Hurricane Florence just got a lot more scary. The latest model runs have the monster storm stalling off North Carolina before tracking to the southwest along the South Carolina coast. This could amplify the destructive impact of the storm as it hammers coastal areas with high winds and torrential rainfalls fueled by moisture and energy from warm ocean waters. Storm surges and high waves will inundate and batter coastal areas while massive rainfall will generate devastating flooding in inland areas.
Why Florence’s Path Changed
Hurricane Florence was previously expected to follow a conventional track hitting the Southeastern coast of North Carolina and tracking to the northwest in fairly a straight line. However, projections of the storm’s path have shifted dramatically between Tuesday evening and Wednesday.
According to Phillipe Papin, an atmospheric scientist with the U.S. Navy Research Laboratory, a high-pressure ridge over the Midwest grew stronger. Meanwhile the high-pressure ridge behind the storm to the east, which had been pushing the storm northwards, was dissipated by a deepening trough over Bermuda. The midwestern high-pressure ridge is now set to block the storm’s northern progress and eventually turn it to the southwest.
Why is Hurricane #Florence taking this rather unusual track to the west over the Atlantic south of Bermuda?
A strong "blocking ridge" to the North prevents the typical "recurvature" or escape poleward or northward.
ECMWF 12z update still arriving (https://t.co/XcmEbEJxko) pic.twitter.com/c2UbK5CjOf
— Ryan Maue | weathermodels.com (@RyanMaue) September 6, 2018
Echoes of Harvey
Hurricane Florence’s projected jog to the southwest is reminiscent of Hurricane Harvey, which made landfall before turning back towards the Gulf of Mexico and tracking back up the coast. This allowed Harvey to draw moisture out of the Gulf of Mexico for an extended period of time and inundate Houston with massive rainfall for days. Some areas received as much as 40 inches of rain. The result was devastating floods and the wettest storm on record. Hurricane Florence is projected to produce similarly intense rainfall, with an estimated 20-30 inches of rain falling across parts of coastal North Carolina and as much as 40 inches in some places.
Why Florence is Such a Strong Storm
If a hurricane were a fire, hot water would be gasoline. Between the tropics where Florence first formed and the Carolina coast, unusually hot water fueled its intensification to a Category 4 storm. Although winds have subsided a bit back to Category 3 levels as it moves into cooler coastal waters, it remains a powerful and life-threatening storm.
It’s well known that hurricanes gain strength from hotter waters. Rising warm air cause storms to churn more violently. Higher sea temperatures generate higher maximum sustained winds in storms, but warmer air and sea temperatures also increase the rainfall potential of storms.
We know from basic physics that warmer air holds more moisture at a given pressure than cooler air. This can be calculated using the Clausius–Clapeyron relation, which shows that at standard pressures, a 1 C rise in temperature yields a 7% increase in the moisture-holding capacity of the air. Warmer sea temperatures increase evaporation off the water’s surface increases as well. The result is more moisture-laden air that generates storms with heavier rainfall. When Florence stalls on or close to shore, the back side of the storm will continue to pull moisture and energy from warmer offshore waters, dramatically increasing rainfall totals.
The Bottom Line
Hurricane Florence is potentially the most dangerous storm to hit the east coast in decades. And, there’s more where that came from. The National Hurricane Center is now tracking four named storms out in the Atlantic, tying the all time record. A fifth storm forming in the Gulf of Mexico may break it.