Florence Expected to Stall Near Coast, Bringing Extreme Rain, Surge, and Wind Damage

September 12, 2018, 10:08 AM EDT

Above: ISS image of Hurricane Florence at 9 am EDT Wednesday, September 12, 2018. Image credit: Ricky Arnold.

Extremely dangerous Hurricane Florence continues to maintain Category 4 strength as it steams west-northwest towards a devasting encounter with North Carolina and South Carolina. Florence is a storm whose strength, impacts, and unorthodox track all point to outcomes that will likely lie outside historical experience for much of the Southeast U.S. The odds continue to increase that Florence will stall on Friday and move slowly west-southwestward along the coast for several days, bringing a devastating rainfall, storm surge, and wind event for a large swath of North and South Carolina.

The Hurricane Hunters and microwave satellite imagery this morning found that Florence had weakened by 10 mph since Tuesday night, thanks to an eyewall replacement cycle (ERC)—the second one the hurricane has undergone. During this phenomenon, common in intense hurricanes, the eye shrinks to such a small diameter (typically around 10 – 15 miles) that the eyewall becomes unstable and collapses. The hurricane then creates a new larger-diameter eyewall out of a spiral band. During this process, the peak winds typically fall by 10 – 15 mph, and the central pressure can rise 10 – 15 mb.

The Hurricane Hunters measured top surface winds of 130 mph Wednesday morning, down from the 140 mph peak winds of Tuesday night, and Florence’s central pressure ranged between 943 - 948 mb, compared to 946 mb on Tuesday night. The eye had expanded from 17 miles in diameter on Tuesday night to a diameter of to 29 miles on Wednesday morning.  The aircraft noted that the southeast side of Florence’s eyewall was missing at 7:30 am EDT. Florence's top sustained winds remained 130 mph as of 11 am EDT Wednesday.

Florence has plenty of time to recover from this eyewall replacement cycle, and hurricanes usually begin to re-intensify within 12 hours of closing off their new eyewall. Florence had closed off its new eyewall as of 9:10 am EDT Wednesday, according to data from the Air Force hurricane hunters. After an ERC is complete, a hurricane’s wind field will expand, subjecting a larger area of the ocean to strong winds and allowing a larger storm surge to build. At 11 am EDT Wednesday, Florence was a slightly above-average sized hurricane, with tropical storm-force winds that extended out up to 175 miles from the center. Hurricane-force winds extended out 70 miles from the center. These numbers will increase today as the hurricane recovers from the ERC.

At 11 am EDT Wednesday, Florence was about 485 miles southeast of Wilmington, North Carolina, moving northwest at 15 mph. Sea surface temperatures (SSTs) were a warm 29° - 29.5°C (84° - 85°F), which is about 2°- 4°F above average. Florence was embedded in an atmosphere with dry air (a mid-level relative humidity of 50%). However, with wind shear a light 5 - 10 knots, the hurricane has successfully walled off this dry air, and has been minimally affected by it. Satellite images on Wednesday morning showed that the hurricane had a very impressive appearance, with a prominent eye surrounded by intense eyewall thunderstorms with cold cloud tops. By tracking the motion of the wispy upper-level cirrus clouds, three upper-level outflow channels were apparent, to the northwest, east, and southeast. High level cirrus outflow clouds from Florence were already streaming over coastal North Carolina.

Florence forecast
Figure 1. HWRF model winds and pressure forecast for 11 pm EDT Thursday, September 13, from the 6Z Wednesday run of the model. The HWRF model was our top intensity model from 2017, and predicted that Florence would make landfall near the South Carolina/North Carolina border as a Category 3 hurricane with winds of 125 mph. Image credit: Levi Cowan, tropicaltidbits.com.

