[Hurricane Basics]

There is nothing like them in the atmosphere. Born in warm tropical waters, these spiraling masses require a complex combination of atmospheric processes to grow, mature, and then die. They are not the largest storm systems in our atmosphere or the most violent, but they combine these qualities as no other phenomenon does.

In the Atlantic Basin, they are called hurricanes, a term that echoes colonial Spanish and Caribbean Indian words for evil spirits and big winds. These awesome storms have been a deadly problem for residents and sailors ever since the early days of colonization. Today, hurricane damage costs billions of dollars. During this century, 23 hurricanes have each caused damage in excess of $1 billion (adjusted for inflation). Damage from Hurricane Andrew (1992) alone was estimated at more than $25 billion in South Florida and Louisiana and undoubtedly would have been higher had the storm hit Miami directly.

Thankfully, the number of people injured or killed during tropical cyclones in the United States has been declining, largely because of improvements in forecasting and emergency preparedness. Nonetheless, our risk from hurricanes is increasing. With population and development continuing to increase along coastal areas, greater numbers of people and property are vulnerable to hurricane threat. Large numbers of tourists also favor coastal locations, adding greatly to the problems of emergency managers and local decision makers during a hurricane threat.

Hurricanes cannot be controlled, but our vulnerability can be reduced through preparedness.

[Hurricane Hazards]

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.

---Storm Surge---

The greatest potential for loss of life related to a hurricane is from the storm surge, which historically has claimed nine of ten victims.

Storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the storm. 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. In addition, wind waves are superimposed on the storm tide. This rise in water level can cause severe flooding in coastal areas, particularly when the storm tide coincides with the normal high tides. Because much of the United States' densely populated Atlantic and Gulf Coast coastlines lie less than 10 feet above mean sea level, the danger from storm tides is tremendous.

The level of surge in a particular area is also determined by the slope of the continental shelf. A shallow slope off the coast (right, top picture) will allow a greater surge to inundate coastal communities. Communities with a steeper continental shelf (right, bottom picture) will not see as much surge inundation, although large breaking waves can still present major problems. Storm tides, waves, and currents in confined harbors severely damage ships, marinas, and pleasure boats.

Additional Comments on Storm Surge

One tool used to evaluate the threat from storm surge is the SLOSH model. The links below provide some altered photos and storm surge maps that show how the intensity of the storm (as given by the Saffir-Simpson Hurricane Scale) affects the possibility of flooding from storm surge at two locations. Storm surge also affects rivers and inland lakes, potentially increasing the area that must be evacuated. You can see some of these effects by looking at storm surge pictures and maps for Brunswick, GA and Long Island, NY.

Obviously, the more intense the storm, and the closer you are to its right-front quadrant, the larger the area you will have to evacuate. The problem is, how do you know what category storm is going to hit you? We'll discuss this more in the Forecasting section, but you know enough about the basics of the hurricane life cycle to know that hurricanes change. And you know from your own experience that forecasts are not perfect. As an emergency manager, you have to balance that uncertainty with the risk of significant economic damage (from evacuating too large an area, particularly if the storm loses strength) and the risk to your population if you can't get them out of the threatened areas in time. One main rule of thumb for emergency management is to plan for a storm one category higher than what is forecast. This is a reasonable precaution to help minimize the loss of life from hurricanes.

Wave and current action associated with the tide also causes extensive damage. Water weighs approximately 1,700 pounds per cubic yard; extended pounding by frequent waves can demolish any structure not specifically designed to withstand such forces.

The currents created by the tide combine with the action of the waves to severely erode beaches and coastal highways. Many buildings withstand hurricane force winds until their foundations, undermined by erosion, are weakened and fail.

In estuaries and bayous, intrusions of salt water endanger the public health and send animals, such as snakes, fleeing from flooded areas.

---High Winds---

The intensity of a landfalling hurricane is expressed in terms of categories that relate wind speeds and potential damage. According to the Saffir-Simpson Hurricane Scale, a Category 1 hurricane has lighter winds compared to storms in higher categories. A Category 4 hurricane would have winds between 131 and 155 mph and, on the average, would usually be expected to cause 100 times the damage of the Category 1 storm. Depending on circumstances, less intense storms may still be strong enough to produce damage, particularly in areas that have not prepared in advance.

