Weather Education

Hurricane Forecast Models and Tropical Weather Words

Do you know the difference between a tropical depression and a tropical disturbance? Do you know that an "invest" has a number but that doesn't make it a storm?! Here's a breakdown of the terms used to describe tropical weather.

TROPICAL TERMS  
Hurricane Wind of 74mph or higher. Hurricanes form in the tropics and they do not have fronts.
Tropical Storm Wind of 39mph to 73mph. Tropical Storms form in the tropics. They do not have fronts. They are given names to make it easier to track them.
Tropical Depression Low pressure in the tropics steady circulation and winds under 39mph. Tropical Depressions are given numbers so that they can be tracked and sorted out.
Tropical Disturbance A cluster of thunderstorms that move together in the tropics for more than a day, without wind circulation. Most of these do not grow any stronger.
Tropical Wave Low pressure and cloudiness, usually moving from east to west in the tropics. These are a normal part of weather and most tropical waves don't become anything more.
SPECIAL TERMS  
Potential Tropical Cyclone A strong tropical disturbance that is very likely to become a tropical depression, tropical storm, or a hurricane, within a short time. It is given a number or name so that countries can issue watches and warnings.
Typhoon The same as a hurricane, in other parts of the world. Typhoons and Hurricanes are also known as Tropical Cyclones.
Invest A tropical disturbance that is being "investigated" to see if it might develop. These are assigned numbers for computer tracking but that number is totally separate from the numbering that tropical depressions and tropical storms get.
Major Hurricane A hurricane that is Category 3 or higher, with winds over 110 mph
Spaghetti Plot A series of computer model projections for where the center of a tropical system is forecast to be. They resemble spaghetti and tell you how similar or different multiple models might be. They do not show wind.
Forecast Cone A forecast for where the center of a tropical system is likely to be. It does not tell you strength or size, and it does not say where the worst impact may be.
Post-Tropical A storm that was a tropical cyclone but no longer is. It can still be as strong as a tropical storm.
Extra-Tropical A storm that was a tropical storm, that has moved out of the tropics or gained a front. It can still be as strong as a tropical storm or hurricane.
ACE Accumulated Cyclone Energy: A measure of the windspeed of a tropical storm or hurricane added up for the total life of the storm.
Cyclone A universal term for a tropical storm or hurricane or typhoon.
Subtropical A tropical storm or tropical depression that is a hybrid of a regular low pressure system. It still can have heavy rain and high wind.

 

"Spaghetti" plots are the center point of a low pressure system track forecast from many weather models. When the multiple models are close together it tells the forecaster that a particular track or location is more likely.

For hurricanes, there are dozens of models and model groups used around the world for forecasting and for creating spaghetti plots. Each model may use different equations and have different resolutions (detail). They all have to solve math problems at grid points both horizontally and vertically. Remember that weather models perform like stocks- No one is always good, that's why many are used; some are good in certain situations or latitudes; and some give the best overall solution when grouped together like a mutual fund. Here are just a few of the models you might see used on TV or online...

HWRF is Hurricane Weather Forecast Model

CMC is Canadian Meteorological Center

UKMET is United Kingdom Meteorology

GFDL is Geophysical Fluid Dynamics Laboratory

GFSO is Global Forecast System Operational

The National Hurricane Center uses many more models and here are what the other abbreviations stand for. It's a long list of different models, with different computing methods.


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