Written by cuweathernerd    Saturday, 25 February 2012 00:00   
Chasing 101: Section 1:8 Forecasting: Skew-Ts Part 2

This is part of the continuing series called chasing 101, a course to help people who are new to chasing learn the fundamental skills to chase productively and safely. They are meant as both information and as a forum for discussion.

This is a continuation of the previous two posts on CAPE and Skew-Ts Part 1, and assumes you have read and comprehend them.

Soundings, as we have mentioned, are at the heart of most people's severe weather forecasting. A lot of what we have talked about so far is something you would do in the forecast stage -- watching models at home, deciding if you wanted to go out on a chase or to sit out this round. That's good, and developing the skills to actually determine a good thermodynamic environment vs a marginal one is one of the more changing and common tasks you will have as a amateur severe weather forecaster.

In reality, what you are looking at are model soundings -- what a computer run looks like. That's nice, because it lets us be prognostic for the future, but on the other hand, it isn't observed data, and therefore guaranteed to be inaccurate. Especially when you are out in the field, we want something better.

As a meteorological community, we launch weather balloons frequently. In fact, 92 balloons are launched twice a day in the US. These soundings are at 12Z and 0Z -- which isn't exactly the most useful to us as severe weather meteorologists, because 1 is more or less at dawn, the other dusk. We tend to want to know what is going on during the early afternoon, so on certain days, extra balloons are launched.

No matter when the balloons are launched, the actual, measured data is turned into Skew-Ts. You can find these observed soundings in lots of places, but perhaps the best is the SPC's sounding analysis page. Go ahead and bookmark that page.

Watch this animation for an explanation of what you're looking at: CLICK HERE FOR HIGHER QUALITY ANIMATION

As you can see, there is a lot of data there. All of the measurements we have already talked about are there, but instead of being a forecast, they are observed values. In the field, such data is amazingly valuable. You'll use it mainly to evaluate if the model soundings are accurate -- do what the models say line up with what is observed? If they don't, you might be skeptical of the model, especially if there is a big difference. This might lead you to call off a chase or to reposition if you need to.

In practice, it is normally a cap, a layer of warm air called an inversion that prevents thunderstorms from forming, that is identified by soundings and of concern for us out in the field. We are interested in this layer's decay, so knowing how the cap is behaving is often the core of the question before thunderstorms initiate.

There are forecasts of cap, and the best I know of is available as a nice, reliable plot here. This takes some of the sounding analysis and makes it into a nice map view, which can help with immanent forecasting. Used in tandem with the HRRR, it is a formidable tool. For now, keep it in your bookmarks.

There are a lot of other things you can do with a Skew-T -- most of which are pretty complex.

One of the most common is a measure called "lifted index", which is a measure of the difference between the calculated parcel and environmental profile at a given level, normally 500mb. In practice, this value is less useful than CAPE because LI is calculated only at one level, not the depth of the atmosphere. Personally, it is not something worth learning or using -- you can be successful without it.

Finally, the 'shape' of the CAPE matters a lot. Short, 'fat' CAPE, where there is a lot of difference between the environment and the parcel, leads to strongest updrafts. So we want to see the mass of our CAPE near the surface. One great product to keep in mind is the 0-3km CAPE found on Earl Barker's page. It makes a nice forecast that looks like the picture below. That map helps me find the most favorable temperature profiles -- just as a map of helicity helps me find the best hodographs.


If you really want to hone your skills with the Skew T, I recommend the excellent course over at METed, called a comet module.

Anyway, you'll need a free username to access it. When you have one, you can find the lesson here.

It is really in depth, but if you take the time to really go through it, you will have a mastery of Skew-T. It has been one of the best resources I've used in my own education at university.


The above is an article written by Reddit user cuweathernerd and has been modified by Ryan Lehms. The original article and discussion can be found here:


Last Updated ( Sunday, 11 August 2013 12:13 )

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