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A Bow Echo and Severe Weather Associated with a Kona Low in Hawaii

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A Bow Echo and Severe Weather Associated with a Kona Low in Hawaii 576 WEATHER AND FORECASTING VOLUME 13 A Bow Echo and Severe Weather Associated with a Kona Low in Hawaii STEVEN BUSINGER AND THOMAS BIRCHARD JR. University of Hawaii, Honolulu, Hawaii KEVIN KODAMA* Joint Institute for Marine and Atmospheric Research, Honolulu, Hawaii PAUL A. JENDROWSKI National Weather Service Forecast Of®ce, Honolulu, Hawaii JIAN-JIAN WANG1 UCAR COMET Outreach Program, Boulder, Colorado (Manuscript received 23 April 1997, in ®nal form 21 November 1997) ABSTRACT On 2 November 1995 a kona low formed to the northwest of Hawaii. During the following 48 h a series of convective rainbands developed on the southeastern side of the low as it slowly moved eastward. On the afternoon of 3 November 1995 Hawaiian standard time (HST), a bow-echo signature was identi®ed in the re¯ectivity observations from the recently installed WSR-88D located on the south shore of Kauai, and led to the ®rst severe thunderstorm warning ever issued by the National Weather Service Forecast Of®ce in Honolulu, Hawaii. Subsequent to the warning, winds of 40 m s21 (80 kt) were observed at Nawiliwili Harbor on the southeast side of Kauai. The goals of this paper were to (i) document, within the constraints of the observational data, the synoptic and mesoscale environment associated with the formation of the bow echo and severe weather in Hawaii and contrast them with investigations of similar phenomena in the midlatitudes and Tropics, and (ii) provide a discussion of the implications of the availability of data from the new WSR-88D radars in Hawaii to operational forecasting of severe weather in the central Paci®c. 1. Introduction Whirlwinds sweep over the earth; Rolling down are the rocks of the ravines; Despite its reputation for benign tranquillity, punc- The red mountain-streams are rushing to the sea. tuated only by brief tropical showers, the atmosphere Here are the waterspouts; over Hawaii can produce surprisingly violent storms. Tumbled about are the clustering clouds of heaven; This is a fact that did not escape the islands' earliest Gushing forth are the springs of the mountains. inhabitants and is much on the minds of operational [ancient Hawaiian chant describing kona low; forecasters working in the Paci®c region (Kodama and Fornander (1996)] Businger 1998, this issue): Most notable of these violent storms are hurricanes Coming is the dark cloud and the rainbow; such as Iniki in 1992. However, from October to April, Wildly comes the rain and the wind; extratropical weather systems can propagate suf®ciently equatorward to substantially alter the dominant, stable trade wind pattern in the vicinity of the Hawaiian Is- lands.1 Among these synoptic-scale disturbances is a *Current af®liation: National Weather Service Forecast Of®ce, Ho- 2 nolulu, HI. class of subtropical cyclones known as kona low pres- 1Current af®liation: Department of Atmospheric Sciences, Uni- sure systems that can produce a wide range of weather versity of Illinois at Champaign±Urbana, Urbana, Illinois. Corresponding author address: Dr. Steven Businger, Department 1 The Hawaiian Islands range in latitude from ;198 to 228N, and of Meteorology, University of Hawaii, 2525 Correa Rd., Honolulu, mean temperatures in summer and winter differ by only a few degrees. HI 96822. 2 Kona is Hawaiian for leeward, with respect to the usual NE trade E-mail: [email protected] wind ¯ow. q 1998 American Meteorological Society SEPTEMBER 1998 BUSINGER ET AL. 577 hazards, including heavy rains, hailstorms, ¯ash ¯oods, been utilized to determine the dynamics of rear in¯ows landslides, high winds, large surf and swell, water- that characterize bow-echo mesoconvection in midlati- spouts, and in the case that is the subject of this paper, tudes (Lee et al. 1992; Weisman 1992, 1993; Johns severe thunderstorms (Simpson 1952; Ramage 1962; 1993; Skamarock et al. 1994; Przybylinski 1995; Knupp Schroeder 1977a,b; Kodama and Barnes 1997; Ramage 1996; among others). Johns and Hirt (1987) observed 1995). that bow echoes associated with widespread damaging Most kona lows form from extratropical cyclones that winds (derechos) east of the Rocky Mountains can occur become secluded at lower latitudes by the blocking ac- with either strong, migrating low pressure systems tion of a warm high. Less frequently, kona lows develop (called the ``dynamic'' pattern) or with weather patterns from cutoff mid- and upper-tropospheric cold-core lows exhibiting relatively weak synoptic-scale features that extend their circulations to the surface. In both these (called the ``warm season'' pattern). Moreover, Johns instances the cold low aloft draws on sources of cold (1993) noted that individual bow echoes and bowing vorticity-rich air from higher latitudes. Kona lows tend line segments can form in a wide variety of synoptic to track in slow and erratic loops, resulting in a period environments. of weather characterized