Tropical storm-force winds are strong enough to be dangerous to those caught in them. For this reason, emergency managers plan on having their evacuations complete and their personnel sheltered before the onset of tropical storm-force winds, not hurricane-force winds.

Hurricane-force winds can easily destroy poorly constructed buildings and mobile homes. Debris such as signs, roofing material, and small items left outside become flying missiles in hurricanes. Extensive damage to trees, towers, water and underground utility lines (from uprooted trees), and fallen poles cause considerable disruption.

Burger King Headquarters' CEO office in Miami after Hurricane Andrew

Damage from Hurricane Frederic (1979)

High-rise buildings are also vulnerable to hurricane-force winds, particularly at the higher levels since wind speed tends to increase with height. It is not uncommon for high-rise buildings to suffer a great deal of damage due to windows being blown out. Consequently, the areas around these buildings can be very dangerous.

Windows fall from high-rise buildings As indicated in the Basics section, the strongest winds usually occur in the right side of the eyewall of the hurricane. Wind speed usually decreases significantly within 12 hours of landfall. Nonetheless, winds can stay above hurricane strength well inland. Hurricane Hugo (1989), for example, battered Charlotte, North Carolina (which is 175 miles inland) with gusts to nearly 100 mph.

The Inland Wind Model, which is now part of the HURREVAC model, can be used by emergency managers to estimate how far inland strong winds extend. The inland wind estimates can only be made shortly before landfall when the windfield forecast errors are relatively small. This makes the program most useful in the decision-making process to decide which people might be most vulnerable to high winds at inland locations.

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---Heavy Rains---

Widespread rainfall of 6 to 12 inches or more is common during landfall, frequently producing deadly and destructive floods. Such floods have been the primary cause for tropical cyclone-related fatalities over the past 30 years. The risk from flooding depends on a number of factors: the speed of the storm, its interactions with other weather systems, the terrain it encounters, and ground saturation. Even storms with relatively light winds can be very damaging?Tropical Storm Claudette dumped 45 inches of rain near Alvin, Texas in 1979, contributing to more than $680 million* in damage.

Rains are generally heaviest with slower moving storms (less than 10 mph). To estimate the total rainfall in inches, one rule of thumb is to divide 100 by the forward speed of the hurricane in miles per hour (100/forward speed = estimated inches of rain). Your local Weather Forecast Office (WFO) may have a more accurate estimation method for your area.

The heaviest rain usually occurs near or along the cyclone track in the period 6 hours before and 6 hours after landfall. However, storms can last for days. Long after the winds of Hurricane Agnes (1972) had died down, its remnants fused with another storm system over the Florida panhandle and produced floods all the way up to the northeastern United States. The result: 122 deaths and $6.9 billion* in damage. Occasionally hurricanes produce little rain where it is expected. Hurricane Inez (1966) resulted in only a little rain in Miami when the city was under the eyewall where torrential rains were likely. "As a result of the absence of rain, the strong winds blew salt spray many miles inland, causing severe damage to vegetation from salt accumulation".1 Scientists are still trying to understand the reasons for these unusual events.

Large amounts of rain can occur more than 100 miles inland where flash floods are typically the major threat along with mudslides in mountainous regions. Tornadoes and high winds generally become less of a threat the farther inland a hurricane moves (although there have been several exceptions), but the heavy rains frequently continue and even intensify as the dying, but still powerful, hurricane is forced up higher terrain or merges with other storm systems in the area. For example, Hurricane Camille (1969) devastated the Gulf Coast, but weakened quickly as it moved northeast. The storm combined with a cold front in the mountains of central Virginia to produce an unexpected 30 inches of rain. As a result, 109 people died.

The possibility for heavy rain after a hurricane moves inland makes monitoring changing weather conditions and maintaining contact with the local WFO very important.

