Debby struck the islands of the northeast Caribbean as a 65-knot hurricane, but the impact was not very significant. It appeared that Debby was going to be a significant threat to south Florida, but the system dissipated rather unexpectedly.
a. Synoptic History
A strong tropical wave moved off the west coast of Africa on 16 August, accompanied by winds to near 50 kt at the 650 mb level over Dakar. The following day, a broad area of low pressure associated with this wave was noted in the vicinity of 10N 30W, but there was insufficient curvature in the associated bands of deep convection for a Dvorak classification of the system. A low-level circulation center was identified by the Tropical Analysis and Forecast Branch (TAFB) at 0000 UTC 18 August, but the system was still considered "too weak to classify". An initial classification was done by TAFB at 1145 UTC on the 18th. By that time the curvature of the convective bands had increased, however the system was still broad and poorly organized. As the disturbance continued westward around 15 kt, it gradually became better organized. By 1800 UTC 19 August the cloud pattern had become consolidated around a well-defined center and it is estimated that the fourth tropical depression of the season developed, about 900 n mi east of the Windward Islands (Table 1, Figure 1).
Vertical shear was weak over the area, with anticyclonic outflow developing aloft. In this environment, the cyclone strengthened and became Tropical Storm Debby around 0600 UTC 20 August. A pronounced mid-level ridge to the north of the cyclone maintained a west-northwestward motion. By midday on the 20th, microwave imagery data began to suggest some southwesterly shearing over Debby, with the low-level center displaced a bit to the southwest of the mid- to upper-level center. Nonetheless the storm strengthened further and is estimated to have become a hurricane by 0600 UTC 21 August. By this time the infrared imagery also showed the signature of a sheared system. Even though Debby was not well organized it strengthened even further, and reconnaissance data indicated that its maximum winds increased to 75 knots, which was the peak intensity, a little later on the 21st.
Radiosonde data from the Lesser Antilles indicated that the southwesterly shearing may initially have been due to strong lower-tropospheric easterly flow, with the low-level center "outrunning" the mid- to upper-level center. By late on 21 August, the strengthening ceased. Debby was a 65-knot hurricane when its center moved across the extreme northern Leeward Islands from 0600 to 1200 UTC on the 22nd. Continuing west-northwestward, Debby's center moved over the British Virgin Islands around 1500 UTC on the 22nd, and passed about 30 n mi off the northeast coast of Puerto Rico a few hours later. By early on the 23rd, southwesterly shearing over Debby increased (dropsonde data from a two-aircraft synoptic flow mission revealed that upper-tropospheric flow was now largely responsible). Although not well organized, the system maintained minimal hurricane strength until around 1200 UTC on the 23rd when the cloud pattern became even more disorganized-looking.
The weakening storm turned toward the west and moved along the northern coast of Hispaniola. Although the mountainous land mass of that island may have played some role in the weakening by restricting inflow from the south and disrupting the southern part of the cyclone's circulation, it appears that vertical shear was the main cause for weakening. Around midday on the 23rd, a distinct low-cloud circulation center was evident just to the north of Hispaniola, displaced well to the west of the main area of deep convection. The cyclone continued westward, entering the Windward Passage around 0000 UTC 24 August. It dissipated near the south coast of eastern Cuba on the morning of the 24th. Debby's remnant, a strong tropical wave, continued to track westward, spreading locally heavy showers and gusty winds over Cuba, the Straits of Florida, and southern Florida over the next couple of days.
b. Meteorological Statistics
Table 1 lists the best track positions and intensities of Debby at six-hourly intervals. Figure 1 is a display of this track. Figure 2 and Figure 3 depict the curves of maximum one-minute average "surface" (10 m above ground level) wind speed and minimum central sea-level pressure, respectively, as a function of time. Also plotted are the observations on which the curves are based. These consist of measurements by reconnaissance aircraft, Global Positioning System (GPS) dropsondes, Dvorak-technique estimates using satellite imagery from the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB), and the U.S. Air Force Weather Agency (AFWA).
Debby's peak intensity of 75 kt on 21 August is based on 85% of the maximum 1500-ft flight level wind speed, 88 kt, measured by aerial reconnaissance. Interestingly, the minimum central pressure was about 1004 mb at that time, well above the values typically associated with even minimal hurricane intensity. A central pressure of 991 mb was measured about 12 hours after the occurrence of Debby's estimated peak winds.
Wind gusts to 48 kt were reported at Antigua late on 21 August during the passage of a band of convection in advance of Debby. Maximum sustained winds there were only 25 kt. At St. Barthelemy, a gust to 66 kt was reported at 0830 UTC on the 22nd (that station was in the eye at 0915 UTC), and at St. Maarten, sustained winds of 33 kt with gusts to 52 kt were reported at 0900 UTC, with a minimum pressure of 998 mb at 1000 UTC. There were no measurements of sustained tropical storm force winds from the U.S. Virgin Islands or Puerto Rico, however, wind gusts of 39 and 37 kt were reported from St. Thomas and St. Croix, respectively, on 22 August.
