During the Northern Hemisphere winter, the North Pacific subtropical high, and the low latitude Inter-Tropical Convergence Zone (ITCZ), migrate south toward the equator. The seasonal migration brings drier subtropical trade winds to the northern portions of Micronesia and ITCZ rainfall to the southern regions. The weather conditions across the U.S.-Affiliated Pacific Islands (USAPI) during this USDM week (1/15/20-1/21/20) reflected this seasonal pattern. A dry season trade-wind regime was firmly entrenched over all the main islands of Micronesia. A near-equatorial trough was to the south, affecting mainly Kapingamarangi and Nukuoro, plus Indonesia and Kiribati. A couple cold front/shear lines penetrated into Marianas and western Federated States of Micronesia (FSM) waters and the northern Marshall Islands (RMI). Weak surface troughs briefly made an appearance in the central FSM but lingered over the RMI, while trade-wind disturbances brought a few showers to the Marianas and the RMI. South of the equator, Tropical Cyclone Tino (04F) moved west then south of the Samoan Islands. The track kept Tino away from American Samoa, but it was close enough that its wind and rainbands affected the islands. Concurrently, the monsoon trough (South Pacific Convergence Zone, SPCZ) lined up over the islands, enhancing rain over American Samoa.
Satellite-based estimates of 7-day precipitation (QPE) showed very little rain across Micronesia. Scattered areas of 1-4 inches were seen in parts of the southern FSM, and a larger area was south and east of the RMI. This larger area was part of an extensive area of rainfall which was along and just south of the equator and which extended southeast toward and past the Samoan Islands. This 4+ inch heavy rain area was associated with the ITCZ and SPCZ. It should be noted that the satellite QPE is able to detect broadscale synoptic rain patterns but does not capture smaller convective showers well.
Dry conditions persisted this week across Palau, receiving only 0.08 inch of rain. This week marked the fourth consecutive week with precipitation totals less than 1 inch. The month of January (as of January 21) had only 0.92 inch of rain. Due to the persistent dryness affecting Palau, abnormal dryness continued this week.
Dry conditions continued to affect the Marianas Islands this week, with all three stations having less than 1 inch of rain. Rota had the most precipitation at 0.70 inch of rain, while Guam had 0.63 inch, and Saipan’s manual gauge had 0.19 inch (ASOS: 0.10 inch; NPS 0.10 inch). During the last two months, Guam’s monthly precipitation totals have been less than 2 inches of rain, which is much less than the monthly threshold of 4 inches to meet most water needs. Guam’s International Airport had only 2.73 inches of rain from December 2019 through January 21, 2020, which is 28.5% of normal precipitation. For this reason, drought classification was deteriorated to moderate drought (D1-S) for Guam. Rota and Saipan continued to be in abnormally dry conditions this week.
All stations across the FSM had a dry week as they all received less than their 2-inches weekly threshold to meet most water needs. The stations with the least precipitation were Yap (0.04 inch; 2 days of missing data), Kosrae (0.09 inch), Pingelap (no rain was observed during this week), and Woleai (0.03 inch), which had less than a tenth of an inch of rain or no rain at all.
Yap, Lukunor, and Woleai had at least three to four weeks of precipitation totals less than 1 inch of rain. For this reason, their drought classification was changed to abnormally dry.
Kapingamarangi, Pohnpei, Chuuk Lagoon, Kosrae, and Pingelap had drought free conditions this week as this was only the first or second week with precipitation totals below the weekly 2-inches threshold.
Nukuoro had 1.25 inches of rain this week and a total of 9.63 inches of rain for the month so far. Since the January precipitation total was more than the monthly threshold of 8 inches, drought free conditions remained in Nukuoro this week. Ulithi only had 0.17 inch this week, however, last week 7.92 inches of rain fell, securing Ulithi’s drought free status this week. Fananu had no data and was set to missing.
Dry conditions were present across the Marshalls Islands, with all stations analyzed receiving less than half of an inch of rain or no rain at all. The stations located in Majuro and Jaluit had no rain during the week, with Mili and Ailinglapalap reporting only 0.01 and 0.05 inch of rain, respectively. Ailinglapalap, Kwajalein, and Jaluit’s drought classification was changed from D-Nothing to abnormally dry conditions as this was at least the third consecutive week with little to no rain.
Three of the last four weeks have been dry for Majuro, receiving less than the weekly minimum of 2 inches of rain to meet most water needs. According to Majuro’s Water and Sewer Co, Majuro’s reservoir levels were at 66.8% of the maximum capacity of 36 million gallons (as of January 21, 2020), which is well below the critical 80% threshold. For this reason, Majuro’s drought classification was also changed to abnormally dry conditions.
Wotje and Mili were the only two stations that had drought free classifications for this week as this was only the second consecutive week with precipitation totals less than the weekly threshold of 2 inches. Utirik was set to missing this week due to insufficient data at the time of analysis.
Very wet conditions were observed in Tutuila during this drought week as 9.21 inches of rain fell in Pago Pago. This is over twice the monthly precipitation total of 4 inches to meet most water needs. The other stations located in Siufaga Ridge and Toa Ridge also saw wet conditions with precipitation totals at 5.18 inches and 5.80 inches, respectively. Dryness or drought development was not a concern for Tutuila and drought free conditions persisted this week.