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Regional Observations during the Landfall of Tropical Juliette (2001) in Baja California,

LUIS M. FARFA N Unidad La Paz, Centro de InvestigacioÂn Cientõ®ca y de EducacioÂn Superior de Ensenada B.C., Baja California Sur, Mexico

(Manuscript received 8 August 2003, in ®nal form 9 January 2004)

ABSTRACT This paper presents an observational analysis of landfall over the eastern Paci®c Ocean. The tropical cyclone that developed during the 2001 season off the Mexican coast was named Juliette and made landfall in the Baja California peninsula. Juliette approached land over the southwestern peninsula, and the evolution of a localized region of cyclonic circulation occurred over the Gulf of California. The storm passage was associated with extensive property damage to the population in the southern peninsula, with most of the damage caused by heavy rainfall and strong winds. The intensity of the circulation associated with Juliette indicates that this was a strong system in the record of landfall during the period 1992±2002. All available sources of observations are collected to investigate characteristics of the circulation motion and structure. In situ observations, including surface and upper-air data, are applied to document the evolution of storm ¯ow while the center was located near and over the southern peninsula. Imagery from a geostationary satellite is used to determine characteristics of convective patterns. The analysis indicates development of distinct bands of deep convection and a period of slow system motion, which are related to large precipitation. Low- level winds are used to identify characteristics of the incident circulation along the Paci®c coast. However, the nocturnal nature of the event and the lack of observations at critical times limit the attempts to explain how the incident circulation is associated with that observed over the gulf the next morning.

1. Introduction in terms of an anticyclone being located over central Mexico. This implies midlevel steering ¯ow with a me- Our current understanding on the life cycle of tropical ridional component forcing northward storm motion in the eastern Paci®c Ocean has progressed along the west coast of Mexico. Smith (1986) docu- signi®cantly in the last decades. It is known that these mented the penetration of cyclones into the southwest- cyclones tend to form in a limited area south of Mexico ern and found that while these storms and west of . While intensi®cation oc- move in the area, synoptic-scale circulations, embedded curs, the systems follow a direction roughly parallel to the large-scale ¯ow and along the southwest coast of in the westerly ¯ow, approached the western coast of Mexico. However, some of them experience substantial North America. deviations and they may acquire a northerly component In order to illustrate general characteristics of tropical to move toward the continent. This situation becomes cyclone landfall in Baja California, storm evolution is a potential threat to the population over western Mexico, analyzed with data compiled from the National Hurri- including the Baja California peninsula. The peninsula cane Center (NHC) for the eastern Paci®c basin. This has a coastal length of approximately 3600 km, which is known as the best-track database and is constructed represents 31% of the total coast in Mexico, and this during a postseason analysis using several observational fact indicates that the peninsula requires special atten- sources. The best track provides center position, max- tion. imum sustained wind speed at 10 m, and minimum sea According to a study performed by Allard and Pe- level pressure for the duration of each system (e.g., terson (1987), 14% of tropical cyclones developing in Avila et al. 2003). According to Jarvinen et al. (1984), the eastern Paci®c made landfall along the west coast this should be considered as the best estimate of the of Mexico, and 58% of them occurred in Baja Califor- large-scale storm motion rather than a precise track of nia. Allard and Peterson explained the northward tracks the storm. Because of a lack of in situ observations over water and coastal areas in the eastern Paci®c, there is strong dependence on satellite products for the esti- Corresponding author address: Dr. Luis M. FarfaÂn, Centro de In- vestigacioÂn Cientõ®ca y de EducacioÂn Superior de Ensenada B.C., mation of storm position and intensity. Unidad La Paz, Mira¯ores 334, La Paz 23050, BCS, Mexico. Figure 1 shows tracks of tropical cyclones that E-mail: [email protected] crossed Baja California during the period 1992±2002.

᭧ 2004 American Meteorological Society

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FIG. 1. Best-track positions for tropical cyclones making landfall in Baja California during the period 1992±2002. The positions are at 6-h time intervals, and labels next to the name indicate date (year/ month/day) in which each system is ®rst located over land. The heavy black line indicates positions for Juliette (2001).

An outstanding feature to note is the existence of a preferred area of landfall over the west coast. This was documented by Latorre and Penilla (1988) in a study of systems that approached the area in the period 1960± 79. The study indicates that the majority of arrivals were from lower latitudes in the Paci®c and the ®rst contact with land occurred in the southern or central peninsula. An exception to the meridional approach in Fig. 1 is Hurricane Calvin (1993), which approached the area from an easterly direction. According to the descriptive FIG. 2. Mean geopotential heights (dam) at the 500-mb level from summary for Calvin (Avila and May®eld 1995), the the NCEP±NCAR reanalysis for selected cases from Fig. 1. Times hurricane hit the southern coast of Mexico and arrived are at (a) 2 days prior to and (b) during the day of landfall. Contour at the peninsula as a weakening tropical storm. interval is 1.5 dam, and the dots indicate mean position of tropical Latorre and Penilla (1988) presented the ®rst publi- cyclones from the best-track data. cation that examines characteristics of storm passage near Baja California. Data from this study were used to determine that 60% of 30 storms occurred in the month search (NCEP±NCAR) reanalysis (Kalnay et al. 1996). of September, 20% in August, and 17% in October. This Note that the reanalysis has a horizontal resolution of is in agreement with results from Allard and Peterson approximately 210 km, which implies that the data are (1987), who concluded that September is the primary able to resolve large-scale circulations and, thus, provide month for recurvature in the eastern Paci®c during the limited representation of tropical cyclones. In order to period 1966±80. In fact, note that the events shown in avoid contamination from nontypical cases and focus Fig. 1 tend to occur in September or late August. In on past events, data relative to Calvin (1993) and Juliette this ®gure, the author indicates the date in which the (2001) have been excluded. The results from our com- system center is ®rst detected over land in the best-track putations are shown in Fig. 2 as geopotential heights 2 records. days prior to landfall (Fig. 2a) and during the day of General features of tracks followed by landfalling the event (Fig. 2b). The 500-mb level is selected because systems may be explained by the structure of the large- of the known relation between storm motion and geo- scale, environmental ¯ow and the application of the strophic ¯ow in the middle troposphere (e.g., Dong and steering concept (e.g., Elsberry 1995). Based on this Neumann 1986). framework, we estimate mean storm motion by using According to Fig. 2a, the location of storms is near data derived from the National Centers for Environ- Socorro Island (18.7ЊN, 110.9ЊW) a couple of days be- mental Prediction±National Center for Atmospheric Re- fore landfall and occurs while a wave trough propagates

