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Tropical Storm Isaias: What would make it a hurricane, anyway?

Maddy Lauria
Delaware News Journal

Editor's note: This story was originally published in 2018 during Hurricane Florence.

Have you ever wondered what those hurricane categories mean? Or what storm surge actually is? How about wind shear?

We have you covered, thanks to the National Oceanic and Atmospheric Administration’s glossary of terms and assistance from Delaware State Climatologist Daniel Leathers.

TROPICAL STORM ISAIAS:Delaware under tropical storm warning, watches

Saffir-Simpson Hurricane Wind Scale: One of the most widely used ways to describe the intensity of a hurricane is by categorizing it. The lower the number, the slower the wind.

Category 1 ranges from 74-95 mph; Category 2 is 96-110 mph; Category 3 is 111-129 mph; Category 4 is 130-156; and Category 5 is anything over 156. All hurricanes can cause damage, but the higher the number, the worse the outcome.

As of Monday, Tropical Storm Isaias was expected to regain some strength, possibly returning the storm to Category 1 hurricane status.

Storm surge: Refers to a rise in tide heights of streams, rivers, creeks or other water bodies that would be above normal. This term does not account for additional wave action caused by a storm, Leathers said.

Shear: A change in the wind speed or wind direction in the atmosphere. Generally, wind shear can weaken tropical cyclones or prevent them from developing.

Tropical cyclone: A storm system that forms over tropical or subtropical waters that mixes air vertically from the lower to upper atmosphere and has a well-defined center. Once formed, a cyclone extracts heat energy from warm water surfaces and exports cooler air from the upper atmosphere. A hurricane is a tropical cyclone.

Tropical depression: A tropical cyclone with winds no greater than 38 mph.

Tropical storm: A tropical cyclone that produces winds from 39-73 mph.

Hurricane: A tropical cyclone with wind speeds reaching 74 mph. In the Western Pacific, these storms are called typhoons. In the Indian Ocean, they are called cyclones. They are all tropical cyclones, but the terminology depends on where they form.

Nor’easter: A type of mid-latitude cyclone, which is a large low-pressure system that usually forms between late fall and early spring when warm and cold air collides. Nor’easters occur along the East Coast and are named for the northeast winds that occur over coastal areas. They can be as strong as a hurricane and are often much larger. The fabled Storm of 1962 was a mid-latitude cyclone.

Warning: There are a variety of warnings related to hurricanes and other storm systems, but the term is generally used to state that the risk – whether that be the arrival of a hurricane or storm surge – is expected within 36 hours in the area under a warning.

Watch: Instead of risk being expected, as with a warning, a watch means that there is a possible risk of dangerous weather conditions in the area under a watch. 

In the case of a tornado, for example, a watch means that conditions are favorable that a tornado could form. A warning would mean that one has been spotted.

This National Weather Service graphic shows the likely arrival time of tropical storm winds, as of forecasts available at 11 a.m. Monday. This shows most of Delaware is facing a more than 50% chance of seeing tropical storm-force winds on Tuesday.

Advisory: Includes all warnings and watches in effect for tropical cyclones.

Eye wall: An organized band of the strongest winds around the center of a hurricane.

During a hurricane’s development, the collection of thunderstorms that make up the system will build an eye wall once they are strong enough. When the eye wall is first built is generally when the storm is the strongest.

“Over time, it goes through this waxing and waning of how well the eye wall is put together,” Leathers said. “The eye wall will break down and the storm will weaken a bit. Typically, if it’s still over warm water and there’s no shear, it will rebuild and regain strength.”

Contact reporter Maddy Lauria at (302) 345-0608, mlauria@delawareonline.com or on Twitter @MaddyinMilford.