by moist south/southwesterly There have also been a considerable number of ob- ¯ow and rain squalls, with episodes of high winds and servational and numerical modeling studies of tropical ¯ooding over the Hawaiian Islands that can last for a squall lines (Aspliden et al. 1976; Houze 1977; Zipser week or more. A signi®cant fraction of the annual total 1977; Barnes and Sieckman 1984; Chong et al. 1987; rainfall on the kona or southwest-facing slopes of the Nicholls 1987; Roux and Ju 1990; Keenan and Carbone islands can be attributed to the occurrence of a small 1992; Trier et al. 1996, 1997; Jorgensen et al. 1997). number of these subtropical lows. The majority of tropical squall lines studied propagate On 2±4 November 1995, a kona low evolved from from east to west, and those exhibiting greater low-level an occluded wave cyclone and spawned a series of wind shear tend to propagate faster than those with squall lines as it approached the Hawaiian Islands from weaker wind shear (Barnes and Sieckman 1984). The the northwest. As one of these convective bands ap- majority of midlatitude squall lines propagate from west proached the island of Kauai, it exhibited a pronounced to east consistent with the prevailing low-level winds bow-echo structure (Nolen 1959; Fujita 1978) in re- in the midlatitude and tropical latitude belts. ¯ectivity observations with the recently installed Weath- The growth, intensity, and dynamics of tropical and er Surveillance Radar-1988 Doppler (WSR-88D) radar midlatitude squall lines have been related to environ- located on the south side of the island of Kauai. Rec- mental thermodynamic instability and vertical shear of ognition of the implications of the radar data led to the the horizontal winds in the lower troposphere (e.g., issuance of a severe thunderstorm warning, the ®rst such Weisman and Klemp 1982; Barnes and Sieckman 1984). warning ever issued by the National Weather Service The observation of a variety of organized mesoscale Forecast Of®ce in Honolulu, Hawaii (WFSO HNL) (see convective systems has led to a classi®cation method appendix A). Subsequent to the warning, winds of 40 that relates storm severity to the bulk Richardson num- 21 2 ms (80 kt) were observed at Nawiliwili Harbor on ber (Ri 5 B/0.5U , where B is the buoyant energy in the southeast side of Kauai and convective-scale wind the storm's environment and U is a measure of the ver- gusts tore the roof off the Nawiliwili Small Boat Harbor tical wind shear). The combination of higher convective of®ce and knocked down trees and power lines across available potential energy (CAPE) and shear values is portions of south and east Kauai. Radar in this case associated with midlatitude supercells and tornadic provided critical lead time for the issuance of a severe storms, whereas tropical squall lines typically exhibit thunderstorm warning that was subsequently veri®ed lower CAPE and shear values (Keenan and Carbone and provides the data for the mesoscale postanalysis 1992). presented here. A squall line over the western tropical Paci®c Ocean The idealized evolution of an isolated, or single-cell documented by Jorgensen et al. (1997) moved west to type, bow echo is described by Fujita (1978). During east and exhibited a bow-shaped radar re¯ectivity struc- the most intense phase, the center of the bow forms a ture. A mesoscale vortex developed as the re¯ectivity spearhead, with cyclonic and anticyclonic rotation at the signature became increasingly bow shaped. The asso- ends of the bow. Rear in¯ow notches along the trailing ciated CAPE value in their case was ;1500 J kg21 and edge of the line segment usually indicate where the the environmental hodograph showed a 12 m s21 west- strongest downburst winds are located. Rear in¯ow erly jet at ;2 km, with little directional shear up to ;5 notches are a result of rear in¯ow jets that appear to km. A recent paper by Alfonso and Naranjo (1996) de- form bow echoes by differentially advecting a part of scribes a serial bow echo, or derecho, that formed in a the squall line or thunderstorm cell forward, and by baroclinic environment of a midlatitude low pressure entraining dry, lower ue air into the rear ¯ank, which system that affected Cuba, demonstrating the penetra- then erodes the back side of the advancing bow echo tion of an essentially midlatitude phenomena to sub- through the evaporation of hydrometeors. tropical latitudes. This paper provides the ®rst docu- Observational studies and numerical simulations have mentation of a bow echo in association with a subtrop- 578 WEATHER AND FORECASTING VOLUME 13 ical or kona low, in an ocean environment far removed A short-wave±jet streak with winds of 70 m s21 on the from continental landmasses. west side of the trough axis aloft (Fig. 1a) causes the The goals of this paper are to discuss the implications trough to propagate equatorward and increases upper- of the availability of data from the new WSR-88D radars level divergence over the Hawaiian Islands during the in Hawaii to operational forecasting of severe weather ensuing 24 h (Fig.
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