Surface and upper-atmosphere weather features brought rain to parts of the U.S.-Affiliated Pacific Islands (USAPI) during this USDM week (6/24/20-6/30/20). Surface troughs, trade-wind convergence, and tropical disturbances developed or migrated across Micronesia, mainly between the equator and 10 degrees North latitude. Upper-level troughs with embedded lows (Tropical Upper Tropospheric Troughs [TUTT cells]) traversed Micronesia, mainly north of 10 N. They provided divergence aloft which enhanced showers generated by the surface convergence. A dry trade-wind flow continued across the Marianas, with the dryness occasionally interrupted by showers triggered by migrating surface troughs and the upper-level features. The surface weather features across Micronesia are a manifestation of the climatological Inter-Tropical Convergence Zone (ITCZ) which straddles the equator across the globe. South of the equator, active surface troughs moved across the Samoan Islands, dropping abundant rainfall. These surface troughs are a component of the climatological South Pacific Convergence Zone (SPCZ).
Satellite-based estimates of 7-day precipitation (QPE) showed the ITCZ as a broken and narrowing band of precipitation across the Pacific mainly between the equator and 12 N. Across Micronesia, a large area of 2+ inches of precipitation was depicted over Palau and western Yap State, with narrower broken bands extending eastward across the Federated States of Micronesia (FSM) and Republic of the Marshall Islands (RMI). The rain bands became sparse farther east in the region. The QPE detected areas of 1+ inches of rain across the far northern Marianas associated with the TUTT cells. The SPCZ was depicted by a broken band of precipitation with embedded 2+ inch areas extending southeast from Indonesia to past the Samoan Islands. The QPE showed areas of 2+ inches of rain near the islands to the north, west, and south, with a heavy band extending across American Samoa.
This week was wet (more than the 2-inch weekly minimum needed to meet most water needs) in the Republic of Palau, with 3.56 inches of rain recorded at the Palau IAP and 2.62 inches at the Koror COOP station. With the monthly total at both stations (14.21 inches and 12.64, respectively) above the 8-inch monthly minimum, and April and May also wet, D-Nothing continued at Palau.
This week was another dry (below the 1-inch weekly minimum) week at Guam (0.67 inch), Rota (0.38), and Saipan. Two-thirds of an inch or less of rainfall was recorded at the Saipan IAP (0.51), Saipan ASOS station (0.63), and Saipan NPS station (0.20). Monthly totals were below the 4-inch monthly minimum at Guam (3.40 inches), Rota (2.33), Saipan IAP (1.96), Saipan ASOS (1.86), and Saipan NPS (1.53). With the continued dryness, D4-SL continued for Saipan, D3-SL continued at Rota, and D1-SL continued for Guam.
In general, the week was wet across western and some central portions of the FSM, with spotty wet conditions in the east and south. More than 2 inches of rain was reported at North Fanif (4.48 inches), Rumung (4.07), Yap (4.16), and Ulithi (2.95) in the west; Kosrae (7.12) and Mwoakilloa (2.62) in the east; Pohnpei (2.13) and Fananu (3.10) in the central region; and Kapingamarangi (3.24) and Nukuoro (5.60) in the south. Lukonor (0.99), Chuuk (1.32), Pingelap (1.40), and Woleai (1.70) had the lowest weekly rainfall reports. June monthly totals were above the 8-inch monthly minimum at most stations. Two changes were made this week compared to last week. Yap was improved from D0-S to D-Nothing and Ulithi was improved from D1-S to D0-S. This week marked the fourth consecutive week that Yap has been wet (more than 2 inches), and the monthly total for June is 18.56 inches and May had 8.22 inches, both above the monthly minimum. The value for June 2020 ranks as the 8th wettest June out of 69 years, and the May-June total (26.78 inches) ranks May-June 2020 as the 16th wettest May-June. At Ulithi, the 2.95 inches this week brings the monthly total to 8.67 inches, which is above the monthly minimum. May was also above with 9.90 inches. The last two weeks were dry at Woleai, so D0-S continues. Fananu recorded 3.10 inches of rain this week, which is above the weekly minimum of 2 inches. But the previous three weeks were dry, and the June monthly total is still only 7.19 inches, which is below the monthly minimum. Local reports indicate that the water supply is normal and food crops look normal. Fananu continued at D0-S to reflect the recent meteorological dryness. The rain this week at Kapingamarangi resulted in a monthly total for June of only 5.87 inches, which is well below the 8-inch monthly minimum, and previous weeks were very dry, so D2-S continued. D-Nothing continued elsewhere in the FSM.
In the RMI, Utirik has been missing for several weeks and thus could not be analyzed. The week was dry at Jaluit (0.91 inch) and Wotje (1.14), but wet at the rest of the regular reporting stations, with amounts ranging from 2.29 inches at Kwajalein to 6.67 inches at Ailinglapalap. The June total at Kwajalein was 6.09 inches, which is below the 8-inch monthly minimum, so D1-SL continued. This week was the fourth consecutive dry week at Wotje, and the monthly total is only 4.66 inches, so D3-SL continued. The monthly total at Jaluit was only 6.81 inches. The last two weeks were dry, but the previous week was wet. If the recent dryness continues, D0-S may be considered for Jaluit, but D-Nothing continued for now. June rainfall totals were above the monthly minimum at the remaining stations, where D-Nothing continued.
This was another wet week in American Samoa. Weekly rainfall totals were 3.83 inches at Pago Pago, 5.74 inches at the automated station at Siufaga Ridge, and 3.50 inches at the automated station at Toa Ridge. June monthly totals were 20.76, 22.19, and 14.73, respectively, which are above the 8-inch monthly minimum. With June continuing a string of wet to very wet months, D-Nothing continued at Tutuila.