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Flash Floods in the South-Central U.S.: What are They and

How Can we Forecast Them?

Matt KelschThursday, 30 March 2000

UCAR Cooperative Program for Operational

Meteorology, Education and

Training (COMET)

kelsch@comet.ucar.edu

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Excessive Precipitation Precipitation Intensity

• Enhanced Precipitation Efficiency– Deep above freezing cloud layer (3-4 km)

– Low-centroid storms (Warm Rain Process)

– Enhanced low-level lift (topographic or meteorological boundary)

• Minimal Condensate Loss– Moist ambient environment with relatively weak shear

(little entrainment of dry air)

– Weak to moderate updrafts (minimal detrainment)

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Excessive Precipitation Precipitation Duration

• Common focus for regeneration of storms– topographic features– meteorological boundaries

• Evolution of mesoscale features in weak flow regime (near upper level ridge)

• Quasi-stationary low-level supply– Equiv Potential Temp Ridge– Low-level jet

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FLASH FLOODS

• Precipitation – Intensity– Duration

• Hydrology– Basin and Runoff Characteristics

• Communications & Public Response

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Hydrological Aspects

• Physical Characteristics Of Basin– basin size– slope/permeability– antecedent moisture– rainfall characteristics

• storm motion with respect to basin and sub-basins

• Land Use/Human Interaction– urbanization– clear-cut/deforestation– fire scars/volcanic activity

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The Omaha, Nebraska Flash Flood of 7 August 1999

• Meteorological boundaries act as atmospheric terrain to enhance low-level lift and focus regeneration

• Modified channels in urban drainages respond rapidly to intense rainfall rates

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7 August 1999 0600 UTC

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0500 UTC 7 Aug 1999

0700 UTC 7 Aug 1999

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The San Antonio area Flash Flooding of 17 October 1998

• High precipitation efficiency associated w/ tropical atmosphere

• Upper level jet dynamics and low level orography help initiate convection

• Triggering mechanism remains for extended duration

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Eta 6-h PW fcst valid 06Z 17 Oct

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17oct98 1728 UTC low level radar velocity image

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24-h accumulation ending 12Z 18 Oct 1998

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24-h accumulation ending 12Z 19 Oct 1998

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The San Antonio area Flash Flooding of 17 October 1998

Warm-rain process increase RATES

Orographic & outflow boundaries focus regeneration to increase DURATION

Rainfall bursts on small, FAST-RESPONSE BASINS

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The Dallas-Ft Worth Severe Weather turn Flash Flood of 5 May 1995

• High-intensity rainfall• Small, fast-response

urban basins• Severe Weather

distractions* Unlike “classic” flash

flood scenarios, this event was rapidly evolving & severe

351800 UTC 5 May 1995

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ETA e and Wind (850 mb) 1800 UTC 5 May 1995

0000 UTC 6 May 1995

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2200 UTC 5 may 95

0300 UTC 6 may 95

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0031 UTC 6 may 95

0100 UTC 6 may 95

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0204 UTC 6 may 95

0230 UTC 6 may 95

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Radar-derived Accumulation 0400 UTC 6 may 1995

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2300 UTC, 5 May 95

0.5-degree tilt

3.4-degree tilt

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0200 UTC, 6 May 19950.5-degree

3.4-degree

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The Dallas, Texas Storm-scale Transformation & Flash Flood

Enhance Precip Rate:

• Becomes low-centroid

• Taps high-e air

Increase duration:

• Mini comma-head moves slowly across Dallas

Hydrologic Response:

• 9 in/hr rainfall burst on urban basins less <12mi2

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The Fort Collins, Colorado Flash Flood of 28 July 1997

• Terrain-locked storm system increased rainfall duration

• Maritime-tropical characteristics increased rainfall intensity

• Urbanization increased hydrologic response (more rapid runoff)

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500 hPa height on satellite 6.7 image 0000 UTC 29 July 1997

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Satellite IR image with 15-min lightning strikes at 0330 UTC 29 July 1997

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Radar accumulation at 0430 UTC 29 July 1997

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October 1999 Hydromet class at Spring Creek

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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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What IS a Flash Flood?

Physical

A sudden and dangerous increase in water momentum that has the same scales of forcing as the short-duration, intense precipitation.

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What are the typical scales of a Flash Flood?

• Almost always < 75 km2 (30 mi2), and sometimes less than 25 km2 (10 mi2).

• Less than 6 hours, and typically the real intense precipitation burst and subsequent flash flood are occurring in the 1-2 hour time frame.

• Rainfall rates >100 mm/h (4 in/h), somewhat greater in more humid, low-elevation climates– Watch for precipitable water values of 1.00-1.50” in

CO/WY

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Flash Floods in the South-Central U.S.: What are They

and How Can we Forecast Them?

• Rainfall Intensity– Maritime, tropical– Low-centroid storm,

deep above freezing layer (~4 km)

– Enhanced low-level lift

• Rainfall Duration– Focus for regeneration

• Hydrologic Response– Know your basins!

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Flash Floods in the South-Central U.S.: What are They and

How Can we Forecast Them?

To Forecast Flash Floods:• At least 15-min & 2 X 2

km resolution– for precipitation– for hydrologic

processes– for guidance

• Local agencies that are responsible for their flood control structures.

• Public Education

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