MONTHLY PRECIPITATION SUMMARY State of Hawaii portion of Honolulu HSA MONTH: January 1999 PREPARED: February 13, 1999 State: A period of cold front/shear line activity that started in mid-December 1998 continued through the first nine days of January 1999. This includes a shear line passage across the state on New Year's Day and a cold front passage on 8 and 9 January. Shower development in convergent pre-frontal flow prompted the issuance of an urban and small stream flood advisory for east Kauai and east Oahu on 7 January. A flash flood watch was issued on 8 January for the island of Oahu, though no significant flooding resulted. Heavy showers that accompanied the front brought minor flooding and the issuance of urban and small stream flood advisories for Maui County and portions of the island of Hawaii on 8 and 9 January. A trade wind weather pattern returned on 10 January and brought relatively dry conditions across the state through 20 January. From 21 to 31 January, a series of upper level troughs moved over the islands and destabilized the local airmass. This helped trigger a period of wet trades, especially over the eastern half of the Hawaiian Islands. Flash flood warnings were issued for portions of east Maui on the afternoon of 22 January based on radar data. No significant damages or casualties were reported. The heavy rain on Maui occurred during a period when the entire state was under a flash flood watch. Urban and small stream flood advisories were required for portions of Kauai, Oahu, and Hawaii during the same afternoon. Hail (3/8 inches), a rare phenomenon in the islands, was also reported over the slopes of Haleakala on the island of Maui. Continued moist and unstable conditions resulted in heavy showers over the eastern portions of the island of Hawaii on 24 and 25 January. Both days required the issuance of urban and small stream flood advisories. Thunderstorms developed over the islands of Hawaii and Maui on 30 and 31 January. Radar data indicate that the storm cells moved too rapidly across the windward slopes of the island of Hawaii to produce large accumulations or flooding problems on the night of 30 January. However, redevelopment of thunderstorms over east Hawaii on the night of 31 January dropped up to five inches of rain over Hilo. A flash flood warning was issued for this event and road closures due to flooding were reported. On east Maui, a train of storm cells moved onshore near the Kipahulu Gulch (south of Hana), prompting a flash flood warning for this area. Radar rainfall estimates show that these cells, aided by the orography, dropped two to four inches of rain in about six hours. Two landslides were reported in east Maui. Island of Kauai: A good start for rainfall across Kauai with all gages above 50% of normal and two gages above normal for January. The higher than normal amounts were observed at Kapahi (10.54 inches, 121 percent of normal) and Wailua (10.85 inches, 145 percent of normal). More than half of the monthly totals at both sites were received during 7 January (pre-frontal convergence over Kauai) and 22 January (moist trades enhanced by an upper level trough). Kalaheo, a site in south Kauai, received 6.85 inches of rainfall which is 96 percent of normal. Island of Oahu: Locations near the Koolau Range saw the most rain during the month. The influence of the terrain in enhancing local rainfall was also very apparent with interior and upslope sites receiving much higher amounts than coastal lowland locations. The interior sites include the Wilson Tunnel gage (12.29 inches, 106 percent of normal) and the Manoa Lyon Arboretum gage (15.02 inches, 114 percent of normal). Coastal lowland sites include the Waimanalo gage with 4.79 inches (59 percent of normal), and the gage at the Punaluu Pump, with 4.08 inches (46 percent of normal). Sites in central Oahu received totals between 60 and 70 percent of normal. Most of the rain in this area occurred during the period of wet trades from 21 to 31 January. Cold front activity during early January brought some relief to portions of west Oahu, though totals from all sites in this area were less than 50% of normal. Kunia was especially dry with only 0.93 inches, or 18 percent of normal. Maui County: Data from the available gages in Maui County show all totals to be below the monthly normals. Gages along the northern coast of the West Maui Mountains reported the highest amounts (Mahinahina, 5.30 inches, 85 percent of normal; and Kahakuloa, 4.33 inches, 70 percent of normal) with most of the rain observed during the aforementioned wet trades period. Sites in central Maui reported between 30 and 50 percent of normal for the month. The Hana and Haiku gages, both along windward Haleakala, likely had the highest totals but maintenance problems resulted in missing data. Hawaii County: Another very wet month for the windward side of the island of Hawaii. All sites in the Hamakua, Hilo, and Puna districts reported higher than normal rainfall for the month. The highest total was recorded at Glenwood (28.15 inches, 152 percent of normal), though the largest anomaly was at Mountain View (188 percent of normal). Most of the windward rainfall occurred during the last eleven days of the month. Showers associated with cold front activity early in the month helped boost rainfall totals to 50 to 75 percent of normal in the Kona and Kau districts. THE FOLLOWING LIST CONTAINS RAINFALL STATISTICS FOR SELECTED LOCATIONS FROM AROUND THE STATE. THE FIRST COLUMN IS THE OBSERVED RAINFALL FOR JANUARY. THE SECOND COLUMN IS THE 30-YEAR AVERAGE FOR THAT LOCATION WHILE THE THIRD COLUMN LISTS THE PERCENT OF AVERAGE RAINFALL FOR THE MONTH OF JANUARY. KAUAI JAN AVG %AVG LIHUE 4.40 5.9 75 HANALEI 8.92 11.3 79 HANAPEPE 3.43 5.0 69 OAHU HONOLULU 2.12 3.6 59 MANOA ARBOR. 15.02 13.2 114 HAWAII KAI M 4.6 M AHUIMANU 7.04 12.4 57 WAIANAE 1.86 3.8 49 WAIALUA 1.77 5.6 32 MOLOKAI KAUNAKAKAI 1.52 3.3 46 LANAI LANAI CITY 1.36 6.1 22 MAUI KAHULUI 2.01 4.1 49 HANA M 9.5 M KIHEI 1.26 4.1 31 LAHAINALUNA 0.77 4.4 18 WAILUKU 2.57 5.2 49 HAWAII HILO 16.68 9.9 168 PAHALA 4.58 7.7 59 HONAUNAU 2.75 3.7 74 KAMUELA (UPPER) 4.05 6.3 64 GLENWOOD 28.15 18.5 152 LAUPAHOEHOE 16.65 13.5 123 Data Sources: First order climate network weather observation 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