Subject: G1) When is hurricane season ? Contributed by Neal Dorst The Atlantic hurricane season is officially from 1 June to 30 November. There is nothing magical in these dates, and hurricanes have occurred outside of these six months, but these dates were selected to encompass over 97% of tropical activity. When the Weather Bureau organized its new hurricane warning network in 1935 it scheduled a special telegraph line to connect the various centers to run from June 15th through November 15th. Those remained the start and end dates of the 'official' season until 1965, when it was decided to start at the begining of the month of June and run until the end of November. The Atlantic basin shows a very peaked season from August through October, 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 then (Landsea 1993). Maximum 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 Subject G12 - "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. NHC's official dates for this basin are from May 15th to November 30th. The Northwest Pacific basin has tropical cyclones occurring all year round regularly. There is no official definition of typhoon season for this reason. There is a distinct minimum in February and the first half of March, and 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 (>33 m/s winds [76 mph]) 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) Last Revised : January 21, 2010 Subject: G2) How does El Ni–o-Southern Oscillation affect tropical cyclone activity around the globe? Contributed by Chris Landsea El Ni–o/Southern Oscillation (ENSO) - During El Ni–o events (ENSO warm phase), tropospheric vertical shear is increased inhibiting tropical cyclone genesis and intensification, primarily by causing the 200 mb (12 km or 8 mi) westerly winds to be stronger (Gray 1984). La Nina events (ENSO cold phase) enhances activity. Recently, Tang and Neelin (2004) also identified that changes to the moist static stability can also contribute toward hurricane changes due to ENSO, with a drier, more stable environment present during El Nino events. Reference: Tang, B. H., and J. D. Neelin, 2004: ENSO Influence on Atlantic hurricanes via tropospheric warming. Geophys. Res. Lett.: Vol 31, L24204. " The Australian/Southwest Pacific shows a pronounced shift back and forth of tropical cyclone activity with fewer tropical cyclones between 145¡ and 165¡E and more from 165¡E eastward across the South Pacific during El Ni–o (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 Ni–a (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 (140¡W to the dateline) has been suggested to experience more tropical cyclone genesis during the El Ni–o year and more tropical cyclones tracking into the sub-region in the year following an El Ni–o (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 160¡E there were reduced numbers of tropical cyclone genesis with increased formations from 160E to the dateline during El Ni–o events. The opposite occurred during La Ni–a events. Again there is also the tendency for the tropical cyclones to also form closer to the equator during El Ni–o 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. Subject: G3) How might global warming change hurricane intensity, frequency, and rainfall ? Contributed by Chris Landsea In November 2006 the global community of tropical cyclone researchers and forecasters as met at the 6th International Workshop on Tropical Cyclones of the World Meteorological Organization in San Jose, Costa Rica. They released a statement on the links between anthropogenic (human-induced) climate change and tropical cyclones, including hurricanes and typhoons. The following is a summary of their report. There have been a number of recent high-impact tropical cyclone events around the globe. These include 10 landfalling tropical cyclones in Japan in 2004, five tropical cyclones affecting the Cook Islands in a five-week period in 2005, Cyclone Gafilo in Madagascar in 2004, Cyclone Larry in Australia in 2006, Typhoon Saomai in China in 2006, and the extremely active 2004 and 2005 Atlantic tropical cyclone seasons - including the catastrophicsocio-economic impact of Hurricane Katrina. Some recent scientificarticles have reported a large increase in tropical cyclone energy, numbers, and wind-speeds in some regions during the last few decades in association with warmer sea surface temperatures. Other studies report that changes in observational techniques and instrumentation are responsible for these increases. Consensus Statements by International Workshop on Tropical Cyclones-VI (IWTC-VI) Participants :Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point. No individual tropical cyclone can be directly attributed to climate change. The recent increase in societal impact from tropical cyclones has largely been caused by rising concentrations of population and infrastructure in coastal regions. Tropical cyclone wind-speed monitoring has changed dramatically over the last few decades, leading to difficulties in determining accurate trends. There is an observed multi-decadal variability of tropical cyclones in some regions whose causes, whether natural, anthropogenic or a combination, are currently being debated. This variability makes detecting any long-term trends in tropical cyclone activity difficult. It is likely that some increase in tropical cyclone peak wind-speed and rainfall will occur if the climate continues to warm. Model studies and theory