Bob Henson ·

Above: Sentinel3 image of Hurricane Florence from Tuesday afternoon, September 11, 2018. Image Credit: Copernicus EU.

Hurricane and storm surge warnings were issued Tuesday afternoon ahead of Hurricane Florence, which is increasingly likely to deliver one of the most destructive and prolonged disasters ever seen in the Carolinas. As of 8 pm EDT Tuesday, a hurricane warning was in effect from South Santee River, South Carolina, to Duck, North Carolina, including the Albemarle and Pamlico Sounds. A storm surge warning covered the same area, as well as the low-lying Neuse and Pamlico Rivers just inland. Hurricane and storm surge watches extended southward to Edisto Beach, SC, and north to the Virginia-NC border. A tropical storm watch covers the southeast Virginia coast up to Cape Charles Light and Chesapeake Bay south of New Point Comfort.

Florence was again on the upswing in intensity late Tuesday after its strength was briefly dented Monday night by an eyewall replacement cycle, as explained by Dr. Jeff Masters in our previous post. A new, larger eye (about 30 miles across) is now in place, and that will allow the hurricane’s wind field to expand as well. Satellite images on Tuesday night showed that Florence was still adjusting to its new eye, and had an asymmetric shape with a relatively thin area of heavy thunderstorms on the southwest side. Florence’s top sustained winds were raised to 140 mph by the National Hurricane Center at 5 pm Tuesday. Sustained hurricane-force winds now extend up to 60 miles out from Florence’s center, and tropical-storm-force winds extend out 175 miles. These numbers may grow even larger as Florence matures and expands. At its current size, Florence rates as an average-sized hurricane.

No hurricane-hunter flights gathered data from Florence on Tuesday afternoon, but an Air Force plane penetrated the eye at 9 pm EDT Tuesday, and found a central pressure near 944 mb, just 1 mb below what NHC was estimating in their 8 pm EDT advisory. The mission will last into the early morning hours Wednesday.

How the ECMWF (europ) EPS has behaved over the last 13 runs...



Major #hurricane #Florence still sits at category 4 and is unfortunately showing more and more signs of hovering around the coast --> https://t.co/p9nzlHQDxm



Evacuations are underway. #ncwx #scwx pic.twitter.com/ScvD5h8cCT — wxcharts (@wxcharts) September 11, 2018

Forecast for Florence: Uncertainty ramps up beyond Friday

Conditions are close to ideal for Florence to strengthen further into Wednesday. Wind shear will be less than 10 knots, and Florence will be passing over a pocket of deep warm water with high oceanic heat content on Tuesday night. Two high-altitude outflow channels—one on each side of the storm’s center—will keep Florence well-ventilated. Dry air has been lurking around Florence, but the newly rebuilt core should keep that dry air at bay. The NHC forecast brings Florence’s sustained winds to 155 mph by Wednesday, just shy of Category 5 strength. With the conditions so supportive, it’s entirely possible that Florence might cross this threshold and become a Category 5 for at least a few hours on Wednesday. If so, it will enter the history books as the northernmost Category 5 on record.

Figure 1. Infrared satellite image of the Southeast U.S. coast (left) with Hurricane Florence (right) at 8:57 pm EDT Tuesday, September 11, 2018. Image credit: NASA/MSFC Earth Science Branch.

As Florence nears the coast on Thursday night, wind shear may increase to a moderate 10 – 20 knots, and the shallower waters near the coast will provide less oceanic fuel. Florence will most likely be a Category 3 near landfall—assuming it crosses the coast, that is. The steering currents driving Florence toward the East Coast will collapse on Friday, and models now agree the storm is likely to stall somewhere within 100 miles on either side of the coast, perhaps for one or two days.

The 12Z Tuesday run of the European model introduced a new and very distressing possibility: Florence stalling just offshore of North Carolina near Wilmington for roughly a day, then moving southwestward along and just off the South Carolina coast on Saturday, and finally making landfall close to Savannah, Georgia, on Sunday—all while still a hurricane. This outlandish-seeming prospect gained support from the 18Z run of the GFS model. It painted a very similar picture, with a landfall a bit farther north, near Charleston, on Sunday. The 18Z track from the experimental GFS FV3 model is very similar to the GFS track.

Figure 2. The 12Z Tuesday, September 11, 2018, track forecast 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. The forecasts agree strongly that Florence will move very near the coast of southern North Carolina, and all but one of the ensemble members then move the storm southwest along or near the coast into South Carolina, then well inland by Sunday/Monday. At right are the projected intensities in knots from each ensemble member; multiply by 1.15 for miles per hour. Image credit: CFAN.

A high-confidence forecast: Biblical rains

Because the steering currents will be so weak from Friday onward, forecast models will struggle to capture their details, which may lead to big track changes from run to run for the period after Friday. Despite this uncertainty, we’re very confident that Florence’s slow movement from Friday into the weekend will lead to titanic rainfall amounts in at least some places. The 5-day rainfall outlook from the NOAA/NWS Weather Prediction Center (see below) includes a patch of 20”+ totals near the southernmost coast of North Carolina, including the Wilmington area. A much larger area of 10” – 20” includes the eastern third of the state. Local amounts could reach 35”, according to NHC; this would exceed the heaviest rains any tropical cyclone has produced in the contiguous U.S. outside of Texas, Mississippi, and Florida.

How far inland and southward the very heaviest rains extend will depend on Florence’s track from Friday onward, but widespread flash flooding will likely be a risk at least through the weekend, and river flooding well into next week. If Florence moves inland and up the crest of the Appalachians early next week, as is now suggested by the GFS and European models, the flood threat would expand even further.

