DECEMBER 2003 NOTES AND CORRESPONDENCE 1295

The Rapid Intensi®cation and Subsequent Rapid Weakening of Hurricane Lili as Compared with Historical Hurricanes

WILLIAM J. FREDERICK NOAA/NWS/WSO, Vicksburg,

25 February 2003 and 25 May 2003

ABSTRACT The purpose of this paper is to compare the rapid intensi®cation and rapid decay of Hurricane Lili with historical records to determine whether similar events have occurred in the past. Data for this study were compiled from the historical hurricane database that consists of all Atlantic basin tropical storms and hurricanes recorded from 1851 to 2001. Maximum 1-min sustained winds at 10 m were used. This study concludes that the rapid intensi®cation/rapid decay processes that occurred as Hurricane Lili moved north across the have been recorded only 11 other times since 1851. The intensi®cation rate of 35 kt (1 kt ϭ 0.5144 m sϪ1)in 24 h for Hurricane Lili ranked in the 11th percentile when compared with the 769 hurricanes in the historical database. In fact, 35.8% of all hurricanes that achieved an intensity of at least 100 kt intensi®ed at a rate similar to Lili. The decay rate of 40 kt within 24 h after completion of rapid intensi®cation ranks in the 1st percentile. This study also shows that all hurricanes in the Gulf of Mexico that rapidly intensi®ed after 15 September proceeded to decay shortly thereafter by an average of 28 kt within 24 h of completion of rapid intensi®cation. On the other hand, three of ®ve (60%) hurricanes that rapidly intensi®ed over the Gulf of Mexico prior to 15 September continued to intensify, by an average of 9 kt.

1. Introduction A consistent listing of minimum central pressure was not available until the 1970s, and so maximum 1-min The basis of this study stems from rapid intensi®- sustained winds were used in this study. It is noted that cation and subsequent rapid weakening of Hurricane Lili wind measurements are suspect prior to the reconnais- over the Gulf of Mexico on 2±3 October 2002. During sance era that began in the 1950s. To limit the population that time span, Hurricane Lili intensi®ed by 35 kt (1 kt sample, only hurricanes that achieved maximum sus- Ϫ1 ϭ 0.5144 m s ) and then weakened by 40 kt less than tained winds of at least 100 kt and an intensi®cation 24 h later. This paper compares Lili's intensity changes rate of greater than 34 kt in 24 h were used. These with historical records to determine whether similar criteria were selected to compare hurricanes of similar changes have occurred in the past. intensity to Lili.

2. Data and methods 3. Results Data for this study were compiled from the historical The historical database contains 769 hurricanes, of database available from the National Oceanic and At- which 257 (33.4%) reached at least 100 kt. Of these mospheric Administration (NOAA)/National Weather 257 hurricanes, 92 (35.8%) matched the initial test cri- Service (NWS)/Tropical Prediction Center in Miami, teria of maximum 24-h intensity increases greater than . The database consists of all Atlantic basin trop- 34 kt with at least 100 kt of intensity. These criteria ical storms and hurricanes recorded from 1851 to 2001 produced an intensi®cation rate in the 11th percentile. and includes the date and time, latitude and longitude, Figure 1 illustrates 48-h track segments for the 92 maximum 1-min sustained wind speed in knots, and matching hurricanes during rapid intensi®cation. The minimum central pressure in millibars at synoptic times. beginning of rapid intensi®cation is represented as the line starting point, the position that the rapid intensi®- Corresponding author address: William J. Frederick, NOAA/ cation ended is de®ned as the midpoint, and locations NWS/WSO, 1400 Walnut St., Rm. 120, Vicksburg, MS 39180. 24 h after rapid intensi®cation are endpoints for each E-mail: [email protected] line in Fig. 1.

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FIG. 1. The 48-h track segments of 92 hurricanes that intensi®ed at a rate of 35 kt or greater within 24 h since 1851.