Unauthenticated | Downloaded 10/06/21 02:11 PM UTC JULY 2004 FARFAÂ N 1577 to the west coast of the United States. In addition, there sequence of events associated with the landfall of Ju- is an anticyclonic circulation located east of the island liette are presented in section 3. Section 4 provides a and centered over the Atlantic Ocean. A weak gradient discussion of data analysis and compares results with of height contours south of Baja California suggests the respect to previous landfall events in the peninsula. Fi- presence of light steering winds, which is consistent nally, a summary is given in section 5. with slow storm motion within the zonal band 18.0Њ± 22.0ЊN in Fig. 1. Mean heights at landfall (Fig. 2b) consist of an anticyclone over the mainland of Mexico 2. Data and the trough west of the peninsula. This de®nes a southwest±northeast orientation of height contours and The geographical area of interest in this study is the stronger gradients, which is associated with faster north- Baja California peninsula, which is surrounded by the eastward motion at landfall (Fig. 1). Furthermore, Paci®c Ocean and Gulf of California. Since the main heights after landfall (not shown) indicate that the effects of Juliette occurred in the southern peninsula, trough has moved to the east during the next few days, we concentrate our attention on the state of Baja Cal- and this results in the advection of storm remnants into ifornia Sur (BCS). This area is shown in Fig. 3 along the southwestern United States. with a representation of the terrain at 4-km resolution. In a study of satellite-derived precipitation in the east- The western portion of BCS is covered with ¯at, low- ern Paci®c, Rodgers et al. (2000) found that signi®cant level elevations, while the east has mountains located amounts of rainfall are estimated off Baja California, along the southern tip (22.8 ±24.2 N) and gulf coast and the amounts have a large contribution from tropical Њ Њ cyclones. This fact suggests that the storm activity has (24.8Њ±27.8ЊN). These mountains have mean elevations impact in rainfall patterns that occur in the peninsula. above 500 m, and maximum altitudes reach the 1500- Normally, Baja California is a relatively dry area and m level. In addition to topographical elements, Fig. 3 receives some precipitation during the summer and fall. includes best-track data provided by the NHC, and this Based on 20 yr of data, Latorre and Penilla (1988) doc- information is discussed in section 3. umented the patterns associated with tropical cyclones Information to study the large-scale ¯ow during the and determined that, in most cases, these systems pro- development of Juliette is derived from the regular net- vide moderate precipitation for several days. In contrast, work of upper-air soundings as well as gridded data from a few storms are strong enough to provide up to 55% the Aviation (AVN) run of the Medium-Range Forecast of the annual amounts received in the area. According Model (MRF). This is a dynamical model operated by to MosinÄo and GarcõÂa (1974), signi®cant events develop NCEP that provides an operational analysis±forecast during the hurricane season and large amounts of rain system for guidance during the hurricane season at the occur over the mountains in September. NHC (Surgi et al. 1998). The AVN coverage is global, The 2001 tropical cyclone season in the eastern Pa- and, because of availability, the author makes use of ci®c Ocean had 15 named systems, and most of them output at spatial resolution of approximately 250 km. developed far away from the continent. However, Hur- The model ®elds are applied to identify mid- and upper- ricane Juliette made landfall over northwestern Mexico level circulations present over the Paci®c Ocean and in late September. The system moved parallel to the Gulf of Mexico, as well as continental areas in Mexico coast and affected Baja California. The landfall had sub- and the United States. stantial impact in the region because it was associated Imagery from the Geostationary Operational Envi- with property damage, disruption to the tourist business, ronmental Satellite-10 (GOES-10) is applied to docu- and loss of communications for several days. A de- ment cloud-cover structure and intensity. The imagery scription on the evolution of this system is documented originates from the infrared, visible, and water vapor by Avila et al. (2003); however, our study strives for a channels available at hourly time intervals. Because of more detailed discussion on the storm passage over the calibration capabilities, a measure of the convective ac- southern peninsula. tivity can be estimated from cloud-top brightness tem- The goal of this paper is to present a comprehensive peratures detected by the infrared channel (e.g., Clark study of the observational data available during the ap- proach, landfall, and emergence of Hurricane Juliette in 1983). Visible imagery is used to identify low-level cir- Baja California. Aspects that are of particular interest culations during the daylight hours, usually from 1300 are 1) to identify characteristics of the storm track and to 0100 UTC during the early fall in Baja California. intensity changes upon landfall, 2) to examine the struc- The water vapor imagery allows a measure of moisture ture of the low-level circulation and precipitation, and content and is useful in monitoring the evolution of 3) to determine limitations associated with the appli- upper-level circulations. cation of the regional observations. Data from the network of surface stations operated This paper is divided into ®ve sections. In section 2, by the Servicio MeteoroÂlogico Nacional (SMN) in BCS the area of study and the sources observational datasets are used. Network stations are located at Cabo San Lu- are described. A description of the best-track data and cas (CBSL), Ciudad ConstitucioÂn (CDCN), Santa Ros-

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FIG. 3. Topography of Baja California Sur. Contour intervals of terrain elevations (m) are shown in the vertical bar. Best-track intensities for hurricane (hurricane symbol), tropical storm (star), and tropical de- pression (dot) are indicated. Times for best-track positions are given in the format time (UTC)/day. Boxes provide location of automatic surface stations, triangles for regional airports; station acronyms are expanded in the text.

alõÂa (STRS), and Gustavo DõÂaz Ordaz (GDOZ). The are periods of missing data due to canceled aircraft op- spatial distribution of the sites is shown in Fig. 3; data erations during the storm passage. are taken by automatic mechanisms at 10-min intervals, Another source of surface observations is the dataset and reported parameters include sustained winds, peak of daily precipitation from rain gauges throughout BCS gusts, air temperature, relative humidity, and station and part of the mainland, east of the gulf coast in Fig. pressure. Alamos (ALMS), the automatic station east of 3. These sites are managed by the ComisioÂn Nacional the gulf, experienced technical problems, and there are del Agua (CNA), and speci®c information associated no observations on 30 September and 1 October at this with these observations is given in section 3c. This sec- site. Surface observations are available from the SMN tion presents an analysis of rainfall during the storm station in La Paz (LAP) at hourly intervals, and the passage, as derived from the CNA network, and com- station is collocated with an upper-air sounding site. pares the information against the convective patterns Data coverage is augmented with the inclusion of ob- derived from satellite imagery. In addition, there are servations from regional airports that issue hourly re- weather radars in northwestern Mexico, and their ob- ports during the day (1300±0300 UTC); however, there servations would be useful to identify storm rainfall

Unauthenticated | Downloaded 10/06/21 02:11 PM UTC JULY 2004 FARFAÂ N 1579 structures associated with Juliette at landfall. One of these radars is located in southern BCS and two are located over the eastern gulf coast; however, the cor- responding products are not available for the period 28± 30 September, and this fact imposes limitations on a detailed study of the event.