---Tornadoes---

Hurricanes can also produce tornadoes that add to the storm's destructive power. Recall from the discussion on hurricane structure that tornadoes are most likely to occur in the right-front quadrant of the hurricane. However, they are also often found elsewhere embedded in the rainbands, well away from the center of the hurricane. Some hurricanes seem to produce no tornadoes, while others develop multiple ones. Studies have shown that more than half of the landfalling hurricanes produce at least one tornado; Hurricane Buelah (1967) spawned 141 according to one study. In general, tornadoes associated with hurricanes are less intense than those that occur in the Great Plains (see the Fujita Intensity Scale). Nonetheless, the effects of tornadoes, added to the larger area of hurricane-force winds, can produce substantial damage.

We have no way at present to predict exactly which storms will spawn tornadoes or where they will touch down. The new Doppler radar systems have greatly improved the forecaster's warning capability, but the technology usually provides lead times from only a few minutes up to about 30 minutes. Consequently, preparedness is critical.

Tornado Facts

When associated with hurricanes, tornadoes are not usually accompanied by hail or a lot of lightning, clues that citizens in other parts of the country watch for. Tornado production can occur for days after landfall when the tropical cyclone remnants maintain an identifiable low pressure circulation. They can also develop at any time of the day or night during landfall. However, by 12 hours after landfall, tornadoes tend to occur mainly during daytime hours.

---Summary---

[Hurricane Forecasting]

Over the past 20 years, improvements in hurricane computer modeling, observational instrumentation, and better training for forecasters have greatly increased forecast accuracy. In particular, the new data systems described in the Basics/Observation section have given forecasters a greater understanding of tropical cyclones and have provided better and more timely input for computer models used to predict hurricane behavior.

Despite these advances, the many complex interactions that occur within the atmosphere are not fully understood or adequately modeled, limiting the accuracy of forecasts. When all is said and done, hurricane forecasting is still a very difficult job.

This section describes the forecasting process that is the joint responsibility of the Tropical Prediction Center¡¯s National Hurricane Center (NHC) and the local Weather Forecast Office (WFO). It also discusses forecast uncertainty and how forecasters and emergency managers make allowances for that uncertainty. Keep in mind that, although no forecast is perfect (and probably never will be), this forecasting process has contributed to a significant reduction in the number of deaths attributed to tropical cyclones and their related hazards. These forecasts are the best information you have on which to base your decisions.

---Process---

Part of the mission of the National Weather Service (NWS) Tropical Prediction Center (TPC) is to save lives and protect property by issuing watches, warnings, forecasts, and analyses of hazardous weather conditions in the tropics. This section provides information about the roles of those responsible for providing hurricane information to emergency managers and decision makers.

The TPC is comprised of the National Hurricane Center (NHC), the Tropical Analysis and Forecast Branch (TAFB), and the Technical Support Branch (TSB). During hurricane season, the latter two provide support to the NHC.

The Hurricane Liaison Team (HLT) is activiated during hurricanes to provide a link between the NHC and emergency managers and decision makers.

The local NWS Weather Forecast Offices (WFOs) in hurricane-prone areas are also important participants in the forecast process.

The NHC and your local WFO have various roles in the forecast process that are closely coordinated. Their activities are summarized in the table below.

Observation Observations using the techniques described in the Basics/Observation section are the basis for all forecast and warning products issued by the NHC. Quality, timeliness, and quantity of remote sensing observations are critical for accurate and timely forecasts and warnings. Analysis The various observations are checked for quality, analyzed, and put into a suite of computer models (described below). Central Model Guidance/Interpretation The computer models take in the observations and perform millions of calculations to generate predictions of hurricane behavior and the general conditions of the atmosphere in which the hurricane is embedded. The model results are packaged as guidance for the appropriate national centers and local offices and for evaluation and use in the NWS¡¯s forecast and warning process.

Information about Computer Models Coordination within the NWS Model results are coordinated between the national centers and local forecast offices to provide consistency, which is critical during severe weather episodes. Product Generation Once the coordination and collaboration process reaches group consensus, the issuing offices generate forecast and warning products for release to the public. Product Dissemination Timely and reliable dissemination of forecasts and warnings is critical to the protection of life and property. The types of products issued and their schedules are described in the Forecasting/Process/products section. Coordination with Customers The NHC and the local WFO work with customers to determine the level of satisfaction with the service provided and, in particular, whether the forecast and warning products issued were useful.

