Flash Flood 1

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Flash Flood Forecasting

COMET

Hydrometeorology 00-3

Matt Kelsch

Wednesday, 17 May 2000

[email protected]



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Flash Flood Forecasting Flash Flood Forecasting has

historically been approached as

a precipitation forecasting

problem.

But the hydrologic response to

the precipitation is equally as

important.

Recent studies are focusing

more on the combination of:

± precipitation intensity

± fast-response basins



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Flash Flood IngredientsPrecipitation

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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Analyzed Flash Floods



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Defining the Flash Flood

A flash flood is asituation where the

causativeprecipitation and thesubsequent runoff areoccurring on thesame scales.

Intense Precipitation

Small, Fast-responsebasins



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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 (all non-mountain) ± 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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Flash Flood Warnings

Flash Floods lack objective definition

and verificationThere isn¶t a reliable

correlation betweenrainfall amount andflash flood severity.

A flash flood storm isoften not the onlygame in town!



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Issues With Flash Flood Warnings

Improvements to the Flash

Flood Watch stage have

occurred since the 70¶sWarnings are often inadequate

in one of two ways:

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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2) In a conscientious

attempt to provide

specific detail inspace and time,

there is little or no

lead time



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In some cases the

hydrology is the

driving force, andsignificant flash

floods can happen

with unimpressive

rainfall.



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Precipitation Duration

Synoptic Scale Pattern for convective storms (what

might start it, what will end it, will they move slowly?)

Evolution of Meso- and Storm-scale Features

± Common regeneration area for new storms

± Instability/Moisture Replenishment ± Will new cell development balance movement away from

area?

Presence of Orographic Features



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Precipitation Intensity

Enhanced Precipitation Efficiency

± Deep above freezing cloud layer: >3 km (>4 km SE)

± Well-defined high- Ue inflow (low-level jet)

± High Precipitable Water values (25-50 mm) ± Low-centroid storms, warm rain process

± Enhanced low-level lift (topo or metr 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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Frontal & Mesohigh Focus Elements for flood-producing rainfall are

focused along meteorological boundaries.

These mesoscale boundaries serve as an

atmospheric ³topography.´

Particularly common between the Rockies

and Appalachians.



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0500 UTC 7 Aug 1999

0700 UTC 7 Aug 1999



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1800 UTC 23 July 1987



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Moisture Convergence (shaded) and

Theta-E at 2100 UTC 23 July 1987.



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0000 UTC 24 July 1987



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1Aug 1985



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