3. Observations According to NHC records, Tropical Cyclone Juliette developed in late September 2001; Fig. 1 shows the corresponding track. Note that when compared with oth- er systems that approached Baja California Juliette ex- perienced an initial stage farther east. The best-track data starts on 21 September, indicates landfall on 30 September, and completes the record on 3 October (Avi- la et al. 2003). This results in a total of 13 days of activity, which is the longest length among all cases FIG. 4. Aviation model analysis of geopotential heights (dam) at considered in Fig. 1. As 7 of these days are spent north the 500-mb level valid at (a) 0000 UTC 27 Sep, (b) 0000 UTC 28 Sep, (c) 1200 UTC 29 Sep, and (d) 1200 UTC 30 Sep 2001. Contour of the 20.0ЊN latitude, Juliette stays near or over the interval is 1.5 dam. Best-track intensity for Juliette uses the same peninsula for a signi®cant portion of its life cycle. convention as in Fig. 3. Observed winds (m sϪ1) from upper-air The initial detection of a well-de®ned circulation as- soundings at 500 mb, with full barb representing 5.0 m s Ϫ1 and half- sociated with Juliette is on 21 September, when tropical barb 2.5 m sϪ1. depression intensity was assigned to the storm. Satellite imagery (not shown) indicates that during the period 21±22 September the system remains south of Mexico northward motion during the next day, with the center and provides deep convection to areas located along the remaining over water. Estimates of sea surface temper- coast. The system reaches hurricane intensity on 23 Sep- ature indicate values above 30ЊC over the central and tember, and a state of maximum strength occurs at 1800 southern portion of this basin (Zamudio et al. 2002). UTC 25 September. At this time, observations from a The descriptive summary for the system (Avila et al. U.S. Air Force (USAF) aircraft ¯ying at 700 mb are 2003) states that the disturbance becomes poorly de®ned used to estimate 10-m winds of 62.5 m sϪ1 (NHC 2001). over the northern gulf and the remnants spread into the While located near Socorro Island on 27 September, southwestern United States. The last record in the best- hurricane strength is sustained and a northward com- track data is at 0000 UTC 3 October when the depression ponent of motion is acquired. The system moves toward is estimated to be over the northeastern coast of Baja Baja California during the following few days and California (Fig. 1). makes landfall along the southwestern coast on 30 Sep- tember. a. Large-scale ¯ow The track followed by Juliette during the approach and passage over BCS is shown in Fig. 3. This ®gure An analysis of the large-scale ¯ow provides relevant presents storm position and intensity prior to landfall elements that are active during the approach of Juliette as well as motion across the peninsula and central gulf. to Baja California. For this purpose, Fig. 4 shows upper- At 0000 UTC 28 September the storm center is located air soundings and the AVN analysis of geopotential southwest of CBSL, and the system has maximum winds heights at 500 mb for the period 27±30 September. At of 41 m sϪ1. Note the gradual weakening from hurricane 0000 UTC 27 September (Fig. 4a) the location of the into tropical storm intensity during the next 24 h while tropical system is over Socorro Island. If this height moving along a northwestward direction. Juliette re- distribution is compared with the mean ®elds from pre- gains hurricane intensity at 0600 UTC 29 September vious events (Fig. 2a), a couple of common features are and remains at this level for a relatively short period of identi®ed: 1) the approach of a wave on the Paci®c coast time before being downgraded into a tropical storm at of the United States, and 2) an anticyclonic circulation 1800 UTC 29 September. over Mexico. This con®guration suggests a southeast- According to best-track positions, the storm landfall erly steering ¯ow over BCS. occurs a few hours before 0000 UTC 30 September While the anticyclonic circulation remains active over along the Paci®c coast, west of CDCN. The tropical the continent, the wave propagates toward the coast as storm moves across low-elevation terrain and then over shown in Fig. 4b, and at 1200 UTC 29 September (Fig. the eastern BCS mountains where it is downgraded to 4c) the wave is located north of the hurricane. This a tropical depression at 0600 UTC. The system emerges scenario provides Juliette a weak steering ¯ow toward over the central gulf near 1200 UTC and experiences the north, and this is consistent with the slow motion

