23 April 2010

Posted by Dan Satterfield

Severe Storm Forecast from the Storm Prediction Center (NOAA/NWS)

It may sound crass and I do not mean it to be that way, but there are likely to be deaths from tornadoes in the next 48 hours.

If only we could only say exactly where!

What we can say is that conditions are coming together to make the development of a few strong tornadoes and they will likely be in Mississippi and Alabama on Saturday. Here in Alabama there is special concern because nearly 100,000 people will be at a major stock car race in Talladega. Here in Huntsville is a major art festival Panoply.

The odds of a tornado touching down at any one spot are minuscule, but the odds of a tornado in the area are rather high. So all we can do is to warn people to be alert and to have a NOAA Alert Weather Radio.

One other thing. If you live in a mobile home, make arrangements to go somewhere else during a tornado watch on Saturday. Believe me when i say that several will be issued. Your odds of being killed or injured from a tornado are many times higher in a mobile home. Leave the windows closed (Opening them can increase damage) and consider a walk in closet or a bathroom for safety.

You might wonder just how we know tornadoes are possible?

Tornadoes require two basic ingredients. Wind shear and instability.

A tornado can develop with a lot of one parameter and a little of another. The big outbreaks happen when you have a LOT of both.

CAPE is a measure of instability. Above 1000 is very unstable. from UIUC.

This weekend looks to have a lot of  both.

Instability can be measured in different ways but one preferred way is to use numerical weather prediction models to estimate the CAPE.

CAPE stand for Convective Available Potential Energy. Remember potential energy from high school physics? Let me just simplify it and say that numbers over 1000 joules/kg are bad.

Wind shear can also be measured (and is) in different ways. The direction of the wind shear that a storm sees as it moves along is most important and meteorologists look at what is called the storm relative helicity as an indicator. This gets even more complicated (can you say calculus?), but the basics are this- over 200 is bad.

Helicity is one measure of wind shear. It's one of the two ingredients for tornadoes.

Here are some actual NWP charts for those parameters on Saturday. I will let you check out the numbers.

There are a lot of meteorologists at NOAA and at TV stations (like me) who will be working together for very long hours to give as much warning as possible.

Take them seriously this weekend, even if you never have before.

Mother nature may be in a rotten mood.