Several synoptic features influenced the weather across the U.S.-Affiliated Pacific Islands (USAPI) during this USDM week (7/29/20-
8/04/20). Upper-level lows or troughs [TUTT (Tropical Upper Tropospheric Trough) cells] hovered over western Micronesia and skirted northern portions; a near-equatorial trough developed in the west with an Inter-Tropical Convergence Zone (ITCZ) developing further east across the Federated States of Micronesia (FSM) and Marshall Islands (RMI); and a monsoon trough extended from the Philippines into western Micronesia at one point. Surface troughs, weak circulations, and tropical disturbances migrated through the ITCZ and near-equatorial trough, while differential trade-wind speeds aided surface convergence. The surface convergence was enhanced by upper-level divergence associated with the TUTT cells. South of the equator, a high pressure ridge and surface front/shear line took turns bringing dry and wet weather to American Samoa.
Satellite-based estimates of 7-day precipitation (QPE) showed a broader ITCZ stretching across the equatorial Pacific between the equator and 11 degrees North latitude from Indonesia to beyond the Date Line, and another band of precipitation associated with the South Pacific Convergence Zone (SPCZ) extending southeastward from Indonesia. The QPE showed areas of 2+ inches of rain across Palau and Yap State, with a band of 2+ to locally over 4 inches of rain stretching across parts of Chuuk, Pohnpei, and Kosrae States into the RMI. Tendrils of one inches or more extended beyond these core areas while lesser amounts spotted the Marianas. A blob of 1+ inches of rain was measured by the satellite QPE just north of the Samoan Islands, with arms of lesser amounts extending across American Samoa.
The Palau International Airport (PIA) received 3.47 inches of rain this week, and the Koror COOP measured 3.44 inches (1 day missing). For the month of July 2020, the PIA measured 10.13 inches, while the COOP received nearly an inch more (11.12 inches). The weekly and monthly precipitation totals at both locations exceeded the minima required (2 inches and 8 inches, respectively) to satisfy most water needs. Three of the past four weeks in Palau have been wet (i.e. rainfall totals exceeded the weekly minimum threshold). The Republic of Palau therefore remains drought-free.
The island of Guam received 1.36 inches of rain this week, which comfortably surpassed the 1-inch minimum weekly threshold considered adequate to meet most water needs. Guam's July rainfall total of 8.26 inches more than doubled the monthly minimum threshold of 4 inches. Guam continues to remain drought-free. Across the CNMI (Commonwealth of the Northern Mariana Islands), 1.04 inches of rain was reported this week at Rota, while 4.48 inches of rain fell during July. Marginally exceeding the weekly and monthly rainfall minimum thresholds helped to offset further degradation. Rota's drought designation therefore remains at D3(SL). Tinian measured only 0.51-inch of rain (1 day missing), resulting in a dry week, and 4.97 inches for the month of July. Significant variations in rainfall were observed across Saipan this week. A manual gauge at the Saipan International Airport reported 1.45 inches, ASOS measured 1.18 inches, and the National Park Service observed 0.45-inch of rain this week. The respective rainfall totals for July were 5.11 inches, 3.61 inches, and 3.18 inches. As was the case with Rota, this rainfall helped to offset additional deterioration, and no change was made to Saipan's current drought designation (D3-SL).
Widespread variations in rainfall were also observed across the FSM (Federated States of Micronesia) this week. Yap, Ulithi, and Rumung, in western Micronesia, measured 1.87 inches, 1.04 inches (1 day missing), and 1.10 inches of rain (2 days missing), respectively. The monthly precipitation totals for July were 7.71 inches, 9.61 inches, and 7.27 inches. The drought designations for Yap and Ulithi remain at D-nothing (drought-free conditions) this week. Nearby Woleai experienced a dry week this week (1.44 inches). In July, 13.81 inches of rain fell (1 day missing), which easily surpasses the 8-inch minimum. Woleai's depiction therefore remains at D-
nothing. In central portions of Micronesia, Fananu, Kapingamarangi, and Chuuk Lagoon reported week/month rainfall pairs (respectively) of 1.44 (2 days missing)/8.79 inches, 1.38/5.59 inches, and 1.67 (1 day missing)/7.77 inches. Kapingamarangi was dry in July, especially during the first half of the month. Most of the received rainfall fell in the second half of the month, mainly due to low-level wind convergence zones and troughs situated between 4N and 10N. No changes were deemed necessary to these station drought designations this week, though Kapingamarangi (D2(S)) requires monitoring. No changes were needed for Lukunor and Nukuoro this week, with week/month rainfall pairs (respectively) of 6.11 (1 day missing)/13.18 inches and 3.26/10.43 inches. Both of these locations easily satisfied the weekly and monthly minimum rainfall requirements of 2 inches and 8 inches. In fact, 9 of the past 12 weeks were wet in Nukuoro. In eastern portions of Micronesia, the week/month rainfall pairs (respectively) for Pingelap, Pohnpei, and Kosrae (airport) were 3.83/12.86 inches, 3.78 (1 day missing)/9.54 inches, and 4.32/14.26 inches. Other monthly rainfall amounts measured on Kosrae included 13.57 inches at the Nautilus Hotel, 17.79 inches at Tofol, and 14.45 inches at Utwa. In addition, 11 of the past 12 weeks have been wet in Kosrae (airport location). All three stations retain their drought-free status.
Across the RMI (Republic of the Marshall Islands), the atoll of Kwajalein measured 1.72 inches of rain this week and 8.82 inches for the month of July. Kwajalein continues with D0(S) conditions this week. Jaluit and Ailinglapalap measured 1.03 inches and 2.24 inches of rain this week, respectively. For July, the totals were 9.23 inches and 10.56 inches. Both stations remain drought-free this week. No
rainfall observations (either weekly or monthly) was available at Utirik, which precluded an analysis for this location. Week/Month rainfall pairs (respectively) at Wotje, Majuro, and Mili were 0.93/3.58 inches, 1.80/12.85 inches, and 5.45 (1 day missing)/12.42 inches of rain. Wotje's drought condition (D3(S)) warrants future monitoring and possible degradation, though no change was rendered to its drought depiction this week. Both Majuro and Mili retain their drought-free status. As of 4 August 2020, Majuro's reservoir freshwater storage was at 31.27 million gallons, which translates to 86.86 percent of maximum capacity.
South of the equator, American Samoa experienced large variations in observed rainfall this week. Pago Pago reported 0.61-inch of rain. For July, Pago Pago reported 11.17 inches of rain, which is almost three times the monthly minimum threshold of 4 inches. Two National Park Service sites, Siufaga Ridge and Toa Ridge, reported 1.51 inches and 4.27 inches of rain (respectively) this week. Accordingly,
drought-free conditions persist at Tutuila in American Samoa.