Hurricane Questions & Answers

Here are some common questions and answers regarding hurricanes that I wrote specifically for readers of the The Picayune Item and The Poplarville Democrat in Pearl River County, Mississippi. They appeared in a special Hurricane Preparedness section of those newspapers published on May 31, 2017.

Both tropical storms and hurricanes have names. What is the difference?

Tropical low pressure systems, known as depressions, are given names and upgraded to tropical storms when maximum sustained winds increase above 39 mph. They become hurricanes when sustained winds reach 75 mph or greater.

What are “sustained” winds?

Winds are turbulent, and winds at the surface are particularly gusty. Winds over land are even more turbulent than over open water. Sustained wind refers to the one-minute average wind speed at a location. For example, during a one-minute period the wind speed may vary from 80 to 100 to 90 to 105 mph, resulting in a “sustained” wind speed of 94 mph.

What do the different categories of hurricanes mean?

The Saffir-Simpson Hurricane Wind Scale’s five categories correspond to the extent and type of wind damage caused at different sustained wind speeds. Category One winds (75-95 mph) are dangerous, but they cause the least amount of damage among the five categories. At the upper end of the scale, Category Five winds (greater than 154 mph) are the worst and can cause catastrophic damage to many many buildings.

A change of one mile per hour can change a hurricane’s wind category. In reality there isn’t much difference in the wind threat from hurricanes in the upper range of one category and the lower range of the next higher category.

Hurricanes are assigned a category based on their maximum sustained winds. Most of the winds in the hurricane will be less than the maximum sustained speed, and some gusts will be higher.

Also, these categories do not say anything about hurricanes’ deadliest threat, which is storm surge. For example, a hurricane with Category Three winds that cover a large area can produce a storm surge that causes catastrophic water damage in coastal flood zones.

What are the eye and eyewall?

The eye is a circular area 5 to 50 miles in diameter in the center of the hurricane’s circulation where the winds are light or calm. There is little or no rain and skies directly overhead may even be clear. The lowest atmospheric pressure in the hurricane is found in the eye.

Surrounding the eye is the eyewall, a ring of extremely intense thunderstorms containing the hurricane’s strongest winds.

Air in the lower levels of the atmosphere spins counterclockwise around the hurricane center while also spiraling inward toward the low pressure of the eye. As the humid air nears the eyewall it moves upward in intense updrafts, releasing heat as water vapor condenses into liquid water in the clouds, a process that can further decrease the pressure in the eye and intensify the storm.

What goes up must come down. Some of the air begins to sink into the center of the storm, drying and warming as it sinks, resulting in the comparatively nice weather in the eye.

Because the eye is small compared to the size of the hurricane, most people who go through a hurricane will not go through the eye. If you do, remember, you will soon be back in the violent eyewall.

How big of an area do the winds cover in a hurricane?

Hurricanes come in many different sizes. The maximum winds reported in hurricane advisories are usually found in a relatively small area in the eyewall.

But, it can be a deadly mistake to equate a hurricane with the pinpoint location of the eye and its forecast track. Intense hurricane force winds can extend up to 100 miles outward from the eye. Winds less than hurricane force (75 mph) but greater than 40 mph can extend hundreds of mile from the center.

The wind field is almost always lopsided, extending further in some directions than others. Usually the strongest winds are found, and and extend out the furthest, in the right, front quadrant of the storm. For a northward moving hurricane this is the northeast quarter of the storm. This is why it can make a tremendous difference whether the eye moves west of us (bad) or east of us (not as bad).

Even if the eyewall comes right over us, won’t the winds in Pearl River County be much less than the maximum winds at the coast?

The sustained winds in Pearl River County definitely will be less than at the coastline. But, gusts could be as strong as the winds at the coast.

Also, tornados can be a serious threat, even in tropical storms and weak hurricanes, especially ahead of and to the right of the track of the eye.

Damage from high winds to mobile homes and other buildings and indirectly due to falling trees is the greatest threat to Pearl River County from hurricanes.

In Pearl River County do we have to worry about water from hurricanes?

Storm surge is not an issue for us. However, torrential rains can cause streams in our county to flood. Rains of 10-25 inches are possible from some tropical systems, including weak depressions or tropical storms.

How much can we trust hurricane forecasts?

The National Hurricane Center’s forecasts are accurate and timely enough that they have saved countless lives and billions of dollars in property damage over the past several decades.

Thanks to advances in satellites, aircraft-based instrumentation, numerical computer modeling, and a better understanding of hurricane dynamics, NHC’s 3-day forecast track has steadily improved from a not-very-helpful 300-mile average error in 1989 to a 75-mile average error last season. The 24-hour forecast average error has improved from 100 miles to 40 miles.