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FIG. 5. Vertical cross section of winds (m sϪ1, barbs) and relative humidity (%, isopleths) from upper-air soundings at 1200 UTC 29 FIG. 6. Aviation model analysis of 200±850-mb horizontal wind Sep 2001. Vertical axis indicates pressure level (mb), and winds use Ϫ1 same notation as in Fig. 4. Contour interval of relative humidity is shear (m s ) valid at (a) 0000 UTC 27 Sep, (b) 0000 UTC 28 Sep, 25%, and areas higher than 75% are shaded gray. Stations included (c) 1200 UTC 29 Sep, and (d) 1200 UTC 30 Sep 2001. Contour intervals are 12.5, 20, 30, and 40 m sϪ1, with values above 12.5 m in this ®gure are Desert Rock, NV (DRA), Yuma, AZ (YUM), and Ϫ1 Empalme (GYM), La Paz (LAP), MazatlaÂn (MZT), and Manzanillo s shaded. Best-track intensity for Juliette uses the same convention (MAN), Mexico. as in Fig. 3. Winds are at the 200-mb level, with full barb representing 5.0msϪ1, half-barb 2.5 m sϪ1, and ¯ag triangle 25.0 m sϪ1. that the storm follows over the west coast of BCS (Fig. 3). An inspection of the analysis at 1200 UTC 30 Sep- from moderate winds at the 200-mb level as the system tember (Fig. 4d) indicates wave dissipation and return approached the southwestern peninsula. In contrast, of the anticyclonic circulation centered over the western there is an extensive area of strong shear (Ͼ20.0 m sϪ1) United States. This fact is related to limited motion over over the northern peninsula and gulf as well as strong the northern gulf and presence of steering winds with westerly ¯ow above the 300-mb level. These conditions a component from the east, which is consistent with the are not favorable for intensi®cation and may be partially track experienced by Juliette during the late stages of responsible for the intensity changes on 28±29 Septem- its life cycle (Fig. 1). ber, as shown in Fig. 3. At 1200 UTC 29 September Information on the structure of the westerly wave and (Fig. 6c) the analysis depicts increased westerly ¯ow tropical cyclone is shown in Fig. 5. This is a vertical and shear over northern Mexico, while the best track cross section of winds and moisture derived from the shows system weakening and motion across the pen- stations included in Fig. 4c. The stations are chosen to insula and central gulf (Fig. 6d). Real-time discussions depict atmospheric conditions along the gulf at 1200 (NHC 2001) recognized that the presence of strong shear UTC 29 September. Note the presence of dry and light played a role in Juliette's weakening and in the storm southwesterly ¯ow over the middle troposphere at De- development being con®ned at low levels. sert Rock, Nevada, and Yuma, Arizona. This corre- sponds to an area within the southern edge of the wave b. Satellite imagery and ahead of the northern quadrants of Juliette. In con- trast, Fig. 5 illustrates a well-de®ned area of strong ¯ow In order to illustrate characteristics of convective and high relative humidity (Ͼ75%) in the southern gulf, structure and intensity associated with the landfall of which is consistent with the approach of the storm core. Juliette, infrared imagery from GOES-10 is analyzed. According to the 1200 UTC sounding released at LAP, Figure 7 displays selected images that represent upper winds with magnitude up to 25 m sϪ1 are present in the sections of convection as detected by the satellite. Since 850±700-mb layer, while the hurricane center is located this imagery is calibrated, there is a practical relation 125 km west of the station (12/29 in Fig. 3). with cloud-top temperatures: colder clouds are associ- The vertical shear of horizontal winds between the ated with higher tops and more intense convection, and lower and upper troposphere are known to in¯uence this is displayed in white tones. Although there are some intensity changes on tropical cyclones (e.g., Zehr 2003). exceptions to this relationship, it is still applicable to In order to identify environmental conditions for inten- perform a study of changes experienced by the tropical si®cation on Juliette, Fig. 6 shows the magnitude of cyclone of interest. 200±850-mb shear derived from the AVN analysis. Note Convection associated with Juliette is ®rst detected the presence of a relative minimum (12.5±15.0 m sϪ1) over the southern peninsula at 1000 UTC 26 September, around the storm center in Figs. 6a and 6b, which results while the best-track center is located 550 km south of

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FIG.7.GOES-10 infrared imagery at (a) 0000 UTC 27 Sep, (b) 0300 UTC 28 Sep, (c) 0000 UTC 29 Sep, and (d) 0000 UTC 30 Sep 2001. Horizontal resolution of the imagery is 4 km, and the vertical bar indicates cloud-top temperatures (ЊC) scale. Tropical cyclone position and intensity are from the best track and use the notation de®ned in Fig. 3.

CBSL, and the author chose to initiate the analysis the tains high cloud tops south of the core while the system next day. The convective structure at 0000 UTC 27 motion is slowing down. Note the presence of the band September (Fig. 7a) shows the presence of a well-de- over portions of the peninsula (23.7Њ±25.2ЊN), across ®ned area of high cloud tops around the storm center, the gulf (24.4Њ±25.4ЊN), and along the mainland coast with maximum intensity from Ϫ65Њ to Ϫ70ЊC. Ac- (22.0Њ±25.5ЊN). At this time, the band has reached max- cording to real-time discussions (NHC 2001), a USAF imum intensity, with temperatures in the range of Ϫ70Њ aircraft reported the existence of multiple, concentric to Ϫ73ЊC, and the area of most active convection is eyewalls that are located near the best-track center. An- over water. Subsequent animation reveals that this fea- imation of subsequent images indicates that, during the ture remains active during the period 0000±1000 UTC following 24 h, the convective rings show ¯uctuations 28 September and moves northward. This occurs while in response to the weakening of the whole system. As another band develops over southern BCS and shows convection near the core decreases, a distinct band of spiral motion around the storm center. active convection develops in the northeastern quadrant Figure 7c shows high cloud tops located near the of the storm. The band propagates toward the peninsula, southern peninsula at 0000 UTC 29 September, with the and its interaction with land results in enhanced con- area of maximum convection located along the Paci®c vection in the region from CBSL to LAP. coast. The image animation indicates warming of cloud At 0300 UTC 28 September (Fig. 7b), Juliette main- tops and reduction in the degree of organization after

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FIG. 8. Rainfall observations from network of rain gauges from 1500 UTC 26 Sep to 1500 UTC 30 Sep 2001. The accumulated rainfall is in units of mm, and ®lling intervals are shown in the vertical bar. Contours for terrain elevation (solid lines) at 250, 500, and 1000 m. Plus signs indicate the location of stations used in this plot. The best-track data and reference stations use the notation de®ned in Fig. 3.

1200 UTC. In addition, a low-level cyclonic circulation and the Sierra Madre Occidental Mountains. According is detected in the visible imagery after 1400 UTC (not to Avila et al. (2003), ¯ooding affected more than shown). The circulation has a radius of about 200 km, 38 000 people in the state of (26.5Њ±31.3ЊN), and the system motion is in a direction parallel to the and this is linked to the area of intense convection that western coast of BCS. This feature is consistent with developed in the state. best-track positions at 1200 and 1800 UTC 29 Septem- ber in Fig. 3. The analysis of subsequent visible imagery c. Rainfall distribution indicates that at 0000 UTC 30 September the low-level circulation is located 75 km south of CDCN. In order to present an analysis of precipitation asso- While the low-level circulation develops in the Paci®c ciated with the passage of Juliette across BCS, the hor- coast, an extensive area of convection occurs over the izontal distribution of rainfall observations is shown in northeast quadrant of the storm center. The initial stage Fig. 8. This ®gure represents accumulated rainfall re- of the area is shown in Fig. 7d as a feature that resembles ceived during the period 26±30 September, and the data a squall line over the central gulf, extending from STRS originate from daily measurements taken at 1500 UTC. to ALMS. This feature moves northward and results in There are a total of 118 stations, and note that the majority a large area of intense convection along the mainland of sites (78%) are located at terrain elevations below 300 during the period 1000±2000 UTC 30 September. An m, while the rest reach altitudes up to 840 m. A similar inspection of the corresponding imagery indicates that pattern occurs with the stations in the mainland, as most the convective system develops over the coastal plains sites are at altitudes below 250 m.

