Severe Weather

Hurricane Movement

Figure 3: Typical areas where hurricanes begin their development and the common paths of storm movement.

Figure 4: Sea surface temperature map for the northern hemisphere summer. The yellow, orange, and red colors show water temperatures warm enough to sustain hurricanes (> 26.5°C).

Hurricanes derive their energy from the warm tropical water and from the latent heat released as water vapor condenses into clouds.

Figure 3 shows where most hurricanes are born and the general direction in which they move. Notice that they form over tropical oceans, except in the South Atlantic and in the eastern South Pacific. The surface temperatures in these areas are too cold for their development (see figure 4).

Hurricanes that form over the North Pacific and North Atlantic are steered by upper level easterly winds and typically move west or northwestward at about 10 knots. Often, they swing poleward, and when they move far enough north, they can become caught up in an upper level mid-latitude trough, which curves them toward the north and northeast.

The text above summarizes the typical paths taken by hurricanes. However, as many of you know the actual path taken by a hurricane can be caotic and difficult to predict. One difficulty lies in the fact that the upper level winds that can steer hurricanes (pressure levels at 500 mb and higher altitudes) are often quite weak in the tropics. When steering winds are weak, the actual path of a hurricane is determined by the structure of the storm and the storm's interaction with the environment. Factors that may contribute to a storms path include: a nearby area of ever so slightly warmer or colder sea temperatures, interactions between the storm and nearby land areas, the relative location and intensity of convection within a storm, or weak and unnoticed disturbances in the upper air wind flow. Thus, it is not all that uncommon for hurricanes to take erratic paths and make odd turns that catch weather forecasters by surprise.

Destruction and Warning at Landfall

Although we usually categorize hurricanes in terms of their windspeed (see Safir-Simpson table below), coastal flooding due to what is called the storm surge usually causes the most damage. A storm surge is an abnormally high rise in sea level along a shoreline caused primarily by high winds pushing and piling up water. With a hurricane (in the Northern Hemisphere), the most damaging storm surge and strongest winds occur to the right of the storm's center. The right side of the storm is where the wind blows directly onshore and where the windspeed is highest for a moving storm. A diagram will be drawn during lecture.

The damage potential associated with a storm surge depends upon the strength of the wind, the shape of the coastline, the slope of the Earth's solid surface near the coastline (the more gradual the slope, the higher the damage potential), and the timing of the surge with relationship to the normal high and low tides (normal high tide adds to the height of the storm surge). Forecasting the storm surge is complicated; forecasters rely on computer models. Besides the flooding or innundation of a coastal area by the storm surge, the storm's winds produce high waves which batter objects on land. Water is quite heavy and destructive often undercutting buildings and bridges.

Flooding is also caused by the heavy rains associated with hurricanes, especially when a slow moving or nearly stationary hurricane sits just offshore causing a prolonged period of heavy rains over nearby coastal areas. This can be especially problematic if the nearby coast has sharply rising mountains, enhancing the heavy rains by orographic lifting, with the runoff causing landslides.

Considerable damage may also occur from hurricane-spawned tornadoes that may form as the hurricane interacts with land areas. Most hurricanes that strike the United States produce at least one tornado. About one-quarter of them produce significant numbers of tornadoes. Tornadoes are most likely to develop in the right front quadrant of the storm, and are more likely to be associated with a spiral rain band rather than the storm's center. They result from the vertical wind shear present in the lower levels of the hurricane's circulation. Most hurricane-spawned tornadoes are of the weak variety, but can still produce significant damage.

Hurricanes weaken rapidly after making landfall since the strom is cutoff from its fuel supply (the warm ocean water). Although winds weaken, heavy and flooding rains may move far inland with the storm remnants.

In summary, the possible dangers associated with a hurricane are:

• storm surge
• strong winds
• heavy rains (and flooding)
• associated tornadoes

With modern techniques of forecasting and tracking hurricane paths, it is always possible to issue warnings about the "probable" locations that will be affected by any given hurricane. Although hurricanes can be easily tracked using satellite data, prediction of their future movement is by no means certain. The National Hurricane Center defines hurricane watches and warnings as follows:

• Hurricane Watch: An announcement of specific coastal areas that a hurricane or an incipient hurricane condition (sustained winds > 74 mph) poses a possible threat, generally within 36 hours.
• Hurricane Warning: A warning that sustained winds of 74 mph or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.

In an effort to estimate the possible damage of a hurricane's sustained winds and the storm surge could do to a coastal area, the Saffir-Simpson scale was developed (see table below).

Saffir-Simpson Hurricane Damage-Potential Scale

Scale Number (Category)	Central Pressure mb (in.)	Winds mi/hr (knots)	Storm Surge ft (m)	Damage
1	>980 (>28.94)	74-95 (64-82)	4-5 (~1.5)	Damage mainly to trees, shrubbery and unanchored mobile homes
2	965-979 (28.50-28.91)	96-110 (83-95)	6-8 (~2.0-2.5)	Some trees blown down; major damage to exposed mobile homes; some damage to roofs of buildings
3	945-964 (27.91-28.47)	111-130 (96-113)	9-12 (~2.5-4.0)	Foliage removed from trees; large trees blown down; mobile homes destroyed; some structural damage to small buildings
4	920-944 (27.17-27.88)	131-155 (114-135)	13-18 (~4.0-5.5)	All signs blown down; extensive damage to roofs, windows, and doors; complete destruction of mobile homes; flooding inland as far as 10 km (6 mi); major damage to lower floors of structures near shore
5	<920 (<27.17)	>155 (>135)	>18 (>5.5)	Severe damage to windows and doors; extensive damage to roofs of homes and industrial buildings; small buildings overturned and blown away; major damage to lower floors of all structures less than 4.5 m (15 ft) above sea level within 500 m of shore

Figure 5: The number of hurricanes (by each category) that have made landfall along the coastline of the United States from 1900 to 1999.

Figure 6: 1899-1996 U.S. Landfalling Major Hurricanes

Figure 7: Average number of Atlantic Hurricanes per month from 1950-2001.

Links

Answers to a long list of frequently asked questions about hurricanes can be found at The NOAA Hurricane Research Center's FAQ

Information about notable hurricanes of the last decade can be found in NOAA Hurricane Research Division and NCEP Hurricane Center.

More on this factors can be found at The University of Illinois web pages.

Animations of recent hurricanes are available from the Atmos. Sciences Weather Imagery