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Pacific ENSO Update

2nd Quarter, 2010 Vol. 16 No. 2


Guam and CNMI text Guam/CNMI:  Throughout Guam and the CNMI, the rainfall during 2009 was split into two regimes: relatively tranquil and dry conditions during the first half of the year, and wet conditions during the second half of the year that included several rainfall extremes (e.g., more than 5 inches in 24 hours) not seen in over three years. El Niño started late in 2009, with the atmosphere exhibiting a La Niña-type pattern of tranquil weather with few extremes during the first half of the year. During July and August, the developing El Niño was accompanied by a reasonably well-developed monsoon trough near the islands. In August 2009, tropical disturbances brought abundant rainfall to most Guam locations, exceeding 25 inches in a month for the first time since August 2005.

During 2009, several tropical cyclones passed near the islands of Guam and the CNMI, with Typhoon Nida making a direct strike on Alamagan. Vegetation and homes on this island were severely damaged, but no serious injury or loss of life was reported. Hazardous surf (10-15 feet) was noted on Guam when Typhoon Nida passed to the southwest in November. Earlier, Saipan was brushed by Typhoon Melor, with hazardous surf, gusty winds, and heavy rainfall affecting the island. Only minor damage to exposed vegetation was noted on Saipan.

During the first quarter of 2010, rainfall was below normal nearly everywhere in Guam and the CNMI, with only Rota experiencing above normal rainfall. February was particularly dry, with less than one inch of rain experienced at many locations. Several shear lines, as well as the passage of Tropical Storm Omais to the south, brought a return of rainfall to near or above normal in March, which is normally the driest month of the year. Overall, dry conditions persist, causing low water levels in many streams on Guam, and wildfires in southern grasslands. One spectacular brush fire scorched the whole Lonfit Valley, and forced the evacuation of homes along the road from Nimitz Hill to Mount Alutom.

Guam and CNMI Rainfall Summary 1st Quarter 2010 and Annual 2009

Station   Jan. Feb. Mar. 1st Qtr
Predicted
2009 Annual
Guam
Guam Intl. Airport (WFO)
Rainfall (inches)
4.69
1.06
4.40
10.15
8.92
97.13
% of Normal
105%
28%
148%
91%
80%
107%
Anderson AFB
Rainfall (inches)
6.31
1.79
4.04
12.14
11.99
86.88
% of Normal
111%
34%
99%
81%
80%
88%
Dedado (Ypapao)
Rainfall (inches)
7.36
0.98
4.31
12.65
12.05
109.46
% of AAFB
129%
19%
105%
84%
80%
111%
Ugum Watershed
Rainfall (inches)
5.48
0.76
3.02
9.26
11.95
111.69
% of WSMO
96%
15%
74%
62%
80%
113%
Sinajaña
Rainfall (inches)
3.97
0.80
3.90
8.67
8.89
107.95
% of WFO
89%
21%
131%
78%
80%
119%
Commonwealth of the Northern Mariana Islands
Saipan Intl. Airport
Rainfall (inches)
2.51
1.12
2.41
6.04
5.35
85.16
% of Normal
73%
47%
120%
79%
70%
114%
Capitol Hill
Rainfall (inches)
2.48
0.94
2.90
6.32
6.60
88.63
% of Normal
62%
31%
116%
67%
70%
98%
Tinian Airport
Rainfall (inches)
4.77
0.73
2.04%
7.54
6.68
77.28
% of Normal
119%
24%
82%
79%
70%
93%
Rota Airport
Rainfall (inches)
13.51
1.91
8.39
23.81
10.88
93.40
% of Normal
256%
41%
227%
175%
80%
99%
Predictions made for 1st Quarter 2010 made in 4th Quarter 2009 Pacifc ENSO Update newsletter.
* The Weather Service Meteorological Observatory (WSMO) (now closed) was located at Finagayan, in the northwest side of Guam.

Climate Outlook: The dry season on Guam and in the CNMI is underway. Although abundant rainfall returned to many low latitude islands in March, mechanisms favoring heavy rain will not reach Guam’s latitude until July. Dry conditions are therefore likely to persist through June 2010. Normal April, May, and June (AMJ) 3-month rainfall ranges from 15 to 18 inches on Guam and 12 to 15 inches on Saipan. During AMJ 2010 totals of 10 to 12 inches and 8 to 10 inches (70 to 80 percent of normal) are expected on Guam and Saipan, respectively. The rainy season should begin nearly on time in early or mid-July. Western North Pacific tropical cyclone activity will likely be delayed, and pushed to the west, reducing the risk of tropical cyclones in Guam and the CNMI for at least the first half of 2010. During the second half of the year the threat of a tropical cyclone should return to near normal. The normal threat of typhoon force winds on Guam and on Saipan is expected to be 15 to 20 percent (between 1 in 7 and 1 in 5) over the course of a year, with the greatest threat in the months of October and November.

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

Inclusive Period

% of Long-Term Average Rainfall /
Forecast Rainfall (inches)
Guam/Rota
Saipan/Tinian
April– June 2010
(2nd Half of Dry Season)
75%
(12.32 inches)
75%
(6.44 inches)
July– September 2010
(Heart of Next Rainy Season)
95%
90%
October – December 2010
(End of Next Rainy Season)
95%
100%
January– March 2010
(Onset of Next Dry Season)
100%
100%

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

source: UOG-WERI



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Page Last Modified: May 29 2010 00:38:27 GMT

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