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Tropical System Forecasting Basics

**If you want basic links fast, try looking at the Tropical Breakdown**

This area of the website will help newcomers with the forecasting tools for tropical weather. If you need a detailed explanation of tropical weather systems, and what makes them tick, check out the hurricane information guide at Hurricane Warning. Please read this guide first as that information will help you understand some of the forecasting information. I will mainly focus on items such as steering currents, shear, water temperature, satellite images, and model information. If you have any suggestions or comments about this page, please e-mail me. This page is meant to be a stepping stone for those that are trying to learn about tropical weather, so as such, it is only a **basic** introduction. Sites with more in depth information will be listed at the bottom. Please listen to all watches, warnings, and statements from the NHC. Their information is official, and nothing on this site should be used in the protection of life and property.

Steering Currents

The intensity of a tropical storm plays a big role in how it is affected by steering currents in the atmosphere. Steering currents are created by other weather systems, such as high pressure areas. In general, the stronger the tropical system, the "deeper" the steering current will be (stronger storms extend higher in the atmosphere). Therefore storms with different intensities will be steered by different wind levels in the atmosphere. The strength of tropical systems is determined by its barometric pressure. The University of Wisconsin - Madison (CIMSS) provides an excellent tool for determining the steering currents for a tropical system. It gives steering currents for different levels of the atmosphere. Its not a given that the storm will follow the logical track, but generally they will behave and follow the steering currents that they interact with. The steering current maps are divided by the strength of the tropical system they would steer. This diagram shows what layers steer a tropical system, based on it's strength. Essentially you look at the pressure of the system you want follow, then select the steering current maps for a system with that pressure.

Below is an a steering current map for Tropical Storm Irene (2005). In this map, you can see two areas of high pressure with TS Irene rotating around the bottom/left side of one of the areas. Ahead of TS Irene, you can see a weakness in the steering currents between the two highs. TS Irene actually went between this weakness in the absence of stronger winds

The Tropical E-wall from PSU also has steering information.

 

Shear

Shear is a change in wind speed or direction with height. Tropical systems need low shear in order to form. The higher the shear, the harder it is for convection (thunderstorms) to form and wrap around the center. Many shear maps exist on the Internet. Again, The University of Wisconsin - Madison (CIMSS) contains good maps.

Below is a shear map showing Tropical Storm Irene (2005). In this image you can see that TS Irene is experiencing shear in the 5-10kt range, which is not enough shear to really hurt the system. The site also contains a shear tendency map. This map shows what the shear has been doing for the past 24 hours (going up, going down, or staying the same).

 

The Weather Underground contains a shear forecast for the tropical Atlantic. Many of the models also include expected shear. The model information is disussed below.

Ocean Temperature

In general, for a tropical system to form and sustain itself, the ocean temperature needs to be at least 26.5C or 80F (to a depth of 50 meters or so). This is not a given though, as tropical systems in 2005 showed us. In general, hotter temperatures can cause stronger storms to develop and maintain their strength. Other factors have to be considered though, including shear. Temperature maps of the ocean are very easy to find on the Internet. I will list some of my favorites below. Others are listed in the link directory.

AOML Tropical Cyclone Heat Potential - Excellent site for ocean temperature maps. Also includes other maps of interest. The map detailing the depth of 26C water is useful for seeing if the ocean has the depth of warm water need to fuel a cyclone.

Ocean Remote Sensing - Contains nice maps of ocean temperatures. Also shows temperature maps for the Gulf Stream, which can help a tropical system intensify quickly.

OSDPD Ocean Page - Contains all kinds of ocean temperature maps

 

Satellite Imagery

Satellite imagery is very useful when forecasting tropical systems. It can show such features as dry air around the system, shear effects, and how the system is interacting with other weather systems. It should be noted that many of tools already listed on this page are derived from satellite data. Satellites are used heavily to forecast weather systems. Below is a water vapor image for Tropical Storm Irene (2005). In this image, you can see TS Irene has some dry air (brown color) around different parts of the circulation. You can generally expect weakening in a tropical cyclone when you see dry air being wrapped toward the center of the storm.

Different types of satellite images exist. Visible satellite images are only available when sunlight is shining on the area the satellite is looking at. For night imagery, infrared imagery is available, as well was water vapor. Listed below are some good sites for satellite imagery. Check the link directory for more satellite imagery sites.

Atlantic Tropical Weather Satellite Imagery - One of the best satellite sites available, provided by the Satellite Services Division of NOAA.

Global Hydrology and Climate Center Imagery - Nice satellite imagery site. Has higher resolution images.

 

Computer Models

Computer models provide a very important part of tropical system forecasting. They are NOT the only means used in forecasting though. Models are prone to VERY large errors, and should not be used exclusively for forecasting. Some models perform better than others, and some will work well in a particular situation, while working poorly in another. Typically, many professional forecasters will use the model consensus. That is, they will use an "average" of the models as a means to determine the best track. Computer models, coupled with experience and other forecasting tools, help meteorologists forecast hurricanes better than ever before. There are many model links available on the Internet. I will list some of the model links below, along with a brief explanation of usage. Some of the more popular models for tropical weather include the GFS, GFDL, UKMET, WRF, and the Navy NOGAPS. That isn't to say they are always the best, and that others are always wrong though. This page is not meant to show you how to interpret each model, rather just to introduce the links to you. The National Hurricane Center provides a page that will help you access model error trends.

FSU Tropical Model Page - This is an excellent model page, and a popular choice among meteorologists and hobbyists. It allows you to view models runs on several major models, and some lesser known ones.

PSU Tropical Model Page - Very similar to the FSU model page, but also includes the WRF model

NCEP Model Page - This model page shows the major operational models that the NWS uses. You can select regions to "zoom" in to a certain area.

Air Resouce Lap Model Animation Page - This page shows animations for models runs (many of the major models)

Colorado State Tropical Model Guidance Page - This page shows many models runs for tropical systems present in the Atlantic. There are early cycle, and late cycle track guidance, as well as intensity forecasts.

Weather Underground Tropical Page - This page will show model runs for active tropical storms, as well as give other tropical weather information. A very good site.

 

Other Resources

This page is just meant to get newcomers started. There are vast resources available on the Internet. Below, I have posted links to other sites with tutorials and other information.

National Hurricane Center FAQ - Frequently asked questions, answered by the NHC

Tropical Weather Links - Links to all kinds of tropical weather sites

Hurricane Warning - A wealth of hurricane information, including storm specific information.

FLHurricane.com - Nice tutorial on hurricane forecasting.

Wiki on Tropical Cyclones - Good information on tropical cyclones

The Weather Prediction - General information on tropical systems

 

 

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