Tropical Cyclone Report

Tropical Storm Hanna

Hanna was a poorly-organized tropical storm that nevertheless produced rip currents responsible for three deaths off the beaches of the Florida panhandle.

a. Synoptic History

Hanna formed in the Gulf of Mexico from a complex interaction of a tropical wave, an upper-level low, and a surface trough. In the days preceding genesis, a broad surface trough in the wake of Hurricane Gustav stretched from the western Atlantic across South Florida and into the central Gulf of Mexico. During this time a westward-moving tropical wave approached the Yucatan peninsula, and when the wave reached the Gulf of Mexico on 10 September a weak 1008-mb low formed on the western end of the surface trough. Initially, there was minimal convection associated with the combination of these two features; however, on 11 September an upper-level short-wave trough over the southern United States cut off over the central Gulf of Mexico, and convection began to develop to the east of the both the upper level low and the tropical wave/surface low. The convection became sufficiently organized to warrant a Dvorak classification at 1800 UTC that day, and over the next 6 hours convection developed closer to the surface low. Shortly before 0000 UTC 12 September a reconnaissance aircraft was able to locate a well-defined low-level circulation center, and with that the ninth depression of the season had formed about 250 n mi south of Pensacola Florida.

The "best track" chart of the tropical cyclone's path is given in Figure 1, with the wind and pressure histories shown in Figure 2 and Figure 3, respectively. The best track positions and intensities are listed in Table 1. The cyclone initially had some non-tropical characteristics, including a westward tilt with height in association with the upper-low. Despite strong southwesterly shear and a disorganized convective structure, the depression became a tropical storm at 0600 UTC 12 September, about 225 n mi south of Pensacola. For the first 24 hours after genesis, the low-level circulation center rotated counter-clockwise around the middle and upper-level centers, first moving northeastward but turning to the southwest by late on 12 September. Moving slowly the following day, Hanna turned to the west and then to the north ahead of an approaching mid-level trough. Hanna strengthened and reached its peak intensity of 50 kt and 1001 mb at 0000 UTC 14 September about 60 n mi south of the mouth of the Mississippi River. In response to the approaching trough, Hanna accelerated northward early on 14 September and its exposed low-level circulation center began to become deformed and elongated. With nearly all the significant weather well to its east, Hanna's center of circulation passed over the extreme southeastern tip of Louisiana near 0800 UTC. Hanna then turned to the north-northeast and made its second landfall near the Alabama-Mississippi border near 1500 UTC. Maximum winds at both landfalls were near 50 kt. Hanna moved northeastward across southern Alabama and weakened rapidly, dissipating by 1800 UTC 15 September. The remnants of the tropical cyclone then produced heavy rains as they moved rapidly across Georgia and the Carolinas.

b. Meteorological Statistics

Observations in Hanna (Figure 2 and Figure 3) include satellite-based Dvorak technique intensity estimates from the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB) and the U. S. Air Force Weather Agency (AFWA), as well as flight-level and dropwindsonde observations from flights of the 53^rd Weather Reconnaissance Squadron of the U. S. Air Force Reserve Command. Selected surface observations from land stations and data buoys are given in Table 2.

At 0600 UTC 12 September, buoy 42003 (at 25.9 N, 86.0 W, about 50 n mi south-southeast of the center) reported an 8-min mean wind of 32 kt. This observation is the basis for the assignment of tropical storm status at this time. Hanna's peak winds are estimated to be 50 kt, based on surface-adjusted flight-level winds of 46 kt at 2346 UTC 13 September and 47 kt at 1303 UTC 14 September, as well as a 47 kt 2-min wind from the Pensacola ASOS station at 1352 UTC 14 September.

The only ship to report tropical storm force winds was KRPP (Nobel Star), which reported winds of 37 kt and a pressure of 1006.0 mb at 0300 UTC 13 September, when it was about 45 n mi west-southwest of the center. Other observations of note include the aforementioned 47 kt report from Pensacola, the strongest sustained wind observation from a surface station. The highest gust reported was 59 kt at Pensacola Beach. An F0 tornado that blew down some trees was reported in south Mobile County Alabama. Gulfport Harbor reported a storm tide of 5.1 ft, and there were other reports in the 3-5 ft range (Table 2). Minor river flooding occurred along Spring Creek near Iron City, Georgia, where the river crested at 15.3 ft, 1.3 feet above flood stage.

