Wisconsin has never experienced a hurricane, however people in the state have been greatly impacted by tropical storms and their remnants which formed as hurricanes weakened over land. This impact is mostly related to high amounts of precipitation and flooding.
Wisconsin experienced its heaviest tropical-cyclone-related rainfall in 1961 when Hurricane Carla entered the United States. Carla broke the record for highest rainfall in three other states as well.
In addition to the above, the Galveston Hurricane of 1900 had a significant impact on the area. The storm maintained tropical characteristics as it crossed the Upper Midwest, with more than 4 inches (100 mm) of rain being reported over most of the southern third of Wisconsin. Wind created problems all over, including over Lake Michigan, and other areas in the path of the storm had similar problems. The Galveston Hurricane was the deadliest on record since 1851. The death toll was 8,000-12,000. This Category4 hurricane occurred August 27 – September 15, 1900 with sustained winds of 145 mph (230 km/h). The only Atlantic hurricanes with a higher death toll were the Great Hurricane of 1780, with over 27,500 deaths and possibly Hurricane Mitch (1998) with 11,000 fatalities.
More recently, Hurricane Sandy influenced wave heights on Lake Michigan. On October 30, 2012, wave heights on Lake Michigan were measured at 21.7 feet. Fortunately for Lake Shore communities, the actual wave heights fell short of forecasted heights. Because of the danger, residents in some areas, including Pleasant Prairie, Wisconsin were urged to evacuate their homes. Shipping on Lake Michigan and the other Great Lakes was brought to a standstill as ships headed for safe harbor until the danger had passed. Hurricane Sandy’s impact was felt by all states from the East Coast where the Hurricane made landfall to the State of Wisconsin.
One source indicates that tropical storms reach Wisconsin and surrounding areas to the east, west, and south once every 50 years on average with others in the 19th century, 1961, and (subject to debate) Hurricane Ike in 2008; tropical depressions hit twice to three times every two years; and subtropical storms and depressions hit equally or more rarely than their tropical counterparts.
As the data indicates, Wisconsin has a low level of risk for being in the path of a hurricane, however heavy rain and flooding occur when the remnants of a hurricane find their way to Wisconsin. One study quantified the risk faced by inland areas. See the following graphic, where darker shades indicate lower vulnerability.
Wisconsin does experience storms with sustained winds approaching 60 mph. These storms do not have tropical origin, and are not classified as hurricanes. These strong storms are known as mid-latitude cyclones since they develop in the middle latitudes of Earth.
What Is a Hurricane?
A hurricane is a type of tropical cyclone, which is a generic term for a low pressure system that generally forms in the tropics. The cyclone is accompanied by thunderstorms and, in the Northern Hemisphere, a counterclockwise circulation of winds near the earth’s surface. Tropical cyclones are classified as follows:
Tropical Depression – An organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of 38 mph (33 knots) or less
Tropical Storm – An organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of 39-73 mph (34-63 knots)
Hurricane – An intense tropical weather system of strong thunderstorms with a well-defined surface circulation and maximum sustained winds of 74 mph (64 knots) or higher
Hurricanes are categorized according to the strength of their winds using the Saffir-Simpson Hurricane Scale.
A Category 1 storm has the lowest wind speeds, while a Category 5 hurricane has the strongest. These are relative terms, because lower category storms can sometimes inflict greater damage than higher category storms, depending on where they strike and the particular hazards they bring. In fact, tropical storms can also produce significant damage and loss of life, mainly due to flooding.
The Birth of a Hurricane
Tropical cyclones form over warm waters from pre-existing disturbances. These disturbances typically emerge every three or four days from the coast of Africa as “tropical waves” that consist of areas of unsettled weather. Tropical cyclones can also form from the trailing ends of cold fronts and occasionally from upper-level lows.
