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Pacific ENSO Update

3rd Quarter, 2009 Vol. 15 No. 3


American Samoa Flag American Samoa:  During the first half of 2009 the rainfall in American Samoa was near normal. Rainfall in American Samoa typically begins to decrease during May and June as the dry season gets underway. However, May was very wet with nearly 20 inches of rain (193% of normal) at Pago Pago International Airport. At A’asufou, a station at 408m elevation in the hills behind the WSO, the May 2009 rainfall was 27.22 inches (177% of normal). This was the wettest monthly total noted at any recording location in the USAPI during the first half of 2009. After the wet May, June was quite dry. American Samoa passed through its hurricane season without any serious affects from any tropical cyclone. Very high surf was observed during June, caused by intense mid-latitude lows to the south, and further exacerbated by strong southeast trade winds.

American Samoa Rainfall Summary 2nd Quarter 2009

Station   April May June 2nd Qtr
1st Half
Pago Pago WSO
Rainfall (inches)
9.28
19.17
3.65
32.10
61.05
% of Normal
77%
193%
49%
109%
93%
A'asufou
Rainfall (inches)
17.40
27.22
N/A
36.14
53.42
% of Normal
96%
177%
N/A
N/A
N/A

Climate Outlook: American Samoa is now within the heart of its dry season (May through November). Nearly all climate models favor slightly below normal rainfall over the next few months, which span the heart of the 2009 dry season.

For the past two years, the focus of South Pacific tropical cyclone activity has been in the Coral Sea from northeastern Australia eastward to Fiji. In the upcoming 2009-10 cyclone season, El Niño conditions may elevate the risk of a damaging impact by a hurricane or strong tropical storm affecting American Samoa. During El Niño, the Australian Northwest Monsoon episodically pushes to the east of the International Date Line in the rainy season, bringing several periods of gusty northwesterly winds to American Samoa. This weather pattern also enhances tropical cyclone activity from Tonga and Samoa eastward into the Cook Islands. A moderate El Niño brings greater hurricane risk to American Samoa than a strong El Niño.

Forecast rainfall for American Samoa from July 2009 through June 2010 is as follows:

Inclusive Period

% of Long-Term Average Rainfall /
Forecast Rainfall (inches)

July– September 2009
(Heart of Next Dry Season)
90%
(17.60 inches - Pago Pago)
October - December 2009
(Onset of Next Rainy Season)
100%
January - March 2010
(Heart of Next Rainy Season)
120%
April - June 2010
(Onset of Next Rainy Season)
90%

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: June 01 2010 22:10:35 GMT

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