Most meteorologists use percentages on a seven day. While they can help tell the weather story, they can also cause some confusion. A percentage on the seven day explains our chances for seeing precipitation.
It takes into consideration both the area of precipitation and forecast confidence. For example, if I am one hundred percent confident it will rain tomorrow for fifty percent of the area, it's like saying one times point five. I could also be only fifty percent sure everyone will see widespread rain. The higher the area the more widespread the rain will be. Keep in mind it is rare for a meteorologist to be one hundred percent sure of something. But we do spend hours analyzing charts and looking at the forecast.
Which means the percentage is more than a roll of the dice, but it does give you an idea of how likely it is too see rain or snow. Keep in mind the more you travel within the area, the better your odds are of seeing precipitation. The percentage does not tell us how heavy the precipitation will be or if it will fall all day. There could be a 90 percent chance of light rain in the morning.
Percentages are usually lower during the summertime with thunderstorms. This is mainly because they are spotty and not widespread. For example I could be 90 percent sure of a few thunderstorms popping up over the weekend. That leads to .9 times .3 which comes out to roughly 27 percent. Thunderstorms can also be heavy rain producers. This extra information is why a full weathercast can be beneficial. A percentage can’t tell you the impact. A 70 percent chance of sticking snow is different than snow that melts when it hits the ground.
Forecast confidence goes down with time, especially after three days. For example, if today is Wednesday, forecast confidence is much lower for the weekend than Thursday. This means percentages usually start out small and go up as the date gets closer. I don’t remember ever putting a 90 percent chance down for something a week away. However, I will probably say on air it’s something to watch.
