What are the chances?
National Weather Service forecasters say there is a 40 percent chance of precipitation in Austin on Thursday night. But what does that mean, exactly? And how does that relate to the more general terms you hear forecasters use a lot, like “scattered showers”?
Troy Kimmel and the National Weather Service can explain. Kimmel, who teaches meteorology at the University of Texas, said you’ve probably got a decent sense of the basics, even if you don’t know exactly what the terminology means. But we should briefly cover this.
For instance, let’s say the forecasters call for a 50 percent chance of rain — that does not mean it’s simply a flip of a coin. Nor does it mean that the forecast calls for rain to be falling 50 percent of the time, or for rain to be falling across 50 percent of the region.
It means that if you stand in a particular spot, there is a 50 percent chance you’ll get rained on at some point over the forecast period (usually 12 hours). Within the five-county Austin metro area, a 50 percent chance of rain means that, if I stand in my yard for the entire night, I have a 50 percent chance of getting wet.
Calculating . . .
So where does that 50 percent come from? This guide from the National Weather Service explains. Basically, a forecaster decides the likelihood rain will fall, then determines how much of the forecast area will probably be rained on.
Let’s say a forecaster thinks that the odds of rain falling are 100 percent, and that rain will drop on 50 percent of the forecast area. From there, it’s multiplication: 100 percent times 50 percent (or 1.0*0.5).
Voila. A 50 percent chance of rain.
Sometimes forecasters substitute descriptions for percentages. But for those of you who like forecasts expressed as percentages can match them up. The Weather Service’s terms go as follows:
0 percent = no chance of rain, anywhere in the forecast area
10 percent = slight chance of isolated showers
20 percent = slight chance of rain
30 percent to 50 percent = a chance of scattered showers
60 percent to 70 percent = showers are “likely” and/or will be “numerous”
80 percent or above = stay inside, otherwise your pant cuffs are gonna be damp
Okay, I made that last one up. The term is actually “categorical.” But that means your pant cuffs are gonna be damp.