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| "I myself have experienced only one real disaster, Hurricane Andrew, and it was considerably different from the disaster movies that I've seen. For one thing, in the movies, there's always some kind of romance interest; whereas after Hurricane Andrew, nobody in the affected area was able to take a shower for approximately two months. Everybody smelled like a cologne named Eau de Dead Goat. The most romantic thing people did during that time was refuel each other's generators." --- Dave Barry, 1997 |
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FAQ: HURRICANES, TYPHOONS, AND TROPICAL CYCLONES
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OUTLINE
Part I:
Section A : BASIC DEFINITIONS
Section B : TROPICAL CYCLONE NAMES
Section C : TROPICAL CYCLONE MYTHS
Part II
Section D : TROPICAL CYCLONE WINDS
Section E : TROPICAL CYCLONE RECORDS
Part III
Section F : TROPICAL CYCLONE FORECASTING
Section G : TROPICAL CYCLONE CLIMATOLOGY
Section H : TROPICAL CYCLONE OBSERVATION
Part IV
Section I : Real Time Information
Section J : Historical Information
Section K : Preparedness Information
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| (c) 2008 Ova Yonda, Inc. |
There are seven tropical cyclone "basins" where storms occur on a regular basis:
The National Hurricane Center in Miami, Florida, USA has responsibilities for monitoring and forecasting tropical cyclones in the Atlantic and Northeast Pacific basin east of 140W. The Central Pacific Hurricane Center has responsibilities for the remainder of the Northeast Pacific basin to the dateline. The Northwest Pacific basin is shared in forecasting duties by China, Thailand, Korea, Japan, the Philippines, and Hong Kong. The North Indian basin tropical cyclones are forecasted by India, Thailand, Pakistan, Bangladesh, Burma, and Sri Lanka. Reunion Island, Madagascar, Mozambique, Mauritius, and Kenya provide forecasts for the Southwest Indian basin. Australia and Indonesia forecast tropical cyclone activity in the Southeast Indian/Australian basin. Lastly, for the Australian/Southwest Pacific basin Australia, Papua New Guinea, Fiji, and New Zealand forecast tropical cyclones. Note also that the USA Joint Typhoon Warning Center (JTWC) issues warnings for tropical cyclones in the Northwest Pacific, the North Indian, the Southwest Indian, the Southeast Indian/Australian, and the Australian/Southwest Pacific basins, though they are not specifically tasked to do so by the WMO. The USA Naval Western Oceanography Center in Pearl Harbor, Honolulu does the same for the Pacific Ocean east of 180E. (Neumann 1993)
Note that on rare occasions, tropical cyclones (or storms that appear to be similar in structure to tropical cyclones) can develop in the Mediterranean Sea. These have been noted to occur in September 1947, September 1969, January 1982, September 1983, and, most recently, during 13 to 17 January, 1995. Some study of these storms has been reported on by Mayengon (1984) and Ernest and Matson (1983), though it has not been demonstrated fully that these storms are the same as those found over tropical waters. It may be that these Mediterranean tropical cyclones are more similar in nature to polar lows.
The following are the addresses of tropical cyclone centers listed above that are responsible for issuing advisories and/or warnings on tropical cyclones (thanks to Jack Beven for these):
The following cities are also mentioned as tropical cyclone warning centers, though I don't have the addresses for them.
