Climate Notes May/June 2008

May 2008 brought a return to relatively dry weather across much of Puerto Rico. While statistically speaking the month of May is the second wettest month of the year, islandwide, May 2008 was significantly drier than average. Only the north coast of Puerto Rico, which includes San Juan, saw above normal precipitation for the month of May, with the remaining 5 sectors of the island receiving well below normal precipitation. At the Luis Muñoz International Airport in San Juan, 5.00 inches of rain fell in May, which was actually -0.71” inches below normal, well shy of the record 14.99” recorded in May 1965. The driest May on record was May 1972, when only 0.44” of rainfall was recorded. Temperatures in San Juan averaged 81.7ºF, which was 0.8ºF warmer than the 52 year average for the month, making May the first above normal month, temperature wise, across San Juan in 2008.

Islandwide, Puerto Rico received, on average, approximately 2.30 inches less rainfall than normal, or only 62% of the normal value, for the month of May. This represents a deficit of rainfall greater than normal rainfall that falls during the entire month of February! At the end of the month, the only section of the island that remained above normal year to date (YTD) was the north coast, with the northern slopes and western interior receiving 86% and 96% of their normal rainfall YTD respectively. The remaining 3 sections of the island had received only 56-78% of their normal rainfall YTD, which helps explain the most recent U.S. Drought Monitor graphic of Puerto Rico which continued to classify almost the entire southern half of Puerto Rico as being abnormally dry (see graphic below or visit the Climate Prediction Center (CPC) for more information). While the northern coast of Puerto Rico received above normal rainfall during the month of May, once again, this was a bit deceiving, as during the last 10 days of the month, very heavy rainfall led to urban and small stream flooding across the northern coast of the island, while the remainder of the month was nearly equally dry across all sections of the island. Some of the highest temperatures recorded for the month were 94ºF, reported in Lajas on May 7 th, 93ºF, reported in Coloso on May 14 th and 92ºF, reported in both San Juan and Truijillo Alto multiple times. The coolest temperature recorded during the month of May was 55ºF, recorded in Adjuntas on May 28 th.

Looking ahead, June is the 5 th driest month across Puerto Rico, with the Luis Muñoz International Airport in San Juan receiving 4.31” of rainfall on average*. A record 10.96” of rain fell in June 1965 and the driest June on record was June 1985, when only 0.29” of rainfall was recorded in San Juan. The average high temperature at the Luis Muñoz International Airport in San Juan for June is 82.3 ºF*, making June the 4 th warmest month of the year. Presently, La Niña conditions appear to transitioning to an ENSO-neutral state (http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_disc_may2008/ensodisc.pdf). This transition has an impact on expectations for the hurricane season, with NOAA currently forecasting a 90% of a near or above normally active hurricane season.

* All climate averages for San Juan taken from 52 continuous year period of 1956-2007 which differ from the 30-year average used in official climate reports *