Hurricane Forecast Cone, and Wind Scale

Hurricane Forecast Cone, and Wind Scale

Look at these three forecast cone images from the National Hurricane Center, for Isaac in 2012. Each is 48 hours apart. They give a lot of information in the image and in the legends but some is often misinterpreted or overlooked. These cones are what we base our cone forecasts on at News 5. The cone shows where the center of the storm is predicted to be in time. It's constructed by using computer models and forecast experience. The cone accounts for a bunch of individual forecast positions and tracks which you often see online as "spaghetti plots" but it's not just a simple average or majority that gives the forecast.

All forecasts become less certain with time, that's why the cone gets bigger with time. In a perfect forecast the storm would travel right down the middle of the cone but no forecast is perfect. Some are pretty close but meteorologists are far from fully understanding hurricanes.

If you study the 3 forecasts for Isaac, you'll notice the cone shifts. That's not unusual and it's not a matter of someone changing their mind. The cone shifts because new information gives updated and usually better data than what the previous forecast used.

It might seem that the cone for the Mobile area gets smaller in time. It does! The reason for that is there is more certainty to where a storm will end up when it is a day away vs. 5 days away. The reality is the cone is not shrinking, it is moving with the storm. It may also appear smaller or bigger, given the adjustment to the map view as a storm nears land.

For a fast-moving storm, the cone will be elongated, while for a very slow storm the cone may be more of a circle.

In these Isaac forecast cones, landfall was within or right on the edge of the cone for the 5 days leading up to it. That's the purpose of the cone; to let an area know that landfall is possible, with a more precise projection leading up to it.

Five critical things to remember about the cone:

1. The cone projects where the center of a storm is likely to be 2/3 of the time, so 1/3 of the time the center may travel outside the cone.

2. Even with a perfect forecast of a storm staying in the center of the cone, the wind, rain, surge, tornado, and rip current impact of the storm can easily be outside of it.

3. The cone tells you absolutely nothing about the wind speed in a storm, or the potential storm surge.

4. The cone is updated every 12 hours when a new dataset arrives. Sometimes the change is minor. At other times the cone may change in location or it may stay in the same spot but extend farther out because a storm may be speeding up in motion.

5. The cone for a fading storm will not always go out five days since the storm may not last that long.

 


 

While the cone does not give the hurricane wind speed, you know the Saffir-Simpson Hurricane Wind Scale as the category of a hurricane. It was named after Herbert Saffir, an engineer, and Bob Simpson, a past Director of the National Hurricane Center. This scale deals with wind but not the size, surge, rain threat, or tornado threat of a hurricane. While you may hear a lot of talk about a hurricane strengthening from one category into another, there is far more variation within wind categories that dramatically changes the force of the wind. In other words, going from a high category 1 to a low category 2 is not as big of a change as going from a low category 2 to a high category 2.

When wind increases, the force of the wind increases with the wind speed squared. For example, if wind increases from 60mph to 100mph the force of it nearly triples. This is why increases in wind speed create so much more damage, especially when gusts and tornadoes are involved.

There is no longer a mention of storm surge in the Hurricane Scale because surge is not always directly related to the landfall wind speed. A hurricane with weakening wind at landfall may still have a high surge while a hurricane with increasing wind at landfall may have a small storm surge. Listen carefully for both parts of the story.



A Category 1 hurricane has winds between 74 and 95 mph. Damage is minimal but winds are still dangerous.
People and animals struck by flying debris could be injured or killed. Older mobile homes could be destroyed. Some weak frame homes will suffer minor to major damage to roofing, siding, coverings, garage doors, and chimneys. Failure of aluminum, screened, and pool enclosures can occur. Apartment buildings, shopping centers and industrial buildings can lose roofing and siding. Large windows, and windows in high-rise buildings may be broken by flying debris, creating danger even after the storm. Tree branches may snap and shallow-rooted trees can fall. Damage to power lines and poles will cause power outages likely lasting several days. Danny in 1997 was a category one but its biggest problem was from the more than 2 feet of rain that fell.



A Category 2 hurricane creates moderate damage with winds of 96-110 mph.
There is risk of injury or death from flying debris. Older mobile homes have a high risk of being destroyed. Newer mobile homes can also be destroyed. Some uncovered windows will be broken by impact of debris. Weak frame homes may lose roof structures. Well-constructed frame homes could sustain major roof and siding damage. Failure of aluminum, screened, and pool enclosures will be common. Roof and siding damage to apartments and industrial buildings will occur. Unreinforced masonry walls can fall. Windows in high-rises can be broken by flying debris, creating a significant danger even after the storm. Commercial signs, fences, and canopies will be damaged. Many shallowly rooted trees will be snapped or uprooted and block roads. Near-total power loss is expected with outages that could last from several days to weeks. Clean water could become scarce as filtration systems begin to fail. In 1995 Erin made landfall as a category 2. In 1998 Georges hit land as a category two, leaving over 2 feet of rain.



A Category 3 hurricane produces damaging wind of 111-129 mph. Category 3 and higher hurricanes are classified as "major."
There is a high risk of injury or death from flying debris. Older mobile homes will be destroyed. Newer mobile homes will be severely damaged. Complete failure of older metal buildings and older unreinforced masonry buildings is likely. Well-built homes will have damage of some sort. Windows will be blown out of high-rises resulting in falling glass, which will be a threat even after the storm. Most commercial signs, fences, and canopies will be destroyed. Trees will be snapped or uprooted, blocking roads. Electricity and water will be unavailable for several days to a few weeks. Opal in 1995, Ivan in 2004 and Dennis in 2005 were marginal category 3 at landfall, while Frederic in 1979 was a high category 3 storm. Katrina in 2005 was also a high category 3 at landfall based on the wind even though storm surge along the Mississippi coast was more like that expected from a much stronger storm.



A Category 4 hurricane produces extreme damage with winds of 130-156mph.
Widespread damage and destruction of homes and buildings occurs. Power outages will last for weeks. Water service will be out. Hardest hit areas will be uninhabitable for weeks or months. Charley in southwest Florida in 2004 and Hugo in South Carolina in 1989 were category 4 hurricanes.



A Category 5 hurricane is catastrophic. The winds exceed 156mph.
It's the rare but worst-case scenario. Near-total destruction of entire neighborhoods happens. Months and years of difficult recovery follow. Camille in 1969 was a category 5 in southern Mississippi and so was Andrew in south Florida in 1992.

 

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