April 2009 Climate Report for Puerto Rico and the U.S. Virgin Islands  

April 2009 Highlights:

 
April 2009 was the first full month of 2009 in which below normal precipitation was recorded across Puerto Rico. While at the end of the month, Puerto Rico had received 119% of its normal rainfall year to date (YTD), only 82% of the normal rainfall accumulated across the island during the month of April. Of the different island sectors, the Western Interior of the island fared the best, receiving 104% of its normal precipitation for the month, while the Northern Slopes and Eastern Interior received only 55% and 60% of their normal precipitation, respectively. Thanks to the above normal rainfall that accumulated over the first three months of the year, no sector of the island was classified as experiencing abnormally dry or drought conditions by the U.S. drought monitor as of May 5th, 2009. In San Juan, 2.79” of rain fell in April, which was 75% of the 30-year average. Across the US Virgin Islands, April precipitation totals on Saint Thomas and Saint Croix varied from 44% to 80% of normal during the month, respectively.

 Temperatures across Puerto Rico ranged from 48ºF in Adjuntas on the 12th of the month to 95ºF in Ponce on the 30th of April. Temperatures across the U.S. Virgin Islands ranged from 62ºF at Beth Upper New Works on Saint Croix, observed on April 2nd, to 89ºF, also at Beth Upper New Works on Saint Croix, observed on April 21st and 27th. Other notable temperatures for April 2009 are listed below:

Other Notable April 2009 High Temps:           
 
93ºF in Rio Piedras on April 21st
91ºF in San Juan on April 16th
91ºF in Corozal on April 17th
91ºF in Juncos on April 24th               
90ºF in Lajas, Levittown, Trujillo Alto, Gurabo and Coloso.
Other Notable April 2009 Low Temps:
 
53ºF in Toro Negro on April 3rd
55ºF in Trujillo Alto on April 13th
58ºF in Aibonito on April 7th
58ºF in Maricao on April 3rd  
59ºF in Lajas on April 2nd & 13th  
60ºF in Gurabo on April 21st
 

The highest 24-hour precipitation total for the month of April was 5.40”, which fell in Negro Corozal on the 19th of the month. The highest weekly rainfall total occurred in Toro Negro, where 6.47” of rainfall accumulated the week ending April 24th. Not surprisingly, both these stations are located in the Western Interior of Puerto Rico, the only island sector to receive above normal precipitation for the month. Island wide precipitation for Puerto Rico, plotted by island sector, is displayed below.

 

 Figure 1 – Island wide precipitation forJanuary-April 2009 

Monthly Weather Summary:

April generally marks the transition to the rainy season here in Puerto Rico. While statistically speaking the month of April is much wetter than March, this is not always the case. In April 1997, only 0.08” of rainfall was recorded at the Luis Muñoz International Airport in San Juan, one of the driest months on record. However in 2005, 15.00” of rainfall fell in San Juan, making April 2005 one of the wettest months on record (a record 16.81” of rainfall accumulated in San Juan in December 1981). On average, 3.71 inches of rainfall accumulates at the Luis Muñoz International Airport in April, with April 2009 bringing an official total of 2.79 inches. By the end of the month, the airport had received a total of 11.82”, or 106% of normal rainfall year to date. This was 2.56” less than had accumulated by this time last year. Temperatures in April averaged 79.6 ºF, which was 0.5 ºF warmer than the 30 year average for the month.

                April 2009 began with a very dry and stable airmass in place over the region. The combination of only patchy passing amounts of low level moisture along with ridging in the mid to upper levels of the atmosphere, helped to limit precipitation across Puerto Rico during the first half of the month. This all changed around the 17th of the month, when a long wave upper trough helped destabilize the atmosphere, allowing for more significant shower activity to develop during the afternoon and early evening hours across the local islands. This shower activity was further enhanced by a pre-frontal trough that formed across Puerto Rico on the 19th of the month, when the most significant rainfall fell, resulting in localized flooding across various sectors of the island. The month of April ended with an upper level low pressure system again enhancing shower activity across much of the interior of the island, which helped contribute to the near normal precipitation that was received island wide the last two weeks of the month. While the precipitation received during the last half of April was not as widespread and significant as the precipitation received with several dissipating cold fronts that affected the local islands during the months of January-March, the corresponding increase in island wide precipitation for the final two weeks of April, associated with generally more favorable atmospheric conditions, can still be seen in Figure 1.
 
