GC Briefing September 13, 2018 Weather Sentinel Hurricane Florence Tropical storm conditions have been reported for areas of North Carolina and will continue to spread inland to the west and south. Hurricane conditions are imminent. Hurricane Florence is now a category two hurricane on the Saffir-Simpson Scale and carries maximum sustained winds of 100 mph according to the National Hurricane Center (NHC). The center of Florence will continue to approach the coast tonight and move inland tomorrow morning according to the latest NHC forecast. The large size of the storm together with its slow forward speed will amplify the threats of flooding due to heavy rainfall, coastal impacts due to storm surge as well as prolonged wind exposure for affected areas. Status at 5 PM EDT (21 UTC) Today (NHC) Maximum Sustained Winds: 100 mph Approximate Location: About 100 miles east-southeast of Wilmington, North Carolina Motion: West-northwest at 5 mph Minimum Central Pressure: 955 mb Extent of Hurricane-Force Winds from the Center: 80 miles Extent of Tropical-Storm-Force Winds from the Center: 195 miles Status: Category-2 Hurricane Discussion Rainbands of Florence continue to move inland over North Carolina, possibly with some embedded tornadoes. Tropical storm conditions have already been reported for certain areas, and onset of hurricane conditions is imminent. A NOAA station located at Cape Lookout, North Carolina recently reported sustained winds of 68 mph with a gust to 85 mph. The center of Hurricane Florence now sits about 100 miles east-southeast of Wilmington, North Carolina according to the National Hurricane Center (NHC). The storm has slowed considerably and is now moving to the westnorthwest at 5 mph. Florence is subject to weakening steering currents between a weakening ridge to the northeast and another ridge over the north-central U.S, which will cause reduced forward speed and a gradual shift in track to the west and then west-southwest into the weekend. This should be followed by a turn to the north into next week as the U.S. ridge slides eastward. Accounting for these factors, the NHC best track forecast has the center of Florence making slow approach to the coast later tonight before moving ashore early tomorrow. The center of Florence should then slowly move over South Carolina through the weekend before turning north through the Appalachians while transforming into a frontal system. Hazards extend well away from the center. Concerning intensity, Florence has struggled with some intrusions of dry air after severe disruption by unexpected wind shear over the last day or so. There is a brief period of time where Florence will be in an environment of reduced wind shear and warmer waters, and during this time some slow intensification is possible. However, escalating shear and increasing land interaction will eventually begin to weaken the storm. Accounting for these factors, the NHC best intensity forecast has Florence retaining strength on approach to the coast, with an accelerated weakening trend after the center of the storm moves inland.
Florence has evolved into a storm of very large diameter and will move very slowly. This will amplify the storm surge threat because the winds will be acting over a larger area of water. In the meantime, heavy rainfall rates over a long duration will bring excessive rainfall totals exceeding 10 inches for large areas of the Carolinas with some local amounts to 40 inches. The threat of prolonged wind duration may also lead to enhanced property damage for certain areas. Hurricane, storm surge and tropical storm watches and warnings are now active for areas under potential or active threat from the Carolinas to the Mid-Atlantic. The U.S. National Weather Service has also issued watches and warnings for inland hazards including flood, flash-flood and tornado, available at www.weather.gov. Some of these extend well inland. Updates will be necessary as the situation evolves. Hazards of concern include: Damaging winds to render variable property and vehicle damage over a widespread area from Georgia to the Mid-Atlantic, along with downed trees and powerlines. Wind impacts will be increasingly severe for areas along and to the right of the storm track and adjacent to the coast. Tropical storm conditions are spreading inland to the west and south, and hurricane conditions are imminent. Coastal flooding due to storm surge will also affect areas near the coast and connected waterways. This will render property and vehicle damage due to seawater inundation, water velocity and wave battering with waterborne debris. The storm surge footprint and severity will be amplified by the large diameter of the storm and prolonged wind duration. Inundation heights above ground may reach as high as 7-11 feet for certain areas according to the NHC. Heavy rainfall will also bring the threat of catastrophic flooding and flash-flooding for areas well inland from the Carolinas to the Mid-Atlantic. Rainfall amounts of 20-30 inches with local amounts to 40 inches are possible for some areas according to the NHC. Saturated soils from the prior heavy rainfall this season may amplify local flood effects. Some tornadoes are also possible. Power outages and infrastructure disruption will be possible for affected areas, with increased severity along and to the right of the storm path, and especially near the coast for areas subjected to prolonged exposure to hazards. Key Messages From The NHC 1. A life-threatening storm surge is highly likely along portions of the coastlines of South Carolina and North Carolina, and a Storm Surge Warning is in effect for a portion of this area. The greatest storm surge inundation is expected between Cape Fear and Cape Hatteras, including the Neuse and Pamlico Rivers and western Pamlico Sound. 2. Life-threatening, catastrophic flash flooding and prolonged significant river flooding are likely over portions of the Carolinas and the southern and central Appalachians through early next week, as Florence is expected to slow down as it approaches the coast and moves inland. 3. Damaging hurricane-force winds are likely along portions of the coasts of South Carolina and North Carolina beginning this evening, and a Hurricane Warning is in effect. Strong winds could also spread inland into portions of the Carolinas. 4. Large swells affecting Bermuda, portions of the U.S. East Coast, and the northwestern and central Bahamas will continue this week, resulting in life-threatening surf and rip currents. Further Sentinel updates will be issued as materially new guidance comes available. Statements from the National Hurricane Center can be found at www.nhc.noaa.gov. Local National Weather Service watches, warnings and advisories can be found at www.weather.gov (type in the location or zip code, or click on the map). Local watches and warnings from official meteorological agencies, and statements from local emergency management agencies supersede this update, and should be closely heeded concerning matters of personal safety.
NHC Position and Best Forecast Earliest Reasonable Arrival Time of Tropical-Storm-Force Winds (Contours) Probability of Tropical-Storm-Force Winds (Shaded)
Probability of Hurricane Force Winds Forecast Rainfall Amounts 5-Days Ending 8PM EDT September 18 (00 UTC September 19)
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