Helene made landfall as a minimal tropical storm near Fort Walton Beach, Florida and redeveloped into a strong tropical storm over the North Atlantic.
a. Synoptic History
Helene developed from a tropical wave that emerged from the African coast on 10 September. The wave lost all of its deep convection the next day as it continued moving westward. There was little sign of redevelopment until 14 September when convection formed near the center of the system. Showers and thunderstorms continued overnight and Dvorak satellite estimates indicate that Tropical Depression Twelve formed on the afternoon of the 15th. The depression appeared to weaken before aircraft reconnaissance first flew into the system the next day. The aircraft could not find a closed circulation, indicating that the depression had degenerated into a tropical wave. It is notable that, even without any center, the plane reported winds in excess of 55 knots at 1500 ft to the north and east of the wave.
The remnants of the depression moved over the Leeward Islands on the 17th, producing heavy rains and gusty winds to tropical storm force in squalls. Upper-level conditions seemed very favorable for regeneration, but the system was slow to redevelop. It was not until late on the 19th that a reconnaissance aircraft found a closed circulation northwest of Grand Cayman Island, while the system was moving more to the west-northwest. The depression was very weak with only a few thunderstorms near the center when it crossed the western tip of Cuba the next day. However, convection redeveloped near the center and it became Tropical Storm Helene early on the morning of the 21st in the southeast Gulf of Mexico. The storm moved northwestward, strengthening under marginally favorable conditions. However, the vertical wind shear increased, preventing Helene from attaining hurricane status. The system became very asymmetric as a result of the shear, with most of its deep convection, winds, and heavy rainfall displaced to the east of the center. It peaked at a maximum intensity of 60 knots about twelve hours before landfall. The shear increased further and weakened Helene to an intensity of 35 knots during landfall near Fort Walton Beach, Florida around 7 am CDT on the 22nd. Helene then moved toward the northeast over the southeastern states as a tropical depression.
Even with strong westerly shear, deep convection began to intensify over the coastal waters of North Carolina when the system approached the east coast. Tropical storm force winds were measured at stations off the coast of North Carolina. A post-analysis of buoy data and satellite imagery indicates that Helene had developed enough tropical characteristics to be considered a tropical cyclone as it emerged from the coast of Virginia. The cyclone began to race northeastward away from the United States toward decreasing shear. Ship observations and satellite images indicate the system was very compact over the Atlantic, no more than 120 n mi wide with the strongest winds in the south and east quadrants. An intense burst of convection formed over the center on the 24th, and it is estimated that Helene reached a second peak intensity of 60 knots early on the 25th before merging with a cold front later that day. The best track is listed in Table 1 and is plotted in Figure 1.
b. Meteorological statistics
Figure 2 and Figure 3 shows the best track curves for maximum sustained 1-min surface winds and minimum central pressure data, respectively, as functions of time. These plots include aircraft reconnaissance, Dvorak satellite classification estimates, and surface observations. Table 2 includes selected surface observations along the path of Helene. The intensity or redevelopment of Helene as it emerged off the Mid-Atlantic coast would likely never have been known if not for the hourly reports of the ship Neptune Olivine. The ship reported sustained winds of 56 knots at 0600 UTC on the 25th as the storm moved nearby. An intensity of 60 knots has been estimated from this ship report. The ship also recorded a lowest pressure of 988.2 mb with a westerly wind of 46 knots. However, it is likely that the ship did not report the minimum pressure as the winds indicate that the ship was displaced to the south of the center and a final estimate of 986 mb has been made. It is notable that Neptune Olivine had reports that were similar to another ship to its southeast, the Global Mariner, that reported sustained winds of 52 knots at the same time that the Neptune Olivine reported 56 knots. Figure 4 displays the hourly wind and pressure data reported by the Neptune Olivine.
c. Casualties and damages
The one casualty associated with Helene was a man killed in a F2 tornado in South Carolina as the tropical depression moved through the region on the 23rd. The storm caused extensive flooding in Tallahassee, Florida where it dumped near nine inches of rain.
d. Forecast and warning critique
Despite the apparently favorable conditions for tropical cyclone development: low shear, enough convection, low pressure, warm ocean, and high ocean heat content, the depression did not strengthen until after it entered the Gulf of Mexico. The expected strengthening from the official forecast as well as the intensity guidance prompted tropical storm watches and warnings for some of the eastern Caribbean Sea islands and then for western Cuba. Once Helene formed in the Gulf of Mexico, watches and warnings were issued for a portion of the northeast Gulf coast. Table 3 includes a watch and warning summary. The final official forecast issued on Helene indicated the possibility of restrengthening over water as an extratropical system. However, the post-analysis indicates that system retained tropical characteristics.
Operationally, Helene was a tropical storm for 36 hours only. Therefore, no forecast verification statistics are presented. It is worth noting that, in general, track models correctly captured the westward motion of the system through the Caribbean and the gradual northward turn over the Gulf of Mexico.
Figure 1. Best track positions for Tropical Storm Helene, 15-25 September, 2000.
Figure 2. Best track maximum sustained wind speed for Tropical Storm Helene, 15-25 September, 2000.
Figure 3. Best track minimum central pressure for Tropical Storm Helene, 15-25 September, 2000.
Figure 4. Hourly wind and pressure reports from vessel Neptune Olivine from 00 to 10 UTC 25 September 2000.
