Pacific ENSO Update

1st Quarter, 2006 Vol. 12 No. 1

Guam and CNMI text Guam/CNMI: Rainfall on Guam during 2005 was near normal at most locations. The 2005 annual total of 89.22 inches at the Guam International Airport (GIA) was close to normal at 98%. In stark contrast to torrential island-wide rainfall on several days in 2004, there were few island-wide heavy rain events in 2005. At the GIA, there were eight days during 2005 with 24-hour rainfall in excess of 2 inches and one day with rainfall in excess of 4 inches. By contrast during 2004 there were 13 days with 24-hour rainfall totals in excess of 2 inches and four days with 24-hour rainfall in excess of 4 inches. The biggest rain event of 2005 spanned the last day of August and the first day of September when a total of 6.82 inches was recorded at the GIA. The greatest 24-hour total during this event was 4.53 inches. This rainfall was in association with the passage of Typhoon Nabi to the north of Guam. The peak wind gust at the GIA during Nabi’s passage was 45 mph, and this was the GIA’s highest wind gust for the year.

During 2005, most of the CNMI rainfall stations reported slightly less total rainfall in inches than on Guam, but similar amounts in terms of percent of normal. The Saipan International Airport (SIA) and Capitol Hill experienced 110% and 102% of the annual normal, respectively. The heavy rains (6-9 inches) from Typhoon Nabi as it passed north of Saipan on the last day of August represented the most extreme 24-hour rainfall for the year. Rota’s annual rainfall of 91.90 inches was similar to that on Guam. The 2005 annual total of 76.61 inches recorded at Tinian was the lowest value recorded at the CNMI, but it was still 92% of the mean annual rainfall.
Typhoon Nabi passed approximately 40 miles to the north of Saipan during the night of 30 August. Nabi was the only typhoon during 2005 to produce typhoon-force winds on an island in Micronesia. Damage to vegetation on the north side of Saipan was consistent with that of a Category 1 typhoon. The SIA, the public utilities, businesses and homes suffered only minimal damage. Saipan and Guam experienced very high surf for several days after Nabi’s passage.

Guam and CNMI Rainfall Summary 4th Quarter 2005

Station   Oct. Nov. Dec. Total 2005
Guam International Airport WSO Rainfall (inches)
12.13
6.06
3.36
21.55
89.22
% of Normal
101%
74%
62%
84%
98%
Anderson Air Force Base Rainfall (inches)
8.45
9.13
4.37
21.95
106.16
% of Normal
66%
100%
73%
79%
108%
University of Guam Rainfall (inches)
12.11
5.38
2.96
20.45
20.43
% of Normal
N/A
N/A
N/A
N/A
N/A
Saipan International Airport Rainfall (inches)
15.84
5.56
7.33
28.73
83.97
% of Normal
147%
96%
190%
140%
109%
Capitol Hill Rainfall (inches)
14.20
6.23
8.84
29.25
86.89
% of Normal
118%
85%
184%
121%
102%
Tinian Airport Rainfall (inches)
12.52
5.25
5.12
22.89
76.61
% of Normal
104%
72%
107%
95%
92%
Rota Airport Rainfall (inches)
16.86
7.81
5.35
30.02
91.90
% of Normal
133%
90%
94%
111%
97%
* Long term normal is not established for these sites

Climate Outlook:

During La Niña years the tropical cyclone season in the western North Pacific basin is often delayed, and the number of tropical cyclones through mid-July is typically below normal. Also during La Niña years, the site of formation of the basin’s tropical cyclones is shifted to the west, reducing the threat of typhoons on Guam and the CNMI. Thus, during all of 2006 (especially during the months of September through December) two or three tropical storms and one or two typhoons should pass within 200 miles of a Guam and CNMI location. The odds of typhoon force winds (or greater) at any location on Guam or in the CNMI during any given year (status of ENSO not considered) are approximately 1 in 7. During El Nino years, the odds of typhoon force winds on Guam or on any individual island in the CNMI rise to about 1 in 3. During non-El Nino years the odds fall back to around 1 in 10. Dangerous surf from typhoons do not require that the typhoons pass close to any location, so it is certain that at least one episode of dangerous typhoon-generated waves will occur. Every year several lives are lost due to hazardous surf and the rip currents produced by them.

Rainfall is anticipated to be near normal to above normal for Guam and the CNMI for 2006. Predicted rainfall for the Mariana Islands from January 2006 through December 2006 is as follows:

 
Inclusive Period

% of Long-Term Average
Guam/Rota
Saipan/Tinian
Jan – May 2006 (Heart of Dry Season) 110% 100%
Jun – Jul 2006 (Onset of Next Rainy Season) 100% 95%
Aug – Oct 2006 (Heart of next Rainy Season) 110% 100%
Nov – Dec 2006 (Onset of next Dry Season) 100% 100%
source: UOG-WERI