Guam/CNMI:  The first half of 2009 was generally drier than normal throughout Guam. Persistent trade winds accompanied especially dry conditions at some Guam locations during the 2nd Quarter months. Despite dry conditions, the weather remained just wet enough to sustain southern Guam’s drinking water sources at the Ugum River pumping stations and the Fena Reservoir, and to reduce the number of dry season grass fires. The rainfall totals during the first half of 2009 were approximately 20-25 inches at all recording locations, ranging from 19.25 inches at WFO Guam to 26.60 inches at the head of the Ugum watershed in the southern mountains. The weather on Guam during the first half of 2009 continued to be mundane, as it has been for much of the past three years, with no unusually heavy rains or strong winds.

The 2nd Quarter 2009 and 1st half of 2009 rainfall totals in the CNMI were generally drier than normal, with six-month totals near 25 inches, similar to Guam. However, a particularly wet June – with 8.25 inches and 8.32 inches recorded at the Saipan International Airport and Rota Airport, respectively – inflated seasonal rainfall totals, masking the true extent of persistent dryness. For the first time in recent history, it was noted that Donni Spring in the central Saipan highlands completely dried up; the combination of low rainfall and new wells in the area were thought to have contributed to this. The weather continued its long period of tranquility throughout the CNMI during the first half of 2009 with no unusually heavy rains or strong winds, some brief periods of heavy showers in June notwithstanding.

Guam and CNMI Rainfall Summary 2nd Quarter 2009

Climate Outlook: The 2009 dry season on Guam and in the CNMI persisted through June. In early July, the monsoon trough developed normally across Micronesia, and the rainy season commenced on Guam and in the CNMI. With El Niño conditions now developing in the Pacific Basin, the tranquil weather of the past three years may be coming to an end. All islands should have abundant rainfall through the end of 2009 and may see an increase in the frequency of heavy rainfall events (i.e., 2 inches or more in 24 hours), with high month-to-month variability. Thereafter, the rainfall amounts will depend on the evolution of El Niño: if El Niño persists into the first half of 2010, near normal rainfall will continue, but if El Niño ends in the first half of next year, tranquil and dry conditions will resume. Very dry conditions in Guam and throughout Micronesia are almost always experienced after a strong El Niño, and sometimes follow a weak or moderate El Niño as well.

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

source: UOG-WERI