Hurricane Arthur Shifts Left, Aims for a DIrect Hit in Eastern North Carolina

Hurricane Arthur has shifted to the left inside its cone of track uncertainty, and is poised to deliver a direct hit to the barrier islands of eastern North Carolina on Thursday night and Friday morning. The hurricane's 90 mph winds and 979 mb pressure from the 5 pm EDT Thursday NHC advisory make Arthur as strong as the strongest hurricane of 2013, Hurricane Humberto. Humberto peaked as a Category 1 storm with 90 mph winds and a central pressure of 979 mb as it traversed the waters a few hundred miles northwest of the Cape Verde Islands off the coast of Africa. The strongest winds measured at a buoy today in Arthur were 52 mph, gusting to 67 mph, measured at buoy 41004 offshore from Charleston, SC, at 12:50 pm EDT. Heavy rains from Arthur brought radar-estimated rainfall amounts of 3 - 4" as of 4:30 pm EDT near Wilmington, in southern North Carolina. Radar out of Wilmington shows that Arthur has developed an imposing area of heavy rains, but dry air is still infiltrating its core, creating a large gap in the eyewall. Satellite loops on Thursday afternoon showed a moderate-sized hurricane with a prominent eye surrounded by intense thunderstorms with very cold cloud tops. An excellent outflow channel has developed on the east side, but outflow is still restricted on the west side, where dry air is interfering with the storm. Wind shear a light 5 - 10 knots. Arthur's core has moved north of the axis of the Gulf Stream, and the hurricane is no longer able to take full advantage of the heat energy this narrow ribbon of very warm waters carries.


Figure 1. MODIS satellite image of Tropical Storm Arthur, taken at approximately 16:30 UTC (12:30 pm EDT) on Thursday, July 3, 2014. At the time, Arthur was a Category 1 storm with 90 mph winds. Image credit: NASA.


Figure 2. Radar out of Wilmington, North Carolina at 4:39 pm EDT July 3, 2014.

Forecast for Arthur
With the eyewall still showing gaps due to dry air infiltration, rapid intensification into a Category 3 hurricane appears unlikely. The 18Z Thursday run of the SHIPS model predicted that wind shear will remain light to moderate, 5 - 15 knots, between now and Friday morning, then rise steeply. The model also predicted a 20% 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 5 pm EDT Thursday wind probability forecast from NHC gave Cape Hatteras a 80% chance of experiencing hurricane-force winds, and Morehead City a 92% chance. The 12Z Thursday runs of our top two track models, the GFS and European (ECMWF), showed the eye of Arthur hitting Cape Lookout, North Carolina between midnight and 1 am, with the strongest winds of the eyewall's right front quadrant affecting Cape Hatteras between 3 am - 5am.


Figure 3. Radar-estimated rainfall from Wilmington, North Carolina as of 4:44 pm EDT July 3, 2014.

Arthur's storm surge
Along with wind damage, the biggest threat from Arthur is coastal flooding due to storm surge. A surge of 2 - 5 feet will peak late Thursday night through early Friday morning from Morehead City, NC, to the North Carolina/Virginia border. Low tide will occur near 6:30 - 7 pm EDT Thursday night, and again at 7 - 7:30 am Friday morning. High tide will be between 12:30 - 1:00 am Friday, and this is when the highest water levels (storm tide) will occur along much of the North Carolina coast south of Cape Hatteras, due the combined effect of the storm surge and tide. Tidal range between low and high tide is about 2 feet along much of the North Carolina coast, though it is only about 0.5' along portions of the Outer Banks. Tidal range at the Hatteras USCG station, which isn't far from the lighthouse, is only 0.3', so it doesn't matter much when the surge arrives there. At 4 pm EDT on Thursday, Arthur was bringing a storm surge in excess of one foot to portions of the South Carolina and North Carolina coasts:

1.7' at Oyster Landing, SC
1.6' at Wrightsville Beach, NC
1.3' at Wilmington, NC
1.1' at Beufort, NC
0.8' at Charleston, SC

I highly recommend NHC's Potential Storm Surge Flooding Map to evaluate how high above high tide the storm surge is likely to inundate the coast.


Figure 4. Observed storm surge from previous Category 1 and 2 hurricanes to hit North Carolina. Isabel of 2003 brought the most dangerous surge of these historic storms, since it was a very large storm that took an unusual north-northeasterly track into the coast near the North Carolina/Virginia border. Image credit: Western Carolina University Program for the Study of Developed Shorelines Storm Surge Database.

Arthur's tornadoes
Tornadoes are another threat from Arthur, and NOAA's Storm Prediction Center recorded three preliminary tornado sightings in North Carolina between 3:20 and 4:40 pm EDT. A tornado watch continues through 2 am EDT Friday for coastal North Carolina.

Arthur's impact on Canada and New England
As Arthur accelerates northeastwards towards Nova Scotia, Canada, large waves of 4 - 5 feet will begin to pound coastal Massachusetts on Friday night. Sustained tropical storm-force winds of 39 mph and higher are likely on Cape Cod and Nantucket, Massachusetts between 8 pm Friday - 2 am Saturday, and the 5 pm EDT NHC wind probability forecast gave Nantucket an 82% chance of seeing tropical storm-force winds. By 8 am EDT Saturday, Arthur will be merging with a cold front and transitioning to a hurricane-strength extratropical storm, and is expected to make landfall in Nova Scotia later that morning. The 5 pm EDT NHC wind probability forecast gives Halifax, Nova Scotia an 80% chance of seeing tropical storm-force winds of 39 mph or higher, and a 9% chance of hurricane-force winds.

Arthur's formation is not a harbinger of an active hurricane season
The first hurricane of the season typically occurs on August 10, so Arthur is quite a bit ahead of schedule. Arthur was able to form so early because it was over the very warm waters of the Gulf Stream Current, and these waters happened to be over 1°F warmer than usual for this time of year. Formation of a June or July hurricane like Arthur off the U.S. coast is typically not a harbinger of an active hurricane season, since these storms do not form from African tropical waves. Arthur spun up from a cluster of thunderstorms and their associated low pressure system that moved off the Southeast U.S. coast, and hurricanes that get their start this way are typically too far north and too close to land to be able to intensify into major hurricanes. The bigger threat are hurricanes that get their start from tropical waves traversing Main Development Region (MDR) for hurricanes (from the coast of Africa to Central America between 10° - 20°N, including the Caribbean Sea.) Tropical waves that traverse the MDR are responsible for 85% of all major (Category 3 and stronger) hurricanes. When June and July hurricanes and tropical storms form in the MDR, it usually does portend an active hurricane season, since it shows that atmospheric and oceanic conditions are primed to assist development of tropical waves coming off the coast of Africa during the peak mid-August through mid-October part of hurricane season.

A better way to evaluate whether or not this will be an active hurricane season is to look at sea surface temperatures (SSTs) in the MDR, and the status of El Niño. MDR SSTs are currently very close to average, and are thus unlikely to contribute to an above-average hurricane season. The very warm equatorial waters currently off the coast of South America suggest that an El Niño event is in the process of developing. When an El Niño event occurs during hurricane season, it tends to create an atmospheric circulation that brings unusually strong upper-level winds to the tropical Atlantic. These strong winds create a shearing action (wind shear) on any tropical storms or hurricanes that may be attempting to form, disrupting their circulation. Thus, the pre-season predictions of a below-average or near-average hurricane season still look good.

Stay safe tonight, all of you in North Carolina!

Jeff Masters