project a 3-5% increase in wind-speed per degree Celsius increase of tropical sea surface temperatures. There is an inconsistency between the small changes in wind-speed projected by theory and modeling versus large changes reported by some observational studies. Although recent climate model simulations project a decrease or no change in global tropical cyclone numbers in a warmer climate, there is low confidence in this projection. In addition, it is unknown how tropical cyclone tracks or areas of impact will change in the future. Large regional variations exist in methods used to monitor tropical cyclones. Also, most regions have no measurements by instrumented aircraft. These significant limitations will continue to make detection of trends difficult. If the projected rise in sea level due to global warming occurs, then the vulnerability to tropical cyclone storm surge flooding would increase. A PDF version of the official report is available here. Revised July 2, 2007 Subject: G4) Why do tropical cyclones occur primarily in the summer and autumn? Contributed by Chris Landsea 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 fa vorable during this time of year: warm ocean waters (at least 26¡C or 80¡F), 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). 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. Subject: G5) What determines the movement of tropical cyclones? Contributed by Chris Landsea 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 - often times 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". Subject: G6) Why doesn't the South Atlantic Ocean experience tropical cyclones? Contributed by Chris Landsea What never ?? Well, hardly ever. In March, 2004 a hurricane DID form in the South Atlantic Ocean and made landfall in Brazil. But this still leaves the question of why hurricanes are so rare in the South Atlantic. Though many people might speculate that the sea surface temperatures are too cold, the primary reasons that the South Atlantic Ocean gets few 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. In addition, the US NHC has documented the occurrence of a strong tropical depression/weak tropical storm that formed off the coast of Congo in mid-April 1991 (McAdie and Rappaport (1991)) . This 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. Penn State University write up on the South Atlantic hurricane Last updated July 13, 2005 Subject: G7) Does an active June and July mean the rest of the season will be busy too? Contributed by Stan Goldenberg Yes and No. The vast majority of Atlantic activity takes place during August-September-October, the climatological peak months of the hurricane season. The overall number of named storms (hurricanes) occurring in June and July (JJ) correlates at an insignificant r = +0.13 (+0.02) versus the whole season activity. In fact, there is a slight negative relationship between early season storms (hurricanes) versus late season - August through November - r = -0.28 (-0.35). Thus, the overall 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. However, as shown in (Goldenberg 2000), if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22¡N and east of 77¡W (the eastern portion of the Main Development Region [MDR] for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during JJ have been at least average and often times above average. So it could be said that a JJ storm in this region is pretty much a "sufficient" (though not "necessary") condition for a year to produce at least average activity. (I.e., Not all years with average to above-average total overall activity have had a JJ storm in that region, but almost all years with that type of JJ storm produce average to above-average activity.) The formation of a storm in this region during June-July is taken into account when the August updates for the Bill Gray and NOAA seasonal forecasts are issued. Subject: G8) Why do hurricanes hit the East coast of the U.S., but never the West coast? Contributed by Chris Landsea 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. Recently (Chenoweth and Landsea, 2004), it was re-discovered that a hurricane struck San Diego, California on October 2, 1858. Unprecedented damage was done in the city and was described as the severest gale ever felt to that date nor has it been matched or exceeded in severity since. The hurricane force winds at San Diego are the first and only documented instance of winds of this strength from a tropical cyclone in the recorded history of the state. While climate records are incomplete, 1858 may have been an El Nino year, which would have allowed the hurricane to maintain intensity as it moved north along warmer than usual waters. Today if a Category 1 hurricane made a direct landfall in either San Diego or Los Angeles, damage from such a storm would likely be on the order of a few to several hundred million dollars. The re-discovery of this storm is relevant to climate change issues and the insurance/emergency management communities risk assessment of rare and extreme events in the region. Subject: G9) How much lightning occurs in tropical cyclones? Contributed by Chris Landsea 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). Subject: G10) What is the 20th century hurricane record for each U.S. coastal county? Contributed by Chris Landsea The NOAA Coastal Services Center provides an on-line revision to the original NOAA technical memorandum by Jarrell et al. 1992. One can query for any U.S. coastal county and obtain a graph with hurricane strikes and population changes. This information lets users know how many and how often hurricanes have struck, what Saffir- Simpson Hurricane Scale category they were, and how the populations have changed during the 20th Century. Subject: G11) What is my chance of being struck by a tropical storm or hurricane? Contributed by Chris Landsea 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. For any particular location the chance that a hurricane will directly affect the area sometime during the whole June to November hurricane season is shown here. 