Arthur Nears Hurricane Strength

By: Dr. Jeff Masters , 9:03PM,GMT on July 2,2014


 

Tropical Storm Arthur has formed a large, 30-mile diameter eye, and appears destined to be a hurricane by early Thursday morning, as the storm heads towards a rendezvous with the North Carolina coast on Friday. An Air Force hurricane hunter aircraft was in the storm Wednesday afternoon, and measured top surface winds of about 70 mph with their SFMR instrument. Arthur's central pressure was 995 mb at 3:10 pm EDT, then dropped to 992 mb during their second eye penetration an hour later, at 4:04 pm. Radar out of Melbourne, Florida and satellite loops on Wednesday afternoon showed that Arthur had closed off an eye, though portions the northwestern portion of the eyewall had a gap due to infiltration of dry air. Arc-shaped bands of low cumulus clouds are no longer spreading out to Arthur's north, indicating that the storm has walled off its center from dry air. An impressive "hot tower"--a powerful eyewall thunderstorm with a high top--developed along the eastern side of the eye at 4 pm EDT. Wind shear continued to be a light 5 - 10 knots. Water vapor satellite loops showed dry air to the north and west of Arthur, and some of this dry air was still filtering into the circulation.


Figure 1. MODIS satellite image of Tropical Storm Arthur, taken at approximately 16:30 UTC (12:30 pm EDT) on Wednesday, July 2, 2014. Note the arc-shaped lines of low clouds to Arthur's north, caused by dry air at mid-levels of the atmosphere getting ingested into Arthur's heavy thunderstorms, creating strong downdrafts that spread out along the ocean surface. This process robs a tropical storm of moisture and energy. Image credit: NASA.


Figure 2. Melbourne, Florida radar image of Tropical Storm Arthur at 4:37 pm EDT July 2, 2014.

Forecast for Arthur
Now that Arthur has mostly closed off an eye and walled off much of the dry air to its north, we can expect some modest intensification, with perhaps a 20 mph increase in strength by Thursday afternoon. The 18Z Wednesday run of the SHIPS model predicted 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, and the SHIPS model predicted a 17% chance of rapid intensification--a 30 mph increase in winds in 24 hours. I put the odds Arthur becoming a Category 3 or stronger storm at 10%. 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 - 95 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 5 pm EDT Wednesday wind probability forecast from NHC gave Cape Hatteras a 26% chance of experiencing hurricane-force winds. Given the degree of model unanimity, the cone of uncertainty is likely thinner than presented. The latest 12Z Wednesday runs of our top two track models, the GFS and European (ECMWF), showed the eye of Arthur passing over Cape Hatteras, North Carolina between 5 am - 8 am EDT Friday, July 4.


Figure 3. Screenshot of the experimental NHC Potential Storm Surge Flooding Map for the North Carolina Outer Banks, generated at 11 am EDT Wednesday July 2. Inundation of the land to a depth of 3 - 6 feet (yellow colors) has a 10% chance of occurring near the vulnerable section of Highway 12 near Rodanthe, and at the Bonner Bridge over Oregon Inlet. The bridge was forced to close in December 2013 for 12 days of emergency repairs due to dangerous scouring around the support pillars. 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.

North Carolina's Outer Banks at risk of getting cut off
Hurricane Sandy pummeled the Outer Banks of North Carolina in late October 2012, causing $13 million in damage, and wiping out the protective dunes along a long section of coast. Coastal Highway 12 that connects North Carolina's Outer Banks to the mainland was cut for nearly two months, until repairs were completed in December 2012. Residents of the Outer Banks were forced to take a 2-hour ferry ride to get to the mainland when Highway 12 was cut. Nor'easters and storms repeatedly caused overwash and road damage to NC-12 during the winter of 2012 - 2013, but a temporary barrier was erected in 2013 that has been successful at keeping the road open during the past year (though minor overwash occurred during storms on October 10, 2013 and March 18, 2014, and Tropical Storm Andrea closed the road to all but 4-wheel drive vehicles on June 7, 2013.) Emergency post-Sandy federal funding of $20.8 million was secured to construct a barrier designed to last three years, but the project is up for bid, and has not yet begun.



Figure 4. Top: Coastal Highway 12 in North Carolina, which connects the Outer Banks to the mainland, as seen at 5:43 pm EST on Tuesday, November 13, 2012, near Rodanthe. Hurricane Sandy wiped out most of the protective dunes along the coast, allowing the ocean to directly pound the road during high tide. Bottom: the same view as seen in June 2014, after a temporary repair was made to the beach destroyed by Sandy. Image credit: North Carolina DOT. Live views of Highway 12 road cams are available from the NCDOT web site.

NC 12 is once again in danger of being cut, due to the storm surge and wave action of Arthur. Also at risk is Highway 12's Bonner Bridge over Oregon Inlet on the Outer Banks, which was forced to close for 12 days in December 2013 for emergency repairs, when scans revealed excessive sand erosion (scour) around some of the support columns. With sea level rise steadily increasing the damage that storms like Arthur can do to the vulnerable Highway 12, some very expensive long-term solutions are being studied for keeping Highway 12 open.


Figure 5. 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