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Weather forecasting and analysis, space and historic events, climate information

10:00 AM | 2015 Tropical and Mid-Atlantic Summertime Outlooks

Paul Dorian

Discussion

Overall Summary

The overall numbers are likely to 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. east coast vulnerable to “home-grown” tropical hits. The major factors involved with this year’s tropical outlook include a strengthening El Nino in the central equatorial Pacific, colder-than-normal waters off of the west coast of Africa, and pockets of warmer-than-normal temperatures in the Gulf of Mexico and also just off the US east coast. As a result, there are likely to be fewer-than-normal “African-wave” type tropical systems that travel long distances across the tropical Atlantic this season and more in the way of “home-grown” type systems that develop much closer to the U.S. Typically, the “African-wave” type storm plays an important role during the peak months of the tropical season (August and September) while “home-grown” systems can be important early and late in a given tropical season.

In a normal 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 normal due to the sea surface temperature pattern in the western Atlantic and Gulf of Mexico.

One final note of interest, amazingly and fortunately, the US mainland has not been struck by a major hurricane (i.e. category 3 or higher) since Hurricane Wilma in October 2005. Although the landfall record gets muddy before the early 20th century, this is the first time since hurricane record-keeping began in 1851 that the United States has gone so long without at least a category 3 landfall. The previous streak was eight years, from 1861 to 1868.

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 equatorial Pacific Ocean, affects global weather patterns and it 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 usually an inhibiting factor for tropical storm formation in the Atlantic Basin as it tends to “rip apart” developing storms. Currently, there are numerous signs for strengthening of the current El Nino in the tropical Pacific Ocean over the next few months and this should inhibit storm formation in the tropical Atlantic Ocean. Specifically, numerous computer forecast models support the idea that the current relatively weak equatorial El Nino strengthens this summer into moderately-strong status.

El-Nino-forecasts.gif

[Computer model forecasts of El Nino; courtesy IRI, Columbia University, NOAA]

SST1.gif

Atlantic Ocean, Gulf of Mexico sea surface temperature patterns

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. Similar to last year, there is a pocket of colder-than-normal sea surface temperatures off 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 pockets of water just off the U.S. east coast and across much of the Gulf of Mexico, and these anomalous regions should aid in the development of “home-grown” type storms in nearby locations such as the Caribbean Sea, Gulf of Mexico or just off the Southeast U.S. coastline. [Sea surface temperature anomaly pattern (orange, yellow = warmer-than-normal; courtesy NOAA]

Mid-Atlantic Summer Outlook

I believe there is little chance for a hot, dry summer in the Mid-Atlantic region and the most likely scenario is for near normal rainfall amounts with a slight leaning towards the cool side of normal when it comes to temperatures in the June, July, August time period The strengthening of El Nino in the tropical Pacific Ocean will play a role in our summertime weather pattern which usually leads to cooler-than-normal conditions, and there are two other factors that should turn out to be meaningful. First, the record-breaking Great Lakes ice cover extent during the past winter season is generally a useful predictor of cooler-than-normal temperatures in the Mid-Atlantic region during the subsequent summer season. In fact, Great Lakes ice cover extent as of late April was 27% - the highest ever so late in the season – and there was still more than 5% coverage in early May across Lake Superior. This finding of generally cooler-than-normal summers in the Mid-Atlantic region following high ice cover winters in the Great Lakes region is not necessarily because of the actual ice cover, but rather due to the overall - and likely still on-going - weather pattern that created the anomalous ice cover in the first place and typically persists beyond the winter season. In addition, soil moisture content is relatively normal around here in the Mid-Atlantic region as we head into the month of May thanks to the snowy winter and recent spring rains. Normal-to-high soil moisture content tends to significantly reduce chances for summertime drought and excessive heat.

Paul Dorian

Vencore, Inc.

Extended Video Discussion

httpv://youtu.be/x8QZ3wxVGJM