The worst storms of the past have tended to come in August and September, but everything conspired this year to make Dennis happen early.
This year, the ocean was warm and the winds were just right -- or wrong.
"It's a double whammy … that makes both the ocean and atmosphere better for hurricanes," said Hugh Willoughby, a senior scientist at the International Hurricane Center of Florida International University.
Hurricanes form in the tropics, mostly because of the steamy water below them. Such storms need ocean temperatures of at least 82 degrees Fahrenheit or so.
In most years, it takes several weeks of summer for the water to get that warm. This year, the Caribbean happens to be a little bit warmer than usual.
Once the storm takes shape, it is carried along by the prevailing winds.
"You can liken that to putting, say, a marble on your dining room table and blowing on the marble," said Sim Aberson of the National Oceanic and Atmospheric Administration's Hurricane Research Division.
Dennis fluctuated in strength because the winds blew it straight over Cuba.
While 20 people died there and in Haiti, the early landfall should have been good news for the mainland United States.
"When the hurricane moves over land, it loses its fuel, the source being the warm waters below it," said Ed Rappaport, deputy director of the National Hurricane Center in Miami. "It also encounters increasing roughness or friction on the surface of land as opposed to the water, and both of those contribute to the weakening or disruption in the circulation."
But again, things went just right for Dennis -- and wrong for the people in its path.
The storm weakened from Category 4 to Category 1 after Cuba -- in other words, from a massive storm to a moderate one -- then revived in the Gulf of Mexico, briefly reaching Category 4 as it approached Alabama and the Florida panhandle.
"Most … Category 4 and 5 storms usually go through these cycles of rapid intensification," said Eric Williford, a private forecaster at Weather Predict Inc., in North Carolina. "We know they occur, it's just very difficult to predict them."
That problem continues to bedevil meteorologists. Their forecasts of a hurricane's path have become more and more precise with time, but a storm's intensity has proven to be much more complicated.
An early hurricane season tends to mean a long one, said forecaster Willoughby.
"This early start is a bad sign we're likely to be doing this at Thanksgiving," he said.