MONTHLY PRECIPITATION SUMMARY State of Hawaii portion of Honolulu HSA MONTH: March 1999 PREPARED: April 5, 1999 State: The month of March saw nearly continuous trade winds over the Hawaiian Islands. The only disruption came at the start of the month when a weak cold front stalled over the island of Kauai. Rainfall from this system was minimal. Although the trades dominated the weather pattern, shear lines and shear line remnants embedded within the trade flow brought relatively wet conditions across many areas of the island chain during 13 to 16 March, 20 to 22 March, and 27 to 29 March. No flash flood warnings were required for any of these events. However, an Urban and Small Stream Flood Advisory was issued on the night of 20 March due to the occurrence of minor flooding in portions the windward side of the island of Hawaii. Island of Kauai: More than half of the sites across the island reported rainfall greater than 50% of normal with most of the rain coming from the shear line on 27 to 29 March. Strong trade winds that accompanied the passage of this system may have helped push rainfall into portions of the drier south-facing slopes. One of the sites in this area is Kalaheo, which received 1.87 inches during the three-day period. The highest March rainfall total, 7.87 inches, belongs to the Wainiha gage, though this is well off the normal amount of 15.80 inches. Island of Oahu A better month than February with respect to rainfall across the island of Oahu. Highest amounts were recorded at the Manoa Lyon Arboretum gage (19.58 inches, 136 percent of normal) and the Nuuanu Water Supply gage (12.56 inches, 94 percent of normal). Other rain gages along the interior sections of the Koolau Range recorded between 60 and 90 percent of normal rainfall for March. Sites near the windward coast of Oahu, such as Punaluu Pump and Waimanalo, reported totals only in the range of 20 to 40 percent of normal. Strong trade winds that accompanied two of the shear lines (20 to 22 March and 27 to 29 March) helped carry shower activity well into the lee of the Koolau Range. This resulted in fairly wet conditions at central Oahu sites such as Waiawa (6.20 inches, 91 percent of normal) and Palisades (6.79 inches, 93 percent of normal). The showers apparently were not able to reach the Honolulu International Airport since this gage reported 0.14 inches, which is only 6 percent of normal. Maui County The persistence of the trade winds in March brought dry conditions for sites along the leeward areas of the island of Maui. The gage at Kihei, which has a normal March rainfall of 2.1 inches, recorded zero rainfall. Other leeward gages include the site at Ulupalakua, which saw only 0.31 inches (normal 4.0 inches), and the Waikapu Country Club site, with 0.50 inches (normal 3.2 inches). Windward sites fared better with the gage at Haiku observing 7.79 inches (110 percent of normal) and the gage at Kahakuloa getting 2.65 inches (55 percent of normal). The highest amount observed throughout the network on Maui was 9.32 inches at the Mahinahina (169 percent of normal). Most of the rainfall at this site occurred during the passage of the second and third shear lines of the month (20 to 22 March and 27 to 29 March). While this gage is not directly exposed to the trade winds, it is suspected that the stronger trade winds that accompanied these shear lines helped push showers over the slopes and into the Mahinahina area. The natural follow-on question then becomes, why did the Lahainaluna gage not receive enhanced rainfall amounts? Is it because the terrain upslope of Lahainaluna is higher and was able to block the showers more effectively? At this point, I have no definitive answer. Hawaii County: March did not bring any relief to the very wet conditions experienced on the windward side of the island of Hawaii. Nearly all sites reported above normal rainfall. In a repeat performance of February, the highest observed total occurred at Glenwood (28.54 inches, 142 percent of normal). The Kamuela Upper gage reported the largest anomaly at 244 percent of normal (16.36 inches observed, 6.7 inches normal for March). Honokaa had the second largest anomaly at 208 percent of normal (20.64 inches observed, 9.9 inches normal for March) and the largest 24-hour rainfall amount at 5.81 inches on 20 March. In contrast, gages in the North and South Kona Districts (leeward side of the island) all received rainfall much below normal with all gages except the Kealakekua site having reported amounts less than 20 percent of normal. THE FOLLOWING LIST CONTAINS RAINFALL STATISTICS FOR SELECTED LOCATIONS FROM AROUND THE STATE. THE FIRST COLUMN IS THE OBSERVED RAINFALL FOR MARCH. 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 MARCH 1999. KAUAI MAR AVG %AVG LIHUE AIRPORT 1.25 4.2 30 HANALEI 6.58 14.0 47 HANAPEPE 0.74 3.2 23 OAHU HONOLULU AIRPORT 0.14 2.2 6 MANOA ARBOR. 19.58 14.4 136 NIU VALLEY 2.61 5.1 51 WAIHEE PUMP 4.98 10.2 49 LUALUALEI 1.15 3.0 38 WAIALUA 0.96 3.8 25 MOLOKAI KAUNAKAKAI 0.45 1.8 25 LANAI LANAI CITY 1.18 3.7 32 MAUI KAHULUI AIRPORT 1.48 2.7 55 HANA 4.17 9.5 44 KIHEI 0.00 2.1 0 LAHAINALUNA 0.86 2.4 36 WAILUKU 1.34 3.6 37 HAWAII HILO AIRPORT 12.13 13.9 87 PAHALA 0.39 6.3 6 HONAUNAU 0.76 5.1 15 KAMUELA (UPPER) 16.36 6.7 244 GLENWOOD 28.54 20.1 142 LAUPAHOEHOE 14.00 19.5 72 Data Sources: First order climate network weather observation stations of the National Weather Service at Lihue, Honolulu, Kahului, 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