3—
Meteorology

INTRODUCTION

The Hawaiian Islands are flood prone. Schroeder (1977) found that an average of five floods are reported each year. Ramage (1971) pointed to three essential factors that contribute to torrential rains and potential flooding: (1) synoptic-scale weather disturbance such as a front, (2) plentiful moisture supply such as an adjacent ocean, and (3) anchoring by some discontinuity in surface roughness such as a mountain range. Synoptic activity in Hawaii is predominantly a cool-season (October through April) feature. Schroeder (1978a) found that Hawaiian torrential rains are primarily cool-season events. Moisture is nearly always available from the surrounding oceans. The mountainous terrain presents plentiful anchoring mechanisms. Fifty percent of the state's land area lies above 2,000 feet elevation and 50 percent lies within 5 miles of the ocean. Most of Oahu's inhabitants live within 5 miles of the ocean and in close proximity to the two mountain ridges. The mountains are the remnants of basaltic volcanoes overlaid with thin soil. These thin soils and fractured rocks are capable initially of absorbing steady rains of several inches per day, but once the porous media is saturated, flooding rapidly ensues.

Prior to the onset of the 1987–1988 rainy season, Hawaii experienced a series of dry years due to the strong 1982–1983 El Niño and a moderate to weak 1986–1987 El Niño. Meteorologists have recognized the correlation between El Niño and Hawaiian drought since the original work of Walker and Bliss (1932). The dry spell ended in early December 1987, as a surface low-pressure system of the type referred to in Hawaii as a "Kona storm" formed west of the islands. The onset of this storm was signaled by continuous thunderstorm activity over Oahu during the night of December 11–12. In the subsequent 10 days, 12 to 18 inches of rain fell over Oahu. The rains were relatively uniformly distributed, and some lowland areas actually received more rain than the adjacent mountains. Flooding was minor. Much of the



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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 3— Meteorology INTRODUCTION The Hawaiian Islands are flood prone. Schroeder (1977) found that an average of five floods are reported each year. Ramage (1971) pointed to three essential factors that contribute to torrential rains and potential flooding: (1) synoptic-scale weather disturbance such as a front, (2) plentiful moisture supply such as an adjacent ocean, and (3) anchoring by some discontinuity in surface roughness such as a mountain range. Synoptic activity in Hawaii is predominantly a cool-season (October through April) feature. Schroeder (1978a) found that Hawaiian torrential rains are primarily cool-season events. Moisture is nearly always available from the surrounding oceans. The mountainous terrain presents plentiful anchoring mechanisms. Fifty percent of the state's land area lies above 2,000 feet elevation and 50 percent lies within 5 miles of the ocean. Most of Oahu's inhabitants live within 5 miles of the ocean and in close proximity to the two mountain ridges. The mountains are the remnants of basaltic volcanoes overlaid with thin soil. These thin soils and fractured rocks are capable initially of absorbing steady rains of several inches per day, but once the porous media is saturated, flooding rapidly ensues. Prior to the onset of the 1987–1988 rainy season, Hawaii experienced a series of dry years due to the strong 1982–1983 El Niño and a moderate to weak 1986–1987 El Niño. Meteorologists have recognized the correlation between El Niño and Hawaiian drought since the original work of Walker and Bliss (1932). The dry spell ended in early December 1987, as a surface low-pressure system of the type referred to in Hawaii as a "Kona storm" formed west of the islands. The onset of this storm was signaled by continuous thunderstorm activity over Oahu during the night of December 11–12. In the subsequent 10 days, 12 to 18 inches of rain fell over Oahu. The rains were relatively uniformly distributed, and some lowland areas actually received more rain than the adjacent mountains. Flooding was minor. Much of the

