Guam, Northern Mariana Islands Unknown Hydrologic Outlook | Effective 08/20/2026 23:41

ESFGUM This Hydrologic Outlook is for Guam, Rota, Tinian, and Saipan. A monsoon surge is developing well south of Tropical Storm Saudel and will lift northward across the Marianas late this weekend and into next week. Some uncertainty remains regarding potential rainfall impacts, but early model ensemble guidance estimates suggest 4 to 8 inches of rainfall falling Sunday night through Thursday across Guam and Rota, and 3 to 7 inches across Saipan and Tinian. Locally higher amounts will be possible, especially if the monsoon surge is stronger than forecasted. Scattered to numerous locally heavy showers and isolated thunderstorms will be possible through the week. Residents on these islands need to closely monitor this developing situation as flash flood watches, flood advisories and flash flood warnings could be issued later. If living near streams and rivers, prepare to move items away from stream and river banks. Make sure storm drains nearby are not clogged, especially If living at low-lying areas. This product will be updated once a day or replaced with a Flash Flood Watch.