Preliminary Report
Hurricane Dennis
24 August - 08 September 1999

1999 PRELIMINARY REPORTS

Tropical Storm Arlene (TS)
Hurricane Bret (4)
Hurricane Cindy (4)
Hurricane Dennis (2)
Tropical Storm Emily (TS)
Hurricane Floyd (4)
Hurricane Gert (4)
Tropical Storm Harvey (TS)
Hurricane Irene (2)
Hurricane Jose (2)
Tropical Storm Katrina (TS)
Hurricane Lenny (4)

Dennis was a larger-than-average western Atlantic hurricane that was erratic in both track and intensity. Although it never made landfall as a hurricane, it affected the North Carolina coast with hurricane force winds, heavy rains, prolonged high surf, and beach erosion. Dennis also produced tropical storm force winds over portions of the Bahamas.

 

a. Synoptic History

The origin of Dennis can be traced to a tropical wave that moved off the coast of Africa on 17 August. The system moved westward with little significant weather until 21 August, when associated shower activity increased a few hundred miles northeast of the Leeward Islands. A low- level circulation developed over the next two days as convective organization increased. An investigative flight by the Air Force Reserve Hurricane Hunters failed to find a surface circulation on the 23rd. However, the aircraft data indicated a circulation was present at 850 mb. Later surface observations showed a closed circulation, and it is estimated that Tropical Depression Five formed at 0000 UTC 24 August about 190 nautical miles east of Turks Island. Reconnaissance data and ship reports indicated further intensification, and the depression became Tropical Storm Dennis at 1200 UTC the same day.

The initial structure was unusual. Dennis was at the east-southeast end of an elongated trough that extended to southern Florida. This and upper-level westerly shear caused an asymmetric pattern of convection and tropical storm force winds, with both confined to the eastern semicircle on 24-25 August. Despite the shear, the cyclone intensified unsteadily and reached hurricane strength early on the 26th.

The unusual structure may have also affected the cyclone's motion. Dennis initially moved at 10 to 14 mph, but slowed to an erratic 3 mph on 25 August as steering currents weakened due to a mid-latitude trough passing to the north. At one time that day, the center appeared to re-form eastward along the trough axis. Once Dennis reached hurricane strength, it began a more steady northwestward motion near or over the eastern Bahamas. This motion continued into the 28th.

Westerly shear persisted, preventing significant strengthening until late on 27 August. After the shear decreased, Dennis reached a peak intensity of 104 mph on the 28th and maintained that intensity until early on the 30th. Even at peak intensity, Dennis never consolidated into a classic tightly-wound hurricane. The eye was 30 to 40 miles wide, and on several center fixes the Hurricane Hunters did not report an eye. The radius of maximum winds was as large as 70 to 85 nm on the 29th and 30th.

A second mid-latitude trough caused Dennis to turn gradually northward on 28-29 August, which was followed by acceleration to the east-northeast on 30th and 31th. This turn kept the center about 60 miles south of the North Carolina coast. The east-northeast motion continued until the trough passed Dennis on the 31st. At that time, steering current collapsed and the cyclone slowed to an erratic drift about 110 nautical miles east of Cape Hatteras, North Carolina. The erratic motion would last into 2 September.

During this time, Dennis became involved with the cold front associated with the mid-latitude trough. A combination of vertical shear and cool dry air entraining into the circulation decreased the convection and weakened the cyclone. Dennis weakened to a tropical storm on 1 September, and on the 1st and 2nd may have been as much a subtropical or extratropical cyclone as a tropical cyclone. Despite the lack of convection, surface observations indicate maximum sustained winds were near 52 mph during 2 September. Some of these winds were due to the combination of Dennis and a strong surface ridge north of the front, which caused 39 mph or greater winds as far north as the New Jersey coast.

A large westerly ridge over the eastern United States forced Dennis southward late on 2 September. This motion toward warmer water probably aided a deep convective burst on the next day. Later that day, Dennis turned northwest toward the North Carolina coast as the ridge moved east into the Atlantic. This motion continued on the 4th along with re-intensification. Dennis was just below hurricane strength when it made landfall over the Cape Lookout National Seashore just east of Harkers Island, North Carolina at 2100 UTC that day. Dennis continued inland and weakened to a depression on the 5th over central North Carolina. Even in dissipation, Dennis continued to move erratically. Figure 1 shows that the cyclone followed a zig-zag course northward for the rest of its life. Dennis became extratropical on the 7th and was absorbed into a larger low on the 9th.

b. Meteorological Statistics

1. Storm Surge Data

Few detailed observations of storm surge are available from areas affected by Dennis (Table 2). Storm tides of 3 to 5 ft above normal were reported along much of the North Carolina coast on both 30 August and 4 September. Areas along the Neuse River reported tides of 8 to 10 ft above normal tide level on 30 August, while areas along the Pamlico River reported similar values on 4 September. Portions of the South Carolina and southeastern Virginia coast experienced 2 to 4 ft above normal tides during Dennis, while amateur radio reports from the Bahamas indicate tides 1 to 3 ft above normal as the eye passed over the Abacos.

Since Dennis meandered off the North Carolina coast for several days, the above normal tides were unusually prolonged. This led to extensive beach erosion along portions of the North Carolina and southeastern Virginia coasts.

2. Rainfall Data

Dennis affected the mid-Atlantic states twice within a week, and other weather systems affected the region during the same period. This makes determination of storm total rainfall in that area difficult. Table 2 shows the storm total rainfalls for Dennis, including the best estimates in North Carolina and Virginia. The maximum reported total was 19.13 inches at Ocracoke, North Carolina, with 6 to 10 inches reported elsewhere over portions of eastern North Carolina. Rainfalls of 3 to 6 inches occurred elsewhere over eastern North Carolina, extreme eastern South Carolina, and over portions of southeastern Virginia. Rainfalls were generally 1 to 3 inches elsewhere over eastern South Carolina and less than an inch in Florida and Georgia.

Dennis and the other weather systems contributed to a wet period over portions of the mid-Atlantic states. Table 2b shows 11-day rainfall totals of 6 inches or more ending at 1200 UTC 8 September. The heaviest rainfalls were observed over eastern North Carolina and central Virginia. While this rainfall broke a prolonged dry spell in the area, it also set the stage for the severe flooding caused by Hurricane Floyd two weeks later.

Official rainfall data from the Bahamas indicates a maximum total of 4.00 inches at Cat Island. Heavier amounts likely occurred on Eleuthera and in the Abaco group near the eye of Dennis.

3. Tornadoes

One tornado was reported with Dennis on 4 September. This F2 tornado in Hampton Virginia caused an estimated $7 Million damage and 15 injuries, 6 of them serious.

c. Casualty and Damage Statistics

There were 4 deaths attributed to Dennis; 4 in Florida.

The deaths reported in Florida were related to high surf spawned by the hurricane. No deaths are known due to winds, rains, storm tides or tornadoes associated with Dennis.

In the United States, the Property Claims Services Division of the Insurance Services Office reports insured losses due to Dennis totaled $60 Million in North Carolina and Virginia. To determine the total property damage, a two to one ratio is applied to the insured property damage based on comparisons done in historical hurricanes. Press reports indicate that agricultural losses in North Carolina and Virginia were $37 Million. Combining these reports gives a total estimated damage from Dennis of $157 Million.

There are no damage reports available from the Bahamas as of this time.