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 (2/12/20-2/18/20) reflected this pattern with dry northeast trades dominating in the north and a near-equatorial trough persisting in the south. Cold fronts approached from the north and west, but they manifested mostly as shear lines as they moved across northern and western Micronesia with dry mid-latitude air masses in their wake. A weak trade-wind disturbance developed in the near-equatorial trough south of Koror, and a weak circulation formed in the trough south of the Marshall Islands (RMI). The near-equatorial trough extended south of the equator and spread southeastward toward the Samoan Islands and beyond as the South Pacific Convergence Zone (SPCZ) monsoon trough. Circulations developed within the SPCZ as it lay near or across the Samoan Islands, with two of the circulations prompting advisories at times. These were Tropical Disturbances 93P and 96P.
Satellite-based estimates of 7-day precipitation (QPE) showed a patchy band of 1-4 inches of rainfall across southern portions of Micronesia, associated with the ITCZ, with little to no rain indicated across northern portions except for a few stripes where the shear lines triggered showers. The ITCZ rain band turned southeastward as the SPCZ toward and past the Samoan Islands which were within the 4+ inch area of a wide band of precipitation associated with the SPCZ. Samoan residents reported continuous heavy rainfall with damaging winds, especially associated with the tropical disturbances.
After two consecutive weeks of precipitation totals above 2 inches in Palau and a month-to-date precipitation total of 6.10 inches, drought classification was improved from moderate drought to abnormally dry conditions.
The Marianas had a dry week. All three stations had less than half an inch of rain for the week. Saipan had the least precipitation, receiving only 0.06 inch (ASOS: 0.03 inch; NPS: 0.09 inch), followed by Guam at 0.14 inch, and Rota at 0.29 inch. This week marked the second consecutive week for Guam and Rota having less than half an inch of rain. However, Rota’s month-to-date precipitation total is 5.15 inches, which is more than the monthly threshold of 4 inches to meet most water needs. Abnormally dry conditions persists for another week across Rota and Guam, while Saipan remains in moderate drought for another week.
During this drought week, much of the Federated States of Micronesia had dry conditions. Several locations had less than half an inch of rain, while only two stations (Kosrae and Pingelap) had a very wet week. Kosrae and Pingelap remained in drought free status as both of these stations had over 4 inches of rain for the week. Lukunoch had the least precipitation total for the week, receiving only 0.21 inch of rain. Yap (0.30 inch), Chuuk Lagoon (0.31 inch), Nukuoro (0.40 inch), and Ulithi (0.31 inch) also had less than half an inch of rain, while Kapingamarangi and Pohnpei had 0.60 inch and 0.81 inch of rain, respectively. Although this was the second consecutive week with less than an inch of rain, Chuuk Lagoon, Nukuoro, Ulithi, and Pohnpei remained in drought free status. Lukunoch’s drought status changed from abnormally dry to moderate drought after precipitation totals during six of seven consecutive weeks had less than an inch of rain. Kapingamarangi, Woleai, and Yap remained in abnormally dry conditions this week.
Another dry week affected the Marshalls Islands. The station with the least precipitation was Mili which reported no rain this week. Meanwhile, Jaluit had the most precipitation with only 0.80 inch of rain. After three consecutive weeks of little to no rain, Mili’s drought classification was changed from drought free to abnormally dry. Ailinglapalap and Jaluit remained in abnormally dry conditions. Although Majuro had 1.82 inch of rain this week, the reservoir levels were at 64.4% of the maximum capacity (as of February 19), which is below the critical 80% threshold. For this reason, Majuro's drought classification was downgraded to moderate drought. Wotje had 1.50 inches of rain this week, which is slightly below the weekly threshold of 2 inches to meet most water needs. However, precipitation totals the last five weeks have been less than an inch, with the last three weeks having no rain at all. For this reason, Wotje’s drought classification was downgraded to moderate drought. Kwajalein's drought classification remained at moderate drought.
Drought free conditions remained across American Samoa as it was another wet week. Pago Pago had 7.97 inches of rain this week, while Siufaga Ridge (4.20 inches) and Toa Ridge (4.34 inches) had over 4 inches of rain this week. Overall, Pago Pago has had 22.15 inches of rain so far for the month of February, with Siufaga Ridge and Toa Ridge having over 11 inches this month.