The process by which a tropical cyclone forms and subsequently strengthens into a hurricane depends on at least three conditions:
1. A pre-existing disturbance with thunderstorms
2. Warm (at least 80ºF) ocean temperatures to a depth of about 150 feet
3. Light upper level winds that do not change much in direction and speed throughout the depth of the atmosphere (low wind shear)
Heat and energy for the storm are gathered by the disturbance through contact with warm ocean waters. The winds near the ocean surface spiral into the disturbance’s low pressure area. The warm ocean waters add moisture and heat to the air which rises. As the moisture condenses into drops, more heat is released, contributing additional energy to power the storm. Bands of thunderstorms form, and the storm’s cloud tops rise higher into the atmosphere. If the winds at these high levels remain relatively light (little or no wind shear), the storm can remain intact and continue to strengthen.
Growth and Maturity
In these early stages, the system appears on the satellite image as a relatively unorganized cluster of thunderstorms. If weather and ocean conditions continue to be favorable, the system can strengthen and become a tropical depression (winds less than 38 mph or 33 kt). At this point, the storm begins to take on the familiar spiral appearance due to the flow of the winds and the rotation of the earth. If the storm continues to strengthen to tropical storm status (winds 39-73 mph, 34-63 kt), the bands of thunderstorms contribute additional heat and moisture to the storm. The storm becomes a hurricane when winds reach a minimum of 74 mph (64 kt). At this time, the cloud-free hurricane eye typically forms because rapidly sinking air at the center dries and warms the area.
During their life span, hurricanes can last for more than two weeks over the ocean and can travel up the entire Atlantic Coast.
The Storm’s End
Just as many factors contribute to the birth of a hurricane, there are many reasons why a hurricane begins to decay. Wind shear can tear the hurricane apart. Moving over cooler water or drier areas can lead to weakening as well. Landfall typically shuts off the hurricane’s main moisture source, and the surface circulation can be reduced by friction when it passes over land. Generally, a weakening hurricane or tropical cyclone can reintensify if it moves into a more favorable region or interacts with mid-latitude frontal systems.
The main parts of a hurricane are the rainbands on its outer edges, the eye, and the eyewall.
Air spirals in toward the center in a counter-clockwise pattern, and out the top in the opposite direction. In the very center of the storm, air sinks, forming the cloud-free eye.
The hurricane’s center is a relatively calm, clear area usually 20-40 miles across. People in the midst of a hurricane are often amazed at how the incredibly fierce winds and rain can suddenly stop and the sky clear when the eye comes over them. Then, just as quickly, the winds and rain begin again, but this time from the opposite direction.
The dense wall of thunderstorms surrounding the eye has the strongest winds within the storm. Changes in the structure of the eye and eyewall can cause changes in the wind speed, which is an indicator of the storm’s intensity. The eye can grow or shrink in size, and double (concentric) eyewalls can form.
The Spiral Rainbands
The storm’s outer rainbands (often with hurricane or tropical storm-force winds) can extend a few hundred miles from the center. Hurricane Andrew’s (1992) rainbands reached only 100 miles out from the eye, while those in Hurricane Gilbert (1988) stretched over 500 miles. These dense bands of thunderstorms, which spiral slowly counterclockwise, range in width from a few miles to tens of miles and are 50 to 300 miles long. Sometimes the bands and the eye are obscured by higher level clouds, making it difficult for forecasters to use satellite imagery to monitor the storm.
Typical hurricanes are about 300 miles wide although they can vary considerably, as shown in the two enhanced satellite images below. Size is not necessarily an indication of hurricane intensity. Hurricane Andrew (1992), the most devastating hurricane of 20th century, was a relatively small hurricane. Hurricane destructive winds and rains cover a wide swath. Hurricane-force winds can extend outward to about 25 miles from the storm center of a small hurricane and to more than 150 miles for a large one. The area over which tropical storm-force winds occur is even greater , ranging as far out as almost 300 miles from the eye of a large hurricane.