Rainfall totals were as high as 12.63 in at Rio Piedras and 10.28 in at Rio de la Plata, Puerto Rico. These heavy rains occurred mainly after the center of Debby had moved northwest of the island.
c. Casualty and Damage Statistics
A man died in Puerto Rico when he fell off the roof of his home while trying to remove a satellite antenna. This was an indirect fatality.
There was moderate damage to the roofs of a few structures in Barbuda. There, and in several of the other islands of the extreme northern Leewards, some damage to fruit trees, utility poles and lines took place. Overall, the damage did not appear to be severe, but there are no monetary estimates available.
In Puerto Rico, the main impact came from heavy rainfall. There were reports of mud slides and damaged or collapsed bridges. Over 400 homes were reportedly "affected" by flood waters. Five homes suffered moderate to severe structural damage. Total damage estimate is 0.5 million dollars.
There was also some damage due to storm surge and wave action along the northern coast of the Dominican Republic.
Debby's rains were beneficial over eastern Cuba, since that region had been suffering from a severe drought.
d. Forecast and Warning Critique
Table 2 lists the average track prediction errors for Debby, for a selection of the numerical models and for the official forecast, as well as the long-term averages of the latter. The average official track forecast errors were lower than the most recent ten-year averages. The average official forecast errors for Debby are generally of comparable magnitude to, or below, the corresponding average model errors. It should be added, however, that several of the official and model forecasts having a northward bias were not verified because of Debby's unpredicted dissipation.
The numerical track predictions for 0000 UTC 23 August are of particular interest, since GPS dropsonde data from a two-aircraft synoptic flow surveillance mission appeared to have a significant impact on some of the models. In comparison to earlier forecasts, there was a significant southward shift in the model tracks. This was particularly noticeable in the U.K. Met. Office global model (UKM), whose 72-h forecast shifted from near or just east of Florida, to near western Cuba. Dropsonde measurements of the steering flow around the above time showed that there was a strong mid-level anticyclone to the north of Debby, and that a trough was bypassing the hurricane. It should be added that later sensitivity runs of the UKM from the above initial time, with and without the dropsonde input, confirmed that the track forecast improvements were attributable to these data.
The Debby case reminds us, all too well, of our limitations in forecasting tropical cyclone intensity change. In spite of the sheared nature of the system, the GFDL model repeatedly predicted that Debby would reach southern Florida area with a central pressure corresponding to a major hurricane. Moreover, the best available intensity guidance, SHIPS, consistently forecasted Debby to intensify (even when it was apparent that the tropical cyclone was dissipating). Given the objective guidance, and the possibility that the vertical shear could diminish, the official forecasts mostly over-predicted the wind speeds. The average bias in the official intensity forecasts ranged from +5.8 knots at 24 h to +28.0 knots at 72 h. These are similar to the biases shown by SHIPS.
Table 3 summarizes the various watches and warnings issued in connection with Debby. The hurricane warning for the northern Leeward Islands was issued 15 to 18 h prior to the arrival of the center in that area.
Watches and warnings are not always necessary to trigger preparedness actions. Based on the official 3-day forecasts which brought Debby near south Florida, an evacuation of non-residents from the Florida Keys was ordered, resulting in a significant loss of tourism revenue for Monroe County.
Table 1. Best track, Hurricane Debby, 19-24 August 2000. Date/Time
Stage Lat. (°N) Lon. (°W) 19 / 1800 12.0 44.5 1010 30 tropical depression 20 / 0000 12.6 45.3 1010 30 " 20 / 0600 13.3 46.8 1009 35 tropical storm 20 / 1200 14.0 48.8 1008 40 " 20 / 1800 14.7 50.6 1007 45 " 21 / 0000 15.1 52.1 1006 55 " 21 / 0600 15.4 54.0 1005 65 hurricane 21 / 1200 15.7 56.3 1004 75 " 21 / 1800 16.1 58.5 1004 75 " 22 / 0000 16.8 60.1 995 70 " 22 / 0600 17.5 61.7 993 65 " 22 / 1200 18.1 63.5 994 65 " 22 / 1800 18.8 65.4 995 65 " 23 / 0000 19.2 66.7 995 65 " 23 / 0600 19.5 68.1 995 65 " 23 / 1200 19.8 69.7 1005 60 tropical storm 23 / 1800 20.0 71.5 1009 50 " 24 / 0000 19.9 73.3 1010 40 " 24 / 0600 19.6 75.1 1011 35 " 24 / 1200 19.5 77.0 1011 30 tropical depression 24 / 1800 dissipated 21 / 1200 15.7 56.3 1004 75 maximum intensity 22 / 0300 17.1 60.9 991 70 minimum pressure Landfalls 22 / 0600 17.5 61.7 993 65 landfall near Barbuda 22 / 0915 17.9 62.8 993 65 landfall near St. Barthelemy 22 / 1500 18.5 64.4 994 65 landfall near Virgin Gorda