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Data shown in Fig. 8 suggest the existence of a large de®ne a point of maximum intensity at 0100 UTC 28 area of signi®cant rainfall in the southern mountains of September, with sustained winds of 39.1 m sϪ1, sea level BCS. The area has sites that report more than 750 mm, pressure equivalent to 982.0 mb (Fig. 9a), as well as and these tend to be located along the south or east side moist ¯ow from the east (Fig. 9b). These data are con- of the mountain range. A maximum of 1011 mm is sistent with the hurricane center being located 100 km reported at CaduanÄo, located 30 km north of San Jose southwest of the station (00/28 in Fig. 3). In addition, (MMSD), where more than 900 mm are received during Fig. 9a depicts the presence of high-frequency ¯uctu- 3 consecutive days. Inspection of daily data suggests ations in speed and moisture content ahead of the storm that the occurrence of the maximum is due, in part, to core, from 0000 UTC September 27 to 0000 UTC 28 the passage of convective bands detected in the satellite September. These features may be due to precipitation- imagery on 27±29 September. A secondary rainfall max- induced downdrafts characteristic of convective bands imum is located over the central mountains (25.0Њ± in mature hurricanes (e.g., Willoughby 1995). Recall 27.0ЊN), where elevations are near the 500-m terrain that the location, structure, and intensity of convective level. The observations are in the range 250±500 mm bands in the southern peninsula are shown in Figs. 7a and de®ne the maximum south and east of Loreto and 7b. After 0000 UTC 28 September, and as the storm (MMLT). Data from sites on the mainland suggest mod- moved away from the station, the magnitude of incident erate rainfall amounts (50±250 mm) in the area where ¯ow decreases. There is also a slow increase in pressure, satellite imagery shows the development of convection and the air loses some moisture. The smooth rates of on 28 September (Fig. 7b). change are associated with decreased convective band Lack of precipitation is observed at sites over the activity behind the storm core, as well as the limited northwestern sector of the state. This is due, in part, to motion experienced by Juliette while located west of limited elevation and reduced density of stations in the the southern peninsula. area. Another reason for the lack of rainfall in north- The station near the landfall region estimated from western BCS is associated with the development of in- the best-track data is CDCN (Fig. 3). This site is located tense convection on the right side of the storm track at 45 km east of the Paci®c Ocean coast, and the cor- and less convection on the left side, north of 26.0ЊN responding time series are shown in Figs. 10a and 10b. and west of 113.0ЊW. An inspection of the GOES-10 As the storm motion is reduced, winds and pressure at water vapor imagery (not shown) indicates the intrusion the site tend to remain unchanged during the ®rst portion of dry air at middle and upper levels over the west coast. of the plot. At 0810 UTC 30 September the observations The intrusion is related to the development of the west- reach maximum (minimum) wind speeds (pressure) erly wave discussed in section 3a that is located over while the best-track center is placed farther north, near the Paci®c Ocean (Fig. 4). As the trough approaches the MMLT (06/30 in Fig. 3). The remaining observations peninsula, dry conditions and increased wind shear (Fig. at CDCN are consistent with the storm departure from 6) are present in the area, providing conditions that in- the area with sustained speeds reaching a limited mag- hibit the development of convective processes. nitude (Ͻ6.0msϪ1) by 1100 UTC and a period of calm conditions occurring after 0500 UTC 1 October. Note that the presence of decreasing winds and a smooth d. Surface data variation of pressure are consistent with the best-track Primary observations of low-level ¯ow during the intensity assigned to Juliette as it was downgraded to passage of Juliette are derived from the SMN network tropical storm (18/29) and tropical depression (06/30). of automatic surface stations. There are four sites in The time series at STRS (Fig. 11a) re¯ects the passage BCS, and their spatial distribution is shown in Fig. 3. of a low pressure system at 1220 UTC 30 September, with Note that the stations are located close to the track fol- northeasterly wind speeds of 15.0 m sϪ1 and gusts up to lowed by the storm center over the southern peninsula, 20.3 m sϪ1. The maximum intensity is higher than that at and a couple of them are on the coast. Time series of CDCN (Fig. 10a), where 11.3 m sϪ1 sustained winds and sustained winds, peak gusts, sea level pressure, and mix- 13.6 m sϪ1 peak winds are observed at 0810 UTC. De- ing ratio are shown in Figs. 9±11. These plots cover tection of the system at STRS is consistent with the 1200 48-h periods and are centered at the time of minimum UTC best-track center (Fig. 3) over the Gulf of California, distance between the storm center and each station. The 50 km southeast of the station, and at tropical depression magnitude of sustained winds at these stations is av- strength. Note that from 0600 to 1800 UTC the time series eraged from 10-min intervals, while the best-track in- (Fig. 11b) shows a cyclonic wind shift, an indication that tensity is given for 1-min means. No adjustment is ap- the cyclone core moves near STRS. There is also a de- plied to the raw data; however, it is known that a factor creasing trend in the mixing ratio as the wind acquires a to apply in a conversion from 10-min data to a 1-min signi®cant component from the west. This fact suggests equivalent is approximately 1.11 (Powell et al. 1996). advection of dry air, more likely from the Paci®c coast. Located in the southern end of the peninsula, CBSL Supporting data to indicate the northward motion of the (Fig. 9) shows increasing winds and decreasing pressure system are provided by the time series at GDOZ (not during the approach of the storm core. These parameters shown). This station is located over the central peninsula

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FIG. 9. Time series of observations at Cabo San Lucas (CBSL) from 0000 UTC 27 Sep to 0000 UTC 29 Sep 2001. (a) Sustained speeds (m sϪ1), peak gusts (m sϪ1), and sea level pressure (mb) are indicated with plus signs, circles, and solid line, respectively. (b) Mixing ratio (g kgϪ1; solid line) and sustained winds (m sϪ1). Winds use same notation as in Fig. 4.

