Florence Threatens to Inflict Unusual and Multi-Dimensional Flood

Main Points about Flood Potential

1) Hurricane Florence is forecast to slow down and drift near coastline. This would inflict a prolonged compound flood event from storm surge and heavy rain.

2) Possible southward drift along coastline would inflict an unusual, rapid storm surge event, as winds immediately reverse from offshore to onshore when Florence's eye passes.

3) Heavy rain impacts are likely for extended distances inland and to the north of Florence's center. The decrease in forward speed will enable Florence to spread torrential rain over a widespread area for many days.

4) Storm surge flood threat most pronounced in the Carolinas. Inland flood impacts extend as far west as the Appalachians and as far north as Pennsylvania; ground from Virginia through Pennsylvania is already saturated.

Action Items

1) If you live along East Coast, from South Carolina through Pennsylvania, stay tuned to the National Hurricane Center, National Weather Service and your local media outlets for updated information, warnings and mandatory evacuations. Florence threatens to inflict disastrous wind and flood impacts across a large region.

2) Two Facebook groups are connecting people people threatened by Hurricane Florence with volunteers. Flood Survivors (Harvey and Others) Helping Hurricane Florence Friends connects flood victims with flood survivors and provides practical discussions on everything from evacuation to mold remediation. Person2Person4Relief directly connects flood victims with volunteers who help with flood relief. 

Meteorological Analysis in Detail

Hurricane Florence is a category-4 hurricane on the Saffir-Simpson Hurricane Wind Scale. Florence will move into a favorable environment with low wind shear and sea surface temperatures exceeding 84 degrees F (29C) over the next day, enabling some strengthening Wednesday. Florence may begin to slightly weaken on Thursday, as the storm encounters increased wind shear, cooler sea surface temperatures from upwelling and drier air drawn in from the continent. Nonetheless, Florence should approach the coastline as a major hurricane with widespread impacts.

Florence to Drift near the Coastline

A major change from yesterday's forecast is that it is becoming more likely that Florence will slow down and drift before making landfall. This is an ominous development, as warm ocean waters fuel hurricanes. A hurricane centered offshore will weaken at a slower rate than one that stalls inland.

Another development in today's forecast is that Florence is expected to drift west, or even southwest, possibly enabling it to track along the North Carolina and South Carolina coastline. A slow-moving hurricane tracking near a coastline is bad news indeed, as it enables the storm to inflict destructive storm surge along an extended area.

The latest run of the European model shows the unbelievable- Florence nearly making landfall near Wilmington, North Carolina on Thursday evening....then slowly tracking along the entire South Carolina coast through the entire weekend and making a landfall near the Georgia/ South Carolina border on Sunday evening. This scenario would devastate the Carolinas.

European model depicting Florence's position on Thursday evening. Source: Tropical Tidbits/ Levi Cowan.

European model depicting Florence's position on Sunday evening. Source: Tropical Tidbits/ Levi Cowan.

It is important not to focus on the exact track, as Florence's extended forecast is still quite uncertain. It becomes difficult to forecast a hurricane's path when there is a drastic decrease in the upper-level steering currents, rivers of air in the upper levels of the atmosphere that drive these storms.

This makes sense to us because we have all experienced a change in steering when riding a bicycle. At a high speed it is easy to choose a straight path, but when we stop pedaling our path becomes less certain as we approach our stopping point. Hurricanes that lose their upper-level steering sometimes wobble or drift in an erratic pattern for extended periods.

Unusual, Rapid Storm Surge Possible

Most hurricanes that track along the East Coast move from south to north. Because circulation around hurricanes in the Northern Hemisphere is counterclockwise, onshore winds steadily increase as the storm approaches. This enables storm surge to gradually build over time.

It is quite unusual for a hurricane to track from north to south along this coastline, and if this happens, as the European model suggests, the wind and storm surge patterns will be unusual, and inflict rapid storm surge flooding in southern North Carolina and along the South Carolina coast.