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 rain was absorbed by the soil before runoff. Mid- to late December is the period of minimum insolation and thus minimum evaporation of soil moisture. It can be assumed that the soils in the watersheds of the Koolau Mountains remained near saturation on New Year's Eve. METEOROLOGICAL CONDITIONS ON DECEMBER 31, 1987 The surface weather conditions on the afternoon prior to the floods are shown in Figure 2. A cold front extending from a low-pressure center at 37°N, 142°W, weakened into a shear line in the vicinity of the islands. A shear line has few of the classical characteristics of a front but is a center of strong low-level convergence between weak east winds or, in this instance, east-southeasterly winds to the south and fresh north or northeast winds to the north. A shear line is a significant cloud Figure 2 Honolulu surface analysis for 0000 GMT January 1, 1988 (2:00 p.m. December 31, Hawaiian Standard Time). Pressure values are given in millibars with 1,000 millibars = 00. The dashed line over the Hawaiian Islands represents the shear line emanating from the cold front NE of Oahu. 1° latitude = 60 nautical miles.

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 and rain producer. The shear line depicted had already caused rains of 2 to 8 inches over windward Oahu and the Koolau Mountains by 8:00 a.m. on New Year's Eve. The shear line had stalled near Oahu. Steady showers fell over windward Oahu during much of the daylight hours. This is not unusual for that region. In the mid-troposphere a 500-millibar trough lay to the west of the islands with a strong jet over the islands. The prognosis indicated that a disturbance embedded in the flow associated with the trough would move over the islands. Forecasters anticipated that this disturbance would trigger heavy showers for New Year's Eve and thus forecasted showers and thunderstorms. An additional possible contributor to the situation over Oahu on New Year's Eve was a small mesoscale vortex (a center of enhanced convergence) that may have formed in the easterly or southeasterly flow over the high mountains (4,000 meters or 12,000 feet) of the island of Hawaii southeast of Oahu. Such vortices have been observed in satellite imagery to be carried in the prevailing flow northwest toward Oahu and Kauai. The vortex on New Year's Eve was indistinct in satellite imagery, but as it approached Oahu and experienced the influence of the mid-tropospheric support it had the potential to become an active shower producer. Stability analysis of radiosonde launched from Lihue and at Hilo indicated only moderate to weak instability. The Lihue Showalter Index at 2:00 p.m. on December 31 was +5°C; at Hilo it was +6°C. This indicates that the atmosphere over the islands would not support the evolution of thunderstorms. The combination of factors described above is typical of conditions that lead to normal winter rains in the islands, but the rains that ensued were exceptional. As has often been shown (Schroeder, 1977, 1981; Cram and Tatum, 1979), the island topography contributes significantly to where and how much rain will fall. RADAR AND SOUNDING ANALYSIS The Department of Meteorology at the University of Hawaii at Manoa has a 3-centimeter marine radar that is used for instruction and research. Reacting to the National Weather Service (NWS) forecast for New Year's Eve, the faculty set the radar in a 72-mile-range surveillance mode. All images were stored on videotape and subsequently analyzed. The 10-degree horizon due to the adjacent mountain ridges and the radar's 20-degree vertical-beam width limit the range at which shallow clouds can be detected; clouds over northern Oahu must exceed 20,000 feet to be detected. Fortunately, the New Year's rains occurred nearby. Figure 3 shows radar images from the University of Hawaii's 3-centimeter radar for the period between 12:00 p.m. (Hawaiian Standard Time) December 31, 1987, and 3:00 a.m. January 1, 1988. The radar cannot detect shallow clouds at azimuths between 300 and 360 degrees due to blockage by mountains. The return from 270 degrees at 12:00 p.m. is a reflection off the southern ridge of the Waianae Mountains. The echo over the radar and extending south over the ocean consists of light showers and ground clutter. The echo west-northwest of the radar over western

