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Discussion

Tropical Outlook Summary The overall numbers may be down this year in terms of the number of Atlantic Basin tropical storms, but the sea surface temperature pattern in the western Atlantic and Gulf of Mexico makes the U.S. quite vulnerable to some late season tropical hits. The two major factors involved with this year’s tropical outlook include a developing El Nino in the equatorial Pacific Ocean and the current sea surface temperature pattern across the Atlantic Ocean which features an area of colder-than-normal waters off of the west coast of Africa, but warmer-than-normal temperatures near the U.S. east and Gulf of Mexico coastlines. As a result, there are likely to be fewer-than-normal “African-wave” type tropical systems this season that travel long distances across the tropical Atlantic Ocean and perhaps more in the way of “home-grown” type systems that develop much closer to the U.S. Typically, the “African-wave” type of tropical storm dominates the first half of the season (June/July/early August) and the “home-grown” type of tropical storm plays more of an important role during the second half of the season (late Aug/Sept/Oct).

In a typical Atlantic Basin tropical season, there are about 12 named storms with 6 or 7 reaching hurricane status and only 2 or 3 actually reaching major status (i.e., category 3, 4 or 5). This year there may be more on the order of 8-10 named storms with 3-5 reaching hurricane status, but despite these expected slightly below normal overall numbers, the U.S. could actually see more tropical activity than usual due to the sea surface temperature pattern in the western Atlantic and Gulf of Mexico. One final note, fortunately, the US mainland has not been struck by a "major" hurricane (i.e. category 3 or higher) since Hurricane Wilma in October 2005 and this is one of the longest stretches ever recorded without a "major" hurricane hit.

El Nino in the tropical Pacific Ocean What goes on in the tropical Pacific Ocean does indeed have an effect on the tropical Atlantic Ocean. El Nino, which refers to warmer-than-normal waters in the central and eastern tropical Pacific Ocean, affects global weather patterns and tends to produce faster-than-usual high-altitude winds over the tropical Atlantic Ocean. This increase in the upper atmospheric winds over the tropical Atlantic Ocean is an inhibiting factor for tropical storm formation in the Atlantic Basin and tends to rip apart growing storms. Currently, there are numerous signs for the development of an El Nino this summer in the tropical Pacific Ocean and this should inhibit storm formation in the tropical Atlantic Ocean.

Atlantic Ocean sea surface temperature pattern The main breeding grounds for Atlantic Ocean tropical systems are in the region between the west coast of Africa to the Caribbean Sea and the Gulf of Mexico. Above normal sea surface temperatures in this region generally help to intensify tropical waves that come off of the west coast of Africa and move westward in the trade winds. This year, however, there is a pocket of colder-than-normal sea surface temperatures off of the west coast of Africa and this should inhibit the formation of tropical storms in that part of the tropical Atlantic Ocean. On the other hand, there are warmer-than-normal waters just off the U.S. east coast and across much of the Gulf of Mexico and this should aid in the development of storms in nearby locations such as the Caribbean Sea, Gulf of Mexico or just off the Southeast U.S. coastline.

Mid-Atlantic Summer Outlook In general, I believe there is little chance for a hot and dry summer in the Mid-Atlantic region, but rather somewhat near normal in terms of precipitation with a slight leaning towards the cool side of normal when it comes to temperatures. The developing El Nino in the tropical Pacific Ocean will play a role in our summertime weather pattern and there are two other factors that should turn out to be meaningful. First, the far higher-than-normal Great Lakes ice cover extent this past winter and spring is generally a useful indicator for normal to cooler-than-normal temperatures in the Mid-Atlantic region (not necessarily because of the ice cover itself, but because of the continuing overall weather pattern which produced the anomalous ice cover in the first place). Second, soil moisture content is rather high around here thanks in part to the very snowy winter and also to the plentiful spring rains that we have experienced. High soil moisture content tends to significantly reduce the chances for summertime drought and excessive heat.