Intensity forecast for Florence: Category 4 strength through Thursday, then weakening

Florence’s environment is very conducive for intensification. The SHIPS model predicts shear will remain low through Thursday night. SSTs will be near 29 - 29.5°C (84 - 85°F) during this period, and ocean heat content will be high, near 30 - 40 kilojoules per square centimeter. The presence of three upper-level outflow channels will help Florence suck in large quantities of moisture-laden air from the surface, helping it maintain its intensity. Our top three intensity models unanimously predict Florence will remain a Category 4 hurricane with 130 - 140 mph winds through Thursday night, and the storm is also expected to increase in size. Florence will still be embedded in a relatively dry atmosphere, so it is possible by Thursday night, when wind shear is expected to increase to a moderate 10 – 15 knots, that this dry air will weaken the hurricane.

The Wednesday morning forecasts by our top three intensity models all predicted that Florence would weaken on Thursday night and Friday morning as it neared land, becoming a Category 3 storm with 115 – 125 mph winds. This weakening is to be expected, since the hurricane will be moving slowly over shallower waters with low heat content, providing less energy to the storm. Florence will also have a portion of its circulation over land, which will cut down the amount of moisture available to the storm; the increased friction of the winds blowing over the land will also slow down the storm’s winds.

Florence forecast
Figure 2. The 0Z Wednesday, September 12, 2018 track forecasts by the operational European model for Florence (red line, adjusted by CFAN using a proprietary technique that accounts for storm movement since the time of the model run), along with the track of the average of the 50 members of the European model ensemble (heavy black line), and the track forecasts from the “high probability cluster” (grey lines)—the four European model ensemble members that have performed best with Florence thus far. These forecasts were very unified on Florence approaching the coast of North Carolina, then moving southwest along the coast and inland over South Carolina. Image credit: CFAN.

Track forecast: a stall near the North Carolina coast, then west-southwest motion

The track forecasts from our top five models agreed on Wednesday morning that Florence would cruise towards the North Carolina coast through Thursday afternoon, as the clockwise flow around the Bermuda High pushed the storm west-northwest to northwest. But on Thursday night, Florence will put on the brakes and screech to a halt as the steering currents collapse. What’s going on is that the clockwise flow of air around a second high pressure system, currently centered over the Midwest near Michigan, will bring northerly winds to the East Coast. The two high pressure systems will battle for dominance, with the northerly flow from the Midwest high opposing the southerly flow from the Bermuda High. The Midwest U.S. high pressure system is expected to win the battle, as it strengthens and build to the east, forming a blocking ridge of high pressure to the north of Florence that will force the hurricane to the west, west-southwest, or southwest, along or just inland from the coast of South Carolina.

Given that the steering currents will be very weak Friday and beyond, slight changes in the balance between these forces guiding the hurricane will make a huge difference in where the hurricane makes landfall. This makes the exact landfall location difficult to predict accurately. The most concerning forecast is from our top model for forecasting hurricanes, the European model, which predicts that Florence’s stall will occur just offshore of the NC/SC border, with the hurricane then traversing most of the coast of South Carolina just offshore, before making landfall in southern South Carolina. This worst-case scenario allows Florence to keep its eye over water, greatly increasing the amount of rain it can dump, and subjecting a very long stretch of coast to extreme winds and near-record levels of storm surge. Our other top models, the GFS, HWRF, UKMET, and HMON, all predict Florence will move ashore near the NC/SC border, then turn to the west or west-southwest over land. On this track, Florence would still keep a large part of its circulation over water and dump extreme rains, but would not bring extreme storm surge and wind damage to a long stretch of coast. The model that will replace the GFS in 2019, the GFS FV3, is between these extremes, showing a west-southwest turn just offshore and a landfall in central South Carolina near Charleston.

Florence rainfall forecast
Figure 3. Predicted 7-day rainfall amounts from Florence. Image credit: NWS.

