Guam/CNMI:  Throughout Guam and the CNMI, the rainfall during the first half of 2010 was below normal, with most locations throughout these islands experiencing 60% to 80% of average rainfall. The island of Rota in the CNMI was the only recording location in the region to experience above-normal rainfall during the first half of 2010, largely a result of a wet January. Guam and the CNMI are typically drier than normal during the first half of a year that follows El Niño. February and May were particularly dry with monthly rainfall at or below one inch. Grassland fires were common. Low rates of stream flow at popular waterfall sites on Guam caused their base pools to become stagnant and fill with strands of algae. Isolated thunderstorm activity in late June provided northern Guam with some much needed rainfall. Elsewhere, it remained quite dry through the second quarter months, with some locations receiving an increase in their June rainfall from isolated thunderstorm activity.

Guam and CNMI Rainfall Summary 2nd Quarter and 1st Half 2010

Climate Outlook: The rainy season has commenced on Guam and in the CNMI. The average rainfall on Guam is at or above 10 inches per month from July through October. Interestingly, the average rainfall at Saipan is roughly 2 inches less than the monthly rainfall on Guam for all months of the year. During the years that follow El Niño, the dry season on Guam and in the CNMI is drier than normal, and is often prolonged into July. The rainy season of years that follow El Niño is also typically shorter and drier than normal in Guam and in the CNMI. These two island groups are among the few places in Micronesia where rainfall does not fully recover to normal or above normal annual amounts following El Niño. Guam and the CNMI depend on tropical cyclone and monsoon trough activity for much of their rainy-season rainfall, and following an El Niño event, the weather becomes tranquil (inhibited monsoon with a reduction of tropical cyclones). The odds are cut in half (to 10%, but not zero) 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. Rainy-season totals during the years that follow El Niño around 85 to 95 percent of normal, with stronger El Niño events tending to have the greater influence (consider the extreme dry conditions of the years that followed both the 1982 and 1997 El Niño events).

Predicted rainfall for Guam and the Mariana Islands from July 2010 through June 2011 is as follows:

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

source: UOG-WERI