PRACTICAL VADEMECUM ON NORTH ATLANTIC HURRICANES
Hurricanes often originate from a tropical disturbance, a system with no strong winds, no close isobars and non-frontal migratory character. At its next stage of developments it becomes a tropical depression (first stage) which often forms around the Cape Verde Islands moving westwards across the North Atlantic.
A tropical depression starts its rotary circulation at the surface when isobars become closer to each other. The system can develop into a larger tropical storm (second stage), taking from half a day to a couple of days for this change to occur and about the same time is required for a tropical storm to become a hurricane (third stage). About 60 per cent of tropical cyclones in the Atlantic develop into hurricanes, but even those that do not become hurricanes can pose serious hazards to mariners.
Hurricanes and tropical storms are classified according to their wind speed. Winds in a hurricane reach at least 64Kts, as shows below:
► Tropical Depression. Winds up to 7 Beaufort (33 Kts)
► Moderate Tropical Storm. Winds up to force 8/9 Beaufort (34-47Kts)
► Severe Tropical Storm. Winds reaching force 10/11 Beaufort (48-63Kts)
► Hurricanes. Winds equal or greater than force 12 Beaufort (64 Kts).
The Saffir-Simpson Hurricane Scale
Hurricanes are classified in five categories by the Saffir-Simpson Hurricane Scale (SSHS) as shown in the table below.
This scale does not distinguish hurricanes in terms of wind speed, but also in terms of the rise of water level, known as surge. The cyclone offshore will force the surge to propagate, and will undergo a marked increase as the surge reaches the coastal areas.
The large height reached by the waves offshore mainly depends by the wind speed, which is the determining factor in the scale. Storm surge values are also dependent on the slope of the continental shelf and the shape of the coastline in the landfall region.
| Hurricane Category One: | Winds of 64-82 kts or 119-153 km/hr. Storm surges generally 4-5 ft above normal. No real damage to building structures. |
| Hurricane Category Two: | Winds of 83-95 kts or 154-177 km/hr. Storm surges generally 6-8 feet above normal. |
| Hurricane Category Three: | Winds of 96-113 kts or 178-209 km/hr. Storm surges generally 9-12 ft above normal. |
| Hurricane Category Four: | Winds of 114-135 kts or 210-249 km/hr. Storm surges generally 13-18 ft above normal. |
| Hurricane Category Five: | Winds greater than 135 kts or 249 km/hr. Storm surges generally 18 ft above normal or over. |
Conditions for Hurricane Development in the Atlantic
The main factors that cause hurricanes are listed below.
If one of these factors is not present, the hurricane can weaken or decay.
1) Ocean water temperature above 26,5°C
2) Distance from the equator of at least 300 nautical miles (i.e. 5° latitude). This is due to the fact that the earth vorticity (also called Coriolis force) would not be high enough to initiate the tropical system. (In fact, the Coriolis force is directly proportional to the sin of the latitude and increases poleward. Since sin0°=0, the earth vorticity is nil at the equator).
3) High relative humidity values from the surface to the middle levels which reduce evaporation in the cloud and maximize latent heat release because there is more precipitation
4) Strong upper level easterly winds and low vertical shear, i.e. little change of wind speed with height
5) A mid-level tropospheric wave originating off the West coast of Africa which expands towards the tropical region of Central America.
The Hurricane Season
Tropical cyclones can occur in the North Atlantic Ocean in any month but are a threat mostly from June through November. This is the Hurricane Season.
August, September and October are the months of highest risk. Between 10 to 15 tropical storms form each season and 6-8 reach hurricane force.
Areas Affected by the Hurricanes
Early or late in the season the tropical storms develop in the tropical region to the West of 050°W, whereas in the middle part of the season they usually move from Cape Verde Island to the Carribeans, The Greater Antilles and the Gulf of Mexico, recurving North-NorthEast’wards across eastern USA. Most tropical cyclones can continue eastward across the North Atlantic, but very rarely reach NW Europe, like the one that hit Southeast England during mid October 1987.
Taking An Early Action
To constantly plot the position and most probable track of the storm, if the action is taken sufficiently early, it is the most important thing for a Captain to take the vessel in a safer position.
In the Northern Hemisphere, were the winds rotate in an anticlockwise direction into a cyclone, the right sector of the storm track is called dangerous semicircle, because of the greater wind speed, due to the pression gradient augmented by the forward motion of the storm.
The part to the left of the storm track is called less dangerous semicircle.
The safest procedure with respect to tropical storm is to avoid them in advance.
The National Hurricane Center (NHC) warnings and the modern communication systems are today the two most important factors for the safety of the navigation.
The Captain should remember that even in port the vessel may be in danger and he has the option of getting ashore to find a more secured pace to ride out of the storm.
To study carefully the coast in respect of the surge risk it is necessary to reach sheltered conditions.
It will be necessary to know constantly where the tropical storm is located and its direction.
Unfortunately it is never certain what a tropical storm is going to do, however access to the internet represents today a very important factor to collect real time data, forecasts, warnings and satellite images.
Direct and accurate observations of the wind force and direction, sea level, waves, cloud systems and pressure variations are very helpul.
