Flash floods defining and forecasting
This presentation is the property of its rightful owner.
Sponsored Links
1 / 42

FLASH FLOODS: DEFINING AND FORECASTING PowerPoint PPT Presentation


  • 112 Views
  • Uploaded on
  • Presentation posted in: General

FLASH FLOODS: DEFINING AND FORECASTING. Matt Kelsch 28 & 30 June 2000 [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:

Download Presentation

FLASH FLOODS: DEFINING AND FORECASTING

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Flash floods defining and forecasting

FLASH FLOODS: DEFINING AND FORECASTING

Matt Kelsch

28 & 30 June 2000

[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


Flash floods defining and forecasting1

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 forecasting2

Flash Floods: Defining and Forecasting

Precipitation

  • Intensity

  • Duration

    Hydrologic Response

  • Natural Basin Hydrology

  • Altered Basins

    Flash Flood forecasting requires more than accurate forecasts of excessive accumulation


Analyzed flash floods

Analyzed Flash Floods


Defining the flash flood

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 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

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 warnings1

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

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 floods1

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


Flash floods defining and forecasting

0600 UTC


Flash floods defining and forecasting

0500 UTC 7 Aug 1999

0700 UTC 7 Aug 1999


Flash floods defining and forecasting

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


Precipitation intensity

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-einflow (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

Timetable of Flood Events


Flash floods defining and forecasting3

Flash Floods: Defining and Forecasting

Precipitation Intensity

Tropical 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 forecasting4

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

IT DOESN’T HAVE TO ROTATE TO KILL YA!


  • Login