(Fig. 3), 75 km west of the gulf coast, and experiences Velden (2002). In general, these winds provide near- maximum sustained winds and peak gusts at 1650 UTC surface coverage in the outer radii of tropical cyclone 30 September. circulations. The spatial distribution of winds at 2100 UTC 29 September (Fig. 12a) allows identi®cation of the cy- e. Low-level circulation clonic circulation along the western coast of BCS. Ex- As described in previous sections, regular observa- amination of winds at earlier times indicates the north- tions indicate the approach, landfall, and exit of Juliette ward displacement of the circulation. The best-track po- over Baja California. However, there are limited data sition at 2100 UTC has been interpolated from data at from surface stations to closely monitor the track of the 1800 UTC 29 September and 0000 UTC 30 September. core during the event. A combination of in situ and Note that the best-track center is in agreement with the satellite-derived winds is applied to augment the spatial circulation identi®ed from the combined dataset. How- coverage of the dataset. This dataset will allow us to ever, the position difference is signi®cant at 0000 UTC, perform an analysis of the structure and evolution for as the best-track center is placed farther north and over the low-level circulation associated with the storm. For land in Fig. 12b while the circulation from cloud motion this purpose, Fig. 12 illustrates the temporal and spatial remains over the ocean. The time series of winds at distribution of winds from the combined dataset. Cloud- CDCN (Fig. 11b) suggests that the location of the storm drift winds are derived from tracking low-level clouds center is south of the station at this time. in sequential, high-resolution (1 km) GOES-10 visible As night arrives, there are no more visible imagery imagery and are provided by the University of Wis- cloud-drift winds to monitor the motion and structure consinÐMadison. The fundamentals, methodology, and of the circulation. In addition, airports stop reporting validation of this product are discussed by Dunion and observations after 0100 UTC 30 September, and only

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FIG. 10. As in Fig. 9, but for time series of observations at Ciudad ConstitucioÂn (CDCN) from 0000 UTC 29 Sep to 0000 UTC 1 Oct 2001. data from the automatic sites and LAP are available. Paci®c Ocean during late September 2001. According These facts become a major limitation to analyze the to Avila et al. (2003) one death was attributed to the subsequent evolution of the circulation. It is not until storm in Baja California, and a regional newspaper re- the next morning when enough observations become ported that estimates of property loss were about $16 available to examine the ¯ow structure. Winds at 1500 million (U.S. dollars) with most of the damage in urban UTC (Fig. 12c) and 1800 UTC (Fig. 12d) show a con- areas over southern BCS. According to satellite and in ®guration consistent with the best track as a low-level, situ analyses from section 3, the main elements affecting cyclonic ¯ow is detected over the eastern peninsula the region were intense precipitation and strong winds. coast. In fact, the combined dataset provides elements This is consistent with typical weather-related hazards to identify a circulation over the central gulf, between experienced during the approach and landfall of cy- STRS and Guaymas (MMGM in Fig. 3). The dataset is clones over coastal regions (e.g., AMS 2000). Further- also useful to detect a large area of southerly ¯ow from more, a revision of individual storm summaries for the the Paci®c Ocean into BCS and the lack of convective other cases shown in Fig. 1 indicates that extreme cells west the central peninsula. The latter is consistent weather conditions were also experienced during the with the presence of dry air that is determined from approach of those systems to the peninsula. water vapor imagery as well as the decrease of mixing- Best-track data from the NHC are used as preliminary ratio levels at STRS (Fig. 11b) and GDOZ (not shown). information to outline position and intensity changes associated with Juliette. As stated in the introductory section, this dataset provides a smooth estimate of storm 4. Discussion motion but not the precise location of the system. Be- Previous sections describe data available during the cause of availability, this study is focused on better es- development of Tropical Cyclone Juliette in the eastern timation of storm center location by applying obser-

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FIG. 11. As in Fig. 9, but for time series of observations at Santa RosalõÂa (STRS) from 1200 UTC 29 Sep to 1200 UTC 1 Oct 2001. vations from satellite imagery and all station reports in ever, the large-scale ¯ow is not able to provide details BCS. Additional data sources, such as land-based radar, of the incident circulation over the peninsula and its ship reports, and aircraft research ¯ights, would be ideal development over the central gulf. The absence of an to enhance our analysis; however, for a variety of rea- enhanced network of observations is a contributing fac- sons, these sources were not available for this area and tor in the problem of determining the physical mecha- period of interest, which is a signi®cant limitation in nisms involved in the event. This fact implies that fur- this case study. In a unique opportunity, research ¯ights ther work is required to identify the dynamics of the were used to determine accurate location of Tropical circulation at landfall, and this work may need to apply Cyclone Paine (1986) while passing just south of BCS methods beyond the observational analysis. (Gunther and Cross 1987; Daida and Barnes 2003). Satellite imagery (Fig. 7) was useful in identifying Upper-air data (Fig. 4) indicate that, during the ap- changes in the convective intensity and development of proach of Juliette to BCS, an anticyclonic circulation distinct bands. As the system approached the peninsula, was established over northern Mexico, and a Paci®c convective bands provided deeper cloud coverage than westerly wave propagated toward the U.S. coast. As that associated with the storm core. The lack of storm discussed in section 3a, this is a ¯ow con®guration con- motion for a couple of days was a key factor in providing sistent with the track followed by previous landfall cases abundant precipitation to the region. An analysis of the in the period 1992±97. Although system motion may CNA rain gauge network (Fig. 8) indicated that maxi- deviate from the environmental ¯ow, it is assumed that mum amounts of accumulated precipitation were in the the application of the steering concept is suf®cient to range 500±1000 mm. These values are higher than those explain general characteristics of tropical cyclone mo- known from previous landfall cases (Fig. 1) as the max- tion. The analysis of vertical shear and upper-level imum rainfall derived from the passage of Lester (1992; winds (Fig. 6) indicates the presence of environmental Lawrence and Rappaport 1994) and Hilary (1993; Avila conditions that are favorable for storm weakening. How- and May®eld 1995) were 220 and 110 mm, respectively.

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FIG. 12. GOES-10 visible imagery, in situ wind observations, and cloud-drift winds at (a) 2100 UTC 29 Sep, (b) 0000 UTC 30 Sep, (c) 1500 UTC 30 Sep, and (d) 1800 UTC 30 Sep 2001. Units for winds are in m s Ϫ1, as in Fig. 4, and best-track intensities use the notation de®ned in Fig. 3. Smaller circles represent missing station reports.

