MONTHLY PRECIPITATION SUMMARY State of Hawaii portion of Honolulu HSA MONTH: February 1999 PREPARED: March 7, 1999 State: The trade winds, strong at times, marked the first ten days of the month across the Hawaiian Islands. These trades brought occasional periods of heavy showers over the windward side of the island of Hawaii, but no damage was reported. Rainfall totals for the rest of the state were relatively modest. The initial period of trade wind weather was followed by the passage of a weak cold front across the island chain on 11 and 12 February. This system was accompanied by rainfall totals of less than 0.50 inches at most locations across the state and a few reports of 1.00 to 1.50 inches from the windward side of the island of Hawaii. The trailing high pressure ridge settled in just to the north of the state bringing stable weather conditions with land and sea breezes through 18 February. On 19 February, a weak shear line pushed across the state and lined itself up with the island of Hawaii the following day. An upper level shortwave trough then dropped in from the northwest and became superposed over the shear line. This combination resulted in an unstable airmass over the eastern end of the island chain and helped trigger heavy showers and thunderstorms on the morning of 20 February. Urban and small stream flood advisories were subsequently issued for the eastern districts of the island of Hawaii. Showers and thunderstorms continued to develop through the day, which prompted the issuance of a flash flood watch for the windward districts of the island. The watch was later expanded to cover the entire island of Hawaii and all of Maui. The axis of the upper level trough moved east of the island chain during the early morning hours of 22 February and the subsiding air to the west of the trough stabilized the airmass. As much as 13 inches of rain fell over the windward side of the island of Hawaii during the three-day event leaving some low-lying areas flooded, though no major damage was reported. Minimal rainfall was observed statewide through 25 February where light to moderate trade winds were accompanied by stable conditions aloft. A weak shear line stalled over the island of Kauai on 26 February and dropped up to 1.50 inches on the windward slopes of the island. The rest of the state remained dry through the end of the month. Island of Kauai: The jump start for 1999 rainfall in January reversed itself in February. Nearly all stations reported less than 50% of normal as Kauai missed out on most of the wet trade wind events and experienced only weak synoptic systems in the westerlies. The highest rain total for the month belonged to the Wainiha gage at 9.22 inches. This amount should be higher but a malfunction on 25 February resulted in a loss of data. The Hanalei gage, which is another windward site, reported 6.91 inches (68 percent of normal). Island of Oahu Like Kauai, the island of Oahu experienced a relatively dry February. Leeward sites, especially those at the lower elevations were especially dry with five gages reporting less than one inch of rain for the entire month. These areas normally receive between two to four inches of rain during this time of year, mainly from synoptic-scale systems such as cold fronts. Windward Oahu was also quite dry, despite periods of sustained trade winds. Noteworthy windward totals include 0.45 inches at Waimanalo (just 9 percent of normal) and 0.71 inches at Olomana (16 percent of normal). Gages along the interior sections of the island fared better with amounts about 60 to 80 percent of normal. The highest amount recorded on the island again went to the gage at the Manoa Lyon Arboretum which observed 7.95 inches, or 78 percent of normal. Note that this amount of rain is just slightly more than half of what it received in January (15.02 inches). Maui County The island of Maui was far enough east to experience the wet and unstable airmass that resulted from the superposition of the upper level low over the shear line cloud band. Windward locations were relatively wet with gages reporting amounts greater than 50 percent of normal. The site at Kahakuloa, along the northeast coast of West Maui, observed 4.66 inches of rain (106 percent of normal) and Kahului Airport received 2.12 inches (73 percent of normal). For the Kahului Airport station, this was the largest monthly total since December 1997. Another windward gage, located at Haiku, observed 7.31 inches or 128 percent of normal. A maintenance problem at this site resulted in a loss of data so the total rainfall was underestimated. Both Molokai and Lanai were very dry with Molokai's gages reporting less than an inch of rain for February (Kaunakakai, 0.34 inches or 13 percent of normal; Kamalo, 0.60 inches or 14 percent of normal). Hawaii County: February marked 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 Puna District was extremely wet with both the highest observed rainfall (Glenwood, 28.77 inches or 198 percent of normal) and the largest anomaly (Pahoa, 23.20 inches or 213 percent of normal). Most of the windward rainfall occurred during the first week of the month and during the period from 20 to 22 February. The leeward areas appeared to have been as dry as the windward areas were wet. The gage at Kamakoa reported a meager 0.10 inches (4 percent of normal) and the gage at Honaunau reported 0.40 inches (13 percent of normal). THE FOLLOWING LIST CONTAINS RAINFALL STATISTICS FOR SELECTED LOCATIONS FROM AROUND THE STATE. THE FIRST COLUMN IS THE OBSERVED RAINFALL FOR FEBRUARY. 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 FEBRUARY. KAUAI JAN AVG %AVG LIHUE 1.12 3.3 34 HANALEI 6.91 10.1 68 HANAPEPE 1.66 3.0 55 OAHU HONOLULU 0.74 2.2 33 MANOA ARBOR. 7.95 10.2 78 NIU VALLEY 1.04 3.9 27 WAIHEE PUMP 2.37 9.5 25 WAIANAE 0.33 2.3 14 WAIALUA 0.74 3.9 19 MOLOKAI KAUNAKAKAI 0.34 2.6 13 LANAI LANAI CITY 1.46 4.1 36 MAUI KAHULUI 2.12 2.9 73 HANA M 6.8 M KIHEI 0.28 2.2 13 LAHAINALUNA 1.66 2.7 61 WAILUKU 2.32 3.8 61 HAWAII HILO 19.34 10.3 188 PAHALA 5.17 6.1 85 HONAUNAU 0.40 3.2 13 KAMUELA (UPPER) 5.31 5.1 104 GLENWOOD 28.77 14.5 198 LAUPAHOEHOE 16.36 13.2 124 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