Hurricane Circulation and Movement
In the northern hemisphere, hurricane winds circulate around the center in a counter-clockwise fashion. This means that the wind direction at your location depends on where the hurricane’s eye is. A boat on the northern edge of the orange area in Hurricane Fran would experience winds from the east, while a boat on the southern edge would have westerly winds. A hurricane’s speed and path depend on complex interactions between the storm with its own internal circulations and the earth’s atmosphere. The air in which the hurricane is embedded is a constantly moving and changing “river” of air. Other features in that flow, such as high and low pressure systems, can greatly alter the speed and the path of the hurricane. In turn, it can modify the environment around the storm. Typically, a hurricane’s forward speed averages around 15-20 mph. However, some hurricanes stall, often causing devastatingly heavy rain. Others can accelerate to more than 60 mph. Hurricane Hazel (1954) hit North Carolina on the morning of 15 October; fourteen hours later it reached Toronto, Canada where it caused 80 deaths. Some hurricanes follow a fairly straight course, while others loop and wobble along the path These seemingly erratic changes are difficult to forecast and will be discussed in more detail in the Forecasting section of this module. Track of Hurricane Gordon, 1994
The Right Side of the Storm
As a general rule of thumb, the hurricane’s right side (relative to the direction it is traveling) is the most dangerous part of the storm because of the additive effect of the hurricane wind speed and speed of the larger atmospheric flow (the steering winds). The increased winds on the right side increase the storm surge. Tornadoes are also more common here. Looking at the figure above, pretend you are standing behind the hurricane with your back to the steering flow. In this case, the right side is the eastern section of the hurricane. (If it were traveling east to west, the right side would be the north section.) The winds around the hurricane’s eye are moving in a counterclockwise fashion. At Point A, the hurricane winds are nearly in line with the steering wind, adding to the strength of the winds. For example, if the steering currents are 30 mph and the average hurricane winds are 100 mph, the wind speed would be 130 mph at Point A. On the other hand, the winds at Point B are moving opposite those of the steering wind and therefore slow to 70 mph (100 – 30 mph). Incidentally, National Hurricane Center forecasts take this effect into account in their official wind estimates.
The video shows many of the features described above. The animation is of Hurricane Sandy, October 23 – 31, 2012.
The main hazards associated with tropical cyclones and especially hurricanes are storm surge, high winds, heavy rain, and flooding, as well as tornadoes. The intensity of a hurricane is an indicator of damage potential. However, impacts are a function of where and when the storm strikes. Hurricane Diane (1955) hit the northeastern U.S. and caused 184 deaths. It was only a Category 1 hurricane but the thirteenth deadliest since 1900. Hurricane Agnes (1972), also a Category 1 hurricane, ranks fifth with damages estimated at 6.9 billion when adjusted for inflation1. A storm surge is a large dome of water, 50 to 100 miles wide, that sweeps across the coastline near where a hurricane makes landfall. It can be more than 15 feet deep at its peak. The surge of high water topped by waves is devastating. Along the coast, storm surge is the greatest threat to life and property.
Hurricane winds not only damage structures, but the barrage of debris they carry is quite dangerous to anyone unfortunate enough (or unwise enough!) to be caught out in them. Damaging winds begin well before the hurricane eye makes landfall.
Tropical cyclones frequently produce huge amounts of rain, and flooding can be a significant problem, particularly for inland communities. A typical hurricane brings at least 6 to 12 inches of rainfall to the area it crosses. The resulting floods cause considerable damage and loss of life, especially in mountainous areas where heavy rains mean flash floods and can also result in devastating mudslides. Tornadoes spawned by landfalling hurricanes can cause enormous destruction. As a hurricane moves shoreward, tornadoes often develop on the fringes of the storm. These hazards can bring other consequences not directly related to the storm. For example, hurricane-related deaths and injuries are often the result of fires started by candles used when the electricity fails. Heart attacks and accidents frequently occur during the clean-up phase. And depending on the industrial facilities in your area, hurricane damage might cause chemical spills that could make the disaster even worse.
When the winds from these storms reach 39 mph (34 kt), the cyclone is given a name. Years ago, an international committee developed six separate lists of names for these storms . Each list alternates between male and female names. The use of these easily remembered names greatly reduces confusion when two or more tropical cyclones occur at the same time. Each list is reused every six years, although hurricane names that have resulted in substantial damage or death are retired.
The season begins on June 1 and ends on November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin, though the formation of tropical cyclones is possible at any time. The peak time of year for Atlantic Hurricanes is normally in September.
Information about current hurricanes is available on the GreendaleWeather.com Hurricane page.
More information about hurricanes can be found at the National Hurricane Center.