                Across the U.S. Virgin Islands, generally dry conditions were observed during the month of April, mainly due to the lesser effect that favorable upper level conditions can have on the local weather there. Saint Thomas received only 1.06” of rain during the month of April and Saint Croix received only 1.59” of rain for the month. Year to Date, rainfall across Saint Thomas remained well above normal, having received 125% of it normal rainfall YTD, while Saint Croix was faring much worse, having received only 51% of its normal rainfall YTD. In fact, the 4.03” of rain that Saint Croix has received YTD, represents the 3rd driest start to a year on record, with only 2001 (3.74”) and 1953 (3.52”) starting off drier.

April 2009 Statistics:

Average April Temperatures/Precipitation (NCDC 1971-2000 averages): 

 
High:     
Low:     
AvgT:   
Pcpn:      
San Juan              
84.9ºF
73.2ºF    
79.1ºF
3.71”
Saint Thomas
87.6ºF    
73.9ºF    
80.8ºF
2.39”
Saint Croix
86.2ºF
74.0ºF
80.1ºF
2.00”

 
April 2009 Observations:

 
High:     
Low:     
AvgT:   
Pcpn:      
San Juan
85.7ºF     (+0.8ºF)
73.5ºF     (+0.3ºF) 
79.6ºF     (+0.5ºF) 
2.79”       (-0.92”) 
Saint Thomas
84.3ºF     (-3.3ºF)  
73.6ºF     (-0.3ºF)  
79.0ºF     (-1.8ºF)  
1.06”       (-1.33”) 
Saint Croix
85.2ºF     (-1.0ºF)
73.2ºF     (-0.8ºF)
79.2ºF     (-0.9ºF)
1.59”       (-0.41”)

 
April 2009 Extremes:

 
High:  
Low:  
San Juan
91ºF – April 16th   
71ºF – April 3rd
 
Saint Thomas
87ºF – April 23rd & 27th
69ºF – April 3rd & 4th   
Saint Croix
88ºF – April 25th
66ºF – April 2nd

 
April Historical Statistics:
 
Highest April Temperatures on record:

San Juan              
97ºF – April 17th, 1983
95ºF – April 22nd & 24th, 1983
94ºF – Numerous Times     
Saint Thomas      
96ºF – April 30th, 1992, April 29th, 1994 & April 4th, 1996
95ºF – March 25th, 1994 &  
March 31st, 1996
Saint Croix
93ºF – April 21st, 1961 & April 30th, 1973
92ºF – April 3rd, 6th & 19th, 1973

 
Lowest April Temperatures on record:

San Juan              
64ºF – April 12th, 1968
65ºF – April 7th, 1960, April 24th & 25th, 1963 & April 8th, 1965
Saint Thomas
58ºF – April 10th, 2001
62ºF – April 11th, 1972 & April 10th, 1984       
65ºF – April 8th & 9th, 1984
Saint Croix
57ºF – April 7th, 1997
62ºF – April 15th, 1974 & April 2nd & 3rd, 1982
63ºF – April 7th, 1982

 
Wettest Aprils:

San Juan              
15.00” April 2005
10.37” April 1988
10.06” April 2006
Saint Thomas      
11.20” April 2003
9.07”   April 1983
6.66”   April 1978
Saint Croix
8.54” April 2003
8.24” April 1983
5.38” April 1952

 
Driest Aprils:

San Juan              
0.08” April 1997
0.28” April 1984
0.50” April 1968
Saint Thomas
0.10” April 1973
0.23” April 1953
0.48” April 1985
Saint Croix
0.00” April 1988
0.02” April 1997
0.10” April 2005

 2000-2009 Decade in Review:

For the city of San Juan, April 2009 was the fourth warmest April of the past decade, with the average temperature of 79.6ºF being 0.5ºF warmer than the 30-year average and a full 2.1ºF warmer than March 2009. Across the U.S. Virgin Islands, April temperatures on both Saint Thomas and Saint Croix were both below average, with readings of 1.8ºF and 0.9ºF below their 30-year averages on Saint Thomas and Saint Croix, respectively. Year to date, temperatures across San Juan have averaged 0.73ºF above normal, while temperatures on Saint Croix and Saint Thomas were running 0.83ºF and 1.85ºF degrees below normal. A graphical depiction of the temperature deviations across San Juan, Saint Thomas and Saint Croix can been seen in Figures 2-4.

 Figure 2 – Temperature deviations for San Juan, PR

 Figure 3 – Temperature deviations for Saint Thomas, VI

Figure 4 – Temperature deviations for Saint Croix, VI 

Statistically speaking, the month of April was a very wet month across Puerto Rico during the 2000’s. An average of 6.32 inches of rain fell in San Juan during the month this past decade, largely influenced by the record 15.00 inches of rain that fell in April 2005 and the nearly 10 inches of rain that fell in 2003, 2005 and 2006. When new 30-year climatological statistics are derived next year to account for the 2000’s, expect April precipitation statistics for San Juan to be skewed significantly higher than the current 30-year average of 3.71”, an average that accounts for the 1970-1999 time period.

