VITAMINS FOR GUSTAV

Tropical Storm Gustav weakend after moving over the southern tip of Haiti Tuesday as a hurricane . Although its winds have dropped to 60 mph it is expected to strengthen once it gets into the Gulf of Mexico.

Intensity forecasts are tricky to predict; much harder than track forecasts, but looking at the sea surface temperature along the forecast path can give clues as to how much the cyclone may strengthen. The SST typically needs to be at least 79 to 80 degrees or warmer for a strengthening tropical cyclone. But more importantly is the depth of the warm water below the surface. Deep warm subsurface water makes it difficult for colder water to upwell or rise to the surface. A cyclone evaporates warm water and extracts the heat to power the intense winds. The graphic above shows the Potential Heat Potential of the Atlantic Basin. This summarizes the depth of warm water and how it could impact the potential fueling of a cyclone. Its like a storm taking vitamins when it moves over an area of red or orange. I have overlayed the NHC track over the current Potential heat map. See how the NHC is strengthening Gustav to a category 3 hurricane once it passes over the red areas. You can see daily updates to this map at http://www.aoml.noaa.gov/phod/cyclone/data/at.html

A storm should intensify when the OHC exceeds 60 joules per kilogram. You can see the legend and pinpoint areas above 60 Kj/cm. Ivan was an example of a hurricane that intensified as it passed south of Cuba. It set a record as a cat 4 for 8 days.

WHAT DOES THE CONE MEAN

We always see the cone of uncertainty or track cone on the weathercast when a tropical cyclone is being tracked by the National Hurricane Center. This familiar product can be misinterpreted. The cone does not indicate certainty. In fact nothing is certain in meteorology which is why forecasts are expressed in probability. The cone is no different. It represents hurricane track errors average over five years for a specific forecast time. The time periods range from 12 hours to 24 hours and out to 120 hours or 5 days.

Hurricanes stay within the cone 2/3rd of the time. The Hurricane Center has to over warn due to the uncertainty that one out of three times a hurricane will track outside of the cone. Hurricane warnings average 417 miles but an average storm's length of hurricane winds are 100 miles. So the probability of hurricane force winds in a warning will be only 25% of the time.

MORE TOOLS TO MEASURE HURRICANE WINDS

This year all the hurricane hunters will use an instrument to measure winds at the surface of the ocean which is more accurate than dropping a measuring device from the plane to the water. A Stepped Frequency Microwave Radiometer, SFMR, measures the ocean roughness caused by wind stress and can compute the magnitude of wind velocity. This will supplement a dropsonde which is ejected form the bottom of the plane and takes measurements of wind speed, direction, humidity, and pressure. Last year only 1 C-130 airplane had the SFMR onboard. In 2008, all eight C-130s will have the instrument in addition to two NOAA P-3's which fly out of MacDill Air force Base.

When we report the wind speed of a hurricane the elevation of the wind is taken at 33 ft. Wind speed is determined by factoring out gusts by averaging all the gusts over one minute to arrive at a sustained wind speed. A tropical storm becomes a hurricane if the wind is 74 mph or higher. Once the winds blow over 110 mph a hurricane becomes dangerous enough to be classified a major hurricane able to cause extensive damage. Hurricane Katrina was a cat 3.

HURRICANE SEASON IS OVER

This season will not be remembered as an active season in the United States but certainly it will leave scars for Caribbean inhabitants. It was a bad season in Jacksonville because of the beach erosion. Northeaster's lashed the coast undermining structure to homes in South Ponte Vedra Beach. Nor'easter's most often result in higher erosion than hurricanes because they last longer.

Only one hurricane made landfall in the US along the upper Texas coast. Humberto was noteworthy because it was the fastest growing storm on record from first advisory to category 1 strength at just 18 hours. Two tropical storms hit the states with little effect. The lack of systems prolonged the drought across the southeast and Florida continues to see a deficit. Actual named storms were 14, with 6 hurricanes, and 2 major. A normal season has 11 named storms, 6 hurricanes, with 2 majors.

Hurricane Dean And Felix were amazingly strong category 5 storms that hit Central America. This was the first year ever to have back to back category five storm make landfall in one season. 27 people died from Dean after it passed through the Lesser Antilles Islands, moving just south of Jamaica, and into Mexico. Felix killed 235 people in Nicaragua.
September was noteworthy for tying the 2002 record of the most named systems during September at 8. But most storms this year barely lasted a day and some almost didn't reach the maximum strength for a named storm. Besides Dean and Felix, the other hurricanes were all within 5 mph above the minimum wind speed for becoming a hurricane. New technology is helping to classify storms which may not have been identified years ago.

Since the season was producing fewer systems than the preseason long range forecasts, there is room to imagine cyclones are getting upgraded to match the predictions. Most storms reached 40 mph in some quadrant of the circulation but internal pressures never dropped below 1000 mb based on aircraft resonance. Lower pressure increases wind. But many of these systems were disorganized as winds were higher than what a typical barometric pressure would support.

The past two years long range seasonal hurricane forecast have been off the mark. Many climate signals pointed to an active 2007 and it was an La Nina year which drives down shear typically increasing storm numbers. So what happened. The CSU team feels an active dust season over the Atlantic may of hampered the numbers. Certainly more research is needed for this type of forecast while near term storm track forecasts improve considerably.