In late August 2017, Hurricane Harvey made landfall in Texas on San Jose Island. Over the following five days, Harvey's forward movement slowed to a glacial pace, essentially stopping over the greater Houston area, weakening in terms of wind speed but retaining an immense amount of moisture that eventually fell as rain in catastrophic amounts.
A year later, residents in the southeast and Mid-Atlantic may face the exact same scenario with Hurricane Florence, and the reason will be eerily similar.
A ridge of high pressure, extreme especially for this time of year, will develop just off the coast of New England, shunting the path of Florence toward the southeast coast. The strength of this ridge will be unprecedented in 30 years, according to forecast models.
With such a strong area of high pressure directly to the north of Florence, the storm has no pathway to curve out to sea as many other tropical systems usually do. Florence will be forced to the west, passing over an environment that is extremely favorable for intensification and on a collision course with the East Coast. But that represents only half of the problem.
As these animations show, Florence is forecast to stall out after the storm makes landfall. The stall is the result of the historic high pressure to the north of storm, refusing to budge and trapping Florence in one location for several days.
Despite being stuck, Florence won't die out easily. The storm will probably lose its hurricane designation rather quickly once it moves over land, but there is still the problem of all that water the storm was carrying. Even when Florence reaches a post-tropical phase, the storm will still be dumping tons of rain over the same locations for several days. It could be similar to Hurricane Harvey's impact on Texas and Louisiana in 2017.
The development, position and ultimate strength of the aforementioned high-pressure system will be the ultimate determining factor on where Florence goes and if (and where) the storm stalls out. Forecast models have generally come into agreement on the strength and placement of the high as Florence approaches the coast — hence, the better agreement on a track forecast.
Regardless of where Florence eventually ends up, it's safe to say that for the second year in a row, a major hurricane is about to hit an atmospheric roadblock, putting millions of people at risk of what could become a catastrophic flooding situation.
What makes this situation even more tenuous is that several inches of rain have already fallen in the region. Whatever falls from Hurricane Florence will be hitting saturated soil. This makes it all the more likely that trees and power lines will come down as winds pick up, and flooding will begin soon after the rain begins.
More than a foot of rain is possible in the Carolinas, and some areas could get much more. From the higher elevations of the Appalachian Mountains to the coast of the southeast and the Mid-Atlantic, streams, creeks and rivers will rise beyond their banks. If these forecasts play out, flash flooding will inundate roads and strand motorists and overwhelm homes and businesses.