Prof. Bill Gray at Colorado State University in Fort Collins, Colorado (USA) has issued seasonal hurricane forecasts for the Atlantic basin since 1984. Details of his forecasting technique can be found in Gray (1984a,b) and Gray et al. (1992, 1993, 1994). Landsea et al. (1994) also provides verifications of the first 10 years of forecasting. A quick summary of the components follows:
Dr. Gray and his forecast team issues seasonal forecasts in late November, early June, and early August of each year with a verification of the forecasts given in late November. To obtain these forecasts, surf to: http://tropical.atmos.colostate.edu/forecasts/index.html
Also available (via unix machines) a finger command to get a table with the latest forecast info and what the observations have been of the season so far. Available via: finger forecast@typhoon.atmos.colostate.edu
Here are the numbers that Dr. Gray has issued for his real-time Atlantic tropical cyclone seasonal forecasting:
Named Storms: 1950 to 1990 Mean = 9.3
| December | April | June | August | ||
| Year | Forecast | Forecast | Forecast | Forecast | Observed |
|
1984 |
--- |
--- |
10 |
10 |
12 |
|
1985 |
--- |
--- |
11 |
10 |
11 |
|
1986 |
--- |
--- |
8 |
7 |
6 |
|
1987 |
--- |
--- |
8 |
7 |
7 |
|
1988 |
--- |
--- |
11 |
11 |
12 |
|
1989 |
--- |
--- |
7 |
9 |
11 |
|
1990 |
--- |
--- |
11 |
11 |
14 |
|
1991 |
--- |
--- |
8 |
7 |
8 |
|
1992 |
8 |
--- |
8 |
8 |
6 |
|
1993 |
11 |
--- |
11 |
10 |
8 |
|
1994 |
10 |
--- |
9 |
7 |
7 |
|
1995 |
12 |
10 |
12 |
16 |
19 |
|
1996 |
8 |
11 |
10 |
11 |
13 |
|
1997 |
11 |
11 |
11 |
11 |
7 |
|
1998 |
9 |
10 |
10 |
10 |
14 |
Hurricanes: 1950 to 1990 Mean = 5.8
| December | April | June | August | ||
| Year | Forecast | Forecast | Forecast | Forecast | Observed |
|
1984 |
--- |
--- |
7 |
7 |
5 |
|
1985 |
--- |
--- |
8 |
7 |
7 |
|
1986 |
--- |
--- |
4 |
4 |
4 |
|
1987 |
--- |
--- |
5 |
4 |
3 |
|
1988 |
--- |
--- |
7 |
7 |
5 |
|
1989 |
--- |
--- |
4 |
4 |
7 |
|
1990 |
--- |
--- |
7 |
6 |
8 |
|
1991 |
--- |
--- |
4 |
3 |
4 |
|
1992 |
4 |
--- |
4 |
4 |
4 |
|
1993 |
6 |
--- |
7 |
6 |
4 |
|
1994 |
6 |
--- |
5 |
4 |
3 |
|
1995 |
8 |
6 |
8 |
9 |
11 |
|
1996 |
5 |
7 |
6 |
7 |
9 |
|
1997 |
7 |
7 |
7 |
6 |
3 |
|
1998 |
5 |
6 |
6 |
6 |
10 |
Major Hurricanes: 1950 to 1990 Mean = 2.3
| December | April | June | August | ||
| Year | Forecast | Forecast | Forecast | Forecast | Observed |
|
1990 |
--- |
--- |
3 |
2 |
1 |
|
1991 |
--- |
--- |
1 |
0 |
2 |
|
1992 |
1 |
--- |
1 |
1 |
1 |
|
1993 |
3 |
--- |
2 |
2 |
1 |
|
1994 |
2 |
--- |
1 |
1 |
0 |
|
1995 |
3 |
2 |
3 |
3 |
5 |
|
1996 |
2 |
2 |
2 |
3 |
6 |
|
1997 |
3 |
3 |
3 |
2 |
1 |
|
1998 |
2 |
2 |
2 |
2 |
3 |
(Track model information contributed by Sim Aberson)
A variety of hurricane track forecast models are run operationally for the Atlantic hurricane basin:
Despite the variety of hurricane track forecast models, there are only a few models that forecast intensity change for the Atlantic basin:
While the Atlantic hurricane season is "officially" from 1 June to 30 November, the Atlantic basin shows a very peaked season with 78% of the tropical storm days, 87% of the minor (Saffir-Simpson Scale categories 1 and 2 - see subject D1) hurricane days, and 96% of the major (Saffir-Simpson categories 3, 4 and 5) hurricane days occurring in August through October (Landsea 1993). Peak activity is in early to mid September. Once in a few years there may be a tropical cyclone occurring "out of season" - primarily in May or December. (For more detailed information, see section G13 - "What is my chance of having a tropical storm or hurricane strike by each month?")
The Northeast Pacific basin has a broader peak with activity beginning in late May or early June and going until late October or early November with a peak in storminess in late August/early September.
The Northwest Pacific basin has tropical cyclones occurring all year round regularly though there is a distinct minimum in February and the first half of March. The main season goes from July to November with a peak in late August/early September.
The North Indian basin has a double peak of activity in May and November though tropical cyclones are seen from April to December. The severe cyclonic storms (>74 mph or >33 m/s winds) occur almost exclusively from April to June and late September to early December.
The Southwest Indian and Australian/Southeast Indian basins have very similar annual cycles with tropical cyclones beginning in late October/ early November, reaching a double peak in activity - one in mid-January and one in mid-February to early March, and then ending in May. The Australian/ Southeast Indian basin February lull in activity is a bit more pronounced than the Southwest Indian basin's lull.
The Australian/Southwest Pacific basin begin with tropical cyclone activity in late October/early November, reaches a single peak in late February/early March, and then fades out in early May.
Globally, September is the most active month and May is the least active month. (Neumann 1993)
The effect of El Nino-Southern Oscillation (ENSO) on Atlantic tropical cyclones is described in subject F2).