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 Oahu is a return off the southern ridge of the Waianae Mountains. Showers persisted in the mountains during the afternoon. Occasionally a few echoes formed to the south of Oahu and seemed to drift northward. These may have been a signature of the mesoscale vortex detected by the NWS. A less glamorous explanation is that the thick altostratus cloud deck associated with the upper-level trough had begun to precipitate. This has happened in the past when extremely deep, moist currents of air have moved out of the tropics over Hawaii. Radar echoes over windward Oahu persisted and spread during the late afternoon. At 7:00 p.m. on December 31, the activity rapidly expanded, covering leeward valleys and the downwind ocean. A few taller clouds were detectable over central Oahu. A characteristic of 3-centimeter radar is that heavy rain over the radar will strongly attenuate the signal. At 11:09 p.m. attenuation precluded the detection of the southern Waianae ridge (compare to the 10:00 p.m. image). The radar pictures failed to reveal a feature evident in the radar storm videotape. As cells grew over Oahu at the height of the storm, they were seen to shear off with the blow off moving north-northeast and quickly dissipated. The radar was not operating in a mode that allowed range-height indicator displays, so no information on cloud top heights was available. A WSR-74 radar at Hickam Air Force Base adjacent to Honolulu International Airport reported no significant tops over Oahu. Hickam did report a few tops at 19,000 to 21,000 feet to the south and west of Oahu during the afternoon and evening. These reports corresponded with the echoes that the university's radar detected. The 2:00 a.m. January 1 sounding for Lihue (Figure 4) shows a saturated layer extending from the surface to 19,000 feet (500 millibars). This would suggest a precipitating cloud layer extending to 19,000 feet. A puzzle is the nearly isothermal layer between 5,000 feet (850 millibars) and 7,000 feet (775 millibars). One interpretation is that the balloon responsible for the sounding passed through two distinct cloud layers. One cloud layer extended to only 5,000 feet, and the second was an altostratus layer from 8,000 to 19,000 feet. At the time the shear line was past Lihue. Shallow clouds such as proposed are common in the stable air behind a shear line. Woodcock (1975) demonstrated that these clouds could produce orographic rains concentrated on the mountain crests of Oahu. This type of cloud system was observed by Schroeder (1978b) over the Koolau Mountains on January 1 and 2. A second striking feature of the 2:00 a.m. sounding at Lihue was the strong vertical shear of the horizontal winds. Between 5,000 and 10,000 feet the winds reversed direction and showed a velocity shear of 90 mph (80 knots). Comparison of these winds with those 12 hours earlier indicates strengthening of the low-level north-northeasterlies and downward penetration of the south-southwesterlies. The strong vertical shear explains the sheared-off echoes observed by the University of Hawaii's radar. This also explains another feature of the storm: no thunder was reported. An inexpensive sferics (atmospheric interference produced by thunderstorms) detector is an AM radio. Scientists monitoring AM radio on New

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 Figure 3a Figures 3a and 3b Reproduced radar images from the University of Hawaii 3-centimeter radar for the period between 1200 (Hawaiian Standard Time) December 31, 1987 and 0300 January 1,1988. The radar

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 Figure 3b cannot detect shallow clouds at azimuths between 300 and 360 degrees due to blockage by mountains. The return from 270 degrees at 1200 is a reflection off the southern ridge of the Waianae mountain range.

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 Figure 4 Atmospheric sounding for Lihue, Kauai at 1200 GMT January 1, 1988 (2:00 a.m. January 1,1988 Hawaiian Standard Time). Plotted curves are air temperature (right) and dew point temperature (left). Lihue lies 80 miles northwest of Honolulu. Year's Eve reported no sferics. From the sounding it is evident that vigorous clouds would be decapitated before they could grow sufficiently tall to glaciate and become strongly charged and produce lightning and thunder. RAINFALL DISTRIBUTION Unlike the mid-December rains, the New Year's Eve rains were concentrated along the windward slopes and crest of the Koolau range (Figure 5). At Waikiki, 6 miles leeward of the crest, the 24-hour accumulation up to 8:00 a.m. on January 1 was only 0.059 inches. However, at the crest, rainfall totals were estimated to be 25 inches (Figure 5). The highest measured 24-hour totals were 22.89 inches at Maunawili (state raingauge no. 787.1), 22.0 inches at Pali Golf Course (state raingauge no. 788.1), 21 inches at Waimanalo Stream USGS raingauge (state no. 794.3), and 20.20 inches at Mokulama (state raingauge no. 784) in Waimanalo. These four standard raingauges lie on the windward slope of the Koolau Mountains. Puuomao (state raingauge no. 725) in Hahaione Valley just leeward of the crest

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 Figure 5 Rainfall analysis for 24 hours from 8:00 a.m. December 31 to 8:00 a.m. January 1, adapted from analyses performed by the Hawaii State Department of Land and Natural Resources. Rainfall contours (isohyets) are in inches.

