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Galveston.com Hurricane Preparation Guide

For up-to-the-minute weather information, visit the Galveston.com Weather Center, from weatherman Stan Blazyk and AccuWeather.

Chart of Atlantic Tropical Storm Formation, with peak of late August / mid September

Hurricanes, Cyclones, Typhoons - by whatever name, tropical systems are some of the most deadly storms known to man. However, as we've learned more about the nature of these storms, we've improved forecasting accuracy and increased warning time to those in the path of the storm.

Hurricane Season in the Atlantic runs from June 1 to November 30. As you can see from the graph above, the activity ramps up in August and peaks once in early September then again in October. Persons travelling to areas near the Atlantic basin should exercise caution during Hurricane Season.

 

The Anatomy of a Tropical Storm

Cross Section of a Hurricane
1. Outflow
The high level clouds moving clockwise out away from the hurricane at heights of over 35,000 feet. These clouds are indicative of air spreading out over the top of the storm, which is essential to its development.

Illustration of a Cross Section of a Hurricane

2. Feeder Bands
These are squally bands of showers characterized by strong gusty winds and heavy rains. These bands become more pronounced as the storm intensifies, and are fed by the warm ocean.

3. The Eyewall
A band of clouds, strong winds and heavy rains surrounding the eye of the storm. At the eyewall, there is rapid movement of air toward the center and upward into the cloud.

4. The Eye
What goes up must come down, so with the violent rising air converging toward the storm center at the eye, sinking air develops within. This air dries out, creating the clear, calm eye. Winds are very light here since the focus of convergence and hence strong winds are in the eyewall.

Illustration of Storm Surge

The Storm Surge
Low pressure in the hurricane can act as a plunger, slightly pulling up the water level. However, the components that contribute to the greatest storm surge affect are the winds blowing to the left side of the storm and the topography of the land as the storm makes land fall. The strongest surge comes ashore just to the right of the eye, where the fierce hurricane winds are blowing toward land. Winds on the left side of the storm might actually cause the water level to run slightly lower than normal. Higher water level allows waves to strike farther inland, causing massive property damage.

 

Tropical Storm Development

Step 1
Tropical Wave: "bump" or disruption of normal tropical easterly flow. Associated turning of wind causes low-level convergence of air; which helps with falling pressure and enhanced showers.

Step 2
This can evolve into a Tropical Depression, which is a closed circulation of air in the low levels. This in turn increases convergence and pressure falls, and wind speeds increase in a Catch-22 effect (i.e. the stronger the wind blows the greater the convergence, the quicker the pressure falls... so the stronger the wind, etc.).

Step 3
Once sustained winds reach 39 mph in the closed circulation a Tropical Storm is named. Usually there are at least 2 closed isobars of 4 mb increments around the center. If atmospheric conditions remain correct the system will evolve into a...

Step 4
Hurricane. There is usually a difference in pressure of at least 0.60 inches of mercury between the center and surrounding pressure field, with the greatest change near the center (eyewall). It is this great difference in pressure, which sometimes can be as great as 2.95 inches of mercury, that causes the wind to be so strong.

Step5
A mature hurricane is a well-oiled meteorological machine, but disruption of the processes that drive the storm (i.e. interaction with land or colder air feeding in) will begin to destroy the storm, and the disintegration of a hurricane can often be quick and dramatic.

 

Hurricane Categories

Hurricanes are evaluated in a number of ways. Storms are assigned a category based on winds, storm surge and barometric pressure, using the Saffir-Simpson scale.

Category 1 storms have winds of 74-95 miles per hour, making them the weakest of hurricanes. Even these storms can generate a storm surge of 4 or 5 feet above normal high tide.

Category 2 storms have winds of up to 110 miles per hour, and can push a storm surge of 6 to 8 feet.

Category 3 storm winds can reach 130 miles per hour. This is the cutoff for "major" hurricanes, with commensurate storm surge potential of 9 to 12 feet.

Category 4 winds can be as high as 155 miles per hour, and such a storm brings a 13 to 18 foot storm surge.

Category 5 storms, with winds greater than 155 miles per hour, are very rare. These monsters can have storm surges of over 20 feet.

 

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