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HOUSTON – With every severe weather threat, meteorologists make predictions about the timing and severity of the storms.
Here’s a look into the process KPRC2 meteorologists go through to make those decisions.
Common parameters we look at are:
1. How much moisture is in the air? This is known as Precipitable Water and helps evaluate the potential for how much water can fall out of that air. Obviously, the more moisture that is there, the higher the predicted inch amounts can go.
2. What will lift that moist air up into the sky where it will cool, condense, form clouds and precipitate? The common methods are a front, whereby cooler air or a different wind direction causes the lift, or good old-fashioned convection — our warm atmosphere supplies enough heat to lift the air mass high enough to begin cooling and raining.
3. Given that we live in a tropical atmosphere, the ‘inch’ amounts forecasted by the computer model can often be doubled, or even tripled, as to what actually falls. Simply put, warm air expands and the farther apart the warm, air molecules go, the more room there is to put in water molecules. So with more water in the air mass, when it does unload, we can see huge amounts of rain over what was predicted.
Tuesday’s heavy rain was a good example of all of this — plenty of moisture in the very warm air. We’ve been in the 80s every afternoon and only around 70 at night. Upper-level winds moved air so that lower level winds could replace it. Think of how you can sweep the water away at the top of your pool, but the water below quickly comes up to replace it. That replacement was the lifting mechanism. But the overall surface winds were light and not enough to move the storms quickly. As a result, they built up, collapsed and by collapsing actually caused a mini-cold front behind them.
This mini-cold front, or gust front, became ANOTHER lifting mechanism causing MORE lifting of the moist air and, therefore, more rain. This is a training effect and while it’s a small train, we still had boxcar after boxcar over the same area. At that point, we can only look at how much rain is falling and predict how much more will fall in the immediate future.
Around these parts, when it will rain is easier than how much it will rain. And to that end, predicting how much flooding will occur because of those rains becomes very difficult due to the urban structures, highways, bayous, creeks and overall flat geography.
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