Guam/CNMI: With the exception of the El Niño year of 2009, the past several years have seen no strong episodes of the SW monsoon, a sharp reduction of tropical cyclone activity, and few notable extremes of rainfall throughout Guam and the CNMI. During the El Niño year of 2009, the northern islands of the CNMI were affected by two intense typhoons: Melor and Choi-Wan, and intense Typhoon Nida passed between Guam and Yap. During 2010, very quiet conditions resumed. Relatively tranquil conditions continued in the 1st Quarter of 2011, but there were hints of change in continuing the quiet character of the climate and weather. For the first time in a long time, it was quite wet at most locations. The rainfall during January 2011 was nearly double (200%) the normal value at all locations – an odd uniformity for rainfall of showery convective origin. During the three-month period January through March, there were some episodes of 2 or 3 inches of rainfall in 24 hours. Distant lightning was observed several times in offshore convective cloud systems. Tropical Depression 02W passed to the north of Guam and the CNMI on April 04 bringing brisk westerly winds to island waters.

Guam and CNMI Rainfall Summary 1st Quarter 2011

Climate Outlook: The evolution of ENSO from La Niña to ENSO-neutral usually brings wetter weather to Guam and the CNMI. This shift of the status of ENSO, coupled with observed increase in extreme rain events and other indicators of greater instability in the regional atmosphere during the 1st Quarter of 2011 portend a more active rainy season than has been seen in the past few years. This could mean the occurrence of a few moderate or strong episodes of the southwest monsoon during the upcoming rainy season, an increase in extremes of rainfall (e.g., 4 in or more during a 24-hour period), and an increase in tropical cyclone activity in regional waters. The anticipated increase of tropical cyclone activity will bring the risk of a typhoon to near normal (a 15 to 20% risk of gales or higher from a nearby TC passage) for Guam and the CNMI, which may seem quite active with respect to the several recent very quiet years. The highest risk of a typhoon in the region occurs during El Niño, when the risk of gales in regional waters increases to 25-35%, or three times the odds for non-El Niño years (1-in-10).

Predicted rainfall for Guam and the Mariana Islands from April 2011 through March 2012 is as follows:

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

source: UOG-WERI