Landfalling Tropical Cyclone Webpage Questions and Answers

1. Why make tropical cyclone spawned high wind event probabilities available on the web?

There is a great curiosity among most coastal residents about the chances of intense hurricane, hurricane and tropical storm force winds occurring in their area. An objective probability of this has yet to be developed, and particularly one that takes into account the varying global climate conditions from year to year and decade to decade. These varying yearly general circulation features cause the various wind speed probabilities to be raised or lowered on seasonal, yearly, and decadal time scales.

2. What is our biggest worry about issuing these landfall probability values?

That coastal residents will not take these tropical storm and hurricane wind speed probabilities as seriously as they should when they learn how small the seasonal probability of high winds is at any individual location.

3. What are sustained winds?

Sustained winds are winds that last for periods of about one hour or longer. Our wind strength probabilities are given for sustained winds only. Sustained winds are to be distinguished from local wind gusts resulting from squall lines, tornadoes and thunderstorms that can be much stronger (50 to 150 percent higher) and very short lived - lasting for a few seconds up to 20-30 minutes. We do not forecast the probability of these short period strong wind events that can at times cause significant additional damage.

4. If seasonal hurricane force wind probability in a particular coastal area is 2 percent, what does that mean?

There is a 2 percent chance, or one chance in 50 that sustained hurricane force winds will impact this region, sub-region, or county in the specified forecast interval of a season.

5. What is the difference between landfall probabilities and sustained wind probabilities?

Due to the broad radius extent of cyclone winds, one can experience hurricane or tropical storm winds from a cyclone making landfall in a neighboring county or sub-region. We only give cyclone landfall probabilities for each of our 11 U.S. coastal regions, not for sub-regions or counties, where we issue three categories of wind speed probability.

6. What are the differences of sustained wind speeds of a tropical storm (TS), a hurricane (H), an intense hurricane (IH) and a named storm (NS)?

TS (40-75 mph), H (≥ 75 mph), IH (≥ 115 mph) and NS (≥ 40 mph).

7. How should one interpret the probability of high wind vicinity probability?

Many tropical cyclones come very near a coastal area and just miss bringing high winds due to changes in their forecast motion or intensity. These systems do not bring the high winds that may have been anticipated. However, tropical cyclone watches and warnings might have to be issued as precautionary measures for the threatened area. Storm preparations may have had to be taken. These near-miss cyclones will always have to be closely monitored. We arbitrarily increased our potential high wind probability by 3 times the actual radius of our actual high winds covering an area increase by a factor of 9 to accommodate the near-miss or vicinity cyclone events.

8. Will these various high wind probabilities vary from year to year and forecast to forecast?

Yes, high wind probabilities vary by year and decade depending on the global atmospheric and ocean conditions which exist during these periods. Variability can be quite large within all speed categories and time periods.

9. How might one utilize these probabilities?

To learn the likelihood of your region being hit by sustained winds or various categories. One might desire this information out of curiosity, judgments on hurricane insurance, decisions on how much hurricane protection to spend on your home, chances of a vacation or special event being impacted, etc. Note how low these probabilities are for specific coastal areas in any one year, but the appreciable high wind probability in most areas if you consider a 50-year period (the typical period you would want a building to last).

10. How were the 11 regions and 55 sub-regions chosen?

The 11 regions were chosen by the frequency of landfalling intense (Cat. 3-4-5) hurricanes. The sub-regions were chosen by the density of coastal population.

11. What is the basis for these three classes ( ≥ 40 mph; ≥ 75 mph; ≥ 115 mph) of high wind probabilities?

They are derived from data on all tropical cyclone US landfalls of the 20^th Century along with combined wind assumptions of the radial distribution of wind strength with cyclone intensity. Cyclone intensity was determined by a combination of maximum sustained wind and minimum central pressure at landfall.

12. What might I learn from these numbers?

How very low are the chances that a tropical storm or hurricane force wind event will occur along any particular coastal area in any individual year. However, if one considers the probability of a high wind event in the vicinity of a coastal location or a high wind event occurring over a 50 year period, the probability can be much higher. Coastal probability of intense hurricane winds (≥ 115 mph) is typically very low. Nevertheless, on a long term normalized basis, intense hurricanes have been found to cause about 80 percent of all tropical cyclone observed damage.

13. If it has been an unusually long time since a coastal area has experienced a landfalling hurricane or high winds, does that mean that they are more likely (or due) to experience one in the coming years?

No, landfall probability does not increase or decrease based on recent past year landfall or non-landfall occurrence. Landfall probability changes occur as a result of seasonal climate conditions that do not have a memory for recent year events.

14. If we have had a lot of landfalling storms in recent years (such as Eastern North Carolina), does that mean that we are less likely or more likely to experience landfall in the next few years?

As in the previous question, recent year activity has no effect on future year occurrences. Landfall probability depends only on the current monthly or seasonal atmospheric and ocean conditions.

15. How much do these high wind and landfall probabilities change from year-to-year and decade-to-decade?

High wind and landfall probabilities can change a great deal with time. Landfall of an intense hurricane along the US East coast has the largest yearly and multi-decadal variability. Weaker intensity systems show less yearly and multi-decadal variability.

16. Is it possible to get hurricane force winds in an area that is experiencing only sustained tropical storm force winds?

Yes, there are often squall lines and tornadoes that occur in sustained tropical storm wind areas. These enhanced winds last for only a short period (seconds to minutes) but bring winds that can be 50-150 percent higher than the longer period sustained wind speeds.

17. Is hurricane damage directly related to hurricane intensity?

Only in a statistical sense. Hurricane damage can vary greatly between tropical storms or hurricanes of similar intensity. Each cyclone has its individual characteristics. Some storms have their greatest damage from winds, some from storm surge, and some from rainfall induced flooding. Also, the types of landfall topography, ocean shoaling characteristics and coastal population can be primary factors in the amount of tropical cyclone-spawned damage. All these factors lead to a broad spread of individual cyclone damage characteristics. Cyclone motion is related to local rainfall. Slow moving or stationary tropical cyclones can bring much more rainfall and flash flooding than do more rapid moving storms.

18. How does hurricane damage typically increase with hurricane intensity?

Although widely varying, there is a general relationship. On average, hurricane damage increases about four times for every increase of Saffir/Simpson category number. A category 3 cyclone will typically have about four times the damage of a category 2 cyclone. A category 4-5 cyclone (no distinction is made between category 4 and 5 cyclones) typically has about 16 times the damage of a category 2 cyclone. But individual cyclones can vary greatly from this relationship.

19. Will individual hurricane landfall ever be able to be predicted weeks, months or seasons in advance?

No. You can only give probabilities.

20. What important information do the smaller sub-region and county landfall probabilities convey?

That the chances of tropical cyclone high wind force wind events in any one year at any location are very small.

21. What is Poisson distribution?

A statistical distribution which we use to specify individual yearly high wind probability based on long period coastal wind and landfall averages. High wind events and landfall occurrences have been observed to closely follow a Poisson distribution. This distribution gives us the probability of 1 or more high wind events per year, 2 or more high wind events per year, etc. Note that the individual year probability of 1 or more high wind events making landfall is always smaller than the long period statistical average. Although there were 73 intense (Cat. 3-4-5) hurricanes to hit the US in the 20^th Century these occurred in only 52 years. Some years had two or more intense landfalling hurricanes.