Table 2. Preliminary forecast evaluation of Hurricane Debby, heterogeneous sample. (Errors in nautical miles for tropical storm and hurricane stages with number of forecasts in parenthesis). Forecast Technique Period (hours) 12 24 36 48 72 AVNI 54 (15) 82 (13) 106 (11) 85 (9) 37 (5) CLIP 48 (15) 91 (13) 138 (11) 172 (9) 378 (5) GFDI 45 (15) 74 (13) 100 (11) 106 (9) 145 (5) NGPI 43 (15) 80 (13) 112 (11) 138 (9) 149 (5) UKMI 37 (14) 75 (12) 96 (10) 80 (8) 87 (5) NHC OFFICIAL 43 (15) 75 (13) 90 (11) 85 (9) 132 (5) NHC OFFICIAL 1990-1999 10-year average 46 (2057) 85 (1842) 122 (1650) 158 (1471) 235 (1164)
Table 3. Watch and warning summary, Hurricane Debby, August 2000. Date/Time
Action Location 20/2100 Hurricane watch issued St. Maarten, Saba, and St. Eustatius 20/2100 Tropical storm watch issued Antigua, Barbuda, and Anguilla 21/0300 Hurricane watch issued U.S. Virgin Islands 21/0300 Tropical storm watch issued British Virgin Islands, St. Martin, St. Barthelemy, Nevis, St. Kitts, Montserrat, and Guadeloupe and its surrounding islands 21/0900 Hurricane watch issued Puerto Rico 21/1500 Hurricane warning issued Guadeloupe northward and northwestward through the British and U.S. Virgin Islands 21/1500 Tropical storm warning and hurricane watch issued Dominica 21/2100 Hurricane warning issued Puerto Rico and its adjacent islands 21/2300 Tropical storm watch issued Dominican Republic 22/0300 Hurricane watch issued Haiti north of Port au Prince 22/0300 Tropical storm warning issued Dominican Republic from Punta Palenque to Cabrera 22/0400 Tropical storm watch issued Southeastern Bahamas and the Turks and Caicos Islands 22/0900 Hurricane warning issued Northern coast of the Dominican Republic 22/0900 Tropical storm watch changed to hurricane watch Southeastern Bahamas and the Turks and Caicos Islands 22/1200 Hurricane warning discontinued Guadeloupe, Antigua, Barbuda, Nevis, St. Kitts, and Montserrat 22/1200 Tropical storm warning discontinued Dominica 22/1500 Hurricane warning issued Southeastern Bahamas and the Turks and Caicos Islands 22/1500 Hurricane watch issued Central Bahamas 22/1500 Hurricane warning discontinued St. Maarten, Saba, and St. Eustatius 22/2100 Hurricane watch issued Northern coast of Cuba for the provinces of Holguin and Las Tunas 22/2100 Tropical storm warning issued Haiti north of Port au Prince 22/2100 Hurricane warning changed to tropical storm warning U.S. Virgin Islands and Puerto Rico 22/2100 Hurricane warning discontinued All islands east of the U.S. Virgin Islands 23/0300 Tropical storm warning discontinued U.S. Virgin Islands and Puerto Rico 23/0600 Hurricane warning issued Central Bahamas 23/0600 Hurricane watch issued Northwestern Bahamas 23/1500 Hurricane warning issued Northern coast of Cuba for the provinces of Guantanamo, Holguin, and Las Tunas 23/1500 Hurricane watch issued Northern coast of Cuba for the provinces of Camaguay and Ciego de Avila and southern coast of Cuba for the provinces of Santiago de Cuba and Granma 23/1800 Hurricane watch discontinued Haiti north of Port au Prince 23/2100 Hurricane warning changed to tropical storm warning Central and Southeastern Bahamas and the Turks and Caicos Islands, northern coast of Cuba for the provinces of Guantanamo, Holguin, and Las Tunas 23/2100 Hurricane watch changed to tropical storm watch Northern coast of Cuba for the provinces of Camaguay and Ciego de Avila and southern coast of Cuba for the provinces of Santiago de Cuba and Granma 23/2100 Hurricane warning discontinued Northern coast of the Dominican Republic 23/2100 Hurricane watch discontinued Northwestern Bahamas 24/0300 Tropical storm warning discontinued Haiti, Turks and Caicos Islands 24/1000 Tropical storm warning discontinued Central and southeastern Bahamas 24/1500 All tropical storm watches and warnings discontinued Cuba
Figure 1. Best track for Hurricane Debby, 19-24 August 2000.
Figure 2. Best track maximum sustained surface wind speed curve for Hurricane Debby, 19-24 August 2000, and the observations on which the best track curve is based. Aircraft observations have been adjusted for elevation using 90%, 80%, and 85% reduction factors for observations from 700 mb, 850 mb, and 1500 ft, respectively. Dropsonde observations include actual 10 m winds (sfc), as well as surface estimates derived from the mean wind over the lowest 150 m of the wind sounding (LLM ) , and from the sounding boundary layer mean (MBL).
Figure 3. Best track minimum central pressure curve and central pressure observations or estimates for Hurricane Debby, 19-24 August 2000.