Chapter 25 cover Flood severe weather facts Duration and Intensity Factors pg 476 (4 th ed.)

Chapter 25 cover

Flood severe weather factsDuration and Intensity Factors pg 476 (4th ed.)

Table 25.1

Table 25.1: Flood probabilities for any single year

“X”-year flood Probability of occurrence in a year

5 20%

10 10%

25 4%

100 1%

500 0.2%

Flood Types

• Flash– Within 3-6 hours of causative event

• River (text: Widespread)– Concept of Watershed

• Coastal– Hurricane Surge– ET Cyclone– Sustained Winds

Figure 25.1

Figure 25.2

Tropical Cyclone Rain

• Houston Avg. Annual Rainfall: 47.84 in.

• TS Allison: 26.5 in.

Figure 25D

Figure 25.3

•MCS or Frontal Squall Heavy Rain Event•Speed of Movement•Training•OMG – Stalled Front!

Figure 25.4

Slow moving MCS

Figure 25.5

Another MCS contribution to 1993 Great Flood

Figure 25.6

FrontalOverunning

Figure 25.7

More on Flash Floods• Simple definition of a Simple definition of a

flash flood: “Too much flash flood: “Too much water, too little time”water, too little time”

• A flash flood is one A flash flood is one where the flooding where the flooding occurs shortly after the occurs shortly after the causative rainfall causative rainfall

• Flash floods kill more Flash floods kill more people than any other people than any other convective-storm convective-storm phenomenon (most are phenomenon (most are vehicle-related)vehicle-related)

Introduction

• It is both a meteorological and hydrological It is both a meteorological and hydrological issue—much depends on basin/watershed issue—much depends on basin/watershed characteristics and previous conditionscharacteristics and previous conditions

• Extreme rainfall and flash floods can result from Extreme rainfall and flash floods can result from a variety of storm systems: MCSs, isolated a variety of storm systems: MCSs, isolated convection, synoptic systems, tropical cyclonesconvection, synoptic systems, tropical cyclones

• Storm types that are important vary from region Storm types that are important vary from region to regionto region

How do we get this much rain?• First, extreme rainfall almost always is caused First, extreme rainfall almost always is caused

by deep, moist convectionby deep, moist convection• So, remember the three ingredients necessary So, remember the three ingredients necessary

for convection:for convection:– MoistureMoisture– InstabilityInstability– LiftLift

How do we get this much rain?• At any point, P=RD (total precipitation equals At any point, P=RD (total precipitation equals

average rainfall rate times duration)average rainfall rate times duration)• In other words: the most rain falls where it In other words: the most rain falls where it

rains the hardest for the longest!rains the hardest for the longest!• This is is sometimes called the “First Law of This is is sometimes called the “First Law of

Quantitative Precipitation Forecasting”Quantitative Precipitation Forecasting”

How do we get this much rain?• Three ingredients for high Three ingredients for high

R: precipitation efficiency, R: precipitation efficiency, upward motion upward motion (convection), water vapor (convection), water vapor content content

R = EwqR = Ewq

How do we get this much rain?• Duration determined by Duration determined by

system speed and size (Slow system speed and size (Slow system movement system movement conducive to extreme conducive to extreme rainfall)rainfall)

From Doswell et al. (1996)

How do we get this much rain?• Duration determined by Duration determined by

system speed and size (Slow system speed and size (Slow system movement system movement conducive to extreme conducive to extreme rainfall)rainfall)

(Corfidi 2003)

Backbuilding/Quasi-stationary (BB)Backbuilding/Quasi-stationary (BB)

Line or cluster of Line or cluster of backbuilding convectionbackbuilding convection

New cells form New cells form upstream of mature upstream of mature ones, cells decay as they ones, cells decay as they move downstreammove downstream

Usually a region of Usually a region of stratiform rain stratiform rain downstreamdownstream

Backbuilding/quasi-stationary MCSs

Example: 7 May 2000

How do these work?

• Individual cells move Individual cells move from left to right, more from left to right, more or less with winds aloftor less with winds aloft

• Outflow boundary Outflow boundary remains stationary or remains stationary or moves to leftmoves to left

• This results in multiple This results in multiple cells forming and moving cells forming and moving over the same location— over the same location— ”echo training”--and can ”echo training”--and can cause flash floodingcause flash flooding

From Doswell et al. (1996)

Figure 25.8

Role of topography – deep canyons: 139 die in Flash Flood (Big Thompson)

Figure 25.9

Chapter 25 cover Flood severe weather facts Duration and Intensity Factors pg 476 (4 th ed.)

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