The Australian/Southwest Pacific shows a pronounced shift back and forth of tropical cyclone activity with fewer tropical cyclones between 145 and 165E and more from 165E eastward across the South Pacific during El Nino (warm ENSO) events. There is also a smaller tendency to have the tropical cyclones originate a bit closer to the equator. The opposite would be true in La Nina (cold ENSO) events. See papers by Nicholls (1979), Revell and Goulter (1986), Dong (1988), and Nicholls (1992).
The western portion of the Northeast Pacific basin (140W to the dateline) has been suggested to experience more tropical cyclone genesis during the El Nino year and more tropical cyclones tracking into the sub-region in the year following an El Nino (Schroeder and Yu 1995), but this has not been completely documented yet.
The Northwest Pacific basin, similar to the Australian/Southwest Pacific basin, experiences a change in location of tropical cyclones without a total change in frequency. Pan (1981), Chan (1985), and Lander (1994) detailed that west of 160E there were reduced numbers of tropical cyclone genesis with increased formations from 160E to the dateline during El Nino events. The opposite occurred during La Nina events. Again there is also the tendency for the tropical cyclones to also form closer to the equator during El Nino events than average.
The eastern portion of the Northeast Pacific, the Southwest Indian, the Southeast Indian/Australian, and the North Indian basins have either shown little or a conflicting ENSO relationship and/or have not been looked at yet in sufficient detail.
The United Nation's Intergovernmental Panel on Climate Change (IPCC) has speculated that climate change due to increasing amounts of anthropogenic "greenhouse" gases may result in increased tropical sea surface temperatures (SSTs) and increased tropical rainfall associated with a slightly stronger inter-tropical convergence zone (ITCZ) (Houghton et al., 1990, 1992, 1996). Because tropical cyclones extract latent and sensible heat from the warm tropical oceans and release the heat in its upper tropospheric outflow to fuel the storm's spin up, early work of the IPCC expressed concern that warmer SSTs will lead to more frequent and intense hurricanes, typhoons and severe tropical cyclones. These concerns prompted the IPCC (Houghton et al. 1990) to suggest in 1990 that:
"There is some evidence from model simulations and empirical considerations that the frequency per year, intensity and area of occurrence of tropical disturbances may increase [in a doubled carbon dioxide world], though it is not yet compelling."
However, any changes in tropical cyclone activity are intrinsically also tied to large-scale changes in the tropical atmosphere. As a result, SSTs by themselves cannot be considered without corresponding information regarding the moisture and stability in the tropical troposphere. What has been identified in the current climate as being necessary for genesis and maintenance for tropical cyclones (e.g. SSTs of at least 26.5C [80F] - Gray 1968) would change in an enhanced doubled CO2 world because of possible changes in the moisture or stability. It is quite reasonable that an increase in tropical and subtropical SSTs would be also accompanied by an increase in the SST threshold value needed for cyclogenesis because of compensating changes in the tropospheric moist static stability (Emanuel 1995). In addition to the thermodynamic variables, changes in the tropical dynamics also play a large role in determining changes in tropical cyclone activity. For example, if the vertical wind shear over the tropical North Atlantic moderately increased during the hurricane season in an increased CO2 world - as what is typically seen during El Nino-Southern Oscillation warm phases (El Nino events), then we would most likely see a significant decrease in tropical cyclone activity. This is due to the Atlantic basin having a marginal climatology for tropical cyclone activity because of its sensitivity to changes in vertical wind shear and lack of an oceanic monsoon trough (Gray et al. 1993). In other less marginal tropical cyclone basins, changes in the vertical shear profile typically result in alterations in the preferred location of development (e.g. Nicholls 1979, Chan 1985, Revell and Goulter 1986, and Lander 1994).
These complications along with conflicting global
circulation modeling (GCM) runs compelled the 1995 IPCC (Houghton et al. 1996)
to express greater uncertainty about the nature of tropical cyclones in an
enhanced CO2
environment:
"The formation of tropical cyclones depends not only on sea surface temperature (SST), but also on a number of atmospheric factors. Although some models now represent tropical storms with some realism for present day climate, the state of the science does not allow assessment of future changes."
