hydrology
• Urbanized
• Fire Scar
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IT DOESN’T HAVE TO ROTATE TO KILL YA!
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