Hanna and its remnants produced heavy rains across much of the southeastern states. These rains were largely confined to the eastern semicircle of the storm, with numerous reports of storm-total accumulations of between 5 and 10 inches. The highest reported storm total, 15.56 in, was from Donalsonville, Georgia.

c. Casualty and Damage Statistics

Three deaths are attributed to rip currents generated by Hanna. An adult male (age 20) drowned in rough surf near Pensacola Beach on the afternoon of the 14^th. Two other adult males (ages unknown) drowned, one at Seagrove Beach (Walton County) on the 14^th, and another at Panama City Beach on the 15^th.

Storm effects were relatively minor, and insured losses did not meet the $25M threshold to be recorded by the American Insurance Services Group. Minor beach erosion was reported from Dauphin Island, Alabama, to Navarre Beach, Florida, as well as in the Florida counties of Walton, Bay, and Gulf. Some storm tide flooding was reported on Dauphin Island and in Mobile County. Roughly 250 homes and 50 businesses were damaged from freshwater flooding in Donalsonville, Georgia. Data from the Georgia Farm Services Agency indicates agricultural damage, primarily to the cotton and peanut crops, amounted to nearly $19 million. There were several other apparently minor flooding events. Well after Hanna had made landfall and weakened to a tropical depression, there was a report of a roof being blown off a house in Donalsonville. Total damage is estimated at $20 million.

d. Forecast and Warning Critique

Average official track errors (with the number of cases in parentheses) for Hanna were 62 (8), 120 (6), 175 (4), and 225 (2), n mi for the 12, 24, 36, and 48 h forecasts, respectively. These errors are roughly 50% larger than the average official track errors for the 10-yr period 1992-2001 (43, 81, 115, and 148 n mi, respectively [Table 3]). Official track forecasts issued from 0000 UTC 12 September through 0600 UTC 14 September are shown in Figure 4. The official forecasts consistently took Hanna northward to the coast too quickly, as they failed to anticipate the westward track of the storm prior to the 14^th. Interestingly, much of the model guidance did capture some of the early counter-clockwise rotation of Hanna around the mid-level low; however, the official forecasts remained, conservatively, on the right-side of the guidance envelope showing a quicker motion toward the coast. Many of the guidance models had mean errors lower than those of the official forecast.

Official intensity forecasts correctly anticipated that Hanna would not strengthen much. Average official intensity errors were 2, 3, 4, and 13 kt for the 12, 24, 36, and 48 h forecasts, respectively. For comparison, the average official intensity errors over the 10-yr period 1992-2001 are 7, 11, 14, and 16 kt, respectively.

Table 4 lists the watches and warnings associated with Hanna. A tropical storm watch was issued at 1500 UTC 12 September, 41 hours prior to the first landfall of Hanna in extreme southeastern Louisiana. A tropical storm warning was issued at 0900 UTC 13 September, 23 hours prior to landfall. Tropical storm conditions were confined to the area under warning.

Acknowledgments

The National Weather Service (NWS) Forecast Offices in Mobile, New Orleans, and Tallahassee contributed surface observations for this report. The best track of Hanna after landfall is based in part on analyses from the NWS Hydrometeorological Prediction Center.

Figure 1: Best track positions for Tropical Storm Hanna, 12-15 September 2002.

Figure 2: Selected wind observations and best track maximum sustained surface wind speed curve for Tropical Storm Hanna, 12-15 September 2002. Aircraft observations have been adjusted for elevation using 80%, 75%, and 80% reduction factors for observations from 850 mb, 925 mb, and 1500 ft, respectively. Dropwindsonde 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). Hanna's landfall near the Alabama-Mississippi border is indicated by the solid vertical line.

Figure 3: Selected pressure observations and best track minimum central pressure curve for Tropical Storm Hanna, 12-15 September 2002. Hanna's landfall near the Alabama-Mississippi border is indicated by the solid vertical line.

Figure 4: Selected official track forecasts (dashed lines, with 0, 12, 24, 36 ,48, and 72 h positions indicated) for Tropical Storm Hanna, 12-15 September 2002. The best track is given by the thick solid line with positions given at 6 h intervals.