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How Do We Forecast?


This is the time of the year where we give tours to the television station, as well as talks to local school classes. One of the questions I always receive is how exactly do we forecast the weather? Do we just look at the National Weather Service? Well, the answer to the latter question is no. Meteorologists, on television or not, create their own forecast from scratch. We use lots of online data to come up with the daily forecast. When forecasting, I start from the top down. That means, I look at what is going on from about 34,000 feet up, all the way down to the surface. the picture above is a surface map of the cold front which will pass through on Saturday. We need to know what goes on at altitudes where airplanes fly because that affects the weather at the surface.

Air at altitudes, such as 34,000 feet, moves very fast and acts as a steering mechanism for waves of energy that drive storm systems in the country. As you work your way down, you see the jet stream level, which begins at about 25,000 feet. We all know that the core of faster wind is at the jet stream and that will help to move the systems, as well. When we start to get to about 9,800 feet, we look at how moist, or dry the air is. Just under 5,000 feet, we can look at temperature profiles to help forecast high temperatures at the surface. Something interesting to note, that level of just under 5,000 feet (which is 850 mb) does not exist in the Rockies. That is the case because they reside above that altitude. So, that level is not available in areas where the elevation is higher than 5,000 feet. The photo below on the left is the chart at about 18,000 feet, or 500 mb level.

Once we look at what's going on above the surface, we look at computer forecast models that depict what will happen at the surface. The photo below on the right is what's called a four panel model. From top left, to bottom right, it shows the wind flow at 500 mb, surface features to 500 mb on the top right, moisture, or dry air at 700 mb (nearly 10,000 feet) on the bottom left and surface precipitation forecast on the bottom right. This particular run is called the GFS model, which is the American forecast model. It goes out to 180 hours. This is a good forecast to see out into the future. However, there is a bias with this model. It tends to overestimate precipitation. So, that's something to take note.

The other models I look at are the European, North American and two Canadian forecast models. I also look at the Texas Tech forecast model, as well as the WRF model when the weather is very active. There are also outputs that give us specific numbers for highs/lows, dew points, wind and precipitation.

If this all sounds confusing, it can be. That can explain why it can be difficult to have an accurate forecast. So, now you know how a forecast comes together. There are also other tools we can look at when there is more active weather. I'll get into that in a later post, perhaps. It can be a bit of inside baseball and can be hard to explain. There is a lot of information out there that meteorologists use to forecast the weather. We all don't use the same things, but we all come up with a reliable forecast.

 

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