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FLOOD FAQs

The 100-year flood is a climatic average, that is, there is a one percent chance a 100-year flood will happen in any given year.

The worst flood in U.S. history resulted from a dam break upstream from Johnstown, Pennsylvania, on May 31, 1889. Although ample warnings were given, they were disregarded by many townspeople, and 2,200 residents died. On February 26, 1972, the Buffalo Creek Dam located in southern West Virginia collapsed sending a black wave of water through one coal mining town after another, and killing more than 100 people. Over 4,000 people were left homeless. A storm in northern Colorado dumped a foot of rain into the Big Thompson Canyon producing a raging torrent of water 19 feet high and resulting in 145 deaths.

Flood Climatology

Where and when do flash floods occur?

Flooding occurs in every U.S. state and territory, and is a threat experienced anywhere in the world that receives rain. In the U.S. floods kill more people each year than tornadoes, hurricanes or lightning.

How do flash floods occur?

Slow moving thunderstorms, thunderstorms that develop or move repeatedly over the same area, or heavy rains from tropical storms and hurricanes can cause floods or flash floods. These floods can develop within minutes or hours depending on the intensity and duration of the rain, the topography, soil conditions, and ground cover.

What are the basic ingredients of a flash flood?

The most basic weather ingredients for a flash flood are high rainfall rates that continue over a long period of time. High rainfall rates occur when air rises quickly and has a large amount of water vapor that condenses easily into rain.

The duration of a heavy rainfall event is related to the system's movement, speed and variations in intensity. Flash floods occur in nearly stationary or slow moving precipitation systems where many convective cells reach maturity and produce their heaviest rainfall over the same areas.

Most flash floods are produced by mesoscale convective systems (MCS's). MCS's are large complexes of thunderstorms that account for 70 percent of the annual warm-season rainfall east of the Rockies .

A "train effect" occurs when cells form and pass repeatedly over the same location.

Certain high precipitation supercells with strong vertically rising winds and high moisture content in low levels also have potential for heavy rains.

Squall lines associated with fronts can create outflows that spread out as a pool of precipitation-cooled air where new updrafts develop.

Not all flash floods come from convective systems. Strong updrafts leading to heavy precipitation can result when mountains or hills force the air upwards.

HOW DOES NSSL CONTRIBUTE?

HydroMet Testbed – Teams are testing and improving streamflow models and learning what is required to make better predictions of where, when, and how much rain will fall. The most recent project took place in the flood-prone American River Basin near Sacramento , CA in 2005-2006. NSSL provided a SMART-Radar to supplement the coverage of other radars, and see if the extra coverage helped improve forecasts.

NAME – The North American Monsoon Experiment was an 8-year joint U.S. Mexican program focused on improving monsoon season precipitation forecasts over the U.S. and Mexico . The seasonal low-level jet and how the monsoon is influenced by the transport of warm water into the Gulf of California were studied, in addition to other aspects of the monsoon.

IHOP_2002 – NSSL participated in the largest-ever weather field experiment in North America , the International H20 Project. Part of the goal was to learn what types of data are needed to make forecasts of thunderstorms and rainfall amounts more specific.

MEaPRS – The MCS Electrification and Polarimetric Radar Study was designed to investigate polarization radar signatures and electrification in Mesoscale convective systems occurring over the Oklahoma-Texas-Kansas region during May, 1999

MAP – Mesoscale Alpine Project – In 1999, NSSL scientists were involved in one of the largest weather research projects in Europe. The Mesoscale Alpine Project studied how wind flowing over the mountains affects precipitation and flooding.

SWAMP – SouthWest Area Monsoon Project – SWAMP was the first cooperative venture between NSSL and private industry, the Salt River Project. SWAMP was a series of meteorological field studies and experimental forecasting exercises to target central Arizona thunderstorm environments, monsoon structure and moisture fluexes, and Mexican convective systems. Flooding is a great threat from monsoon thunderstorms.

PACJET – The Pacific Landfalling Jets Experiment had a mission to improve short-term forecasts and warnings of floods, damaging winds and other severe weather spawned by storms originating over the data-sparse Pacific Ocean.

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