The weather conditions across the U.S.-Affiliated Pacific Islands (USAPI) during this USDM week (3/25/20-3/31/20) consisted of a continuation of a dry trade-wind regime across northern portions of Micronesia and convection associated with the Inter-Tropical Convergence Zone (ITCZ) in southern portions. The dry trade-wind regime is associated with the North Pacific Subtropical High which normally shifts southward during this time of year, with the ITCZ normally also shifting more toward the equator. Also, unusually warmer-than-normal sea surface temperatures near the Dateline the past several months have enhanced convection near the Dateline and suppressed convection farther west over much of Micronesia, especially far western Micronesia. The ITCZ took the form of a near-equatorial surface trough across southern portions of central to western Micronesia and trade-wind convergence across southern portions of central to eastern Micronesia. Weak circulations inhabited the ITCZ in the western sections early in the week while trade-wind disturbances developed in the eastern sections later in the week. A cold front/shear line early in the week, and trade-wind disturbance later in the week, brought isolated showers to the Marianas. South of the equator, a shear line early in the week created showers across the majority of the Samoan Islands, with the showers enhanced by orographic lift and daytime heating. Drier, more stable air moved over the islands later in the week.
Satellite-based estimates of 7-day precipitation (QPE) showed a rain band hugging the equator in the western Pacific, forming the ITCZ. The band split in two further east, with one arm stretching to the northeast north of the equator and another arm extending southeast south of the equator as the South Pacific Convergence Zone (SPCZ). In Micronesia, the satellite QPE depicted areas of 4 inches or more of rain across southern Micronesia, with amounts rapidly decreasing between 5 and 8 degrees north latitude. Little to no rain was depicted by the satellite analysis over northern portions of Micronesia. The satellite QPE depicted a broken band of 2 to 4 inches of rainfall extending northwest to southeast across the Samoan Islands, with little precipitation evident on either side of the band. SPCZ rain could be seen further to the west.
In the Republic of Palau, the last 4 weeks have been dry (less than the 2-inch weekly minimum needed to meet most water needs) at Koror, with 1.19 inches recorded this week and 4.66 inches of rain for the month. This ranks March 2020 as the 13th driest March in the 69-year record, which translates to the 19th percentile. February-March had 15.16 inches which is the 31st driest February-March. According to local reports, vegetation from Koror and Airai’s stand point is looking more lime green and yellow. D0-S continued for Palau this week, but D1-S may be considered in the future if the dryness continues or impacts worsen.
Dry (less than the one inch weekly minimum) conditions continued this week for most of the Marianas. Weekly rainfall totals for the airport stations included 0.78 inch at Guam, 0.84 inch at Rota, and 0.90 inch at Saipan. But a few automated rain gauges on Guam recorded around 0.75 inch from Tuesday’s showers, and Tinian reported 1.10 inches for the week. The NPS automated station on Saipan recorded only 0.35 inch for the week. With 1.33 inches for the month, Guam had the 8th driest March in the 64-year record, with December-March ranking fifth driest. The KBDI (Keetch-Byram Drought Index) at Guam was well past 700, indicating an extreme fire risk. D2-S continued on Saipan and D1-S continued on Guam and Rota.
It was feast or famine again in the Federated States of Micronesia. Fananu has been missing for several months, so this station could not be analyzed. Weekly rainfall totals for the reporting stations ranged from zero at Woleai, 0.04 inch at Rumung, and 0.06 inch at Chuuk, to 6.05 inches at Lukonor. D-Nothing continued at Kosrae, which reported 4.38 inches for the week and 11.33 inches for the month, and at Pohnpei where 3.35 inches was reported for the week, bringing the March total to 9.05 inches. This week had 1.90 inches of rain at Kapingamarangi, but the previous 2 weeks were wet and the month totaled 14.52 inches, so D-Nothing continued. Even though Nukuoro recorded 3.38 inches for the week and 17.40 inches for March, D0-S continued to reflect lingering agricultural impacts. D1-S continued at Woleai which had the fifth driest January-March. Chuuk recorded 0.06 inch for the week (with one day missing), but 5.26 inches for the month which ranks as the 15th driest March. February-March ranked as the 7th driest, so D0-S continued. Ulithi had 0.70 inch of rain for the week, marking the 8th consecutive dry week. With 1.74 inches for the month, March 2020 ranked as the 4th driest March in the 37-year record and February-March ranked as the 8th driest such 2-month period. The status at Ulithi was worsened to D2-S. With 6.05 inches of rain for the week and 12.04 inches for the month, the status at Lukonor was improved to D0-S. Yap reported 0.18 inch of rain for the week and 1.14 inches for the month. Yap had the third driest March and fourth driest January-March. D2-S continued at Yap this week, but the status could be worsened to D3-S if the dry conditions continue. Pingelap had 1.10 inches for the week and 4.90 inches for the month. This ranked March 2020 as the 9th driest March. February-March ranked 7th driest, January-March 6th driest, and December-March 5th driest. D0-S continued at Pingelap, but the status could be worsened to D1-S if dry conditions continue.
In the Marshall Islands, the week was wet in the south and dry in the north. Mili recorded 4.10 inches for the week, keeping the status at D-Nothing, and Jaluit 3.00 inches, which improved the USDM status to D-Nothing. But it was dry at the rest of the reporting stations, with weekly rainfall totals ranging from 0.26 inch at Kwajalein to 1.09 inches at Majuro. Wotje received 0.85 inch of rain on the 28th, but that was the only day with measureable rainfall this month, so D3-S continued. Utirik reported 0.96 inch for the week, bringing the monthly total to 2.27 inches. With each of the last 4 months dry, D2-S continued at Utirik. D1-S continued at Kwajalein, which reported 0.26 inch of rain this week and 1.54 inches for the month. With the Majuro reservoir at 26.65 million gallons on March 30, this is 74% of maximum, which is below the threshold for concern. Majuro reported 1.09 inches of rain for the week, and Ailinglapalap 0.60 inch. Both of these stations continued at D0-S, but their status could be worsened to D1-S if the dryness continues.
In American Samoa, Pago Pago (3.96 inches) and the automated station at Toa Ridge (1.12 inches) recorded more than the weekly minimum rainfall of 1 inch, while less than an inch was measured at the automated station at Siufaga Ridge (0.76). The March rainfall total at Pago Pago was 8.19 inches, twice the monthly minimum needed to meet most water needs. With a wet week and month, February-March and December-March ranking second wettest in Pago Pago’s 54-year record, and January-March wettest on record, D-Nothing continued at Tutuila.