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FGMY70 PGUM 210641
ESFGUM
GUC010-MPC100-110-120-220645-

Hydrologic Outlook
National Weather Service Tiyan GU
441 PM ChST Fri Aug 21 2026


...A MONSOON SURGE MAY BRING HEAVY RAINFALL, FLODDING, AND MUDSLIDE
POTENTIAL TO THE MARIANAS BEGINNING SUNDAY AFTERNOON...

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.

$$

Williams