The development of a tropical storm requires several ingredients: a disturbance in the atmosphere (often a tropical wave), warm ocean water, a plentiful supply of heat and humidity at lower levels and relatively weak upper level winds. Strong winds in the upper atmosphere sheer away the building storm tops, disrupting the vertical development that is essential to the heat engine of a tropical cyclone.
While both tropical storms Ana and Bill developed in a similar fashion (organizing through the sequence of tropical wave and tropical depression), Ana's life as a tropical storm was short, a consequence of her encounter with upper level sheer. Now a disorganized tropical depression in the Caribbean, she may redevelop in the southern Gulf of Mexico as upper winds abate; should that happen, she could be reborn as a hurricane.
Meanwhile, Bill, following in Ana's wake, has strengthened into a hurricane and is projected to intensify over the coming days. Fortunately, an approaching cold front is forecast to nudge its path the the north, keeping the heart of the storm off of the U.S. coast; nevertheless, Bermuda, the Outer Banks and, potentially, New England remain at risk from this strengthening hurricane.
