Predicting the weather is hard, but communicating what you think will happen is also more challenging than you might expect. Often the most misunderstood part of a forecast is the probability of precipitation - the percent chance of rain. 

Probability of precipitation describes:

1. The chance of precipitation
2. At a particular point
3. Over a certain period of time

How do forecasters arrive at this value?

• how confident we are that precip will fall
• how much of our area it will cover
• how much of your day it will affect
• heavy rain vs. light rain 
• how it will impact you (middle of the day vs. middle of the night)
• precip type (rain vs. snow)

Let's look at an example: if I am 100% confident 30% of our area will see rain, using this equation, that would be a 30% POP (1.0 x 0.3 = 0.3 = 30%). A computer can figure that much out. But if I know this will be heavy rain and it will fall all day in that area, 30% is not a high enough POP to represent that. I would raise that to probably 60% and make a note that this is only for that specific part of the area. And remember, we are forecasting for 33 counties, not just your house. That's why areal coverage is one of the biggest factors to consider when making a PoP. 

Clear as mud, right? So much of forecasting the weather is a judgement call in the end, and the Probability of Precipitation is no different. There is no specific formula we plug numbers into that gives us all the answers. 

Another element to consider: your weather app. If the app you use is pulling raw model data (as opposed to being updated by a human meteorologist), the POP is NOT including all of those considerations. It is only looking at the confidence of you seeing rain at that moment. Say you're looking at the forecast for 8 PM in Elizabethtown, Kentucky, and the app tells you there's a 32% chance of rain. That's the model's confidence (based on different runs, ensemble members, etc.) that it will be raining in Etown at 8:00 PM.

• When forecasting snow, we use bigger percentages than if it were rain: FACT. Snow will cause more of an impact than rain will, so we use higher percentages to communicate that. 
• There is direct benefit to us in using higher numbers to scare you: FICTION. If we throw big percentages out every time it rains or snows, you will stop trusting us and that benefits no one. 
• The percentage will grow as we get closer to that event starting: FACT. As we become more certain about what will happen, we increase the percentage.

DISCUSSION IDEAS:

• When you see 30% chance of rain in the forecast, what do you think that means? 
• What factors should forecasters consider when making a PoP? Which ones should they ignore? 
• What is a better way to communicate how likely you are to see rain?