In a previous study, Latorre and Penilla (1998) found ¯ow. In particular, they were important in determining that 1960±79 cyclones provided maximum total rainfall circulation intensity over land during system approach in the range of 250±350 mm. A comparison of these to the southern peninsula, to monitor evolution over the facts ranks Juliette as an extreme system in the record west coast at landfall, and to detect a circulation over of storm rainfall at landfall over BCS. the central gulf. Automatic station observations from The location of surface data, with respect to the storm the SMN (Figs. 9±11) allowed evaluation of strong track, has been valuable for the analysis of low-level winds during the storm approach as maximum sustained

Unauthenticated | Downloaded 10/06/21 02:11 PM UTC 1588 MONTHLY WEATHER REVIEW VOLUME 132 speeds observed at CBSL were 39.1 m sϪ1 and gusts physical parameterizations, nested grids at high spatial reached 46.7 m sϪ1 (Fig. 9). This compares with max- resolution, and sensitivity applications to identify the imum winds of 31 m sϪ1 and gusts of 39 m sϪ1 reported effect of topography in the evolution of the incident at MMSD (Fig. 3) during the landfall of Hurricane Faus- circulation. to in 1996 (May®eld and Rappaport 1998). Data from The analysis of circulations associated with the land- CBSL also showed equipment resistance against dam- fall of Juliette illustrates a practical application of the age from strong winds in Juliette as the station survived available network of regional observations. This in- at least 42 h under tropical storm conditions (sustained cludes upper-air and surface platforms to determine en- speeds Ͼ 18.0 m sϪ1) and a couple of hours of hurricane vironmental ¯ow con®gurations as well as satellite im- intensity (Ͼ33.0 m sϪ1). agery to identify convective features. Assuming avail- The combined dataset of in situ and cloud-drift winds ability of this dataset, a similar study may be performed (Fig. 12) indicated the presence of a well-de®ned, cy- to document previous landfall events in Baja California clonic circulation prior to landfall over southwestern and derive more general conclusions. Therefore, re- BCS. However, it is not clear whether this circulation maining cases that occurred during the period 1992± moved across land or another, secondary circulation de- 2002 (Fig. 1) can be investigated on an individual basis. veloped over the central gulf. The problem originates, However, there are some limitations that are important in part, from a couple of elements that occurred during to consider. For example: 1) automatic stations started the nighttime: 1) the lack of adequate distribution of in operations in the spring of 2000, which prohibits their situ observations to resolve small-scale features and 2) application for earlier events; 2) for safety reasons air- the limitations from visible satellite imagery to estimate ports close operations upon storm approach, and these cloud-track motion. In order to provide a reasonable observations tend to be missing at landfall; and 3) vis- explanation of the events that occurred during and after ible imagery is not available at night, which reduces landfall, Avila et al. (2003) speculate on the possibility availability of products to periods of approximately 12 of an incident circulation that dissipates at landfall and h dayϪ1. As stated above, an appropriate way to over- the subsequent formation of another center in the gulf. come these problems consists in the application of the The relative paucity of surface observations over land modeling approach. The combination of results from and the lack of maritime platforms make the testing of this approach and the observational analysis will allow this hypothesis a dif®cult task. Therefore, it is reason- a better understanding of motion and structure changes able to investigate the landfall event through numerical during tropical cyclone landfall in Baja California. model guidance. The landfall activity did not stop in 2001, and an The application of numerical models has been pro- early assessment of eastern Paci®c systems during the posed by Marks et al. (1998) as a tool to develop im- 2003 season reveals additional cases affecting the area. proved techniques for the study of storm changes during Two events occurred in Baja California, with one in late tropical cyclone landfall. Recommended models are August and another in September. The corresponding those available for use in operational and research ap- best tracks show early development near Socorro Island plications. Among the latter, the use of the Regional and northward motion over the western Gulf of Cali- Atmospheric Modeling System (RAMS), Advanced Re- fornia. In particular, Tropical Cyclone Marty (Franklin gional Prediction System (ARPS), and the ®fth-gener- 2003) moved across the southern peninsula, and an in- ation Pennsylvania State University±National Center for spection of the data recorded at CBSL (not shown) in- Atmospheric Research (PSU±NCAR) Mesoscale Model dicates the occurrence of strong ¯ow that is comparable (MM5) is suggested for research simulations. with that from Juliette. This fact reinforces the need to In a particular application, MM5 was used by FarfaÂn continue analyzing regional observations to better un- and Zehnder (2001) to study changes in the motion and derstand the impact of circulations that affect BCS, an structure of Tropical Cyclone Nora (1997; see track in area of potential damage each tropical cyclone season. Fig. 1) over northern Baja California. The storm ap- proached the west coast and induced a localized area of 5. Summary mesoscale convection over the northern gulf. The model was used to determine that orographic modi®cation of This study documents the development of Tropical the low-level ¯ow was involved in the development of Cyclone Juliette (2001) over the eastern Paci®c Ocean. a secondary circulation over the gulf. An inspection of Our ®ndings supplement data from the NHC best-track terrain features in the area of Juliette's landfall (Fig. 3) dataset during the storm approach and landfall over the indicates the presence of mountains with dimension and southern Baja California peninsula. All available data orientation similar to those documented by FarfaÂn and sources, including satellite products and in situ reports, Zehnder (2001). The performance of mesoscale simu- are used to identify the evolution of convective patterns lations to determine dynamic mechanisms active in the and structure of low-level circulation. development of the gulf circulation is a direction of Satellite imagery shows the limited motion experi- research currently under way. These simulations include enced by the system, and, as recorded by the regional realistic initialization of the model cyclone, adequate network of rain gauges, signi®cant amounts of precip-