Most recently, Henderson-Sellers et al. (1998) addressed a few of the tropical cyclone-greenhouse warming problems. The first is that "there is no evidence to suggest any major changes in the area or global location of tropical cyclone genesis in greenhouse conditions." This conclusion is based upon Holland's (1997) thermodynamic tropical cyclone model which does show that in a greenhouse-warmed climate there is an upward alteration in the minimum SST from 26.5 to 28C (80 to 83F) needed for tropical cyclogenesis. The additional conclusion from Henderson-Sellers et al. (1998) suggests "an increase in [maximum potential intensity] MPI of 10%-20% [in central pressure or 5%-10% in maximum sustained winds] for a doubled CO2 climate but the known omissions (ocean spray, momentum restriction, and possibly also surface to 300 hPa lapse rate changes) all act to reduce these increases." This second finding is also based upon the thermodynamic models of Emanuel (1986) and Holland (1997), which also appear to corroborate similar findings for Northwest Pacific typhoons from a "downscaled" GCM to mesoscale model approach by Knutson et al. (1998). Henderson-Sellers et al. (1998) does not provide guidance for possible changes in tropical cyclone frequency, mean intensity, or area of occurrence.
The most helpful paper that may predict changes in hurricane and typhoon frequency with some realism is the recent work by Royer et al. (1998). Based upon alterations to the large scale atmospheric and oceanic conditions (vertical shear, vorticity and thermodynamic stability), they suggest that only small changes to the tropical cyclone frequencies may result: up to 10% increase in numbers in the Northern Hemisphere (primarily in the Northwest Pacific) and up to a 5% decrease in numbers in the Southern Hemisphere. These values should be considered very preliminary.
To summarize, our current assessment of how global warming may alter hurricanes, typhoons and tropical cyclones is as follows (from Henderson- Sellers et al. 1998, Knutson et al. 1998, and Royer et al. 1998):
Globally, no. However, for the Atlantic basin we have seen an increase in the number of strong hurricanes since 1995. As can be seen in section E9, we have had a record 33 hurricanes in the four years of 1995 to 1999 (accurate records for the Atlantic are thought to begin around 1944). The extreme impacts from Hurricanes Marilyn (1995), Opal (1995), Fran (1996), Georges (1998) and Mitch (1998) in the United States and throughout the Caribbean attest to the high amounts of Atlantic hurricane activity lately.
As discussed in the previous section, it is highly unlikely that global warming has (or will) contribute to a drastic change in the number or intensity of hurricanes. We have not observed a long-term increase in the intensity or frequency of Atlantic hurricanes. Actually, 1991-1994 marked the four quietest years on record (back to the mid-1940s) with just less than 4 hurricanes per year. Instead of seeing a long-term trend up or down, we do see a quasi-cyclic multi-decade regime that alternates between active and quiet phases for major Atlantic hurricanes on the scale of 25-40 years each (Gray 1990; Landsea 1993; Landsea et al. 1996). The quiet decades of the 1970s to the early 1990s for major Atlantic hurricanes were likely due to changes in the Atlantic Ocean sea surface temperature structure with cooler than usual waters in the North Atlantic. The reverse situation of a warm North Atlantic was present during the active late-1920s through the 1960s (Gray et al. 1997). It is quite possible that the extreme activity since 1995 marks the start of another active period that may last a total of 25-40 years. More research is needed to better understand these hurricane "cycles".
For the region near Australia (105-160E, south of
the equator), Nicholls (1992) identified a downward trend in the numbers of
tropical cyclones, primarily from the mid-1980s onward. However, a portion
of this trend is likely artificial as the forecasters in the region no longer
classify weak systems as "cyclones" if the systems do not possess the
traditional tropical cyclone inner-core structure, but have the band of maximum
winds well-removed from the center (Nicholls et al. 1998). These changes
in methodology around the mid-1980s have been prompted by improved access to and
interpretation of digital satellite data, the installation of coastal and
off-shore radar, and an increased understanding of the differentiation of
tropical cyclones from other type of tropical weather systems. By
considering
only the moderate and intense tropical cyclones, this artificial bias in the
cyclone record can be overcome. Even with the removal of this bias in the
weak Australian tropical cyclones that the frequency of the remaining moderate
and strong tropical cyclones has been reduced substantially over the years
1969/70-1995/96. Nicholls et al. (1998) attribute the decrease in moderate
cyclones to the occurrence of more frequent El Nino occurrences during the 1980s
and 1990s.
For the Northwest Pacific basin, Chan and Shi (1996) found that both the frequency of typhoons and the total number of tropical storms and typhoons have been increasing since about 1980. However, the increase was preceded by a nearly identical magnitude of decrease from about 1960 to 1980. It is unknown currently what has caused these decadal-scale changes in the Northwest Pacific typhoons.
For the remaining basins based upon data from the late 1960s onwards, the Northeast Pacific has experienced a significant upward trend in tropical cyclone frequency, the North Indian a significant downward trend, and no appreciable long-term variation was observed in the Southwest Indian and Southwest Pacific (east of 160E) for the total number of tropical storm strength cyclones (from Neumann 1993). However, whether these represent longer term (> 30 years) or shorter term (on the scale of ten years) variability is completely unknown because of the lack of a long, reliable record.
