In Florence's Grip, No Relief for North Carolina

September 13, 2018, 10:57 PM EDT

Above: Infrared-wavelength satellite image of Hurricane Florence at 0402Z (12:02 am) Friday, September 14, 2018. Image credit: NASA/MSFC Earth Science Branch.

Hurricane Florence ground its way along the southern coast of North Carolina on Thursday night, delivering torrential rains, high winds, and an already-dangerous storm surge. As of 11 pm EDT, Florence was about 60 miles east-southeast of Wilmington, drifting northwest at 6 mph with top sustained winds of 90 mph. Although Florence had become a Category 1 hurricane on the Saffir-Simpson wind scale, it remained 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 Florence’s outer spiral bands twirling across much of southern and eastern North Carolina on Thursday night. Atlantic Beach, just south of Morehead City, recorded 12.73" through mid-evening Thursday, noted weather.com, and the preliminary total was nearing 18" by midnight. Several Personal Weather Stations on the coast near Morehead City reported 5 - 10 inches of rain by 6 pm EDT Thursday, but later went silent.

Some of the highest winds with Florence were produced by a secondary ring of thunderstorms encircling the storm's main eye. That ring of intense convection lay parallel to the coast on Thursday afternoon, slowly translating inland and dumping huge amounts of rain. A flight-level wind of 117 mph was reported by hurricane hunters Thursday night in this outer band. Surface winds gusted to 106 mph at Cape Lookout and 105 mph at Fort Macon late Thursday.

Intensity forecast for Florence: little change before landfall

Florence’s environment was still conducive for intensification on Thursday night, and very strong thunderstorms were blossoming around the center of the hurricane. However, Florence was running out of time to parlay this convection into any organized strengthening of its inner-core winds. 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). Together with Florence's large and robust circulation, these factors should help keep the storm from decaying quickly. Instead, 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.

NHC predicts that Florence's center will pass close to Wilmington at midday Friday, and that Florence will remain a Category 1 hurricane centered less than 50 miles west of Wilmington at 8 PM Friday. This will give the city an unusually prolonged assault by the inner core of a large hurricane.

Track forecast: little change from Thursday morning

There remained some differences in landfall timing among our top track models, but some of those differences pertained to how far north they placed the center of Florence as it moved west near the concave stretch of coastline near Wilmington. The most concerning forecast from Thursday came 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. UPDATE: The 0Z Friday run of the European model is now more in line with the other models, and does not have an extended track for Florence over water.

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. The NHC forecast is close to this group consensus.

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 go well past one or more 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 17.45” from Hurricane Beryl of 1994, Virginia’s is 27.00” from Hurricane Camille of 1969. (Note: David Roth from NOAA's Weather Prediction announced on Thursday that the previously reported 17.45" from Tropical Storm Jerry in 1995 was found to be erroneously high.)

The 11 pm 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.

Prolonged surge event to chew on the Carolina coast

Florence’s wind field has put a large region of ocean into motion, which was rapidly pushing up water levels on Thursday night along the coast of North Carolina. As of 11 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.

NHC warned on Thursday night that the highest surge from Florence may occur in the Neuse, Pamlico, Pungo, and Bay Rivers, perhaps exceeding 11 feet in some areas. The city of New Bern reported widespread flooding on Thursday night, with 9.6 feet of inundation recorded by a USGS river gauge near midnight. A storm surge of 10 feet was reported late Friday at the Cherry Branch Terminal on the Neuse River, about halfway between New Bern and Pamlico Sound. This terminal is located near a sharp bend in the Neuse River, which allowed the strong northeast winds of Friday to pile up water near the bend. As winds gradually switch to southeasterly on Friday, they will become more aligned with the section of river extending north-northwest to New Bern, so they could keep surge levels high across this section of the river, especially during Friday's midday high tide. An interactive map from the City of New Bern shows the current state of flooding.

As Florence passes west of Wilmington, NC, on Friday afternoon and evening, offshore flow will switch to onshore flow, and surge into the barrier islands near Wilmington and the Cape Fear River may increase sharply. The worst storm surge flooding in Wilmington can be expected occur near the high tide of 2:16 pm Friday EDT, with significant surge perhaps continuing into another peak on Friday night. The difference between high and low tide is about five feet there. As we discussed in last Friday’s post, 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 into Friday northeast of Florence’s center

Tornadoes will be yet another dangerous threat from Florence into 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.

Connie track
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.
Ione track
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! In addition to our posts, see weather.com for frequent updates on Florence's impacts.

One more tip: if you encounter jaw-dropping images or outlandish-seeming claims on social media that are attributed to Florence, please make sure they're real and Florence-relevant before sharing. To paraphrase Smokey Bear, only you can prevent social media fires.

Dr. Jeff Masters co-wrote this post.




 

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Bob Henson

WU meteorologist Bob Henson, co-editor of Category 6, is the author of "Meteorology Today" and "The Thinking Person's Guide to Climate Change." Before joining WU, he was a longtime writer and editor at the University Corporation for Atmospheric Research in Boulder, CO.

bob.henson@weather.com

@bhensonweather

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