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Pacific ENSO Update

2nd Quarter, 2011 Vol. 17 No. 2


American Samoa Flag American Samoa: Hurricane Wilma passed directly over American Samoa at the end of January 2011. It appears that Wilma was of severe tropical storm intensity when it passed; peak measured winds were 50 kt with gusts to 70 kt. Coastal inundation was experienced on the North Shore of Tutuila, with some damage to roads and bridges. There was also some loss of sheet metal roofs in locations of greater exposure, such as at higher elevations, in valleys where winds were funneled, or where down-slope winds accelerated. Wilma intensified after passing American Samoa, and became a very powerful 115 kt (Saffir-Simpson Category 4) hurricane two-and-one-half days later when it was located well to the south of the Fiji Islands. The months of December 2010 and January 2011 were very wet at Pago Pago with 28.30 and 25.72 inches, respectively. An extreme amount (44.12 inches) of rainfall was seen at Aasuofou during the month of January, which was the third-highest monthly total seen there in its climate record (previous extremes include 47.86 inches in January 2005 and 44.89 inches during February 2006). This higher elevation station on the island of Tutuila often gets far more rain than stations along the coastal perimeter of the island. In contrast, the 4.99 inches of rainfall in February was a very dry value for this site (a record February minimum, but not an all-time low), and was attributed by local observers to be a result of a substantial change of wind and weather patterns between the two months. After the unusually wet months of December and January, the SPCZ shifted to the west to lie across the region from Vanuatu through Fiji, and conditions became much drier in February and March.

American Samoa Rainfall Summary 1st Quarter 2011

Station   Jan. Feb. Mar. 1st Qtr
Predicted
Pago Pago WSO
Rainfall (inches)
25.72
13.39
5.77
44.88
43.93
% of Normal
204%
105%
51%
123%
120%
A'asufou
Rainfall (inches)
44.12
4.99
13.43
62.54
N/A
% of Normal
238%
26%
76%
113%
N/A
Predictions for 1st Quarter made in 4th Quarter 2010 Pacifc ENSO Update newsletter.

Climate Outlook: Computer forecasts and a consensus of outlooks from several regional meteorological centers indicate that rainfall in American Samoa is likely to be slightly below normal for the next few months as the next rainy season wanes and the dry season becomes established. Normal monthly rainfall is typically below 10 inches per month for the period May through September at Pago Pago. Thereafter for the remainder of 2011 and early 2012, the rainfall and tropical cyclone activity in American Samoa will depend on the evolution of ENSO. If the climate system is ENSO-neutral or moving toward El Niño in latter half of 2011, the rainfall and tropical cyclone activity in American Samoa should be normal to above normal. If La Niña returns, the rainfall and tropical cyclone activity should be near normal to below normal.

Forecast rainfall for American Samoa from April 2011 through March 2012 is as follows:

Inclusive Period

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

April - June 2011
(Onset of Dry Season)
90%
(24.78 inches - Pago Pago)
July - September 2011
(Heart of Dry Season)
95%
October - December 2011
(Onset of Next Rainy Season)
100%
January - March 2012
(Heart of Next Rainy Season)
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 20 2011 05:35:23 GMT

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