This coastline would experience increasing offshore winds, blowing from land to water, as Florence's center approaches. This would create chaotic seas, as Florence is still displacing tremendous amounts of water towards land, but a powerful offshore wind would serve to mitigate water levels.

Super Typhoon Haiyan generated a rapid storm surge in the Philippines as winds in the typhoon's eyewall shifted from offshore to onshore. Haiyan's surge hit more like a tsunami than a typical surge. Footage of this event is available in the first minute of this video from the PBS/NOVA documentary Killer Typhoon.

However, as soon as Florence's center passed a location, powerful winds in the hurricane's eyewall, the most intense part of the storm, would immediately shift from offshore to onshore, producing a destructive storm surge in the matter of minutes.

This phenomenon was observed in Super Typhoon Haiyan in the Philippines in 2013. Haiyan's storm surge rushed in more like a typical tsunami than a hurricane storm surge. See the video above (first minute of footage).

It should be noted that Haiyan was among the most intense tropical cyclones to ever make landfall, and the sharp boundaries of the bay near Tacloban exacerbated this surge event. I do not expect a Florence's surge would move in as rapidly.

Nonetheless, a southward moving hurricane tracking along the coastline, would produce a sudden reversal of winds from offshore to onshore, rapidly pushing a storm surge into the coastline. This is unfortunate, as rapidly-moving flood events kill more people than gradual floods.

Compound Flood Threat Increasing 

While Florence's storm surge and rainfall forecasts are constantly updated, it is important to realize that Florence threatens to inflict widespread compound flooding, from the combination of storm surge and heavy rain, particularly near the coastline.

During compound flood events, prolonged onshore winds elevate the ocean and fill bays to capacity with salt water, reducing the efficiency for heavy rainfall to drain. This phenomenon enables less rain to produce serious flooding, as the drainage is slowed considerably.

In 2012, Hurricane Isaac generated a widespread compound flood in southeast Louisiana, flooding some areas that were not flooded by Katrina. This surprised many people, as Isaac was a category-1 hurricane. However, Isaac stalled along the coast, pumping saltwater into Lake Pontchartrain for days, which slowed the drainage of 10-15 inches of rain in the region.  

Hurricane Isaac (2012) inflicted widespread compound flooding in southeast Louisiana. The prolonged nature of the event enabled storm surge and heavy rain to flood some areas not flooded by Katrina. Image: https://www.youtube.com/watch?v=ZMSe75GIo98.

Thomas Wahl, Assistant Professor in the Department of Civil, Environmental and Construction Engineering at University of Central Florida has studied this phenomenon extensively. He is lead author on a recent paper published in EOS Earth and Space Science News that investigates such complex environmental relationships and takes a broad viewpoint on how multiple factors can amplify the effect of severe storms and other extreme events.

Wahl was also lead author on a paper in Nature Climate Change that found the threat of compound flooding from storm surge and heavy rainfall was increasing for major U.S. cities.

Wahl expects impacts from compound flooding in Florence and stated that his biggest concerns come from, "A possibly stalling system that could bring unprecedented rainfall and storm surge to the area, not just in terms of intensity but also duration." 

Rainfall Forecast

National Hurricane Center rainfall forecast from this morning's advisory.

Widespread and long-duration rainfall will likely generate unprecedented flooding through the Carolinas. The graphic above, from this morning's National Hurricane Center advisory, depicts a broad area with 6-10 inches of rain, with rainfall exceeding 20 inches near the North Carolina coast. The advisory states that 20-30 inches could fall in this area, and mentions the possibility of 40-inch totals.

Keep in mind that the rainfall patterns will be highly dependent on Florence's track, which is difficult to pinpoint for a hurricane that stalls near the coastline. Localized maximum rainfall can considerably exceed such forecasts, as training of squalls over the same areas can produced extraordinary rainfall totals for slow-moving tropical systems.

I expect rainfall forecasts may increase considerably across central and western North Carolina, if Florence does indeed drift west, with the center of circulation passing south of this area. Prolonged east winds, which would gain lift as they upslope up the eastern side of the Appalachians, could generate phenomenal rainfall totals.

