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

Table 25.1 Table 25.1: Flood probabilities for any single year “X”-year floodProbability of occurrence in a year 520% 1010% 254% 1001% %

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: 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 Frontal Overunning

Figure 25.7

More on Flash Floods Simple definition of a flash flood: “Too much water, too little time”Simple definition of a flash flood: “Too much water, too little time” A flash flood is one where the flooding occurs shortly after the causative rainfallA flash flood is one where the flooding occurs shortly after the causative rainfall Flash floods kill more people than any other convective-storm phenomenon (most are vehicle-related)Flash floods kill more people than any other convective-storm phenomenon (most are vehicle-related)

Introduction It is both a meteorological and hydrological issue—much depends on basin/watershed characteristics and previous conditionsIt is both a meteorological and hydrological issue—much depends on basin/watershed characteristics and previous conditions Extreme rainfall and flash floods can result from a variety of storm systems: MCSs, isolated convection, synoptic systems, tropical cyclonesExtreme rainfall and flash floods can result from a variety of storm systems: MCSs, isolated convection, synoptic systems, tropical cyclones Storm types that are important vary from region to regionStorm types that are important vary from region to region

How do we get this much rain? First, extreme rainfall almost always is caused by deep, moist convectionFirst, extreme rainfall almost always is caused by deep, moist convection So, remember the three ingredients necessary for convection:So, remember the three ingredients necessary for convection: –Moisture –Instability –Lift

How do we get this much rain? At any point, P=RD (total precipitation equals average rainfall rate times duration)At any point, P=RD (total precipitation equals average rainfall rate times duration) In other words: the most rain falls where it rains the hardest for the longest!In other words: the most rain falls where it rains the hardest for the longest! This is is sometimes called the “First Law of Quantitative Precipitation Forecasting”This is is sometimes called the “First Law of Quantitative Precipitation Forecasting”

How do we get this much rain? Three ingredients for high R: precipitation efficiency, upward motion (convection), water vapor contentThree ingredients for high R: precipitation efficiency, upward motion (convection), water vapor content R = Ewq

How do we get this much rain? Duration determined by system speed and size (Slow system movement conducive to extreme rainfall)Duration determined by system speed and size (Slow system movement conducive to extreme rainfall) From Doswell et al. (1996)

How do we get this much rain? Duration determined by system speed and size (Slow system movement conducive to extreme rainfall)Duration determined by system speed and size (Slow system movement conducive to extreme rainfall)

(Corfidi 2003)

Backbuilding/Quasi-stationary (BB) Line or cluster of backbuilding convection Line or cluster of backbuilding convection New cells form upstream of mature ones, cells decay as they move downstream New cells form upstream of mature ones, cells decay as they move downstream Usually a region of stratiform rain downstream Usually a region of stratiform rain downstream

Backbuilding/quasi-stationary MCSs Example: 7 May 2000

How do these work? Individual cells move from left to right, more or less with winds aloftIndividual cells move from left to right, more or less with winds aloft Outflow boundary remains stationary or moves to leftOutflow boundary remains stationary or moves to left This results in multiple cells forming and moving over the same location— ”echo training”--and can cause flash floodingThis results in multiple cells forming and moving over the same location— ”echo training”--and can cause 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

Popular Topic: “Atmospheric Rivers”

Figure 25.13

NWS Products and Role in Floods Local Office: Flash Flood watches and warnings, Coastal Flood warning RFC: River Flood Warnings National precipitation forecasting support: NCEP/WPC (formerly HPC)

Figure 25.15

River

Figure 25B

Figure 25C

Figure 25A

Great Flood of 1993 – Part of reading assignment, expect questions on next test.

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Figure 25H

Figure 25I