Hurricane Florence achieved Category 4 status over the open Atlantic Ocean, but the storm weakened before it made landfall as a Category 1 with a very large wind circulation. Florence is an example of what meteorologists have come to understand: Category at landfall is not necessarily related to total rainfall.
How does this storm compare with Hurricane Harvey, from 2017? Harvey struck the Southeast Texas coast as a Category 4 but weakened to tropical storm intensity over several days. It dumped as much as 61 inches over the Houston region. Total rain volume was estimated at 33 trillion gallons.
There are striking similarities between both storms beyond generating rainfall in the trillions of gallons. First, both storms had low-end wind ratings when they began intense rainmaking. Second, they lingered for days over the same region, at times barely crawling at a few miles per hour or remaining nearly stationary (Slo Flo?). Third, they stuck close to the coast, having access to an unlimited supply of water vapor evaporating off very warm ocean waters (the warm ocean also rendered the atmosphere unstable enough for rains to fall from intense thunderstorms). And fourth, they both generated a singular, broad rain band that remained fixed in location for days, allowing a sequence of convective cells to “train” repeatedly over the same spots.
Next we look at a series of graphics generated by the Weather Prediction Center, which portray the processes contributing to Florence’s heavy rains. Florence had a lengthy and dynamic period of rain production, and the principal rain generating mechanisms shifted depending on the geography.
I have selected one graphic for each of the five days that storm generated rain over the Carolinas and Virginia, highlighting the key process on each day. The graphics contain a bit of meteorological jargon, but the broad airflows, rain band and storm center locations are evident.
Sept. 13: The outer rain bands begin to rotate onshore over eastern North Carolina, followed by the western eye wall of the storm. Meteorologists calculate the water vapor content of air from a quantity called total precipitable water (TPW), based on measurements from weather balloons. The very highest values we have observed in the D.C. region this past summer have been 2.3 inches to 2.4 inches. But within the core of Florence, TPW values were estimated at a whopping 3 inches — essentially off the charts.
Sept. 14: The storm’s center hugs the coastline, drifting slowly southward. This keeps half of the storm over the ocean, where it can continue to feed off warm Gulf Stream water, maintaining intensity. An extreme rain zone sets up along the southeastern coast of North Carolina, north of the storm’s center. This is where converging winds and oceanic vapor set up a primary feeder band, which will remain anchored in this general location for the next two days.
Sept. 15: The storm center has now migrated to coastal South Carolina, and the stationary feeder band has become well established to the north. The band is essentially a conduit or pipeline directing a continuous train of tropical thunderstorms off the warm Gulf Stream. Fast, moisture-laden winds abruptly slow moving off water onto land, a process called frictional convergence, forcing the air to rise in chimney-like thunderstorms along the coast. Rain falls at rates of two inches to three inches per hour.
Sept. 16: The storm center has moved westward, deep into South Carolina, bringing a swath of heavy rain with it. Meanwhile, the feeder band continues to dump heavy rain along the North Carolina coast — although it begins breaking up late in the day. A new zone of heavy rain develops inland, along the Tennessee-North Carolina border, where fast easterly winds slam into the rampart of the Appalachians. This orographic (mountain-induced) rain zone sets up far to the north of the slowly moving center, which has weakened to a tropical depression.
Sept. 17: The orographically generated zone of rain moves farther north, as the tropical depression begins to accelerate northward, under the influence of upper-level winds. Early in the day, the heaviest rain is along the West Virginia-Virginia border. Later in the day, the depression is centered over Kentucky. The storm’s circulation, while weak, has expanded, such that a corridor of southerly flow develops over the North Carolina-Virginia Piedmont, far to the east. While a surge of dry air from the southwest shuts down rainfall over the mountains, a new rain band sets up over the Piedmont. Convective cells train from south to north from the Research Triangle of North Carolina, into the Richmond region. A mini outbreak of tornadoes (six in all) erupts around Richmond during the afternoon, embedded in this rain band.
By Sept. 18, the tropical depression has rocketed to the northeast, across Pennsylvania, dragging the rain band offshore, back over the Atlantic. Finally, after nearly a week, the Mid-Atlantic can begin to recover. It will take weeks for water to drain out of the Carolinas, and for swollen rivers to subside. And another tropical cyclone enters the history books in the Mid-Atlantic.