 While April 2009 was below normal, precipitation wise, across Puerto Rico, seventeen weeks into 2009, Puerto Rico was still running well above normal, having received 122% of its normal rainfall YTD. All six sectors of the island have enjoyed above near or above normal precipitation thus far, ranging from 95% along the northern slopes of the island to 144% along the south coast. Across the Virgin Islands, Saint Thomas has received 125% of its yearly rainfall, while Saint Croix has been significantly drier, having received only 51% of their normal rainfall YTD. As noted in the Monthly Weather Summary, the 51% YTD rainfall received on Saint Croix represents the 3rd driest start to the year on record. 

Figure 5 – April precipitation deviation 2000’s 

Figure 6 – Year-to-Date precipitation deviations for San Juan, Saint Thomas, and Saint Croix

Decadal Observations for San Juan, Puerto Rico:
 
Decadal April High Temperatures:
2000’s
 
1990’s
 
1980’s
 
1970’s
 
1960’s
 
1950’s*
85.8ºF
 
86.3ºF
 
87.1ºF
 
85.7ºF
 
84.6ºF
 
84.7ºF
 
Decadal April Low Temperatures:
2000’s
 
1990’s
 
1980’s
 
1970’s
 
1960’s
 
1950’s*
73.5ºF
 
73.3ºF
 
73.3ºF
 
73.7ºF
 
71.5ºF
 
69.9ºF
 
Decadal April Average Temperatures:
2000’s
 
1990’s
 
1980’s
 
1970’s
 
1960’s
 
1950’s*
79.6ºF
 
79.8ºF
 
80.2ºF
 
79.7ºF
 
78.1ºF
 
77.3ºF
 
Decadal April Precipitation:
2000’s
 
1990’s
 
1980’s
 
1970’s
 
1960’s
 
1950’s*
6.32”
 
2.90”
 
4.71”
 
3.50”
 
3.56”
 
3.19”
* Records for San Juan began on January 1st, 1956
 
Looking ahead toward the month of May: 
 
May is generally one of the wetter months across Puerto Rico as it falls during the springtime wet season. On average, 5.29” of rainfall accumulates in San Juan during the month of May, with Saint Thomas generally receiving 3.36” and Saint Croix 3.62” during the month. The 5.29” that accumulates on average in San Juan marks the 3rd wettest month of the year, with only September (5.60”) and November (6.17”) receiving more rainfall. May is the 5th wettest month across the Virgin Islands, with the months of August-November all receiving more rainfall than May. The average temperature in San Juan jumps to 80.6ºF during the month of May, which will peak at 82.4ºF during the month of August.

 The end of La Niña conditions across the tropical Pacific and forecast of developing El Niño conditions by the CPC will have some implications on the upcoming hurricane season. NOAA/NWS forecasters will issue their 2009 Atlantic Basin Tropical Cyclone outlook on May 21st

Average May Temperatures/Precipitation (NCDC 1971-2000 averages):
 

 
High:     
Low:     
AvgT:   
Pcpn:      
San Juan              
86.3ºF
74.9ºF    
80.6ºF
5.29”
Saint Thomas
87.9ºF    
75.9ºF    
81.9ºF
3.36”
Saint Croix
87.5ºF
75.9ºF
81.7ºF
3.62”

Moon/Sun/Tide Data for the month of May:

                The sun rises at 5:57 am AST on the 1st of May and sets at 6:46 pm AST, in San Juan, Puerto Rico. This represents a total of 12 hours and 49 minutes of daylight. By May 31st, the sun will rise at 5:47 am AST and set at 6:57 pm AST, providing a total of 13 hours and 10 minutes of daylight, or a total increase of 21 minutes over the course of the month. At Charlotte Amalie on Saint Thomas, the sun will rise at 5:53 am AST and set at 6:41 pm AST on the 1st of the month. By the end of the month, the sun will rise at 5:43 am AST and set at 6:52 pm AST, also representing a total increase of 21 minutes of daylight for the month.
 
                For the month of May, a full moon will be observed on the 9th of the month at 12:01 am AST. There will also be a new moon on the 24th of the month at 8:11 am AST. The first quarter of the moon will be observed on the 1st and the last quarter of the moon will be observed on the 17th.
 
                Links to additional sunrise/sunset data, phases of the moon, as well as tide data can all be found by clicking on the climate link of the National Weather Service San Juan homepage (www.weather.gov/sanjuan) and then clicking on the Astronomical tab.

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