FLASH FLOODS: DEFINING AND FORECASTING

Matt Kelsch28 & 30 June 2000kelsch@ucar.edu

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

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?

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

Analyzed Flash Floods

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

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.

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

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

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)

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

0600 UTC

0500 UTC 7 Aug 1999

0700 UTC 7 Aug 1999

KLOT (Chicago) Radar Storm Total Accumulation as of 1400 UTC 18 July 1996

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)

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

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

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

IT DOESN’T HAVE TO ROTATE TO KILL YA!