As described in subject G1), the primary time of year for getting tropical cyclones is during the summer and autumn: July-October for the Northern Hemisphere and December-March for the Southern Hemisphere (though there are differences from basin to basin). The peak in summer/autumn is due to having all of the necessary ingredients become most favorable during this time of year: warm ocean waters (at least 26C or 80F), a tropical atmosphere that can quite easily kick off convection (i.e. thunderstorms), low vertical shear in the troposphere, and a substantial amount of large-scale spin available (either through the monsoon trough or easterly waves - see subject A4)). While one would intuitively expect tropical cyclones to peak right at the time of maximum solar radiation (late June for the tropical Northern Hemisphere and late December for the tropical Southern Hemisphere), it takes several more weeks for the oceans to reach their warmest temperatures. The atmospheric circulation in the tropics also reaches its most pronounced (and favorable for tropical cyclones) at the same time. This time lag of the tropical ocean and atmospheric circulation is analogous to the daily cycle of surface air temperatures - they are warmest in mid-afternoon, yet the sun's incident radiation peaks at noon.
Tropical cyclones - to a first approximation - can be thought of as being steered by the surrounding environmental flow throughout the depth of the troposphere (from the surface to about 12 km or 8 mi). Dr. Neil Frank, former director of the U.S. National Hurricane Center, used the analogy that the movement of hurricanes is like a leaf being steered by the currents in the stream, except that for a hurricane the stream has no set boundaries.
In the tropical latitudes (typically equatorward of 20-25 N or S), tropical cyclones usually move toward the west with a slight poleward component. This is because there exists an axis of high pressure called the subtropical ridge that extends east-west poleward of the storm. On the equatorward side of the subtropical ridge, general easterly winds prevail. However, if the subtropical ridge is weak - oftentimes due to a trough in the jet stream - the tropical cyclone may turn poleward and then recurve back toward the east. On the poleward side of the subtropical ridge, westerly winds prevail thus steering the tropical cyclone back to the east. These westerly winds are the same ones that typically bring extratropical cyclones with their cold and warm fronts from west to east.
Many times it is difficult to tell whether a trough will allow the tropical cyclone to recurve back out to sea (for those folks on the eastern edges of continents) or whether the tropical cyclone will continue straight ahead and make landfall.
For more non-technical information on the movement of tropical cyclones, see Pielke and Pielke's _Hurricanes: their Nature and Impacts on Society. For a more detailed, technical summary on the controls on tropical cyclone motion, see Elsberry's chapter in _Global Perspectives on Tropical Cyclones_. Both books are detailed in Part II of the FAQ.
Though many people might speculate that the sea surface temperatures are too cold, the primary reasons that the South Atlantic Ocean gets no tropical cyclones are that the tropospheric (near surface to 200mb) vertical wind shear is much too strong and there is typically no inter-tropical convergence zone (ITCZ) over the ocean (Gray 1968). Without an ITCZ to provide synoptic vorticity and convergence (i.e. large scale spin and thunderstorm activity) as well as having strong wind shear, it becomes very difficult to nearly impossible to have genesis of tropical cyclones.
However, in rare occasions it may be possible to have tropical cyclones form in the South Atlantic. In McAdie and Rappaport (1991), the USA National Hurricane Center documented the occurrence of a strong tropical depression/weak tropical storm that formed off the coast of Congo in mid-April 1991. The storm lasted about five days and drifted toward the west-southwest into the central South Atlantic. So far, there has not been a systematic study as to the conditions that accompanied this rare event.
No. The number of named storms (hurricanes) occurring in June and July correlates at an insignificant r = +0.13 (+0.02) versus the whole season activity. Actually, there is a slight _negative_ association of early season storms (hurricanes) versus late season - August through November - r = -0.28 (-0.35). Thus, early season activity, be it very active or quite calm, has little bearing on the season as a whole. These correlations are based on the years 1944-1994.
Hurricanes form both in the Atlantic basin (i.e. the Atlantic Ocean, Gulf of Mexico and Caribbean Sea) to the east of the continental U.S. and in the Northeast Pacific basin to the west of the U.S. However, the ones in the Northeast Pacific almost never hit the U.S., while the ones in the Atlantic basin strike the U.S. mainland just less than twice a year on average. There are two main reasons. The first is that hurricanes tend to move toward the west-northwest after they form in the tropical and subtropical latitudes. In the Atlantic, such a motion often brings the hurricane into the vicinity of the U.S. east coast. In the Northeast Pacific, a west-northwest track takes those hurricanes farther off-shore, well away from the U.S. west coast. In addition to the general track, a second factor is the difference in water temperatures along the U.S. east and west coasts. Along the U.S. east coast, the Gulf Stream provides a source of warm (> 80 F or 26.5 C) waters to help maintain the hurricane. However, along the U.S. west coast, the ocean temperatures rarely get above the lower 70s, even in the midst of summer. Such relatively cool temperatures are not energetic enough to sustain a hurricane's strength. So for the occasional Northeast Pacific hurricane that does track back toward the U.S. west coast, the cooler waters can quickly reduce the strength of the storm.
