Typically, around this time of year, sea surface temperatures in the Atlantic reach their warmest levels for summer. At the same time, wind shear is ebbing across much of the tropical Atlantic, leading to an outbreak of tropical activity.
Rarely has the outbreak been as severe as the current explosion of three hurricanes in the Atlantic with the possibility of a fourth tropical storm developing in the Gulf of Mexico this week. The most immediate concern is Hurricane Florence, which has increased rapidly overnight and now poses a rare and substantial threat to the Carolinas and beyond from high winds, storm surge and, for much of the eastern seaboard of the United States. US, the potential for extremely heavy rainfall.
In the Atlantic hurricane record, which goes back about 150 years and has limited reliability before 1950, there were only 11 years (including this one) in which three hurricanes were active in the Atlantic basin, including the Caribbean Sea and the Gulf of Mexico. Only twice, in 1893 and 1998, have four hurricanes been active at once, according to Colorado State University hurricane scientist Phil Klotzbach.
This contains an assessment of the risks of Florence and the other storms.
Unfortunately, this storm exploded overnight, as hurricane forecasters expected. It has become a Category 2 hurricane with winds of 105 mph and should become a major hurricane later today.
There appears to be little to slow the storm’s further intensification in the coming days as it crosses warm waters (29° Celsius) and encounters little to no wind shear. It’s basically like skating on sharp blades over freshly smoothed ice. In its latest forecast, the National Hurricane Center expects Florence to hit 150mph winds, nearly Category 5 intensity, before making landfall late Thursday night or early Friday.
This is a rare hurricane for the Carolinas. As University of Miami scientist Brian McNoldy, noticed on Twitter, has never made a Category 4 hurricane make landfall in the United States north of the South and North Carolina border. (The closest storm was Hazel, in 1954). Even more unusual is the number of Florence. No hurricane in recorded history has ever made a US landfall from the position of Florence two days ago; those storms tend to follow a northerly track as they get swept up by the mid-latitudes.
As potentially damaging as Florence’s winds and swells are to the coastal areas of the Carolinas, however, the inland rainfall should be equally devastating, if not worse. That’s because, somewhat like Hurricane Harvey in 2017, the steering currents likely appear to weaken once Florence moves inland and comes under high pressure over West Virginia and Kentucky.
This could lead to extremely heavy rainfall for parts of the eastern United States, causing significant and widespread flooding. The effects of this flooding could be a long way from where Florence makes landfall.
We’re going to ignore Hurricane Helene because that system is unlikely to affect landmasses. However, another Atlantic hurricane, Isaac, could also eventually cause problems if it moves westward across the Atlantic to the Caribbean Sea.
As of Monday 8 a.m. ET, Isaac will have 120 km/h winds, but the storm should strengthen a bit over the next few days. However, by Thursday or so, as the storm approaches the Caribbean Sea, it should begin to turn into an atmosphere disrupted by the passage of Hurricane Florence. This should lead to some weakening.
However, Isaac still poses a wind and heavy rainfall threat to the Caribbean islands. And, very far down the line, could pose a threat to Central America, Mexico or the United States. The smart money is probably dying out on the system over the Caribbean, but as this is September, low pressure systems going ashore should be watched carefully.
Finally, there is a low pressure area in the northwestern Caribbean Sea that should cross the Yucatan Peninsula and the Gulf of Mexico Tuesday evening or Wednesday morning. There it could sail more smoothly and perhaps become a tropical depression or storm. The National Hurricane Center rates this as a 40 percent possibility.
Most worrisome is the tropical moisture that this system will carry with it. Coastal areas of Texas have already received a lot of precipitation for August, and the soil there is usually soggy. A wave of tropical moisture could bring at least some localized flooding to parts of Texas this weekend, but it’s too early to say for sure where that might be.