Figure 3. NOAA’s 5-day precipitation outlook for the period starting at 8 pm EDT Tuesday, September 11, 2018. If Florence ends up moving southward along the South Carolina coast, some of the heavier amounts shown here would shift in that direction. See below for discussion of the heavy rains along the Texas coast, which will be produced by a separate disturbance. Image credit: NOAA/NWS Weather Prediction Center.

The bottom line: There’s very high confidence that Florence will reach the southern coast of North Carolina on Thursday night as a very dangerous major hurricane. It now looks increasingly possible that Florence will stay near the coast (either inland or just offshore) for a day or longer, perhaps moving southwest along the coast. In this case, the coastline north of the center could experience storm surge during at least two 12-hour tidal cycles, rather than just one, and at least some surge could spread southwest with Florence. Florence’s track beyond Friday is still subject to major change as the weak steering currents evolve.

Preparing for a siege

Mandatory and voluntary evacuation orders were in place late Tuesday for large parts of the East Coast from South Carolina to Virginia. See weather.com’s state-by-state evacuation roundup for frequent updates. Surge flooding may push well inland through rivers across the coastal plain of North Carolina. As Florence’s rains pile up, inland flooding will become an increasingly serious threat. Referring to the expected rainfall totals, NHC warned: “This rainfall would produce catastrophic flash flooding and significant river flooding.”

If the latest GFS and European tracks came to pass, and Florence lingers just offshore for a day or more, surge damage and beach erosion could be truly devastating for parts of North Carolina. Even people living well inland, within 50 – 100 miles of the coast, could experience huge amounts of rainfall and a long period of tropical-storm-force winds capable of bringing down trees and power lines. Residents in these areas would be well advised to prepare for multiple days of power outages and limited mobility, even if they’re not in mandatory evacuation zones.

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.

Time-lapse video of a #NOAA WP-3D Hurricane Hunter (#NOAA42) flight into Hurricane #Florence on Sept. 10, 2018. Get the latest on the storm at https://t.co/MlZk25kG0d. Credit: Nick Underwood/NOAA pic.twitter.com/FQ3RJMKVUU — NOAA Aircraft Operations Center (@NOAA_HurrHunter) September 11, 2018

Other imminent landfalls around the tropics

We’d be remiss not to take a quick look at several other tropical systems—most of them far weaker than Florence—threatening various parts of the Northern Hemisphere.

Olivia: Tropical storm warnings were up for all of Hawaii’s major islands on Tuesday afternoon as Tropical Storm Olivia barreled toward a Wednesday encounter with the islands. The good news is that Olivia is a discombobulated storm: its mid-level circulation is being torn away by strong wind shear. The beheaded low-level circulation now appears likely to track a bit further north than expected, moving near Maui or nearby islands and perhaps crossing Oahu on Wednesday morning. By then, Olivia’s 60-mph sustained winds will have decreased even further, but localized rains of 5” – 15” and tropical-storm-force winds are possible, especially on whichever island(s) Olivia hits first. No named storm on record has made landfall on any Hawaiian island other than Kauai and the Big Island, so Olivia has a good chance of making history even if its impacts end up on the modest side.

Isaac: A hurricane watch was in effect for Guadeloupe, Martinique, and Dominica, and a tropical storm watch for Antigua, Montserrat, St. Kitts, and Nevis, in advance of Tropical Storm Isaac. Located about 600 miles east of the Lesser Antilles at 5 pm EDT Tuesday with top sustained winds of 70 mph, Isaac continued to cruise west at about 17 mph, a pace that would bring it across the Lesser Antilles early Thursday. Isaac is facing a mixed bag of influences on its intensity, including outflow from Florence. As a small system, Isaac could easily strengthen or weaken quickly, so the best course is to assume Isaac will hold firm and reach the islands as a strong tropical storm or perhaps a Category 1 hurricane.

The European and GFS models keep Isaac heading west into the central Caribbean into Saturday, while several other models peel the storm off toward the north. Some runs of the higher-resolution HWRF regional model have Isaac strengthening sharply on such a northward track, but it’s too soon to trust such outlooks at this point, especially with the global models much weaker.

95L: The area of disturbed weather in the far northwest Caribbean continues to percolate. This system is likely to form into a tropical depression by Thursday or Friday as it moves across the western Gulf. It will encounter very warm sea surface temperatures around 30°C (86°F) and deep ocean heat content, with light to moderate wind shear of 5 – 15 knots. 95L would only have a day or two to develop before steering flow takes it into south Texas or northern Mexico coast by Friday or Saturday. About 30% of the 12Z Tuesday European model ensemble members develop 95L into a tropical cyclone, as do about 20% of the GFS ensemble members. NHC gives this system a 50% chance of development by late Thursday and a 70% chance through Sunday. Regardless of whether it becomes a tropical cyclone, 95L will add to the moisture supply for persistent showers and thunderstorms that have plagued southern Texas all week. See Figure 3 above for projected rainfall totals over the next five days.

RSMC Tokyo has the estimated minimum pressure in Super Typhoon #Mangkhut down to 905mb. Very intense tropical cyclone in the West Pacific. pic.twitter.com/BbQgdYx9Pp — Alex Lamers (@AlexJLamers) September 12, 2018

Mangkhut: Now a Category 5-strength super typhoon with top sustained winds of 160 mph, Manghut will be churning the waters of the Northwest Pacific over the next several days as it moves west-northwest. Mangkhut is projected by the Joint Typhoon Warning Center to move very near the northern tip of the Philippines on its way to a landfall around Sunday on the southeast coast of China, perhaps near Hong Kong. By that point, Mangkhut is expected to weaken significantly, perhaps below Category 3 strength.

Levi Cowan has an excellent Tuesday night video discussion (15 minutes) of the action in the tropics.

Jeff Masters contributed to this post.