Hurricane Lili began rapid intensi®cation on 2 Oc- for the 92 hurricanes. If the rapid intensi®cation cycle tober 2002 while traveling north-northwest through the occurred across 2 months, the month containing the Gulf of Mexico. Table 1 depicts historical geographic endpoint of rapid intensi®cation was used. August, locations of maximum 24-h intensi®cation rate for the September, and October account for over 93% of all 92 matching hurricanes. In cases in which two geo- rapidly intensifying hurricanes, which period also cor- graphic areas were present in the track of the hurricane, responds to peak frequency. locations that depicted the ends of rapid intensi®cation, The most prominent and fortunate characteristic of the midpoint of the line, were used. Note that 21 of Hurricane Lili was that it weakened almost as quickly the 92 hurricanes (27.2%) rapidly intensi®ed over the as it intensi®ed. This process was achieved over the Gulf Gulf of Mexico. Upon further analysis of these 21 hur- of Mexico prior to . According to Emanuel ricanes, 11 made landfall and 10 (5 prior to 15 Sep- (2000), the average decay rate for a hurricane in the tember and 5 after) remained over water. Three of the Atlantic basin over water is roughly two-thirds of its ®ve hurricanes that occurred prior to 15 September prior intensi®cation rate. Applying this formula to Hur- continued to intensify (ϩ15, ϩ50, and ϩ30 kt) during ricane Lili (35-kt intensi®cation in 24 h), a 24-h decay the next 24 h, and two decayed (Ϫ20 and Ϫ20 kt), rate of approximately 23 kt should have occurred. The netting an average 9 kt increase. On the other hand, 24-h decay rate for Hurricane Lili was 40 kt, much all ®ve hurricanes that occurred after 15 September greater than the statistical average. In fact, Lili's 24-h decayed (Ϫ25, Ϫ35, Ϫ30, Ϫ20, and Ϫ30 kt) within decay rate ranks in the 1st percentile. Table 3 depicts 24 h of completion of rapid intensi®cation, with an intensity 24 h after the end of rapid intensi®cation of average decrease of 28 kt. the 92 hurricanes that had intensi®ed at a rate of 35 kt Table 2 depicts the months of rapid intensi®cation

TABLE 2. Month of occurrence of 92 hurricanes that intensi®ed at TABLE 1. Geographic locations of 92 hurricanes that intensi®ed at a rate of 35 kt or greater within 24 h. a rate of 35 kt or greater within 24 h. No. of Percent No. of Percent Month hurricanes of tot Geographic location hurricanes of tot Jun 2 0.2% Tropical Atlantic (S of 20ЊN, E of 60ЊW) 18 19.6% Jul 1 1.1% Atlantic (N of 20ЊN) 27 29.3% Aug 21 22.8% Eastern Caribbean (E of 75ЊW) 10 10.9% Sep 47 51.1% Western Caribbean (W of 75ЊW) 16 17.4% Oct 18 19.6% Gulf of Mexico 21 27.2% Nov 3 3.3%

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FIG. 2. The 48-h track segments during intensi®cation/decay for 11 hurricanes that intensi®ed at a rate of 35 kt or greater within 24 h and decayed at a rate greater than 24 kt within 24 h of rapid intensi®cation.

or greater. Landfall was declared if the hurricane re- compared with statistics in Table 1. Note that the highest mained over land for a period of at least 24 h or if it frequency (19%, or 4 of 21 hurricanes) occurred in the passed over a signi®cant mountainous region such as Gulf of Mexico, the geographic area of Hurricane Lili's Hispaniola. demise. After reviewing test conditions of rapid intensi®cation Table 5 provides comparisons between months of oc- coupled with a subsequent rapid decay over water within currence of the 11 ®nal hurricanes, set with the 92 orig- 24 h of that intensi®cation (Table 3) of greater than 2/ inal hurricanes listed in Table 2. Although September 3, the intensity suggested by Emanuel (2000), 11 hur- contained the highest probability for rapid intensi®ca- ricanes were identi®ed. Of those 11, 4 were in the Gulf tion (51.1%), the highest probability of occurrence by of Mexico at the time of intensi®cation/decay (1909, month for rapid decay to follow the rapid intensi®cation 1946, Florence 1953, Opal 1995), 3 were in the tropical is October (16.7%). Hurricane Lili occurred in this Atlantic approaching the eastern Caribbean (Baker month. 1950, Betsy 1956, Georges 1998), and 4 were in the Table 6 shows maximum 24-h intensi®cation for these Atlantic Ocean (Gladys 1975, Ella 1978, Claudette 11 hurricanes and maximum decay within 24 h of the 1991, Floyd 1999). Figure 2 depicts the 48-h track seg- intensi®cation. All 11 hurricanes weakened by an ment for these 11 hurricanes, including the month and amount less than their intensi®cation. Hurricane Lili year of occurrence. weakened (40 kt) at a greater rate than its intensi®cation Table 4 depicts geographic locations of hurricanes (35 kt); Lili is unique in this aspect. that satis®ed rapid intensi®cation/rapid decay criteria along with the percentage for that geographic region as

