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Weather Academy

Learn all about the weather!

Weather Models

Weather models are one of the chief ways meteorologists predict the weather. Supercomputers proccess thousands of weather atributes and use them to predict weather conditions up to two weeks out. 

S.L.I.M

S.L.I.M stands for the four main weather ingredients needed for Severe Weather. Shear, Lift, Instability, and Moisture.

Shear: Shear is the change of winds with height. There are two types of shear, directional shear where the direction of winds changes with height, and speed shear, where wind speeds change with height. Shear is what causes thunderstorms to rotate and eventually cause tornadoes. It can also help sustain a thunderstorm

Lift: Lift is was initiates thunderstorms. Lift can come in many forms such as warm fronts, cold fronts, dry lines, and surface lows. However not all forms of lift are as likely to cause severe weather, the two forms of lift that are responsible for most severe weather is cold fronts, and dry lines.

Instability: In order for lift to create thunderstorms it needs to go through an area of instabilty. Instability is one of the main things that detirmens how bad storms will get.

Moisture: Humidity in the air that is one of the main things that factors into instability.

CAPE

CAPE which stands for Convective Available Potential Energy is one of the weather ingrediants forecasted by weather models. It is a measure of instability and is a big part of detirming how bad storms could get. Generally CAPE values of 1000+ can support severe weather, 1500+ can support decent severe weather,  values of 2500+ can support significant severe weather, and 4000+ though rare can be an extremely dangerous situation. However keep in mind that all these values are generalized and can be very different depending on other severe ingrediants.

Helicity

Helicity is a weather ingrediant forecasted by weather models. It is a measure of directional shear. Generally values of 100+ can support supercells, 200+ is a good enviorment for tornadoes, and 400+ is rare and extremely dangerous.  However keep in mind that all these values are generalized and can be very different depending on other severe ingrediants.

Bulk Shear

Bulk Shear is a weather ingrediant forecasted by weather models. It is a measure of speed shear. Generally values of 25+ can support severe weather, 50+ can support supercells,100+ is a good enviorment for tornadoes, and 175+ is rare and extremely dangerous. However keep in mind that all these values are generalized and can be very different depending on other severe ingrediants.

EHI

As was said earlier all ingredients are relatice to each other, for example you can have extremely high Shear but low instablity and end up with nothing because you need a combination of the ingrediants to cause severe weather. To make forecasting easier there are some products that combine to ingrediants, one of these is called the EHI (Energy Helicity Index). The EHI combines helecity shear and CAPE into one product. EHIs of under 1 usually will not suport supercells, 1-2 will support supercells, 3-4  will support tornadoes up to EF-3s and 5+ is supportive of EF-4s and EF-5s.

Supercell Composite

Another product that combines severe weather ingrediants is the Supercell Composite. The Supercell Composite combines helicity shear, bulk chear, and cape. 4-7 is supportive of supercells,  8-16 can support a rich atmosphere for tornadoes, and 17+ is when it becomes extreme.

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