Guam/CNMI: Throughout Guam and the CNMI, the weather and climate of 2010 was dull and uneventful. Rainfall throughout the region was generally below normal for most months. During the rainy season (roughly July through November) there was an unusual lack of tropical cyclone activity, and there was a notable lack of episodes of southwest monsoonal winds and squalls. An episode of hazardous surf at the end of December was responsible for a tragic accident at the reef line of Tumon, Guam, which claimed the life of two canoe paddlers. Also on Guam during 2010, there were two separate freak accidents in which weakened tree limbs broke and fell on individuals, resulting in death in both cases.

The dry season is now underway on Guam and in the CNMI. Dry weather began early, with November and December having well below normal rainfall totals. Two slow-moving shear line passages produced some heavy showers that pushed the rainfall total in January 2011 above the totals seen for both November and December. The average monthly rainfall on Guam follows a sinusoidal pattern with a peak of near 15 inches during September, and a minimum of approximately 3 inches during March. At the CNMI locations north of Guam, the average rainfall tracks the same sinusoidal curve as on Guam, but each month has less rainfall (e.g., at Saipan, the average monthly rainfall is 1-2 inches less than it is on Guam).

Guam and CNMI Rainfall Summary 4th Quarter 2010

Climate Outlook: During years that begin in La Niña and then transition towards ENSO-neutral, the spring rainfall is typically near normal or above normal. Guam and the CNMI are now entering the heart of their dry season. March is typically the driest month of the year in this region, with 3 to 4 inches of rainfall on Guam and only about 2 to 3 inches on Saipan. During April through June, the average rainfall increases by about 1 inch per month (e.g. on Guam: March = 3inches, April = 4 inches, May = 5 inches, and June = 6 inches). In July, the rainy season usually begins and the monthly rainfall jumps to 9 or 10 inches. These typical values are anticipated this year during these months.

Guam and the CNMI depend on tropical cyclone activity for much of their rainy season rainfall. Following an El Niño event, and during most La Niña years, the weather is tranquil (inhibited monsoon with a reduction of tropical cyclones). The odds are reduced (i.e., from 20% to 10 %) for the occurrence of a tropical cyclone. The chances for extremes of rainfall (4 or more inches of rain in 24 hours) are similarly reduced. Rainfall is reduced the most during the years that follow El Niño; it is near normal during La Niña; and, the wettest years tend to occur when La Niña wanes to ENSO-Neutral or the beginning of another El Niño. During 2011, it is anticipated that the most likely scenario would be for the current La Niña conditions to slowly fade back to ENSO-neutral by June or July. In this scenario, Guam and the CNMI would probably have above normal rainfall during the 2011 dry season (i.e., January through June).

Predicted rainfall for Guam and the Mariana Islands from January 2011 through December 2011 is as follows:

Forecast rainfall quantities represent BEST ESTIMATES given the probabalistic forecast for each particular season and station.

source: UOG-WERI