Florence Slows to a Crawl Off Coast of North Carolina

Hurricane Florence has begun its long-awaited slow down, and was crawling at just 5 mph to the west-northwest towards Wilmington, NC as of 5 pm EDT Thursday. Heavy rains, a significant storm surge, and high winds from the Category 2 storm are already hitting the coast of North Carolina. Florence may be a Category 2 hurricane by the Saffir-Simpson wind scale, but it is a Category 5 heavy rain and inland flooding threat, and a Category 3 storm surge threat.

Radar data from the Morehead City, NC radar showed that Florence’s outer spiral bands began dumping heavy rains over the Outer Banks and much of Eastern North Carolina on Thursday morning. Rainfall amounts of 0.5 – 1.0 inch per hour were common, with a few heavy cells generating higher rainfall rates. Several Personal Weather Stations on the coast near Morehead City reported 5 - 10 inches of rain by 6 pm EDT Thursday, but have now gone silent. Cedar Island Ferry Terminal near the Outer Banks reported 6.36” of rain as of 6 pm EDT Thursday, and the Morehead City Airport reported 2.49” of rain.

Intensity forecast for Florence: little change before landfall

Florence’s environment is still conducive for intensification, but the storm is not well-structured to do so, and the hurricane is quickly running out of time for any intensification to occur. The SHIPS model predicts shear will remain a moderate 10 – 15 knots through Saturday, and Florence will be over the warm waters of the Gulf Stream, where SSTs are a warm 29°C (84°F). The 4:30 pm EDT Thursday eye report from the Hurricane Hunters found that Florence was missing the eastern portion of the eyewall, something also seen from Morehead City radar. The radar data shows the hurricane has concentric eyewalls. This configuration is unfavorable for intensification, and we can expect the hurricane to gradually weaken as land interaction increases and the slower motion of the storm upwells cold water from the depths. Our top three intensity models predict Florence will be a borderline Category 1/Category 2 hurricane with 95 - 100 mph winds at landfall.

The inner core does look markedly better defined, but does not seem to be producing the highest winds in #Florence anymore. In fact, its the outer eyewall that has had the higher doppler #radar estimated winds so far. pic.twitter.com/VIlrjDyGnq

— Philippe Papin (@pppapin) September 13, 2018

Track forecast: little change from Thursday morning

The latest 12Z Thursday runs of our top five track forecasting models were similar to their previous 0Z runs, agreeing that Florence would move very slowly to the west-northwest or west at about 5 mph, until a landfall near Wilmington, NC occurs between 8 am and 2 pm EDT Friday. The most concerning forecast continued to be from our top model for forecasting hurricanes, the European model, whose 12Z Thursday run predicted that Florence would move southwest along the northern half of the coast of South Carolina, just offshore, then make landfall Saturday night near Charleston. This would allow Florence to keep its eye over water, greatly increasing the amount of rain it can generate, and would subject a very long stretch of coast to high winds and a destructive storm surge. Our other top track models--the GFS, HWRF, UKMET, and HMON--all predicted Florence would move ashore near the NC/SC border, then turn to the west-southwest over land. On this track, Florence would still keep a large part of its circulation overwater and dump extreme rains, but would not bring a devastating storm surge and hurricane-force winds to a long stretch of coast. By Sunday, Florence should be headed due west towards the Appalachian Mountains.

Due to record-breaking rains in May-July, we have had 62.60" of rain this year at ILM. With #Florence stalling near our coast and catastrophic flooding expected, 21.05" would break the record for most rainfall in a calendar year...... with 3+ months to go. #ncwx pic.twitter.com/sak7ZZGclf

— NWS Wilmington NC (@NWSWilmingtonNC) September 12, 2018

Catastrophic flooding expected from extreme rainfall

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. The 11 am EDT Thursday advisory from NHC warns of the potential for isolated totals of 40" near the coast of North Carolina and far northeast South Carolina, which would smash the current record in either state. 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.

