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Hazards associated with flash floods First law of Quantitative Precipitation Forecasting (QPF)

Principles of Flash Flood Development: An Introduction to Hydrometeorology Anthony Phillips GEOG 490/590 Ball State University. Hazards associated with flash floods First law of Quantitative Precipitation Forecasting (QPF) Atmospheric and hydrologic contributors. Objectives.

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Hazards associated with flash floods First law of Quantitative Precipitation Forecasting (QPF)

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  1. Principles of Flash Flood Development: An Introduction to HydrometeorologyAnthony PhillipsGEOG 490/590Ball State University Hazards associated with flash floods First law of Quantitative Precipitation Forecasting (QPF) Atmospheric and hydrologic contributors

  2. Objectives • Definition of a flash flood • Recognize the deadliest convective weather hazards, in order of average annual fatalities, and identify trends over time • Identify whether a hydrological factor (e.g., soil depth) will increase or decrease the potential for flash floods • Know the first law of QPF • Understand what factors favor precipitation duration, “D” • Understand what factors favor average precipitation rate, “”

  3. What is a Flash Flood? • What is the difference between a flood and a flash flood? • The term flood usually refers to a hydrological event that takes place over many hours or days. • Often associated with main-stem rivers and floodplains. • A flash flood often occurs within 6 hours of a convective rainfall event, or when a dam or levee fails. • Small streams and creeks are most susceptible.

  4. What is a Flash Flood? • The NWS defines a flash flood as: • “Within six hours (often within one hour) of a causative event such as intense rain, dam break, or ice jam formation, one or more of the following occurs: • River or stream flows out of banks and is a threat to life or property. • Person or vehicle swept away by flowing water from runoff that inundates adjacent grounds. • A maintained county or state road closed by high water. • Six inches of fast-flowing water over a road or bridge. This includes low water crossings in a heavy rain event that is more than localized (i.e., radar and observer reports indicate flooding in nearby locations) and poses a threat to life or property. • Dam break or ice jam causes dangerous out-of-bank stream flows or inundates normally dry areas creating a hazard to life or property. • Any amount of water in contact with, flowing into, or causing damage to a residence or public building as a result of above ground runoff from adjacent areas. • …”

  5. Why Study Flash Floods? • Which hazard is most deadly? • Wind & hail • Hurricanes • Tornadoes • Lightning • Flash floods • Flash floods are the deadliest hazard associated with con- vection worldwide. • Flash floods cause more fatalities than tornadoes, wind, and hail combined!

  6. Severe Weather Trends • Decrease in tornado- and hurricane-related deaths over the past 50 years. • Better preparedness • Better technology • Increase in lead-times • However, fatalities from flash floods have not steadily decreased • Interaction of meteorology with hydrology • Flash flood risk is increasing • Extreme (>2”) precipitation events • Urbanization • Population growth

  7. Meteorological Factors • The development of flash floods depends on several factors, a few include: • Storm type and precipitation • Antecedent precipitation • Drainage basin size • Land use characteristics • Soil composition • We’ll begin by reviewing the meteorological factors that can lead to the development of flash floods

  8. First law of QPF • The greatest precipitation accumulations occur where it rains the hardest for the longest • Known as the first law of Quantitative Precipitation Forecasting (QPF) • Can be stated as • P, precipitation accumulation • , average precipitation rate • D, duration of precipitation • Average precipitation rate is directly proportional to the duration of the precipitation • How can this be related to flash flooding?

  9. First law of QPF • Maximum precipitation usually occurs only with intense or long duration convectivesystems • Single- or multi-cell convection • MCCs or MCSs • Squall lines • Supercells • Stratiform precipitation is often too light, but can result in riverine flooding • Long duration (D)

  10. First law of QPF • Maximum precipitation usually occurs only with intense or long duration convective systems ( • Usually, rainfall duration (D) increases with decreasing storm translation speeds • Mean wind speed decrease • MCSs: maximum precipitation occurs when cell motion is opposed by propagation of new cells • Results in quasi-stationary system motion

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