Guam/CNMI:  Overall the 1st Quarter of 2008 was wetter than normal at most Guam and CNMI locations. The month-to-month variability was high, however, with a very wet February followed by a dry March. February’s rainfall was 8 to 10 inches across all of Guam, keeping the island green and its streams running full. After the occurrence of roughly 2 inches of rainfall on Guam during a 24-hour period spanning the 8th and 9th of March, the rest of the month was very dry. Grasslands became desiccated to such a point that frequent grass fires erupted. Stream flow slowly declined, with seasonal algal growth matting the stream beds and choking up popular swimming holes. The highest 1st Quarter rainfall on Guam fell on locations in the central and northern part of the island, where a nearly 3-inch 24-hour extreme event helped push February’s total over 8 inches. Three-month rainfall totals at all Guam locations ranged from approximately 13 inches in drier locations to nearly 15 inches at the wetter locations.

Persistent showers yielded 3 to 5 inches of rainfall each month of the 1st Quarter in the CNMI. Though not excessive amounts of rain, these values were generally above the typical values of rainfall for the heart of the dry season. For example, the 12.98 inches of rainfall at the Saipan Coast Guard Station during the 1st Quarter of 2008 was 164% of the 3-month normal. Due to their unique geology, the islands of the CNMI retain a green and well-watered appearance, whereas on Guam, the dry conditions of March and early April have resulted in a noticeable browning of the grasslands and partial defoliation of some of the tangantangan trees that are the dominant cover in much of Guam’s extensive shrub lands.

Guam and CNMI Rainfall Summary 1st Quarter 2008

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. These effects of La Niña can already be seen in this year’s tropical cyclone distribution: below normal activity and a westward shift of that activity (both of the 2008 Tropical Cyclones to date have been in the South China Sea). For the remainder of 2008, two or three tropical storms and one typhoon may pass within 200 miles of any location within Guam and the CNMI. This represents an overall slightly lower than normal risk of adverse impact by a tropical storm or typhoon as compared to normal years. As La Niña wanes in the latter half of 2008, the typhoon activity may return to near normal in the western North Pacific basin, providing Guam and the CNMI with the greatest threat of a damaging tropical cyclone in the fall. Rainfall is anticipated to be near normal to slightly above normal for Guam and the CNMI for the remainder of 2008 (including the rest of the 2008 dry season and the upcoming rainy season), and be near normal through the dry season of 2009.

Forecast rainfall for Guam and the CNMI from May 2008 through April 2009 is as follows:

source: UOG-WERI