As a sample we show here a few images of hurricanes and few graphical and text warnings, concerning the recent passage of the Tropical Storm Fay across the Florida which brought record flooding during mid August (courtesy of Noaa ).
How a hurricane is represented on a synoptic chart
NASA image of Hurricane Fay (Courtesy of NASA)
Here below you the NOAA Forecast/Advisory bulletin for hurricane Fay. Each advisory bulletin has be read carefully by a captain in order to check the intensity of the cyclone or hurricane and to position them on his navigation charts.
000 WTNT21 KNHC 190250 TCMAT1 TROPICAL STORM FAY FORECAST/ADVISORY NUMBER 14 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL062008 0300 UTC TUE AUG 19 2008 A HURRICANE WARNING IS IN EFFECT FOR THE SOUTHWESTERN COAST OF FLORIDA FROM FLAMINGO TO ANNA MARIA ISLAND. A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED WITHIN THE WARNING AREA WITHIN THE NEXT 24 HOURS. PREPARATIONS TO PROTECT LIFE AND PROPERTY SHOULD HAVE BEEN COMPLETED. AT 11 PM EDT...0300 UTC...THE HURRICANE WATCH FOR THE FLORIDA WEST COAST NORTH OF ANNA MARIA ISLAND TO TARPON SPRINGS IS DISCONTINUED. A TROPICAL STORM WARNING REMAINS IN EFFECT FOR THIS AREA. A TROPICAL STORM WARNING IS IN EFFECT ALONG THE FLORIDA EAST COAST FROM FLAGLER BEACH SOUTHWARD...INCLUDING LAKE OKEECHOBEE. A TROPICAL STORM WARNING REMAINS IN EFFECT FOR THE FLORIDA KEYS FROM OCEAN REEF TO KEY WEST...INCLUDING THE DRY TORTUGAS...AND FOR THE FLORIDA MAINLAND EAST OF FLAMINGO TO CARD SOUND BRIDGE. A TROPICAL STORM WATCH REMAINS IN EFFECT FOR THE FLORIDA EAST COAST NORTH OF FLAGLER BEACH TO FERNANDINA BEACH. A TROPICAL STORM WATCH IS IN EFFECT FOR THE NORTHWESTERN BAHAMAS. INTERESTS ELSEWHERE IN FLORIDA AND THE EASTERN GULF OF MEXICO SHOULD MONITOR THE PROGRESS OF FAY. TROPICAL STORM CENTER LOCATED NEAR 25.3N 81.9W AT 19/0300Z POSITION ACCURATE WITHIN 15 NM PRESENT MOVEMENT TOWARD THE NORTH OR 360 DEGREES AT 8 KT ESTIMATED MINIMUM CENTRAL PRESSURE 995 MB MAX SUSTAINED WINDS 50 KT WITH GUSTS TO 60 KT. 50 KT....... 60NE 0SE 0SW 0NW. 34 KT.......110NE 90SE 30SW 30NW. 12 FT SEAS.. 0NE 120SE 30SW 60NW. WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT. REPEAT...CENTER LOCATED NEAR 25.3N 81.9W AT 19/0300Z AT 19/0000Z CENTER WAS LOCATED NEAR 25.0N 81.9W FORECAST VALID 19/1200Z 26.6N 81.7W...INLAND MAX WIND 55 KT...GUSTS 65 KT. 50 KT... 50NE 50SE 0SW 0NW. 34 KT...100NE 100SE 30SW 30NW. FORECAST VALID 20/0000Z 28.1N 81.3W...INLAND MAX WIND 40 KT...GUSTS 50 KT. 34 KT... 75NE 75SE 30SW 30NW. FORECAST VALID 20/1200Z 29.2N 81.0W...ON COAST MAX WIND 30 KT...GUSTS 40 KT. FORECAST VALID 21/0000Z 30.3N 81.0W...OVER WATER MAX WIND 40 KT...GUSTS 50 KT. 34 KT... 60NE 60SE 0SW 0NW. FORECAST VALID 22/0000Z 31.0N 81.5W...ON COAST MAX WIND 45 KT...GUSTS 55 KT. 34 KT... 75NE 75SE 0SW 0NW. EXTENDED OUTLOOK. NOTE...ERRORS FOR TRACK HAVE AVERAGED NEAR 225 NM ON DAY 4 AND 300 NM ON DAY 5...AND FOR INTENSITY NEAR 20 KT EACH DAY OUTLOOK VALID 23/0000Z 32.0N 82.5W...INLAND MAX WIND 30 KT...GUSTS 40 KT. OUTLOOK VALID 24/0000Z 32.0N 84.0W...INLAND...REMNANT LOW MAX WIND 20 KT...GUSTS 30 KT. REQUEST FOR 3 HOURLY SHIP REPORTS WITHIN 300 MILES OF 25.3N 81.9W NEXT ADVISORY AT 19/0900Z $$ FORECASTER PASCH
Recommended web site www.noaa.gov
by Gianfranco Meggiorin and Daniele Vitri
Here below you the NOAA Forecast/Advisory bulletin for hurricane Fay. Each advisory bulletin has be read carefully by a captain in order to check the intensity of the cyclone or hurricane and to position them on his navigation charts.