TABLE 4. Geographic location of 11 hurricanes that intensi®ed at TABLE 3. Intensity change categorization for 92 hurricanes 24 h a rate of 35 kt or greater within 24 h and decayed at a rate greater after rapid intensi®cation. than 24 kt within 24 h of rapid intensi®cation. No. of Percent No. of Percent Intensity change hurricanes of tot Geographic location hurricanes of tot Decreased Ͼ 24 kt 11 11.9% Tropical Atlantic (S of 20ЊN, E of 60ЊW) 3/18 16.7% Decreased 1±24 kt 18 19.6% Atlantic (N of 20ЊN) 4/27 14.8% Increased 0±24 kt 29 31.5% Eastern Caribbean (E of 75ЊW) 0/10 0.0% Increased Ͼ 24 kt 8 8.6% Western Caribbean (W of 75ЊW) 0/10 0.0% Made landfall 26 28.3% Gulf of Mexico 4/21 19.0%

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TABLE 5. Month of occurrence of 11 hurricanes that intensi®ed at 4. Conclusions a rate of 35 kt or greater within 24 h and decayed at a rate greater than 24 kt within 24 h of rapid intensi®cation. Rapid intensi®cation and rapid decay that occurred as Hurricane Lili moved across the Gulf of Mexico has No. of Percent Month hurricanes of tot been documented only 11 other times since 1851. Lili's intensi®cation of 35 kt in 24 h ranks the hurricane in Jun 0/2 0.0% Jul 0/1 0.0% the 11th percentile when compared with 769 other hur- Aug 2/21 9.5% ricanes in the historical database. In fact, 35.8% of all Sep 6/47 12.8% hurricanes above 100 kt intensi®ed at a rate similar to Oct 3/18 16.7% Lili. Therefore, the intensi®cation rate for Hurricane Nov 0/3 0.0% Lili is not considered to be an exceptional event. How- ever, a decay rate of 40 kt in 24 h over water ranks Hurricane Lili in the 1st percentile. Lili is the only TABLE 6. Maximum intensi®cation and decay of 11 hurricanes that intensi®ed at a rate of 35 kt or greater within 24 h and decayed at a hurricane to have decayed at a greater rate than it in- rate greater than 24 kt within 24 h of rapid intensi®cation. tensi®ed. Hurricane Max 24-h Max within name intensi®cation 24-h weakening REFERENCES

1909 ϩ35 Ϫ25 Emanuel, K. A., 2000: A statistical analysis of hurricane intensity. 1946 ϩ40 Ϫ35 Mon. Wea. Rev., 128, 1139±1152. Baker 1950 ϩ45 Ϫ40 Florence 1953 ϩ60 Ϫ30 Betsy 1956 ϩ55 Ϫ25 Gladys 1975 ϩ40 Ϫ35 Ella 1978 ϩ50 Ϫ30 Claudette 1991 ϩ55 Ϫ30 Opal 1995 ϩ50 Ϫ30 Georges 1998 ϩ40 Ϫ30 Floyd 1999 ϩ40 Ϫ30 Lili 2002 ϩ35 Ϫ40

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