The weather conditions across the U.S.-Affiliated Pacific Islands (USAPI) during this USDM week (5/20/20-5/26/20) were dominated by dry trade winds in the north and an Inter-Tropical Convergence Zone (ITCZ) further south. Faster trade winds converging upon slower trade winds enhanced the precipitation across the Federated States of Micronesia (FSM) and Marshall Islands (RMI), while a near-equatorial trough and other surface troughs brought rain to parts of western Micronesia. A weak circulation briefly developed in central Micronesia. The ITCZ rains extended south but ended just short of Kapingamarangi. South of the equator, a surface trough lingered just southwest of the Samoan Islands. The trough teased American Samoa with moisture and unstable air for part of the week, competing against high pressure with a drier and more stable air mass.
Satellite-based estimates of 7-day precipitation (QPE) showed the ITCZ as a band of precipitation extending from Micronesia eastward across the Pacific between the equator and 10 degrees North latitude. A broken band of precipitation extended from Indonesia southeastward past the Samoan Islands as the South Pacific Convergence Zone (SPCZ). Two-plus inches of rain were indicated on the QPE across the FSM and southern RMI, with embedded areas of 4+ inches, and surrounded by lesser amounts to the north and south. Areas with 1+ inches of rain stretched westward across Palau, while little to no rain was indicated across the Marianas and northern RMI. South of the equator, the QPE showed a large area of 4+ inches of rainfall south of the Samoan Islands, with a band of 2+ inches stretching to the west of the islands and weaker fingers extending across the islands.
A surface trough continued a wet pattern across the Republic of Palau this week, dumping over 4 inches of rain on Koror (5.47 inches was recorded at the Palau IAP and 4.31 inches at the Koror COOP station). The monthly totals were well above the 8-inch monthly minimum: 23.71 inches at the Palau IAP and 22.22 inches at Koror COOP, both through May 25. As reported by the National Weather Service, the May monthly total for the weather station at the Palau Airai IAP was a record, surpassing the previous May record of 22.71 inches in 2005. The record May precipitation for the Koror COOP station is 27.46 inches which occurred in May of 1954. With April and May wet, D-Nothing continued at Palau.
It was another dry week for the Marianas, with a third of an inch or less of rain measured at all of the main reporting stations. Reports ranged from 0.07 inch at the Saipan IAP and 0.09 inch at the Guam airport station to 0.33 inch at Tinian and 0.37 inch at Rota (Saipan’s value had one day missing this week). The last 2 weeks were dry (less than the 1-inch weekly minimum needed to meet most water needs) at Guam, but beneficial showers fell early in the month. However, the showers did not reach the Commonwealth of the Northern Marianas Islands, with this week marking the 16th consecutive dry week at Saipan & Rota. With 0.58 inch of rain so far in May, the Saipan IAP has the driest May, April-May, March-May, February-May, and January-May on record. With 0.84 inch so far in May, Rota has the driest May, April-May, and March-May. Southern portions of Guam have had a few wildfires recently but relatively light winds and recent rains have limited spread. Wildfires have been a daily occurrence across Saipan. D3-SL continued for Saipan, D3-S continued at Rota, and D1-S continued for Guam.
The ITCZ created a wet band that split the FSM in half. The week was wet (more than the 2-inch weekly minimum) across central and eastern portions, but dry in parts of the north, west, and south. More than 2 inches of rain was reported this week at Chuuk (2.45 inches), Pohnpei (4.81), Kosrae (8.28), Lukonor (3.42), and Nukuoro (5.65). But it was dry in the far south, where Kapingamarangi measured 0.74 inch for the week, and in western Yap State, where Yap had 1.05 inches, North Fanif 1.43, Ulithi 1.04, and Woleai 1.59 (Woleai’s value had one day missing this week). With this week marking the third consecutive dry week, and the monthly total (so far) of 3.62 inches ranking as the third driest May on record, the status at Kapingamarangi was changed to D0-S. Even though Woleai received less than the 2-inch weekly minimum this week, the previous weeks have been wet and the monthly total of 10.50 inches is above the 8-inch monthly minimum, so the status was improved to D1-S. Woleai still has the seventh driest January-May and fourth driest June-May. The last 2 weeks have been dry at Yap and the reservoir still remains low, so D2-S continued for Yap. February-May ranked 6th driest and January-May 8th driest, so D0-S continued at Chuuk. D1-S continued at Ulithi, where the last 2 weeks have been dry. D-Nothing continued at the rest of the FSM stations.
In the RMI, no data was received this week so an analysis could not be made for Utirik. ITCZ rain gave southern portions of the Marshall Islands a wet week, with Jaluit reporting 6.31 inches of rain, Mili 3.54, and Majuro 2.53. It was dry at Wotje (0.15 inch for the week), Kwajalein (0.03), and Ailinglapalap (1.18). With May wet (10.02 inches of rain so far) and the reservoir level at 85% of maximum (above the 80% threshold for concern), D-Nothing continued at Majuro. D-Nothing also continued at Jaluit and Mili, where May has been wet. Drought continued at Wotje (D3-SL), Kwajalein (D1-SL), and Ailinglapalap (D1-S).
A nearby surface trough lent some moisture to convective instability across parts of American Samoa, where weekly rainfall totals ranged from 0.51 inch and 1.10 inches at the automated stations on Toa Ridge and Siufaga Ridge, respectively, to 1.30 inches at the Pago Pago Airport station. With the May monthly total at Pago Pago (6.03 inches) and Siufaga Ridge (4.67) above the 4-inch monthly minimum, and this month continuing a string of wet to very wet months, D-Nothing continues at Tutuila.