Florence: an extreme rainfall threat

Florence’s stall near the coast and slow motion will result in prodigious amounts of rain. If a significant portion of the storm’s circulation remains over water, as occurred last year with Hurricane Harvey’s stall over Southeast Texas, the rain from Florence may break all-time state records for rainfall from a hurricane or tropical storm. North Carolina’s state rainfall record from a hurricane is 24.06” from Hurricane Floyd of 1999, South Carolina’s is 18.51” from Tropical Storm Jerry of 1995, Virginia’s is 27.00” from Hurricane Camille of 1969. Soils are near saturation in some areas of Florence’s likely heavy rain zone, thanks to a record-wet summer. Heavy rains run off of wet soils and create bigger floods. There is also the danger that Florence could make landfall, then emerge back over water and re-intensify, increasing its rainfall potential. In any case, Florence’s rains will bring catastrophic flooding to a large region along the track of the storm.

Florence size
Figure 4. Size of Hurricane Florence (in degrees of latitude) as a function of time. During the period September 1 – 10, Florence was below average in size (about one standard deviation below average, roughly the 30th – 40th percentile in size). By Wednesday morning, September 12, 2018, the wind field expanded to the point where Florence was a little above average in size (approximately the 60th percentile). This size is expected to increase through Thursday, which will increase the storm surge potential of the storm. Storm size is estimated using IR satellite imagery and storm latitude. Image credit: NOAA/RAMMB.

Expect a storm surge of 6 - 13 feet near and to the right of where the eye makes landfall--and some unusual surge behavior

Although Florence is an average-sized hurricane right now, the hurricane is expected to increase in size, creating a large storm surge that will likely bring a 6- to 13-foot inundation of the coast along an 80-mile-wide stretch near and to the right of where the eye comes ashore, according to the 5 am EDT Wednesday NHC advisory. The highest surge will come over a 10- to 40-mile-wide stretch where the right-hand eyewall makes landfall.

As I discussed in detail in the post, Expect a Storm Surge of 15 - 20 Feet in a Landfalling Category 4 Storm in the Carolinas, two of the three historical Category 4 hurricanes that have hit the Carolinas have generated a storm tide of 18 - 20 feet: Hugo of 1989 and Hazel of 1954. If Florence expands in size to become a large hurricane, and hits as a Category 4, it could potentially bring a 15 – 20’ maximum storm tide to the coast. The storm tide is the combination of the storm surge and the normal lunar tide, measured in height above sea level. The National Hurricane Center uses the terminology “height above ground level” when discussing the storm tide, meaning the height the surge (plus tide) gets above the normal high tide mark.

It usually matters critically when the storm surge hits in relation to the tidal cycle, since the range between low and high tide along much of the coast of North Carolina is 4 – 5 feet. However, in the case of Florence, the storm is expected to slow down so much near landfall that the coast in the highest surge zone is almost guaranteed to get a near-peak surge during at least one high tide and low tide. Near Wilmington, NC, the worst storm surge flooding will occur near the high tides at 1:38 am Friday and 2:16 pm Friday. The difference between high and low tide is about five feet there. As I discussed in Friday’s post, Friday: Increased Storm Surge Damage From Florence Due to the Moon’s Phase, the tides late this week are higher than usual, due to the phase of the moon. In Morehead City, the high tides peak about two hours earlier than at Wilmington, and tidal range between low and high tide is about four feet.

Storm surge expert Dr. Hal Needham has an excellent discussion of why we might expect Florence to have a lower peak storm surge than the ones produced by Hugo or Hazel. In a follow-up post on Wednesday, Needham also discusses the extremely unusual possibility of a southward-spreading storm surge if Florence should move on a path hugging the South Carolina coast, as discussed above. According to Needham: "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. 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."

If you are under a storm surge watch and are asked to evacuate for the storm surge, get out! Tropical storm-force winds may begin along the coast as early as Thursday morning, making it difficult or impossible to evacuate. Above all, residents of the coastal and near-coastal Carolinas should keep in mind that Florence is a historic storm. Its strength, size, and potentially unorthodox track all point to outcomes that may lie outside historical experience.

Bob Henson contributed to this post.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Dr. Jeff Masters

Dr. Jeff Masters co-founded Weather Underground in 1995, and flew with the NOAA Hurricane Hunters from 1986-1990.

jeff.masters@weather.com

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