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 (elevation 400 feet) reported 23.5 inches in 14 hours, plus some loss due to gauge overflow. In addition to the standard raingauges mentioned above, 14 recording raingauges were in the area of the heaviest rains. Some are telemetered to the NWS's Forecast Office at Honolulu International Airport. Of these gauges, five malfunctioned—three due to overflow of a finite-capacity gauge and two due to mechanical stoppage. The recording raingauge that received the highest rainfall intensity was at Maunawili, which is adjacent to the Maunawili standard gauge (Figure 6). These totals are shown in Table 3. Gauge no. 794.3 had the most complete rainfall data for the New Year's Eve storm. This raingauge is located next to streamgauge station no. 2490 (Figure 7). A comparison of rainfall accumulation at raingauge no. 794.3 and the NWS's Waimanalo telemetered raingauge (no. 795.1) demonstrates the spatial and temporal variability of the storm's rainfall (Table 4). The raingauge separation is approximately 2 miles. Prior to 6:00 p.m. on December 31, 1987, the NWS gauge recorded 4.70 inches of rain, contrasted with 0.9 inches at the U.S. Geological Survey's gauge (no. 794.3). In the later stages (i.e., 8:00–9:00 p.m.), the heavier rain fell at raingauge no. 794.3. The rainfall accumulations were impressive. These were not thunderstorm rains Figure 6 Rainfall accumulation for recording raingauge at Maunawili (state raingauge no. 787.1).

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 TABLE 3 Maiwaiwili Rainfall Data, December 31, 1987 (state raingauge no. 787.1) Duration (hours) Amount (inches) 1 4.0 2 7.4 3 10.4 4 13.0 5 14.8 6 15.7 Note: The raingauge overflowed at 10:00 p.m. but continuous orographic rains. The spatial distribution of the storm rain can be explained in terms of the topography. The southern Koolau Mountains form a curve from a northwest-southeast orientation along their northern and central portions to a more west-east orientation. North and north-northeast surface winds encountered a barrier oriented perpendicular to the flow; orographic lifting was accentuated. A shorter-lived episode similar in many ways to that on New Year's Eve occurred on May 6, 1981. During that storm, 9 inches of rain fell in 6 hours, flooding Waimanalo and sending some boulders down Hahaione Stream. In that instance, another shear line moved over Oahu but with less mid-tropospheric support and shorter-duration rains. Schroeder (1981) demonstrated that many Hawaiian storm rain distributions are inherently connected to the interaction of low-level winds with local topographic features. RAINFALL RECURRENCE INTERVALS Rainfall frequency analyses for Oahu have been prepared by the U.S. Weather Bureau (U.S. Weather Bureau, 1962) and by Giambelluca et al. (1984). The former analyses used daily rainfall accumulations and estimated shorter-duration amounts based on empirical relationships derived for continental rains (i.e., those that occur at 100-year intervals). The latter incorporated autographic raingauge data with standard gauges. The U.S. Weather Bureau's analyses were based on empirical distributions. Giambelluca et al. (1984) fitted the Gumbel Type I extreme probability distribution to annual maximum series for each station. Table 5 compares the observed rainfalls at Maunawili to estimates for 100-year recurrence intervals using the U.S. Weather Bureau and the Giambelluca analyses. Totals for 2, 3, 6, and 24 hours exceed the 100-year recurrence value for both sets of estimates. However, the 1-hour rainfall total does not exceed 100-year recurrence values. The estimated recurrence interval for the recorded 6-hour Maunawili rainfall (15.7 inches) is approximately 200 years (Figure 8). The same is true for raingauge no. 794.3 (15.3 inches in 6 hours).

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 Figure 7 Map of southeast Oahu showing the location of raingauges (X) and streamgauges (filled circles) discussed in the text.

