Hurricane Arthur: Unusual For The Time Of Year And An Extra-Tropical Connection


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The forecast track of Hurricane Arthur. Courtesy of NOAA

The forecast track of Hurricane Arthur. Image courtesy of NOAA

Tropical Storm Arthur has turned into Hurricane Arthur at 8 a.m. EDT.

The average dates of the first Atlantic Basin named storm and hurricane are July 9 and August 10 respectively. And Arthur is only the fifth Atlantic storm to form off the Florida coast between June 21 and July 10 in over 150 years.

Arthur will hit the Outer Banks of North Carolina today with category one strength, but then steer far enough out to sea to spare the major cities of the northeast from all but the fringe effect of some gusty winds.

Beaches will have good surfing conditions, but possibly dangerous rip currents.

The Interaction Of Tropical And Mid-Latitude Weather

More attention has been focused on the interplay between the tropical and middle latitude weather regimes since Superstorm Sandy turned into a non-tropical system just before landfall, causing the National Hurricane Center to discontinue all alerts.

The fact is that the atmosphere likes spinning systems, and one kind converts to the other rather easily. Arthur, though now completely a hurricane in its structure, began as an extra-tropical system and will become extra-tropical again as it approaches Nova Scotia.

During the height of hurricane season — mid-August to mid-September — most hurricanes are purely tropical, though even at that time of year a storm can recurve into the middle latitudes and transition to an extra-tropical system. But on the cusp of the season — before August and after September — the mid-latitude westerly weather regime is stronger and can influence the formation of storms. This was the case with Arthur, which was incubated as a minor low pressure center on a cold front that drifted off the Carolina coast, sagged southward, and finally transitioned to a tropical depression.

The Difference Between Tropical And Extra-Tropical Storms

All weather on earth is rooted in the heat of the sun:

  • The sun heats the air at the equator more than the air at the poles.
  • The sun heats the tropical oceans more than those at higher latitudes.

The differential heating of the air between equator and pole creates potential energy in the form of cold air next to warm. Warm air over cold is a lower potential energy state, and the atmosphere relieves the constantly building potential energy by converting it to kinetic energy of wind, then back to heat by dissipation through friction. The dissipation occurs at a higher latitude, and heat has been transferred northward (in the northern hemisphere); the constant buildup of potential energy is constantly being relieved.

The heat stored in the ocean is transferred to the atmosphere when the sun’s radiation rips water molecules from the surface of the sea and they become water vapor in the atmosphere. The heat supplied to tear the water molecule from the ocean is called latent heat of evaporation, and is stored in the water molecule. When the water vapor condenses, the heat is released into the atmosphere, converted to wind, and later dissipated by friction. Again, there is a transfer of heat northward to relieve the pressure of building potential energy in the ocean.

Mother Nature doesn’t seem to care which way the heat imbalance in the ocean-atmosphere system is corrected — through release of potential energy of adjacent air masses or latent heat. Spinning systems can transition from one to the other quickly.

Locations of tropical storm formation from July 1-10 from 1851 to 2009. Courtesy of NOAA

Locations of tropical storm formation from July 1-10 from 1851 to 2009. Courtesy of NOAA

Warm Water And Light Wind

Tropical cyclones require two things:

  • Warm Water
  • Low vertical wind shear (change of wind speed or direction with height)

The Atlantic Ocean, deep and wide, is slow to warm as summer proceeds. Most early-season tropical activity takes place in the warmer Gulf of Mexico or Caribbean Sea. The shallow coastal zone and the Gulf Stream are the only places in the Atlantic where the water is warm enough for tropical storm formation in July.

Conditions of vertical wind shear are generally hostile to tropical storm formation over the Atlantic Ocean at this time of year. Arthur took advantage of a weak flow throughout the atmosphere.

The Future Of Arthur

All the forecasting models agree that Arthur will brush Cape Hatteras and continue northeast, sparing the northeastern US megalopolis of its worst effects. Some wind from Arthur’s outer circulation could affect Fourth of July activities in New York and Boston, but the brunt of Arthur’s 85 mile-per-hour winds will be felt in eastern North Carolina, which is accustomed to hurricanes and is prepared. Evacuations have taken place and coastal roads are closed.

Rip Tides

Hurricanes can cause rip tides and strong currents hundreds of miles from the major part of the wind field. Even when skies are clear and the ocean looks placid, swimmers should be careful to obey warnings of rip tides. Rip tides from Arthur could affect the entire Atlantic coast today and Friday.

Is Arthur An Indication That This Will Be A Busy Hurricane Season?

All of the long-range hurricane forecasts call for a below-normal season in numbers and intensity because of an expected El Niño. Decoded Science has bucked the trend with a forecast of above-normal activity because of warm ocean water.

Arthur puts us a little ahead, but residents of coastal areas should pay scant attention to general forecasts — even Decoded Science’s. When the window opens for a hurricane to form, it can do so in an otherwise inactive season — and it can be a big one. Category Five Hurricane Andrew in 1992, a very inactive season, did massive damage in the Miami metropolitan area.

All coastal residents should be ready, with evacuation routes chosen, and other plans made to protect life and property.

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