MONTHLY PRECIPITATION SUMMARY State of Hawaii portion of Honolulu HSA MONTH: October 1998 PREPARED: November 2, 1998 State: Flash Flood Warning issued for the North and South Hilo districts on the island of Hawaii on 15 October. Urban and Small Stream Flood Advisories issued for portions of the island of Hawaii on 13, 15, and 16 October. The trade winds remained in place over the Hawaiian Islands for almost the entire month of October. However, an upper level low pressure system moved over the island chain from the east and destabilized the airmass during the period from 12 October through 17 October. This resulted in a period of heavy shower formation, mainly over the island of Hawaii and, to a limited extent, on the island of Maui. The rest of the state did not experience enhanced shower activity from this upper level low. October is considered a transition month between the warm and cool seasons. The first shear line or cold front passage of the cool season usually occurs during the latter half of October. These types of systems sometimes produce heavy rainfall over the state and are important sources of rain for the leeward sections of the islands. No fronts or shear lines were able to push far enough south to reach the Hawaiian Islands during the entire month of October. Island of Kauai: All sites reported less than normal rainfall for the month with totals consistently around 50 percent of the October averages. Not surprisingly due to the persistent trades, the windward side of the island received the highest rainfall totals with the Hanalei gage recording the most rain at 5.10 inches. The lowest amounts were at Anahola (0.82 inches, 16 percent of normal) and at the WSO Lihue (0.98 inches, 22 percent of normal). Both sites are along the lower elevations of the windward side of Kauai. These data reflect the strong influence of the orography on rainfall and the lack of large scale weather systems over or near the Hawaiian Islands. Island of Oahu: Like Kauai, all sites on the island of Oahu recorded below normal rainfall. Most of the totals were below 50 percent of the October normals. One noteworthy exception was the Manoa Lyon Arboretum site which saw the highest total for Oahu at 10.53 inches, or 96 percent of normal. This gage is just to the lee of the Koolau Mountains and can receive high monthly totals during persistent trade winds as was the case during October. Not surprisingly, persistent trade winds also meant very low rainfall totals for sites along the lower elevations of the leeward sections. These sites include Kunia (0.14 inches, 6 percent of normal) and Aloha Tower (0.20 inches, 10 percent of normal). Maui County: All of the sites in Maui County reported below normal rainfall. Some sites over the windward and higher elevations of Haleakala on the island of Maui reported close to normal rain such as Kula (1.29 inches, 92 percent of normal) and Haiku (3.68 inches, 72 percent of normal). Leeward sites on the island of Maui were very dry with several reporting less than 10 percent of normal rainfall. Hawaii County: In contrast to the rest of the state, the island of Hawaii experienced a wet month, largely due to the heavy showers that developed in response to the aforementioned upper level low. All gages over the windward districts reported above normal rainfall with the highest total being from Mountain View (21.84 inches, 180 percent of normal). Leeward gages reported amounts that were below normal. The largest 24-hour total came from the gage at Pahoa, which saw 4.60 inches on the evening of 16 October during the passage of a line of thunderstorms over the lower elevations of the Puna District. Thunderstorm development over the slopes of the Kona District on 13 October produced 2.10 inches at Waiaha and helped bring rainfall for the month up to 70 percent of normal. BELOW ARE RAINFALL STATISTICS FOR SELECTED LOCATIONS FROM AROUND THE STATE. THE FIRST COLUMN IS THE RAINFALL FOR OCTOBER. THE SECOND COLUMN HAS THE 30 YEAR AVERAGE FOR THAT LOCATION, WITH THE THIRD COLUMN THE PERCENT OF AVERAGE RAINFALL FOR THE MONTH OF OCTOBER. KAUAI OCT AVG %AVG LIHUE 0.98 4.4 22 HANALEI 5.10 7.0 73 HANAPEPE 1.24 2.5 50 OAHU HONOLULU 0.13 2.3 6 MANOA ARBOR. 10.53 11.0 96 HAWAII KAI 0.79 2.7 29 AHUIMANU 4.19 8.4 50 WAIANAE M 0.3 M WAIALUA 0.36 2.7 13 MOLOKAI KAUNAKAKAI 0.38 1.0 38 LANAI LANAI CITY 1.86 2.8 66 MAUI KAHULUI 0.10 1.2 8 HANA 4.29 7.0 61 KIHEI 0.05 0.7 7 LAHAINALUNA 0.13 1.0 13 WAILUKU 0.17 1.7 10 HAWAII HILO 16.01 9.6 167 PAHALA 1.40 4.2 33 HONAUNAU 2.72 5.2 52 KAMUELA (UPPER) 6.76 3.7 183 KAMUELA 3.48 3.3 105 LAUPAHOEHOE 14.63 9.3 157 Data Sources: First order climate network weather observations stations of the National Weather Service at Lihue, HI, Honolulu, HI, Kahului, HI, and Hilo, HI. Additional data were taken from the Hydronet state network of automated rain gages. Data collected in the automated gages are not certified and information used above should be used for information purposes only. Kevin R. Kodama, Senior Service Hydrologist National Weather Service Forecast Office Honolulu, Hawaii