Guam/CNMI:  For all of June, July, August and September, the weather on Guam has been rather tranquil. Winds have been very light (easterly for the most part, with a few periods of light westerly wind flow), only one tropical cyclone (Man-Yi) adversely affected the island (it generated near-gale winds for one day in early July), and there have been no extreme island-wide heavy rainfall events. In association with La Niña, monsoon southwest winds remained mostly to the west of Guam, and tropical cyclone development was also pushed to the west and north of Guam. In an environment of light wind, rainfall was produced on-island by short-lived (6-12 hr) mesoscale convective systems, and by isolated (mostly daytime) thunderstorms that affected only parts of the island on any given day. Daytime thunderstorms have produced isolated heavy rainfall totals near 2 or 3 inches in a few hours at some locations, however, and over the course of a month, rainfall amounts in this weather pattern tend to average near to slightly below normal. Surprisingly, the highest 3rd Quarter rainfall on Guam was at the WSO where the 3-month total was 37.00 inches (98% of normal). Three-month rainfall totals at other locations were lower: 32.71 inches at Ypapao, 34.15 inches at the UOG, 35.70 inches at AAFB, and 36.91 inches at the upper Ugum watershed in the southern mountains.

A similar tranquil weather pattern dominated the CNMI for the heart of the summer rainy season (June through mid-October). Most rainfall stations reported about ten inches less total rainfall than on Guam. In terms of percent of normal rainfall, it was drier in much of the CNMI than on Guam. Among all Guam and CNMI reporting stations, the island of Rota in the CNMI topped the 3rd Quarter rainfall list with 37.38 inches (101% of normal). Elsewhere in the CNMI, rainfall totals for the 3rd Quarter averaged around 25 inches.

Guam and CNMI Rainfall Summary 3rd Quarter 2007

Climate Outlook: During La Niña years, the tropical cyclone season in the western North Pacific basin is often delayed, and the number of tropical cyclones through mid-July is typically below normal. Also during La Niña years, the site of formation of the basin’s tropical cyclones is shifted to the west. There is a tendency for typhoon activity to return to near normal in the latter part of the year, although the westward shift continues. This reduces the threat of typhoons on Guam , the CNMI, and for those islands from Chuuk eastward to the RMI. For the remainder of 2007 (and January 2008), one or two tropical storms and one typhoon may pass within 200 miles of any Guam and CNMI location. (This represents a lower than normal risk of adverse impact by a tropical storm or typhoon for this time of year.) Rainfall is anticipated to be near- to above- normal for Guam and the CNMI for the remainder of 2007, and to be near normal through the dry season of 2008.

Forecast rainfall for Guam and the CNMI from November 2007 through October 2008 is as follows:

source: UOG-WERI