Tropical Storm Warnings for North Carolina as Arthur Heads North

Tropical Storm warnings are up for the coast of North Carolina as Tropical Storm Arthur heads north at 7 mph. The heaviest weather associated with Arthur is on its east and south sides, and a buoy 128 miles east of Cape Canaveral, Florida, on Arthur's east side, recorded sustained winds of 47 mph, gusting to 60 mph at 6:50 am EDT on Wednesday. Significant wave heights at the buoy were an impressive 17.4' this morning. Radar out of Melbourne, Florida on Wednesday morning showed that Arthur had built about 1/2 of an eyewall, on the northeast to southwest side of the center. Bands of heavy rain were affecting the east coast of Florida near Cape Canaveral Wednesday morning, but rainfall amounts from the storm have generally been less than one inch over Florida. Satellite loops on Wednesday morning showed a slow but steady increase in the intensity and areal coverage of Arthur's heavy thunderstorms, with the potential beginning of a Central Dense Overcast (CDO) of high cirrus clouds forming on the east side of the center. A CDO is a classic sign of an intensifying tropical storm that is approaching hurricane strength. However, the visible satellite loops also showed signs that Arthur is struggling with dry air. Arc-shaped bands of low cumulus clouds were racing away from the storm to the north of the center, indicating that dry air had gotten ingested into Arthur's heavy thunderstorms, creating strong downdrafts that were spreading out along the ocean surface. This process robs a tropical storm of moisture and energy, and means that Arthur has considerable work to do in order to moisten its environment before the storm can close off an eyewall and attain hurricane strength. The clockwise circulation of an upper level high pressure over Florida was bringing northerly winds over Arthur at high altitude, and these winds were creating light wind shear of 5 - 10 knots. Water vapor satellite loops showed dry air to the north and west of Arthur.


Figure 1. Tropical Storm Arthur as seen from the International Space Station at 6:42 am EDT Wednesday, July 2, 2014. Image credit: Reid Wiseman.


Figure 2. Melbourne, Florida radar image of Tropical Storm Arthur at 10:47 am EDT July 2, 2014.

Forecast for Arthur
The 12Z Wednesday run of the SHIPS model predicts that wind shear will remain light to moderate, 5 - 15 knots, between now and the time Arthur makes is closest approach to North Carolina on Friday morning. There will still be dry air to the storm's north and west, so I put the odds of rapid intensification into a Category 3 or stronger storm at just 10%. The presence of surface-based arc clouds radiating away from Arthur this morning likely means that the storm will not achieve hurricane strength on Wednesday. However, the four main intensity models used by NHC--the LGEM, SHIPS, GFDL, and HWRF--continue to be in remarkable agreement, predicting that Arthur will reach hurricane strength on Thursday, and peak at maximum sustained winds between 80 - 100 mph about the time Arthur is making its closest pass to the Outer Banks of North Carolina, on Friday morning. The models are also in good agreement on the track of Arthur. A trough of low pressure passing to the north will turn the storm northeastwards by Thursday, and cause Arthur to pick up speed. The Outer Banks of North Carolina will be the land area at greatest risk of a direct hit, and the 11 am EDT Wednesday wind probability forecast from NHC gave Cape Hatteras a 21% chance of experiencing hurricane-force winds. Given the degree of model unanimity, the cone of uncertainty is likely thinner than presented. The latest 0Z Wednesday runs of our top two track models, the GFS and European (ECMWF), showed Arthur passing very close to Cape Hatteras, North Carolina between 2 am - 8 am EDT Friday, July 4.

Rip Current Danger
Rip currents are typically the biggest killer in shore-hugging tropical storms like Arthur, as the big waves pounding the coast create very strong currents flowing away from the shore that can drown even strong swimmers. High surf of 6 - 8 feet was affecting portions of the east coast of Florida from West Palm Beach to Jacksonville on Wednesday morning, and these waves were creating dangerous rip currents; five swimmers stranded by strong currents were rescued in Fort Pierce, Florida. The high surf will reach the South Carolina coast by Wednesday night, and begin affecting North Carolina on Thursday morning. The Wednesday morning run of NOAA's Wavewatch III model predicted that high surf of 6 - 8 feet capable of generating dangerous rip currents would affect the Mid-Atlantic coast from Virginia northwards beginning on Friday morning, and reach New England late Friday night.


Figure 3. Screenshot of the experimental NHC Potential Storm Surge Flooding Map for the North Carolina coast. Inundation of the land to a depth of 3 - 6 feet (yellow colors) has a 10% chance of occurring near Wilmington and Beaufort. The image was generated using using NOAA's Probabilistic Hurricane Storm Surge (P-Surge 2.0) model. P-Surge 2.0 uses multiple runs of the NWS Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model to create an ensemble of possible inundations, by varying the hurricane's landfall location, intensity, size, forward speed, and angle of approach to the coast. The image shows where the storm surge has a 10% chance of inundating the coast at 3, 6, and 9 feet above ground level. The model does not take into account wave action, freshwater flooding from rainfall, and breaching or overtopping of levees.

New NHC Potential Storm Surge Flooding Maps Being Generated
Arthur is the first storm for which a fantastic new experimental National Hurricane Center product is being issued, the Potential Storm Surge Flooding Map. This is an interactive zoomable map that shows where the storm surge has a 10% chance of inundating the coast at 3, 6, and 9 feet above ground level. The map is available only on the NHC web site.


Figure 4. A small boy plays with a toy donated by Portlight on September 5, 2012. His home in Pearlington, Mississippi was demolished by Hurricane Isaac. Image credit: Portlight.org.

Portlight disaster relief charity ready to respond during the 2014 Atlantic hurricane season
The Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, is asking for donations as the 2014 hurricane season gets underway. This year, Portlight has already deployed staff to help victims of the April 27 EF-2 tornado that devastated Quapaw and Baxter Springs, Oklahoma, and is still active in New Jersey in the aftermath of Hurricane Sandy. This hurricane season, they hope to deploy a disaster shelter trailer which will help them assist shelter operators in making their facilities fully accessible to people with all types of disabilities. This will include ramping, railings, cots, dinner- and drinkware, and assistive technologies for those with vision, hearing, cognitive and developmental delays. Check out the Portlight Blog , and consider a donation to Portlight's disaster relief fund at the portlight.org website.

Jeff Masters