Why are many tropical storms trending north once they reach the Lesser Antilles?

It mainly depends on the synoptic-scale weather patterns. Typically, tropical cyclones in the Atlantic track westward off the coast of Africa due to being on the southern edge of the Bermuda High, which rotates clockwise. In addition, the general flow at lower latitudes tends to be east to west. Once the storm begins to turn northward around the edge of the Ber. High, it begins to encounter mid-latitude flow, which is mainly west to east.

What has been true, however, this summer is that in general, the Ber. High has been a bit more east than usual due to a La Nina event that is strengthening. In general, this has meant that the upper air flow has kept a persistent trough in the eastern part of the United States. Being that the flow of the jet stream is not a line, but a streamflow, this has placed flow which has been going from deep in the south towards the New England states. Hurricanes tend to get caught in upper air flow to help them steer, which means if the air is moving in that direction, so will the hurricane (look at Lee, Irene, et al....they have all been pulled north by the troughiness in the East.

Contrawise, if you wanted to have Cape Verde-bred hurricanes, you would need a decent size ridge over the eastern US, and a weak Ber. High which is farther west than usual to prevent recurvature to the north. This would tend to keep tropical cyclones towards the lower latitudes. El Nino would assist with this, because it tends to promote weaker flow and ridging along the Atlantic and Gulf of Mexico, which essentially makes the storms fend for themselves.

"In the tropics, tropical cyclones usually move toward the west with a slight poleward component. This is because there exists an axis of high pressure called the subtropical ridge that extends east-west poleward of the storm. On the equatorward side of the subtropical ridge, general easterly winds prevail. However, if the subtropical ridge is weak - often times due to a trough in the jet stream - the tropical cyclone may turn poleward and then recurve back toward the east. On the poleward side of the subtropical ridge, westerly winds prevail thus steering the tropical cyclone back to the east. These westerly winds are the same ones that typically bring extratropical cyclones with their cold and warm fronts from west to east."

It's not really "typical" for storms that form east of the Lesser Antilles to enter the Gulf. They often turn northwest and hit eastern Florida, the Carolinas, or Bermuda. Sometimes affect the Northeast, like Irene.

In very general terms, cyclones form in the Gulf of Mexico or near the Cape Verde Islands off Africa. Gulf storms tend to form earlier in the season and move generally north, while Cape Verde storms form later in the season and move west, then northwest, north and northeast.

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