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%.) 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 is shown here. 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 the possible impacts that a hurricane could have on their vacation. If so, please check with your hotel, cruise company, etc. to find out how they 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. Subject: G12) What is my chance of having a tropical storm or hurricane strikeby each month? Contributed by Chris Landsea The following figures show that for All Named Storms Hurricanes Major Hurricanes June June None July July None August August August September September September October October October November November None These figures were created by Todd Kimberlain. at any particular location what the chance that a tropical storm or hurricane hwill affect the area sometime during an individual month. 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). Many folks are are concerned about the possible impacts that a hurricane could have on their vacation. If so, please check with your hotel, cruise company, etc. to find out how they 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. Subject: G13) What is the average number of tropical storms and hurricanes to affect my town? Contributed by Chris Landsea The figure here (created by Todd Kimberlain) 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). Subject: G14) What is the peak number of tropical storms and hurricanes to affect my town? Contributed by Chris Landsea The figure here (created by Todd Kimberlain) 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). Subject: G15) I'm vacationing in the Caribbean/the Bahamas/ Central America/Miami or elsewhere in the tropics during hurricane season. What's my chance of getting hit by a hurricane?" Contributed by Chris Landsea Typically, for someone visiting the tropics during June through November, the chance to experience (or even be threatened by) a hurricane is very small. As an example, this figure shows the chances to have a direct hit by a hurricane during the month of September, which is usually the busiest month. If we look at Puerto Rico, the chance is 8% of experiencing a hurricane, if you are there for the WHOLE month. If you are there for, say, only a week, then the chance would be one fourth of that - or only about 2% chance. To put this into perspective, if you made 50 one week trips to Puerto Rico in September, you would only experience a direct hit in ONE of those 50 visits. So the chances to get impacted by a hurricane are quite small for relatively short trips. (And the case chosen here is the WORST possible, as all other locations in all other months have smaller chances of being hit by a hurricane.) If you would like to see chances of hurricane strikes in other months, see Question G12. Despite the chance being small, one should know in advance what your hotel's, cruise company's, etc. policy is for guests when a hurricane is coming, what actions they plan and what refund policies they have (if any). As is described above, a direct hit by a hurricane is a very rare event for a short visit 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. Last updated August 13, 2004 Subject: G16) What is the average forward speed of a hurricane?" Contributed by Neal Dorst (HRD) The forward speed of hurricanes is very latitude dependent. Typically, Atlantic hurricanes track along the western side of the subtropical ridge in the western Atlantic. As they recurve (turn more northerly) from their westward track they usually slow down. If they reach the midlatitudes, they can interact with upper-level troughs and pick up speed. In the table below, the forward speed of hurricanes in the HURDAT database have been averaged in 5 degree latitude bins : Forward speed of Atlantic hurricanes averaged by 5 degree latitude bins Latitude bin Speed No.Cases km/hr knt mph 0o- 5oN 25.9 14.0 16.1 186 5o-10oN 22.0 11.9 13.7 4678 10o-15oN 19.2 10.4 11.9 7620 15o-20oN 17.4 9.4 10.8 7501 20o-25oN 17.5 9.4 10.8 8602 25o-30oN 20.1 10.8 12.5 6469 30o-35oN 27.1 14.6 16.9 3397 35o-40oN 39.0 21.0 24.2 1120 40o-45oN 49.3 26.6 30.6 264 45o-50oN 51.5 27.8 32.0 34 50o-55oN 51.4 27.8 32.0 15 55o-60oN 55.8 30.1 34.7 1 While there are many cases where the forward speed over the 6 hour interval in the hurricane database is zero, such as Mitch in 1998, the highest speed in the database is for unnamed Tropical Storm #6 in 1961. As it got caught up by a midlatitude trough over the midatlantic states, it went speeding off northeastward over Maine and New Brunswick at a maximum speed of 112.25 km/hr (60.57 kt or 69.75 mph). The fastest hurricane in the record was Emily in 1987, whose maximum speed reached 110.48 km/hr (59.61 kt or 68.65 mph) as it raced over the North Atlantic, before it turned extratropical. Last updated May 29, 2014 h h