Surprisingly, not much lightning occurs in the inner core (within about 100 km or 60 mi) of the tropical cyclone center. Only around a dozen or less cloud-to-ground strikes per hour occur around the eyewall of the storm, in strong contrast to an overland mid-latitude mesoscale convective complex which may be observed to have lightning flash rates of greater than 1000 per hour (!) maintained for several hours. Hurricane Andrew's eyewall had less than 10 strikes per hour from the time it was over the Bahamas until after it made landfall along Louisiana, with several hours with no cloud-to-ground lightning at all (Molinari et al. 1994). However, lightning can be more common in the outer cores of the storms (beyond around 100 km or 60 mi) with flash rates on the order of 100s per hour.
This lack of inner core lightning is due to the relative weak nature of the eyewall thunderstorms. Because of the lack of surface heating over the ocean ocean and the "warm core" nature of the tropical cyclones, there is less buoyancy available to support the updrafts. Weaker updrafts lack the super-cooled water (e.g. water with a temperature less than 0 C or 32 F) that is crucial in charging up a thunderstorm by the interaction of ice crystals in the presence of liquid water (Black and Hallett 1986). The more common outer core lightning occurs in conjunction with the presence of convectively-active rainbands (Samsury and Orville 1994).
One of the exciting possibilities that recent lightning studies have suggested is that changes in the inner core strikes - though the number of strikes is usually quite low - may provide a useful forecast tool for intensification of tropical cyclones. Black (1975) suggested that bursts of inner core convection which are accompanied by increases in electrical activity may indicate that the tropical cyclone will soon commence a deepening in intensity. Analyses of Hurricanes Diana (1984), Florence (1988) and Andrew (1992), as well as an unnamed tropical storm in 1987 indicate that this is often true (Lyons and Keen 1994 and Molinari et al. 1994).
(Figures in process)
What is my chance of being struck by a tropical
storm or hurricane?(Figures in process)
The figure here shows for any particular location what the chance is that a tropical storm or hurricane will affect the area sometime during the whole June to November hurricane season. We utilized the years 1944 to 1999 in the analysis and counted hits when a storm or hurricane was within about 100 miles (165 km). This figure is created by Todd Kimberlain. For example, people living in New Orleans, Louisiana have about a 40% chance (the green-orange color) per year of experiencing a strike by a tropical storm or hurricane. For the U.S., the locations that have the highest chances are the following: Miami, Florida - 48% chance; Cape Hatteras, North Carolina - 48% chance; and San Juan, Puerto Rico - 42% chance.
The second figure (click to show) presents for any particular location what the chance is that a hurricane will directly affect the area sometime during the whole June to November hurricane season. We utilized the years 1944 to 1999 in the analysis and counted hits when a hurricane was within about 60 miles (110 km). This figure is created by Todd Kimberlain. (For example, the chance for Miami, Florida is about 16%.)
The third figure (click to show) presents for any particular location what the chance is that a major hurricane (Category 3, 4 or 5) will directly affect the area sometime during the whole June to November hurricane season. We utilized the years 1944 to 1999 in the analysis and counted hits when a hurricane was within about 30 miles (50 km). This figure is created by Todd Kimberlain. (For example, the chance for Miami, Florida is about 4%.)
Many folks are are concerned about a vacation and the possible impacts that a hurricane could have. If so, please check with your hotel, cruise company, etc. to find out how they plan to inform their guests when a hurricane is coming, what actions they plan and what refund policies they have (if any). Keep in mind that a direct hit by a major hurricane is an extremely rare event and if I had a chance - for example - to go on a cruise in the Caribbean Sea during hurricane season, I would go without hesitation.
What is my chance of having a tropical storm or
hurricane strike by each month?(Figures in process)
The following figures show for any particular location what the chance that a tropical storm or hurricane will affect the area sometime during an individual month. We utilized the years 1944 to 1999 in the analysis. For tropical storm conditions (> 39 mph/18 m/s), we counted tropical storms and hurricanes within about 100 miles (165 km). For hurricane conditions (> 73 mph/33 m/s), we counted hurricanes within about 60 miles (110 km). For major hurricane conditions (> 111 mph/50 m/s), we counted major huricanes within about 30 miles (55 km). (Note that for major hurricanes, the chances everywhere in June, July and November are all less than 1%, so no maps are provided.) These figures are created by Todd Kimberlain.
