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Pacific ENSO Update

3rd Quarter, 2011 Vol. 17 No. 3


Guam and CNMI text Guam/CNMI: Throughout Guam and the CNMI, the rainfall during the first half of 2011 was generally above normal, with most locations throughout the region experiencing [an unusually uniform] 130% to 140% of average rainfall. Rainfall associated with the monsoon depression precursors to some of the early season tropical cyclones contributed to the widespread above normal rainfall. It does not take much rainfall to be produce above normal values in the dry season of Guam and the CNMI. Saipan’s relatively modest 6-month rainfall total of 26.24 inches was one of the lowest readings in the region, but was still 135% of normal. The 6-month total of 35.85 inches of rainfall at the Guam Weather Service Forecast Office was 130% of normal. No extraordinary short period rainfall events were experienced, with but 2 inches or more rainfall in 24 hours was experienced twice at the Guam WSO during January to June, and only once at Saipan during the same time interval. During mid-July, the southwest monsoon swept into Guam and the CNMI for a few days, and the year’s top 24-hour rainfall of 3.37 inches was experienced at the Guam WSO. Heavy monsoon showers and squalls largely spared Saipan where only about 1 inch of rainfall fell on the 15th, the same day of Guam’s 3-inch-plus 24-hour rainfall. Wind on that day gusted to 44 mph from the southwest at Saipan and to 38 mph a day earlier on Guam. During some most years without tropical cyclones, wind gusts over 40 mph are usually near the extreme value for the year!

Because of recent dry years, there is a public perception that this year’s dry season was exceptionally wet. Lawns stayed green, wildfires were infrequent and small, and brush cutters were continually busy keeping parks and roadways clear of rapidly growing weeds and sword grass. The rainy season has now begun, and is off to a reasonably wet start, but nothing out of the ordinary has occurred yet.

Guam and CNMI Rainfall Summary 2nd Quarter and 1st Half 2011

Station   April May June 2nd Qtr
1st Half
Guam
Guam Intl. Airport (WFO)
Rainfall (inches)
5.56
5.77
5.96
17.29
35.85
% of Normal
142%
95%
92%
105%
130%
Anderson AFB
Rainfall (inches)
7.95
6.65
5.34
19.94
45.37
% of Normal
163%
101%
84%
112%
138%
Dedado (Ypapao)
Rainfall (inches)
5.88
6.59
6.33
18.80
42.26
% of Normal
121%
100%
100%
106%
129%
Ugum Watershed
Rainfall (inches)
4.25
5.76
7.15
17.16
42.86
% of Normal
90%
87%
110%
96%
138%
Sinajaña
Rainfall (inches)
5.48
8.09
5.22
18.79
37.80
% of Normal
140%
134%
81%
114%
137%
Commonwealth of the Northern Mariana Islands
Saipan Intl. Airport
Rainfall (inches)
4.96
4.87
5.72
15.55
26.24
% of Normal
177%
111%
123%
131%
135%
Capitol Hill
Rainfall (inches)
5.27
7.96
6.90
20.13
33.34
% of Normal
151%
145%
119%
136%
137%
Tinian Airport
Rainfall (inches)
5.48
2.86
5.91
14.25
33.58
% of Normal
156%
52%
102%
96%
138%
Rota Airport
Rainfall (inches)
7.25
5.68
4.47
17.40
41.75
% of Normal
160%
90%
72%
102%
136%

Climate Outlook: The rainy season has begun on Guam and in the CNMI. An episode of the southwest monsoon in mid-July provided a good boost to July’s rainfall totals, especially at the Guam WSO, where two nights of heavy thunderstorms provided nearly 6 inches to the monthly total. Guam and the CNMI depend on tropical cyclone activity and the southwest monsoon for much of their rainy season rainfall. We anticipate at least two or three additional week-long episodes of the southwest monsoon to sweep across Guam and the CNMI sometime from now through October. Also, two or three tropical storms and one typhoon are forecast to pass within 180 n mi of Guam and Saipan, with the greatest cyclone risk later in the year (October through December). This is normal. Recent years of reduced tropical cyclone activity in the western North Pacific basin have dramatically reduced the numbers of cyclones affecting Guam and the CNMI. A normal number of cyclones passing through regional waters will likely be perceived as unusually active. A degree of complacency sets-in when the weather has been so pleasant and uneventful for so long.

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

Inclusive Period

% of Long-Term Average Rainfall /
Forecast Rainfall (inches)
Guam/Rota
Saipan/Tinian
July – September 2011
(Heart of Rainy Season)
120%
(45.05 inches)
120%
(38.56 inches)
October – December 2011
(End of Next Rainy Season)
125%
125%
January – March 2012
(Onset of Next Dry Season)
100%
100%
April – June 2012
(2nd Half 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



Pacific ENSO Applications Climate (PEAC) Center
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2525 Correa Road, suite 250
Honolulu, HI 96822
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Web Master's email: peac@noaa.gov
Page Last Modified: August 09 2011 02:01:40 GMT

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