Unauthenticated | Downloaded 10/06/21 02:11 PM UTC JULY 2004 FARFAÂ N 1589 itation occurred over the peninsular mountains. The in- R. Stewart, and J.-G. Jiing, 2003: Annual summary: Eastern tensity derived from rain and wind data ranks Juliette North Paci®c hurricane season of 2001. Mon. Wea. Rev., 131, 249±262. as a signi®cant event when compared with other land- Clark, J. D., 1983: The GOES user's guide. NOAA/NESDIS, 169 pp. falling systems that developed during the period 1992± Daida, S. K., and G. M. Barnes, 2003: Hurricane Paine (1986) grazes 2002. Fortunately, this system is not associated with the high terrain of the Baja California peninsula. Wea. Fore- signi®cant loss of human life as only one death was casting, 18, 981±990. Dong, K., and C. J. Neumann, 1986: The relationship between tropical reported in the area. cyclone motion and environmental geostrophic ¯ows. Mon. Wea. There is an aspect of this study that was not accom- Rev., 114, 115±122. plished: an understanding of the circulation evolution Dunion, J. P., and C. S. Velden, 2002: Application of surface-adjusted over the Paci®c Ocean, Baja California peninsula, and GOES low-level cloud-drift winds in the environment of Atlantic Gulf of California. This fact is due to the limited density tropical cyclones. Part I: Methodology and validation. Mon. Wea. Rev., 130, 1333±1346. of observational platforms upon storm passage and sat- Elsberry, R. L., 1995: Tropical cyclone motion. Global Perspectives ellite-derived products that are not available at night. In on Tropical Cyclones, R. L. Elsberry, Ed., World Meteorological order to overcome these limitations, numerical model Organization, 106±197. applications are recommended as a tool of further re- FarfaÂn, L. M., and J. A. Zehnder, 2001: An analysis of the landfall of Hurricane Nora (1997). Mon. Wea. Rev., 129, 2073±2088. search to improve our understanding of tropical cyclone Franklin, J. L., cited 2003: Tropical cyclone report: Hurricane Marty: landfall in Baja California. 18±24 September 2003. [Available online at http:// In conclusion, since the peninsula has a coastal length www.nhc.noaa.gov/2003marty.shtml.] covering one-third of the total coast, this area requires Gunther, E. B., and R. L. Cross, 1987: Annual summary: Eastern special attention in Mexico. A couple of landfall events North Paci®c tropical cyclones of 1986. Mon. Wea. Rev., 115, 2507±2523. that occurred during the 2003 season support the need Jarvinen, B. R., C. J. Neumann, and M. A. S. Davis, 1984: A tropical to continue studying storm activity in the region. The cyclone data tape for the North Atlantic basin, 1886±1983: Con- network of automatic, surface observations is a good tents, limitations, and uses. NOAA Tech. Memo. NWS NHC- tool to monitor the approach and passage of tropical 22, 22 pp. Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Re- cyclones at high temporal resolution. In fact, most of analysis Project. Bull. Amer. Meteor. Soc., 77, 437±471. the data sources used in the study are available in real Latorre, C., and L. Penilla, 1988: In¯uence of cyclones in the pre- time via the Internet. This capability offers meteoro- cipitation of Baja California Sur (in Spanish). AtmoÂsfera, 1, 99± logical centers (e.g., SMN and NHC) information to 112. closely monitor the development of systems and provide Lawrence, M. B., and E. N. Rappaport, 1994: Eastern North Paci®c hurricane season of 1992. Mon. Wea. Rev., 122, 549±558. valuable information to issue more accurate assessments Marks, F. D., L. K. Shay, and PDT-5, 1998: Landfalling tropical of motion and intensity. If available, additional plat- cyclones: Forecast problems and associated research opportu- forms of observation (e.g., land-based radar, ship re- nities. Bull. Amer. Meteor. Soc., 79, 305±323. ports, and aircraft ¯ights) should be used to augment May®eld, M., and E. N. Rappaport, 1998: Eastern North Paci®c hur- ricane season of 1996. Mon. Wea. Rev., 126, 3068±3076. data coverage, and this should allow major improve- MosinÄo, P. A., and E. GarcõÂa, 1974: The climate of Mexico. World ments in the analysis of systems that may threaten the Survey of Climatology, H. E. Landsberg, Ed., Elsevier, 345±404. population in Baja California. NHC, cited 2001: Hurricane Juliette: Advisory archive. [Available online at http://www.nhc.noaa.gov/archive/2001/JULIETTE. Acknowledgments. Partial support for this research html.] was provided under CICESE-CONACYT Grant Powell, M. D., S. H. Houston, and T. A. Reinhold, 1996: Hurricane's Andrew landfall in south Florida. Part I: Stantadarizing mea- 020397. The author wishes to thank the following in- surements for documentation of surface wind ®elds. Wea. Fore- dividuals for providing assistance during the collection casting, 11, 304±328. of various datasets: Michael Leuthold (satellite imagery) Rodgers, E. B., R. F. Adler, and H. F. Pierce, 2000: Contribution of of the University of Arizona Department of Atmospher- tropical cyclones to the North Paci®c climatological rainfall as observed from satellites. J. Appl. Meteor., 39, 1658±1678. ic Sciences, Fortino GarcõÂa (rainfall observations) and Smith, W., 1986: The effects of eastern North Paci®c tropical cyclones Manuel Colima (automatic station reports) of CNA, and on the Southwestern United States. NOAA Tech. Memo. NWS Jason Dunion (cloud-drift winds) of NOAA/AOML/ WR-197, 229 pp. HRD. The comments and suggestions from the anon- Surgi, N., H.-L. Pan, and S. J. Lord, 1998: Improvement of the NCEP ymous reviewers helped to improve the manuscript. global model over the Tropics: An evaluation of model perfor- mance during the 1995 hurricane season. Mon. Wea. Rev., 126, 1287±1305. REFERENCES Willoughby, H. E., 1995: Mature structure and evolution. Global Perspectives on Tropical Cyclones, R. L. Elsberry, Ed., World Allard, R. A. and R. E. Peterson, 1987: Characteristics of northeast Meteorological Organization, 21±62. Paci®c tropical cyclones. Rev. Geo®s., 26, 33±52. Zamudio, L., H. E. Hurlburt, E. J. Metzger, and O. M. Smedstad, AMS, 2000: Policy statement: Hurricane research and forecasting. 2002: On the evolution of coastally trapped waves generated by Bull. Amer. Meteor. Soc., 81, 1341±1346. Hurricane Juliette along the Mexican west coast. Geophys. Res. Avila, L. A., and M. May®eld, 1995: Eastern North Paci®c hurricane Lett., 29, 2141±2144. season of 1993. Mon. Wea. Rev., 123, 897±906. Zehr, R. M., 2003: Environmental vertical wind shear with Hurricane ÐÐ, R. J. Pasch, J. L. Beven, J. L. Franklin, M. B. Lawrence, S. Bertha (1996). Wea. Forecasting, 18, 345±356.

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