Many folks are are concerned about a vacation and the possible impacts that a hurricane could have. If so, please check with your hotel, cruise company, etc. to find out how they plan to inform their guests when a hurricane is coming, what actions they plan and what refund policies they have (if any). Keep in mind that a direct hit by a major hurricane is an extremely rare event and if I had a chance - for example - to go on a cruise in the Caribbean Sea during hurricane season, I would go without hesitation.
(Figures in process)
The figure here shows for any particular location what the average number of tropical storms and hurricanes is that affect the area sometime during the whole June to November hurricane season. We utilized the years 1944 to 1997 in the analysis and counted hits when a storm or hurricane was within about 100 miles (165 km). This figure is created by Todd Kimberlain.
(Figure available only available on the Web version.)
The figure here shows for any particular location what the highest number of tropical storms and hurricanes is that affect the area sometime during the whole June to November hurricane season. Blue indicates a peak of just 1 storm, orange is 2 storms, brick red is 3 storms, green is 4 storms and red is 5 storms. We utilized the years 1944 to 1997 in the analysis and counted hits when a storm or hurricane was within about 100 miles (165 km). This figure is created by Todd Kimberlain.
The Dvorak technique is a methodology to get estimates of tropical cyclone intensity from satellite pictures. Vern Dvorak developed the scheme using a pattern recognition decision tree in the early 1970s (Dvorak 1975, 1984). Utilizing the current satellite picture of a tropical cyclone, one matches the image versus a number of possible pattern types: Curved band Pattern, Shear Pattern, Eye Pattern, Central Dense Overcast (CDO) Pattern, Embedded Center Pattern or Central Cold Cover Pattern. If infrared satellite imagery is available for Eye Patterns (generally the pattern seen for hurricanes, severe tropical cyclones and typhoons), then the scheme utilizes the difference between the temperature of the warm eye and the surrounding cold cloud tops. The larger the difference, the more intense the tropical cyclone is estimated to be. From this one gets a data "T-number" and a "Current Intensity (CI) Number". CI numbers have been calibrated against aircraft measurements of tropical cyclones in the Northwest Pacific and Atlantic basins. On average, the CI numbers correspond to the following intensities:
| CI Number |
Maximum Sustained |
Central Pressure (mb) |
|
|
|
One Minute Winds (kt) |
(Atlantic) |
(NW Pacific) |
|
0.0 |
<25 |
---- |
---- |
|
0.5 |
25 |
---- |
---- |
|
1.0 |
25 |
---- |
---- |
|
1.5 |
25 |
---- |
---- |
|
2.0 |
30 |
1009 |
1000 |
|
2.5 |
35 |
1005 |
997 |
|
3.0 |
45 |
1000 |
991 |
|
3.5 |
55 |
994 |
984 |
|
4.0 |
65 |
987 |
976 |
|
4.5 |
77 |
979 |
966 |
|
5.0 |
90 |
970 |
954 |
|
5.5 |
102 |
960 |
941 |
|
6.0 |
115 |
948 |
927 |
|
6.5 |
127 |
935 |
914 |
|
7.0 |
140 |
921 |
898 |
|
7.5 |
155 |
906 |
879 |
|
8.0 |
170 |
890 |
858 |
Note that this estimation of both maximum winds and central pressure assumes that the winds and pressures are always consistent. However, since the winds are really determined by the pressure gradient, small tropical cyclones (like the Atlantic's Andrew in 1992, for example) can have stronger winds for a given central pressure than a larger tropical cyclone with the same central pressure. Thus caution is urged in not blindly forcing tropical cyclones to "fit" the above pressure-wind relationships. (The reason that lower pressures are given to the Northwest Pacific tropical cyclones in comparison to the higher pressures of the Atlantic basin tropical cyclones is because of the difference in the background climatology. The Northwest Pacific basin has a lower background sea level pressure field. Thus to sustain a given pressure gradient and thus the winds, the central pressure must accordingly be smaller in this basin.)
The errors for using the above Dvorak technique in comparison to aircraft measurements taken in the Northwest Pacific average 10 mb with a standard deviation of 9 mb (Martin and Gray 1993). Atlantic tropical cyclone estimates likely have similar errors. Thus an Atlantic hurricane that is given a CI number of 4.5 (winds of 77 kt and pressure of 979 mb) could in reality be anywhere from winds of 60 to 90 kt and pressures of 989 to 969 mb. These would be typical ranges to be expected; errors could be worse. However, in the absence of other observations, the Dvorak technique does at least provide a consistent estimate of what the true intensity is.