The bottom line: Florence’s rains are likely to bring catastrophic flooding to a large region along the track of the storm.

Hurricane Ike, 2008. 6th costliest hurricane in U.S, history, $30 billion. 21 direct deaths in TX, LA, AR and 64 indirect deaths in Texas. Remnants killed another 28 people in the Midwest. Deadly storm surge, trees falling onto homes, carbon monoxide, electrocution. Category 2. pic.twitter.com/8rMqiXSiHk

— Dr. Rick Knabb (@DrRickKnabb) September 13, 2018

Prolonged surge event to chew on the Carolina coast

Florence’s wind field has put a large region of ocean into motion, which is already increasing water levels along the coast of North and South Carolina. As of 6 pm EDT Thursday, surge levels of 1 - 2 feet were being observed along the coasts of North and South Carolina (see the SURGEDAT storm surge update page or NOAA Quicklook page for live storm surge values along the path of Florence). The surge threat from Florence is higher than you might expect from its Saffir-Simpson category, because the storm is large and will be affecting the coast for a prolonged period. For example, Hurricane Ike in 2008, another former Category 4 storm, also weakened to Category 2 before landfall on Galveston Island, but it produced a devastating and deadly storm surge.

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 we 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.

Tornado threat ramping up tonight and Friday northeast of Florence’s center

Tornadoes will be yet another dangerous threat from Florence, especially Thursday night and Friday over southeast North Carolina. As Florence crawls to the west during this period, winds flowing into the storm across eastern North Carolina will back slightly, due to friction over land. This will increase the vertical wind shear, making it easier for cells within rainbands to develop rotating updrafts that spawn tornadoes--somewhat like supercell thunderstorms. Because of Florence’s angle of approach, the northeast quadrant of the hurricane—typically the most tornado-favored—will cover a large swath of land for a fairly long period while Florence remains at hurricane strength. The NOAA/NWS Storm Prediction Center (SPC) has a slight risk of severe weather across the coastal lowlands of North Carolina for Thursday evening, with a slightly larger slight-risk area on Friday. The greatest threat is expected Friday morning, before Florence undergoes more significant weakening.

Some hurricanes have been known to spawn dozens of tornadoes, such as Ivan (2004) across Alabama, Florida, and Georgia. In the case of Florence, though, SPC notes that the amount of instability over North Carolina may not be especially high. A dry intrusion several miles above the surface can actually increase instability and favor tornadoes in hurricanes, but the atmosphere over eastern North Carolina should remain moist at all levels.

Two historical analogues from 1955: Connie and Ione

This section courtesy of Hugh Cobb, retired branch chief (TAFB) from NHC.
The sluggish movement of Hurricane Florence brings to mind two hurricanes from the 1955 season--Connie and Ione--which stalled over NC for a period of time, dumping massive amounts of rain and producing high storm surges. Both hurricanes were once Category 4 storms, but made landfall as Category 2 hurricanes. Both caused massive destruction and flooding over eastern NC. The big difference with Florence vs. these two analogues is that Florence is moving from east to west, so the surge potential (especially shoving surge upriver) is much higher.

Figure 1. Hurricane Connie, once a formidable Category 4 hurricane on August 6, 1955, lumbered ashore on August 12, 1955 as a Category 2 storm with 100 mph winds and a central pressure of 962 mb. Connie produced storm surges of 7 to 10 feet in North Carolina’s Pamlico Sound. Surprisingly, tides were higher at Swansboro, North Carolina than during Category 4 Hurricane Hazel of October 1954.

Figure 2. Hurricane Ione, also once a Category 4 storm (on September 18, 1955), crept ashore as a much weaker Category 2 storm with 105 mph winds and a central pressure of 955 mb on September 19, and drifted for 7 - 8 hours between Morehead City and New Bern, NC. Prolonged easterly winds forced the tides to increase up to 10 feet above normal, which resulted in one of the largest storm surge inundations in North Carolina history.

Stay safe if you are in the path of this dangerous storm!

Bob Henson contributed to this post.