However, these are errors averaged over each hurricane season. Each hurricane is different, and the forecast track accuracy varies from storm to storm. In other words, 3-day forecasts for a particularly difficult-to-predict storm could still be several hundred miles.

Predicting intensification or weakening of tropical storms and hurricanes has turned out to be much harder than forecasting where the storms are headed.

When is the peak of hurricane season?

Hurricane season extends from June through November. Tropical storms and hurricanes have impacted the north-central Gulf Coast during each of those months. Our most serious threat is in August and September. With the one exception of an early July hurricane in 1916, all the major hurricanes impacting Pearl River County since the mid-1800s have hit between August 12th and October 3rd.

Will the 2017 hurricane season be a bad one?

People who get hit by a hurricane in 2017 will consider the season a bad one, even if that hurricane is the only one that forms all season long.

A better question might be, “How many hurricanes are there likely to be in the Atlantic, Caribbean and Gulf  of Mexico this season?” Several meteorological organizations have shown skill in answering this question in recent years. Their pre-season outlooks issued in March and April this year are that four to six hurricanes are likely to form in 2017. That is at or slightly below the long-term average. [UPDATE: I wrote that in mid-May; since then, several organizations have issued their early June updates. Those updates have been trending toward a more active season, in large part due to the decreasing likelihood that El Nino will kick in. The Barcelona Supercomputing Center and Colorado State University keep an updated summary of seasonal outlooks here.]

Should we evacuate?

The primary reason that people should evacuate ahead of an approaching hurricane is storm surge. Fortunately, storm surge is not an issue in our county.

However, there are still five reasons that residents of Pearl River County should consider for evacuating in advance of an approaching tropical storm or hurricane.

  1. You live in a mobile home. For all tropical storms and hurricanes, people in mobile homes should move to a stronger building away from the danger of falling trees. This might mean leaving the area or just riding out the storm with a neighbor down the street.
  2. You have trees close enough to fall on your house. Check the official National Weather Service forecasts and Hurricane Local Statements, both of which will be issued by the New Orleans/Baton Rouge Forecast Office in Slidell. If sustained winds higher than 75 mph are predicted for Pearl River County, and trees can reach your house, you should consider evacuating. The stronger the hurricane and the closer it is forecast to approach our county, the more seriously you should consider leaving your home for a safer location.
  3. Someone in your household has special medical needs that would be affected by an extended power outage. After major hurricanes, power might not be restored for days or even weeks.
  4. You live in an area prone to flooding from local creeks or branches. Heavy rains could cause flash flooding trapping you in your home.
  5. If a Category Four or Five hurricane is forecast to hit our area, even those with confidence in their well-built homes who do not have trees close enough to fall on the house, may want to consider evacuating the area. The strongest hurricanes can cause damage to even well-constructed buildings. Additionally, riding out the fury of a Category Four or Five hurricane can be a harrowing experience, even if your home suffers no major damage. And, the aftermath of power outages, gas shortages, and spotty phone service could prove difficult.

If you evacuate locally, which means you simply move to a local shelter or someone’s well-built house that you deem safer than your own, make sure to be in that safer location several hours before tropical storm force winds are forecast to arrive and trees may start falling.

If you do decide to evacuate outside the local area, the safest course of action is to move at least one hundred miles inland. Also, the further to the west of the forecast track you go, the lower the risk of experiencing severe weather. Keep in mind that for intense hurricanes, inland hotels can fill up quickly.

Once a tropical storm or hurricane is in the Gulf of Mexico, residents of Pearl River County should closely follow reliable weather information sources such as the websites for the National Hurricane Center and the National Weather Service Forecast Office in Slidell.

If a hurricane watch is issued for the Mississippi Gulf Coast, it means that hurricane conditions are possible in our area. It is possible, but still far from a certainty, that tropical storm force winds could reach our area in approximately 48 hours. As soon as the watch is issued is the time to finalize your plan of action. If that plan involves moving to a hotel inland, you may need to make reservations at this time.

If we are lucky, the hurricane watch will be dropped. If we are not so lucky, the hurricane watch will be changed to a hurricane warning for the Mississippi Gulf Coast. This means that forecasters have decided that hurricane conditions for the warned area are not just possible, but are likely to occur. Ideally, the National Weather Service issues a warning 36 hours before winds of 40 mph or greater are expected to reach the area. However, in some cases we might have less warning lead time, so pay close attention to the official warnings available on the National Weather Service websites.

Regardless, when a hurricane warning is issued, residents need to “pull the trigger” and execute their plan of action, whether that involves evacuation or riding out the storm in their homes.

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