While the Dvorak technique was calibrated for the Atlantic and Northwest Pacific basin because of the aircraft reconnaissance data ground truth, the technique has also been quite useful in other basins that have limited observational platforms. However, at some point it would be preferable to re-derive the Dvorak technique to calibrate tropical cyclones with available data in the other basins.
Lastly, while the Dvorak technique is primarily designed to provide estimates of the current intensity of the storm, a 24 h forecast of the intensity can be obtained also by extrapolating the trend of the CI number. Whether this methodology provides skillful forecasts is unknown.
(Contributed by Neal Dorst.)
In the Atlantic basin (Atlantic Ocean, Gulf of Mexico, and Caribbean Sea) hurricane reconnaissance is carried out by two government agencies, the U.S. Air Force Reserves' 53rd Weather Reconnaissance Squadron and NOAA's Aircraft Operations Center. The U.S. Navy stopped flying hurricanes in 1975.
The 53rd WRS is based at Keesler AFB in Mississippi and maintains a fleet of ten WC-130 planes. These cargo airframes have been modified to carry weather instruments to measure wind, pressure, temperature and dew point as well as drop instrumented sondes and make other observations.
AOC is presently based at MacDill AFB in Tampa, Florida and among its fleet of planes has two P-3 Orions, originally made as Navy sub hunters, but modified to include three radars as well as a suite of meteorological instruments and dropsonde capability. Starting in 1996 AOC has added to its fleet a Gulfstream IV jet that will be able to make hurricane observations from much higher altitudes (up to 45,000 feet). It has a suite of instruments similar to those on the P-3s.
The USAF planes are the workhorses of the hurricane hunting effort. They are often deployed to a forward base, such as Antigua, and carry out most of the reconnaissance of developing waves and depressions. Their mission in these situations is to look for signs of a closed circulation and any strengthening or organizing that the storm might be showing. This information is relayed by radio to the National Hurricane Center for the hurricane specialists to evaluate.
The NOAA planes are more highly instrumented and are generally reserved for when developed hurricanes are threatening landfall, especially landfall on U.S. territory. They are also used to conduct scientific research on storms.
The planes carry between six to fifteen people, both the flight crew and the meteorologists. Flight crews consist of a pilot, co-pilot, flight engineer, navigator, and electrical technicians. The weather crew might consist of a flight meteorologist, lead project scientist, cloud physicist, radar specialist, and dropsonde operators.
The primary purpose of reconnaissance is to track the center of circulation, these are the co-ordinates that the National Hurricane Center issues, and to measure the maximum winds. But the crews are also evaluating the storm's size, structure, and development and this information is also relayed to NHC via radio and satellite link. Most of this data, which is critical in determining the hurricane's threat, cannot be obtained from satellite.
The most incredible sight that I've ever seen is in the middle of a strong hurricane. One might not believe this, but most hurricane flights are fairly boring. They last 10 hours, there are clouds above you and clouds below - so all you see is gray, and you don't feel the winds swirling around the hurricane. But what does get interesting is flying through the hurricane's rainbands and the eyewall, which can get a bit turbulent. The eyewall is a donut-like ring of thunderstorms that surround the calm eye. The winds within the eyewall can reach as much as 200 mph [325 km/hr] at the flight level, but you can't feel these aboard the plane. But what makes flying through the eyewall exhilarating and at times somewhat scary, are the turbulent updrafts and downdrafts that one hits. Those flying in the plane definitely feel these wind currents (and sometimes makes us reach for the air-sickness bags). These vertical winds may reach up to 50 mph [80 km/hr] either up or down, but are actually much weaker in general than what one would encounter flying through a continental supercell thunderstorm.
But once the plane gets into the calm eye of a hurricane like Andrew or Gilbert, it is a place of powerful beauty: sunshine streams into the windows of the plane from a perfect circle of blue sky directly above the plane, surrounding the plane on all sides is the blackness of the eyewall's thunderstorms, and directly below the plane peeking through the low clouds one can see the violent ocean with waves sometimes 60 feet high [20 m] crashing into one another. The partial vacuum of the hurricane's eye (where one tenth of the atmosphere is gone) is like nothing else on earth. I would much rather experience a hurricane this way - from the safety of a plane - than being on the ground and having the hurricane's full fury hit without protection.
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Hurricane FAQ is Provided Courtesy of:
***********************************************************
Chris Landsea .
NOAA AOML/Hurricane Research
Division
Voice: (305) 361-4357
4301 Rickenbacker
Causeway
Fax: (305) 361-4402
Miami, Florida
33149
Internet: landsea@aoml.noaa.gov
http://www.aoml.noaa.gov/hrd/Landsea/landsea_bio.html
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