GRAND RAPIDS, Mich. (WOOD) — This is an excellent question. There is a definitive answer, but to be honest, almost every meteorologist has their own spin on how they decide what number to write down for a precipitation chance on a given day.
WHY POPS ARE CONFUSING
There has been a lot of talk in the meteorological community about moving away from “POPS,” or possibility of precipitation numbers. Not only can it be confusing, it also can lead people to thinking a forecast is less accurate than it actually is because the way they are interpreting POPS is different from how the forecaster is interpreting it. Here at Storm Team 8, we will often times try to use phrases to help clarify, like ‘widespread rain in the morning” instead of just “60%.”
HOW ARE THEY CALCULATED?
Still, there is a scientific way that POPS are calculated. It is an equation. It is found by taking the forecaster’s certainty of precipitation and multiplying it times the area impacted. Stay with me! It’s not too complicated once you start looking at it.
Let’s say I am 80% sure that rain will impact 60% of West Michigan. Well, simple math shows us that 0.8 x 0.6 = .48, or about 50%! This means there is a 50% chance you will see rain at your location in West Michigan that day.
Here is another example: Let’s say I’m only 20% confident that rain will impact 50% of our area. Putting that into math form 0.2 x 0.5 = .10, or 10%! So, there is a 10% chance it will rain across West Michigan that day!
FOR A POINT FORECAST
For a point forecast, like a single city, it is a little different. POPs are a way to show how many times the atmosphere would produce rain on days when the conditions are exactly the same. For example, a 30% of rain would mean that the atmospheric conditions would yield rain three out of every 10 times. Or, another way to put it, if you had 10 days that were exactly the same conditions, it would rain three out of those 10 days.
USUALLY 1 TO 3 DAYS OUT, WE ARE FAIRLY CERTAIN
Forecast accuracy has improved a lot in the last few decades due to massive upgrades in technology. This means that usually, forecasters are pretty certain how many showers will form over their forecast area in the next one to three days.
If we are 100% sure that rain will impact 50% of the area, that gives us a POP of 50%. This means that showers will hit about 50% of the area.
If we are 100% sure that rain will only impact 20% of our area, math will give us a POP of 20% for that day. This means only 20% of our viewers will see showers on that given day.
CERTAINTY IS LOWER WHEN FORECASTING 4 TO 8 DAYS OUT
Think of it this way: If you are driving from your house to a location five minutes down the road, you’re less likely to encounter slow-downs or road blocks that would affect your expected arrival time. But if you are going on a drive that is five hours long, you would expect to not arrive in exactly five hours. There are increased chance for slow-downs, construction and bathroom breaks.
The same is true for the weather. When forecasting storm systems that are several days out, we like to give them some wiggle room. That inherently means lower certainty. Here is a great example: Next Wednesday in West Michigan, our two long-range models are painting very different scenarios for rain.
When two forecast models are showing very different forecasts, meteorologists will lower their certainty. With the above scenario, I am only 50% certain that rain will impact 50% of our area. Using our formula, 0.5 x 0.5 = 0.25 or 25%. We usually round up to percentages of 10. This means right now for Wednesday, if you were to look at the 8-day forecast, you would see a shower chance of 30% for next Wednesday.
As we get closer to next Wednesday, the forecast models will change. This will change our certainty (we will get more certain) and depending on what direction the shower chances go, this could increase our chance, or decrease it.
Since we are in a period of drought, I would not be surprised if we become more certain that less rain will hit us that day. I expect this number will drop for Wednesday over the next few days.