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 TABLE 4 Rainfall at U.S. Geological Survey Raingauge No. 794.3 and National Weather Service Telemetered Raingauge No. 795.1 Time Period 795.1 (NWS) 794.3 (USGS) 12:00 p.m. – 6:00 p.m. 4.70 0.90 6:00 p.m. – 8:00 p.m. 6.40 4.10 8:00 p.m. – 9:00 p.m. 2.20 3.00 9:00 p.m. – 10:00 p.m. 3.10 3.60 10:00 p.m. – 11:00 p.m. out 1.90 11:00 p.m. – 12:00 a.m. out 2.00 12:00 a.m. – 1:00 a.m. out 2.30 1:00 a.m. – 2:00 a.m. out 1.00 Note: Time periods are adjusted to account for irregular sampling of the NWS raingauge. TABLE 5 Rainfall at Maunawili (State Raingauge No. 787.1) Duration (hours) Rainfall (inches) 100-year-TR 43 100-year-DOWALD 1 4.0 5.0 4.5 2 7.4 7.0 6.2 3 10.4 8.5 7.8 6 15.7 12.0 11.0 24 22.9 19.0 18.0 Note: The 24-hour total is from an adjacent standard raingauge. TR 43 = U.S. Weather Bureau (1962); DOWALD = Giambelluca et al., (1984). All estimates are for a recurrence interval of 100 years. When the New Year's Eve 24-hour isohyetal map is superimposed over the 100-year, 24-hour rainfall map of Oahu from Giambelluca et al. (1984), the rainfalls in the upper half of Niu, Kuliouou, and Hahaione watersheds, as well as much of the Maunawili and Waimanalo watersheds, exceed the 100-year recurrence interval values (Figure 9). At the Kailua fire station, 5.5 miles windward of the crest of the Koolau range, 6-hour rainfalls were less than those expected every 10 years. The same is true of the raingauge at the Hawaii Kai Golf Course, which is located 1 mile southeast of the eastern edge of the Koolau crest. The upper Manoa and Palolo valleys, usually

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 Figure 8 Type-I Gumbel frequency distribution for 1965–1986 annual maximum 6-hour rainfall at Maunawili (state raingauge no. 787.1). Rainfall totals such as that from the New Year's Eve storm are expected to recur only every 100–200 years. areas of heavy rainfall, received only 2-year and 10-year rains, respectively. The extreme rains fell in a localized region over and immediately windward of the Koolau ridge. A MODEL OF THE NEW YEAR'S EVE STORM Figure 10 depicts the situation along the windward slope of the Koolau range on New Year's Eve. Freshening north-northeasterly winds associated with a slowly moving shear line lifted along the southern rampart of the Koolau Mountains above Waimanalo. In the presence of strong vertical shear of the horizontal winds, the orographic cumuli that developed sheared off at about 12,000 feet. Steady rains of 2 to 4 inches per hour fell over the presoaked mountains. The shear precluded the thunderstorm activity that was forecasted. Torrential rains have occurred from clouds of similar vertical extent (Schroeder, 1978a, 1981; Cram and Tatum, 1979).

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 Figure 9 24-hour rainfall accumulation for southeastern Oahu for 8:00 a.m. December 31 through 8:00 a.m. January 1, 1988. (Solid curves). Superimposed by dashed curves are 100-year 24-hour rainfall estimates (based on Giambelluca, 1984).

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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988 Figure 10 Schematic of meteorological conditions over the southern Koolau mountains on New Year's Eve, 1987. Low-level north-northeast winds are lifted over the mountains, then sheared by strong south-southwesterly winds. WIND DAMAGE As the shear line moved over Oahu, north-northeasterly winds increased in strength to 40 to 50 mph over the Koolau and Waianae crests and funneled through leeward valleys. Unofficial reports of 60- to 80-mph gusts were received at the NWS's Forecast Office. The sources of these reports were anemometers on the tops of tall buildings in the Honolulu area. Damages included loss of roofs and power line outages. The most significant damage was to the roof of St. Patrick's School in eastern Honolulu.