Natural Hazards

Flood

Flooding is a coast-to-coast threat to the United States and its territories in all months of the year. Flooding typically occurs when prolonged rain falls over several days, when intense rain falls over a short period of time, or when an ice or debris jam causes a river or stream to overflow onto the surrounding area. The most common cause of flooding is water due to rain and/or snowmelt that accumulates faster than soils can absorb it or rivers can carry it away. Approximately seventy-five percent of all Presidential disaster declarations are associated with flooding.

Tropical Cyclones (Hurricanes)

Conditions Which Must Be Present in the Development of a Tropical Cyclone 

1. Low-pressure system 
2. Warm temperatures over the Ocean 
3. Moist environment (precipitation) 
4. Tropical wind patterns over the equator

Anatomy of a Hurricane

Hurricanes are a triple threat bringing high winds, soaking rain, and flying debris. Hurricanes can cause devastation and property destruction on a massive scale, making proper disaster mitigation and well-planned response all the more important. While all coastal areas are at risk for hurricanes, Louisiana is particularly vulnerable.

In addition, hurricanes are capable of causing flooding and damage hundreds of miles inland.

• Eye: The eye is the "hole" at the center of the storm. Winds are light in this area. Skies are partly cloudy, and sometimes even clear. 
• Eye wall: The eye wall is a ring of thunderstorms. These storms swirl around the eye. The wall is where winds are strongest and rain is heaviest.
• Rain bands: Bands of clouds and rain go far out from a hurricane's eye wall. These bands stretch for hundreds of miles. They contain thunderstorms and sometimes tornadoes.

Tropical Cyclone Terms

Tropical Depression: A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 38 mph (33 knot) or less.

Tropical Storm: A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) ranges from 39 mph (34 knots) to 73 mph (63 knots). Hurricane: A tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 74 mph (64 knots) or more.

Storm Surge: An abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide. Storm surge can reach heights well over 20 feet and can span hundreds of miles of coastline.   This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level 15 feet or more. This rise in water level can cause severe flooding in coastal areas, particularly when the storm tide coincides with the normal high tides. Storm surges are the most deadly portion of a hurricane, having caused 50 percent of the fatalities from hurricanes from 1963 to 2012. 

Thunderstorms

The figure above shows the average number of thunderstorm days each year throughout the U.S. The most frequent occurrence is in the southeastern states, with Florida having the highest number 'thunder' days (80 to 100+ days per year).  

It is in this part of the country that warm, moist air from the Gulf of Mexico and Atlantic Ocean is most readily available to fuel thunderstorm development.

Hail is precipitation that is formed when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere. Hail can damage aircraft, homes and cars, and can be deadly to livestock and people. One of the people killed during the March 28, 2000 tornado in Fort Worth was killed when struck by grapefruit-size hail.

Damaging wind from thunderstorms is much more common than damage from tornadoes. In fact, many confuse damage produced by "straight-line" winds and often erroneously attribute it to tornadoes.