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Pacific ENSO Update

4th Quarter, 2012 Vol. 18 No. 4


Guam and CNMI text Guam/CNMI: Almost all locations throughout Guam and the CNMI were wetter than normal during the 3 rd Quarter of 2012, with a very wet August contributing most to the 3-month surplus. August rainfall totals over 20 inches occurred at most locations on Guam and in the CNMI. The 32.05 inches recorded at Tinian was an all-time August record and second only to the 37.85 inches recorded there during June 2004. The highest monthly total on Guam during the 3 rd Quarter was the August total of 26.42 inches (163%) at the Guam Weather Service Forecast Office. The Saipan International Airport also had a big month during August 2012, with 26.73 inches (214%). The heavy rain during August was associated with the western North Pacific monsoon trough. During the last week of July into the first week of August, the monsoon trough extended eastward into the western North Pacific basin to the north of Guam and the CNMI. Several tropical cyclones formed in association with this monsoon activity, all to the north and west of Guam and the CNMI. This helped to usher in the monsoonal southwesterly surface winds. Some tropical disturbances and many mesoscale clusters of showers and thunderstorms contributed to almost daily episodes of heavy showers and thunderstorms. The daily rainfall totals were particularly heavy during the first week of August, with a peak rainfall total of 6.35 inches on Guam during a 24-hour period straddling the 7 th and 8 th. An extreme hourly rainfall total of 3.25 inches occurred in central Guam on the 7 th. This caused local street flooding and pushed water levels in some of the rivers of central Guam to heights not seen since the extreme rains during the typhoons of 2002. Flood waters reworked streambeds of the Sigua, Lonfit, Maguagua and Tarzan rivers and uprooted trees on the river banks. Sporadic heavy daily rainfall events (e.g., 2 inches or more in 24 hours) continued on Guam and in the CNMI through mid-October, then came to an abrupt end as the trade winds of the oncoming dry season became established after the 15th.

Guam and CNMI Rainfall Summary 3rd Quarter 2012

Station   Jul. Aug. Sep. 3rd Qtr
Predicted
Guam
Guam Intl. Airport (WFO)
Rainfall (inches)
6.74
26.42
15.98
49.14
45.30
% of Normal
64%
192%
118%
130%
120%
Anderson AFB
Rainfall (inches)
8.29
17.60
10.98
36.87
45.19
% of Normal
76%
131%
82%
98%
120%
Dedado (Ypapao)
Rainfall (inches)
7.76
26.93
16.60
51.29
45.30
% of Normal
71%
200%
123%
136%
120%
Ugum Watershed
Rainfall (inches)
8.78
18.90
14.67
42.35
45.30
% of Normal
77%
135%
101%
106%
120%
Sinajaña
Rainfall (inches)
11.51
25.39
12.68
49.58
45.30
% of Normal
109%
189%
94%
131%
120%
University of Guam
Rainfall (inches)
7.64
25.01
12.92
45.57
45.30
% of Normal
73%
182%
96%
121%
120%
Commonwealth of the Northern Mariana Islands
Saipan Intl. Airport
Rainfall (inches)
5.19
26.73
10.41
42.33
40.92
% of Normal
64%
214%
77%
124%
120%
Capitol Hill
Rainfall (inches)
11.84
23.14
10.48
45.46
42.00
% of Normal
132%
185%
78%
130%
120%
Tinian Airport
Rainfall (inches)
7.80
32.05
15.95
55.80
42.00
% of Normal
87%
256%
118%
159%
120%
Rota Airport
Rainfall (inches)
7.34
18.34
9.26
34.94
44.40
% of Normal
70%
139%
69%
94%
120%
Predictions made in 2nd Quarter 2012 Pacifc ENSO Update newsletter.

Climate Outlook: El Niño typically has two notable effects on the weather of Guam and the CNMI: (1) an increased risk of a damaging typhoon, particularly during October of the El Niño year through the January of the follow-on year to El Niño; and (2) a high risk of very dry conditions during the first 6-months of the year that follows a moderate or strong El Niño (i.e., January through June). The dryness is exacerbated in two ways: (1) a reduction in the monthly values of rainfall; and (2) and extension of the dry season into June and July. For Guam and the CNMI, the severity of the typhoon threat and the drought is closely related to the severity of the El Niño event. For weak events, the effect is not nearly as severe or persistent as for moderate and strong events. Because we are now only at the borderline of El Niño and it is very late in the year, it is likely that the ENSO status will remain neutral. Even if El Niño were to develop, it would be short-lived and would likely have little significant effect on the behavior of typhoons and rainfall. Tropical cyclones can affect the southern Mariana Islands in November and December, even in ENSO-neutral conditions. Thus, it would be prudent at this time to expect some tropical cyclone activity during the next couple of months, and to prepare for normal dry season weather conditions during the six-month period January-June 2013. An extreme drought of the magnitude that occurred during the first half of 1983 and of 1998 is not anticipated. The risk of a typhoon on Guam and in the CNMI is cautiously set to 15-20% over the course of the next 2 months.

Predicted rainfall for Guam and the Mariana Islands from October 2012 through September 2013 is as follows:

Inclusive Period

% of Long-Term Average Rainfall /
Forecast Rainfall (inches)
Guam/Rota
Saipan/Tinian
October – December 2011
(End of Rainy Season)
100%
(25.63 inches)
100%
(22.06 inches)
January – March 2012
(Onset of Next Dry Season)
90%
85%
April – June 2012
(2nd Half of Next Dry Season)
90%
85%
July– September 2012
(Next Rainy Season)
100%
95%

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: January 15 2013 03:49:58 GMT

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