Guam/CNMI:  Most Guam and CNMI locations have been relatively dry through April 2009, with 1st Quarter rainfall totals rainfall less than 10 inches at many locations. The 1st Quarter total of 6.00 inches (76% of normal) at Tinian was the driest reading in the region, and the 14.77 inches (108% of normal) on the island of Rota was the wettest 1st quarter total for the region. There was a general lack of any large-scale rainproducing weather systems and no early season tropical cyclone activity. Streams on Guam are running slow, and algal growth has proliferated in stagnant low flow. Most stream channels are filled with debris (dead palm fronds and other leafy waste). This debris and algal growth will be flushed to the sea with the first significant rainfall of the season, which may not occur until June. Grassland wildfires have occurred throughout much of southern Guam, burning away vast swaths of multi-year sword grass stands. The burns have not yet been as extensive as those ignited during the drought of 1998 when 12% of Guam’s land area was scorched.

Almost half of the quarterly rainfall total on Guam occurred during a heavy rainfall event on the 4th of January. On that day, 4 to 5 inches of rainfall fell island-wide. This was the heaviest 24-hour rainfall on Guam in over 2 years! The cause was an outbreak of convection along a shear line that was moving slowly southward through Guam and the CNMI. Embedded convection along the east-west oriented shear line passed continually over the island in a phenomenon known as “training”, analogous to the passage of the many cars of a freight train over any place along the railway. The heaviest of rainfall events in Guam and in the CNMI almost always involve training of showers embedded along a band (such as tropical cyclone rain bands, the monsoon cloud band, and shear lines). A lack of such training situations (i.e., no monsoon, no tropical cyclones, and few shear lines) is the reason for the lack of extreme rainfall events in Guam and in the CNMI since the latter half of 2006.

Guam and CNMI Rainfall Summary 1st Quarter 2009

Climate Outlook: Rainfall is anticipated to be slightly below normal for Guam and the CNMI for the remainder of the dry season (through June), and continue to be slightly below normal as the rainy season gets underway during July through September. The months of October through December may be wetter than normal if tropical cyclone activity returns to near normal in the region. The tranquil weather patterns of the past two years is expected to continue at least through the beginning of the rainy season in July, and no extreme of heavy rainfall (i.e., 4 inches or more in a 24-hours) is anticipated until tropical cyclone activity returns to a more normal distribution some time in the fall.

The 2009 typhoon season of the western North Pacific is already experiencing a delay. The number of tropical cyclones is anticipated to be near normal in the western North Pacific basin during the typhoon season of 2009, but will still exhibit a displacement of the distribution of the activity to the west. This displacement of cyclone activity should not be as extreme as that noted for the past two years. For all of 2009, one or two tropical storms and one typhoon may pass within 200 miles of any Guam or CNMI location (this represents a slight reduction of risk). The greatest risk of a damaging tropical cyclone will be during the months of September through December.

Predicted rainfall for Guam and the Mariana Islands from April 2009 through March 2010 is as follows:

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

source: UOG-WERI