MONTHLY PRECIPITATION SUMMARY State of Hawaii portion of Honolulu HSA MONTH: May 1999 PREPARED: 7 June 1999 State: The month of May started off with light trades and very little rainfall across the Hawaiian Islands. After 10 May, persistent troughing in the large-scale weather pattern near the Dateline began to slowly push eastward, resulting in a veering of the low level winds to a southeasterly direction. A few rain bands, possibly formed by convergence in the lee of the island of Hawaii, moved over Kauai and Oahu and produced brief heavy showers on 12 May. By 15 May, large-scale troughing had deepened and pushed far enough south to develop a late- season subtropical cyclone (a.k.a. "kona storm") to the west of Kauai. Rainfall from this system mainly affected the islands of Kauai and Oahu starting in the evening hours of 15 May and lasting through 17 May. A Flood Potential Outlook was issued on the morning of 16 May, followed by a Flash Flood Watch that night. No flash flooding was reported, though several Urban and Small Stream Flood Advisories were issued to cover minor flooding over portions of Kauai and Oahu. Trade winds resumed at light to moderate intensities on 18 May and continued through 27 May with relatively low rainfall totals. Trades once again veered to a southeasterly direction on 28 May through the end of the month with no significant rainfall observed by our network during this period. Island of Kauai: The mid-month kona storm brought some relief to the island of Kauai with 3-day rain totals of 3 to 4 inches. These totals accounted for more than half of the monthly rain at 10 out of 11 sites. Since May is a transition month between the cool and warm seasons, rainfall totals are usually low and the occurrence of such a late-season storm is bound to push rain amounts above normal at one or more locations. For May 1999, five sites reported higher than normal rainfall (Anahola, Kapahi, Lihue Airport, Kalaheo, Hanapepe) and three (Kokee, Wailua, Omao) reported amounts in the 90 to 100 percent of normal range. Hanapepe showed the largest anomaly with its 3.76 inches being 251 percent of normal. For the year, rainfall totals remain below normal across the island, mainly in the 60 to 80 percent of normal range. Island of Oahu Like Kauai, the island of Oahu received widespread rainfall from the kona storm that was noteworthy for its relatively even spatial distribution. Amounts were mainly in the 2 to 3 inch range across the entire island. The even distribution suggests that rainfall occurred from the stratiform precipitation region of offshore thunderstorms and/or large-scale rainfall processes rather than orography-induced showers. Rains from the kona storm helped push May totals above normal at 16 out of 28 gages on the island. Anomalies were largest over west Oahu, which is normally dry during this time of the year. For example, the site at Waianae collected 2.86 inches during May, or 409 percent of the normal 0.70 inches. This was the first time the Waianae gage reported above normal rain since September 1997. Another interesting aspect of May's rainfall was that the Manoa Lyon and Nuuanu sites, normally the wettest in Oahu's network, reported totals only 47 and 44 percent of normal, respectively. Both sites normally benefit from the orographic enhancement of trade wind showers, and aside from the kona storm rains, May's trade winds were relatively dry. Available year-to-date data show that rainfall remains below normal across the entire island, though the Manoa Lyon site is close at 99 percent of normal. Most of the remaining sites are in the 40 to 60 percent range. Maui County For the most part, Maui County was not a beneficiary of the kona storm rains as the 3-day storm totals for all sites were less than one inch. However, at dry sites such as Kihei in the lee of Haleakala volcano, any rain this time of year is significant. The gage at Kihei received 0.59 inches for May, which is 295 percent of normal (0.20 inches). All of this rain fell on 16 and 17 May during the kona storm event and marked the first measurable rain at this site since 21 February of this year. The only other above normal site was Kaunakakai on the island of Molokai with 1.27 inches for the month, or 141 percent of normal (0.90 inches). Otherwise, rainfall over the rest of Maui County was unremarkable. Year-to-date data show all Maui County sites with below normal rainfall through May. Note that this statistic is not available for sites along the windward slopes of Haleakala due to significant data gaps. Island of Hawaii: May continued the general drying trend that started back in mid-April. For the first time since March 1998, all network gages on the island of Hawaii reported below normal rainfall. Even windward sites, which consistently reported higher than normal totals the last nine months, saw only 20 to 30 percent of normal rainfall. The exception is the gage at Glenwood, which reported 7.48 inches, or 62 percent of normal. Despite the dry month of May, windward year-to-date totals are mostly above normal. The aforementioned Glenwood gage has seen 114.36 inches through 31 May, or 139 percent of normal (82.30 inches). Gages in the Kona and Kau Districts reported continued rainfall deficits with year-to-date values in the 30-40 percent of normal range. THE FOLLOWING LIST CONTAINS RAINFALL STATISTICS FOR SELECTED LOCATIONS FROM AROUND THE STATE. THE FIRST COLUMN IS THE OBSERVED RAINFALL FOR MAY. 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 MAY. THE FOURTH AND FIFTH COLUMNS ARE THE YEAR-TO-DATE TOTAL AND THE PERCENT OF AVERAGE, RESPECTIVELY. YTD KAUAI MAY AVG %AVG YTD %AVG LIHUE AIRPORT 4.39 3.1 142 14.28 71 HANALEI 3.48 8.1 43 32.45 58 HANAPEPE 3.76 1.5 251 10.35 69 OAHU HONOLULU AIRPORT 2.14 1.1 195 5.82 55 MANOA ARBOR. 5.75 12.2 47 62.62 99 NIU VALLEY 3.60 2.1 171 12.47 64 WAIHEE PUMP 4.91 9.7 51 26.49 53 LUALUALEI 2.62 1.2 218 7.14 55 WAIALUA 3.59 1.7 211 8.76 49 MOLOKAI KAUNAKAKAI 1.27 0.9 141 3.73 38 LANAI LANAI CITY 1.11 2.9 38 8.02 40 MAUI KAHULUI AIRPORT 0.51 0.8 64 6.46 53 HANA 0.97 6.2 16 N/A N/A KIHEI 0.59 0.2 295 2.13 22 LAHAINALUNA 0.49 0.7 70 3.96 33 WAILUKU 0.33 1.2 28 6.88 41 HAWAII HILO AIRPORT 2.84 9.9 29 67.03 113 PAHALA 0.61 3.8 16 12.14 42 HONAUNAU 1.30 7.0 19 8.43 34 KAMUELA (UPPER) 0.47 3.9 12 28.90 100 GLENWOOD 7.48 12.0 62 114.36 139 LAUPAHOEHOE 1.13 11.7 10 58.81 77 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