Table 1. Best track for Tropical Storm Helene, 15-25 September, 2000. Date/Time
(UTC)Position Pressure
(mb)Wind Speed
(kt)Stage Lat. (°N) Lon. (°W) 15 / 1200 14.9 52.2 1010 25 tropical depression 15 / 1800 15.3 53.0 1010 25 " 16 / 0000 15.6 53.6 1010 25 " 16 / 0600 15.8 54.4 1010 25 " 16 / 1200 16.1 55.9 1010 30 " 16 / 1800 16.4 58.0 1010 30 tropical wave* 17 / 0000 16.6 59.9 1010 30 " 17 / 0600 16.6 61.7 1010 30 " 17 / 1200 16.4 63.6 1010 30 " 17 / 1800 16.7 65.6 1010 30 " 18 / 0000 17.0 67.1 1010 30 " 18 / 0600 17.1 68.7 1010 30 " 18 / 1200 17.2 70.6 1010 30 " 18 / 1800 17.4 72.5 1010 30 " 19 / 0000 17.6 74.4 1010 30 " 19 / 0600 18.3 76.3 1010 30 " 19 / 1200 18.9 78.3 1010 30 " 19 / 1800 19.4 79.6 1010 30 tropical depression 20 / 0000 19.9 81.0 1010 30 " 20 / 0600 20.7 82.6 1010 25 " 20 / 1200 21.8 84.3 1010 25 " 20 / 1800 23.0 85.4 1010 25 " 21 / 0000 23.9 86.1 1008 25 " 21 / 0600 24.9 86.6 1007 35 tropical storm 21 / 1200 26.1 87.0 1006 45 " 21 / 1800 27.1 87.1 999 60 " 22 / 0000 28.4 87.2 996 60 " 22 / 0600 29.5 87.2 1001 50 " 22 / 1200 30.5 86.6 1006 35 " 22 / 1800 31.6 85.4 1010 25 tropical depression 23 / 0000 32.9 83.5 1011 25 " 23 / 0600 33.6 81.7 1012 25 " 23 / 1200 34.4 80.0 1011 25 " 23 / 1800 35.4 78.0 1010 35 tropical storm 24 / 0000 36.4 76.1 1008 40 " 24 / 0600 37.2 74.7 1005 45 " 24 / 1200 38.0 72.5 1001 45 " 24 / 1800 39.2 70.1 997 45 " 25 / 0000 40.1 66.8 993 55 " 25 / 0600 41.6 62.2 986 60 " 25 / 1200 44.0 55.5 988 55 " 25 / 1800 46.1 48.8 990 45 " 26/ 0000 absorbed by a front 25 / 0600 41.6 62.2 986 60 minimum pressure 22 / 1200 30.5 86.6 1006 35 Landfall near Fort Walton Beach, FL *estimated location of the cloud system center tracked by satellite
Table 2. Tropical Storm Helene, selected surface observations, September, 2000. Minimum
sea-level
pressureMaximum surface wind speed
(kt)Location Press.
(mb)Date/
time
(UTC)Sust.
winda
(kts)Peak
gust (kts)Date/
timeb
(UTC)Storm
surgec
(ft)Storm
tided
(ft)Rain
(storm total)
(in)Lesser Antilles Guadeloupe (TFFR) 48 17/Unk. Antigua 3.14 Gulf of Mexico Buoys 42003 32 39 21/1049 42039 31 41 22/0043 Florida Perdido Key 20 34 22/0730 Panama City Beach 23 39 22/0900 1 Pensacola Beach 23 38 22/0930 1 Destin Airport (DTS) 24 35 22/1118 1 Cape San Blas CMAN 26 46 22/1211 Apalachicola (KAAF) 9.56 Sopchoppy 9.50 Tallahassee (KTLH) 7.86 North Carolina Frying Pan Shoals CMAN 40 47 23/1825 Cape Lookout CMAN 36 45 23/1902 Duck Pier CMAN 38 45 23/2215 Diamond Shoals CMAN 51 61 23/2243 aASOS and CMAN's are 2-minute averages, buoys are 10 minute averages, all others are 1-minute averages.
bDate/time is for wind gust when both sustained and gust are listed.
cStorm surge is water height above normal astronomical tide level.
dStorm tide is water height above National Geodetic Vertical Datum (1929 mean sea level).
Table 3. Watch and warning summary, Tropical Storm Helene, September, 2000. Date/Time
(UTC)Action Location 16/0300 Tropical storm watch issued St. Maarten, Saba, St. Eustatius 16/0900 Tropical storm watch issued Antigua, Anguilla, Barbuda, Montserrat, Nevis, St Kitts 16/1700 Tropical storm watch discontinued St. Maarten, Saba, St. Eustatius, Antigua, Anguilla, Barbuda, Montserrat, Nevis, St Kitts 20/0300 Tropical storm warning issued Cuban provinces of Isle of Youth, Havana, Pinar Del Rio, and the city of Havana. 20/1500 Tropical storm warning discontinued Cuban provinces of Isle of Youth, Havana, Pinar Del Rio, and the city of Havana. 21/1500 Tropical storm warning issued The mouth of the Pearl River on the Louisiana-Mississippi border eastward to the mouth of the Aucilla River, Florida 21/2100 Hurricane watch issued The Florida-Alabama border eastward to the mouth of the Aucilla River, Florida 22/0300 Tropical storm warning discontinued The mouth of the Pearl River on the Louisiana-Mississippi border eastward to west of Pascagoula, Mississippi. 22/0900 Hurricane watch discontinued The Florida-Alabama border eastward to the mouth of the Aucilla River, Florida. 22/1500 Tropical storm warning discontinued Pascagoula, Mississippi eastward to just west of Destin, Florida 22/1800 Tropical storm warning discontinued Destin, Florida eastward to the mouth of the Aucilla River, Florida
