Some commenters on CP have asked a similar question. Meteorologist and former Hurricane Hunter Jeff Masters has the answer on his must-read WunderBlog. His conclusion:
A tropical cyclone in its formative stage–as either a tropical depression or a tropical storm with 40 mph winds–might be adversely affected if it encountered the Gulf of Mexico oil slick, due to the reduction of evaporation into the storm. However, a full-fledged hurricane would mix the oil into the ocean to such a degree that the storm would probably not see any significant reduction in evaporation. It remains unknown how the reduction of sea spray by oil might affect a hurricane. If the oil slick expands to a much larger size, there might be a significant reduction in strength of the hurricane, if theory of how a reduction of sea spray will decrease a hurricane’s winds is correct.
However, the oil slick is currently Delaware-sized, while a hurricane tends to be Texas-sized, and I doubt that the oil slick at its current size is large enough to have a significant impact on a hurricane’s intensity. The slick is about 60 miles across, and it would take a hurricane about four hours to traverse the spill at a typical hurricane forward speed of 15 mph. Furthermore, the slick is within 50 miles land, and interactions with land will dominate the behavior of a hurricane that gets that close to the coast.
Unfortunately, there is a decent chance that we’ll get a real-world opportunity to see what will happen. June tropical storms tend to form in the Gulf of Mexico, and we’ve been averaging one June storm every two years since 1995. This year, the odds of a June Gulf of Mexico storm are probably a little lower than usual, shear from our lingering El Ni±o may bring wind shear levels a bit above average. I expect there is a 20% chance that we’ll see a June tropical storm in the Gulf of Mexico that would interact with the oil spill.