Typically, around this time of year, sea surface temperatures in the Atlantic reach their warmest levels for the summer. At the same time, wind shear across much of the tropical Atlantic ebbs, leading to an outbreak of tropical activity.
Rarely is the outbreak so furious as the current explosion of three hurricanes in the Atlantic Ocean with the possibility of a fourth tropical storm developing in the Gulf of Mexico this week. Of most immediate concern is Hurricane Florence, which has rapidly strengthened overnight and now presents a rare and substantial threat to the Carolinas and beyond due to high winds, storm surge and, for much of the US East Coast, the potential for extremely heavy rainfall.
In the Atlantic hurricane record, which goes back about 150 years and has limited reliability before 1950, there have just been 11 years (including this one) in which there have been three hurricanes active in the Atlantic basin, which includes the Caribbean Sea and Gulf of Mexico. Just twice, in 1893 and 1998, have four hurricanes been active at once, according to Colorado State University hurricane scientist Phil Klotzbach.
Herein is an assessment of the risks posed by Florence and the other storms.
This storm has, unfortunately, blown up overnight as hurricane forecasters anticipated. It has become a Category 2 hurricane with 105mph winds, and should become a major hurricane later today.
There appears to be little to slow down further intensification of the storm over the next few days as it crosses warm waters (29° Celsius), and encounters little to no wind shear. It is effectively like skating on sharp blades across recently smoothed ice. In its latest forecast, the National Hurricane Center anticipates that Florence will reach 150mph winds—nearly Category 5 intensity—before making landfall late Thursday night or early Friday morning.
This is a rare hurricane for the Carolinas. As University of Miami scientist Brian McNoldy noted on Twitter, no recorded Category 4 hurricane has ever made landfall in the United States north of the border between South and North Carolina. (That storm was Hazel, back in 1954). Even more unusual is its track. No hurricane in recorded history has ever made a US landfall from Florence’s position of 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 wind and surge are for coastal areas of the Carolinas, however, its inland rainfall should prove equally destructive, if not worse. That is because, somewhat like Hurricane Harvey in 2017, steering currents seem likely to weaken once Florence moves inland and runs into high pressure over West Virginia and Kentucky.
This could lead to extremely heavy rainfall for parts of the eastern United States, which will induce significant and widespread flooding. The effects of this flooding could be far removed from wherever Florence makes landfall.
We are going to ignore Hurricane Helene because that system is unlikely to affect any landmasses. However, another hurricane in the Atlantic, Isaac, also may eventually cause problems as it moves westward across the Atlantic Ocean toward the Caribbean Sea.
As of 8am ET Monday, Isaac has 75 mph winds.but the storm should strengthen some over the next several days. However, by Thursday or so, as the storm nears the Caribbean Sea it should begin to run into an atmosphere disturbed by the passage of Hurricane Florence. This should lead to some weakening.
However, Isaac still presents a wind and heavy rainfall threat for the Caribbean Islands. And, very far down the line, could menace Central America, Mexico, or the United States. The smart money is probably on the system dying out over the Caribbean, but as this is September, low pressure systems moving toward land must be watched carefully.
Finally, there is a low pressure system in the northwestern Caribbean Sea that should move across the Yucatan Peninsula and into the Gulf of Mexico by Tuesday night or Wednesday morning. There, it could find smoother sailing and perhaps become a tropical depression or storm. The National Hurricane Center rates this a 40 percent possibility.
What Is most concerning is the tropical moisture that this system will drag with it. Coastal areas of Texas have already received a lot of precipitation for the month of August, and soils there are mostly waterlogged. A surge of tropical moisture may well bring at least some localized flooding to parts of Texas this weekend, but it's too early to say with any confidence where that might be.