Heavy Rainfall Displacement in Stalled-out Hurricanes

We all make assumptions when we jump to a conclusion that seems obvious. If your friend tells you she lost 10 pounds, the assumption is that she has become healthier. But maybe she has been stressed and not eating, or began smoking heavily. A dramatic increase in salary with a job in a new city may imply increased wealth, until we realize the cost of living in the new city is double.

A major assumption people make with hurricanes is that the flood impacts will be worse the closer we are located to the eye of the storm. This seems intuitive. However, did you realize someone 100 miles from the storm's center may experience more severe flooding than someone 20 miles from the center?

Hurricane Harvey (2017) produced heavy rainfall that extended far from the center of circulation on the onshore side of the storm, but dry conditions near the eye on the offshore side. Adapted from Youtube Channel: Washington 1067. Markups by Hal Needham.

The counter-clockwise flow around Northern Hemisphere hurricanes that stall out near the coastline, produce onshore winds to one side and offshore winds on the other side.

Heavy rainfall commonly extends for more than 100 miles on the onshore side, but can rapidly diminish on the offshore side.

I have personally experienced this phenomenon during both Hurricanes Isaac (2012) and Harvey (2017).

The image above shows the rain pattern in Texas as Harvey was centered inland, between Corpus Christi and San Antonio. Harvey whipped bands of torrential rain more than 100 miles east of the circulation center for several days, while areas near the circulation center, on the "left" side of the storm stayed relatively dry.

People in Corpus Christi were shocked by the relative lack of flooding, as hurricane maps predicted a category-4 hurricane to strike not far east of them, while people displaced more than 100 miles to the east, near Houston/ Galveston and Beaumont/ Port Arthur, were taken off guard that a storm that "missed them" could inflict unprecedented flooding. 

NOAA radar loop from Hurricane Isaac (2012). Image courtesy Charles Kuster Youtube Channel.

The image above is taken from a Hurricane Isaac (2012) radar loop. Note the extent of squalls wrapped around the "right" (onshore) side of circulation, while areas just west of the eye were observing light to moderate showers.

Isaac drifted slowly northwest and the center of circulation passed not far from my home in Baton Rouge. However, Isaac's powerful squalls were displaced considerably to right side of the storm, inflicting more flooding in coastal Mississippi, more than 100 miles to the east, than at my location near the center of the storm.

Rainfall map from Hurricane Isaac (2012) shows heavy rainfall displaced to the right of the storm track

A map of Isaac's observed rainfall shows a substantial displacement of heavy rainfall to the "right" of the storm track. In fact, it appears that Isaac's track provides a western boundary on the area of heavy rain.

We cannot generalize that all hurricanes produce this rain pattern. However, when a hurricane stalls out near the coast for prolonged periods, the side with onshore winds is pulling warm, moist air off the ocean, while the side with offshore winds is pulling drier air off the continent.

The application to Florence is that areas well north of the storm track could observe surprising and unprecedented rainfall totals. At the same time, people in places like South Carolina and even Georgia should remain vigilant...although the southern boundary of the rain shield will probably have a sharp gradient, we cannot know where those rain totals will drop off because of uncertainty in the storm track.

Saturated Soils will Enhance Any Rainfall across Virginia and Mid-Atlantic States

Finally, a look at recent precipitation and soil moisture content is important for us to get a picture of the ability for the soil to retain heavy rainfall. Fortunately, September has been relatively dry so far for much of the Carolinas and Georgia, as depicted in this PRISM rainfall map below. Northern Virginia and the Mid-Atlantic States, however, have had a wet month so far, with flooding recently reported in northern Virginia, Maryland and Pennsylvania.

Cumulative rainfall map for September, from PRISM.

A wet September has unfortunately led to saturated soils from northern Virginia through Pennsylvania and New Jersey. The map below from the Climate Prediction Center/ NCEP/ NOAA shows that soil moisture in this region is unusually wet. Therefore, it will not take much rainfall in these areas to induce rapid flooding. 

Map of Soil Moisture Anomalies. Climate Prediction Center/ NCEP/ NOAA