-----Local Level: The Weather Forecast Office (WFO)----

When the NWS's modernization program is finished, there will be 121 local WFOs. All of the offices are staffed 24 hours a day and produce

Watches and warnings for severe local storms, floods, flash floods, as well as local and zone public forecasts Local aviation forecasts, watches, and warnings Marine warnings and forecasts for coastal areas Hydrologic services such as support for flood and run-off forecasts Offices affected by hurricanes analyze the products created by the NHC and fine tune them for their own locale in order to provide local officials with the necessary information to make timely and efficient decisions. The WFOs produce local weather statements to inform the public about current and anticipated storm effects in their area and to augment NHC advisories and releases. The local statements are highly specific and are designed to keep the media, local decision makers, and the public current on present and anticipated storm effects.

Local forecasters initiate or participate in inter-site coordination between NHC and other local WFOs to ensure forecast and warning consistency. Following product delivery, the local office coordinates with local officials, the media, and the emergency management community. These coordination calls focus on the pending weather threat and what implications the forecast or warning has for the local area. Following the storm, the local Warning Coordination Meteorologist evaluates the service with the forecast users.

----Forecast Products----

The table below lists the main hurricane forecast products issued by the NWS's NHC and the local WFO. Click any of the products listed in the table to go to an explanation and example of that product.

In general, the NHC provides products that have a broad view of the hurricane and its potential impacts, while the local forecast office (the WFO) takes the information from NHC and tailors it to their specific locale, providing local emergency managers with additional information about the hazards expected in their area.

Issuing Agency Frequency Text Product NHC Every 6 hours Tropical cyclone public advisories are intended for the general public and describe the storm and its expected hazards. They are also used to issue watches and warnings.

NHC Every 6 hours Tropical cyclone forecast/advisories provide forecasts of the wind fields around the storm.

NHC Every 6 hours Tropical cyclone discussions are meant primarily for other forecasters, but are useful because they provide insight into the forecaster's reasoning and confidence.

NHC Every 6 hours Tropical cyclone strike probability forecasts describe the probability of a cyclone coming within 65 NM (75 mi.) of various locations during the next 72 hours

NHC Every 3 hours Intermediate public tropical cyclone advisories are issued every 3 hours once a watch or warning is in effect. Similar in format to 6-hour product.

NHC Every 2 hours Intermediate public tropical cyclone advisories are issued every 2 hours when a watch or warning is in effect and land-based radars have identified a reliable storm center. Hourly radar position estimates are issued between the 2-hourly public advisories. Similar in format to 6-hour product.

Local WFOs As needed Hurricane local statements describe expected local impacts.

Local WFOs As needed Inland high wind watches and warnings provide information about hurricane-force winds at inland locations

---Uncertainty---

To a forecaster, a good hurricane forecast is getting the location right within 50 miles and the wind speeds within 8 mph in the period 12 hours before landfall. As an emergency manager, your idea of a good forecast would be to know the location and intensity with 100% accuracy 72 hours ahead of time. That gap is the reality that underlies decision making.

Why is it that forecasts are imperfect, and how do forecasters and emergency managers take that uncertainty into account? By choosing Sources of Uncertainty in the diagram below, you will learn what contributes to errors in today's forecasts and how large these errors are. Choose Solutions: Dealing with Uncertainty to explore information and practices that can help you prepare for a hurricane threat.

---Summary---

Who Produces Hurricane Forecasts

The NHC (National Hurricane Center) is responsible for providing information on the current status of the storm and future forecasts of its behavior. Producing a forecast requires input from other experts, including the CARCAH (Chief, Aerial Reconnaissance Coordination, All Hurricanes), which provides support for aerial reconnaissance, the TAFB (Tropical Analysis and Forecast Branch), which provides support for satellite and radar analyses, the TSB (Technical Support Branch), which provides support for the various models, and other NWS (National Weather Service) groups. The local WFOs (Weather Forecast Offices) fine tune NHC status reports and forecasts for their particular area. The HLT (Hurricane Liaison Team) does not produce forecasts. However, it is the liaison between emergency managers at all levels, the NHC, local WFOs, and other agencies.