When there is only a 20% chance or precipitation I think the chance for rain should be left out. 30% makes a little more sense but even that is a bit low. You rarely ever are going to see it rain at 20% just food for thought.
From the way I have always understood rain forecasts…
the 20% means that there is a 20% chance that it will rain somewhere in the coverage area… it doesn’t mean that it will rain 20% of the time…
I see what you mean, but I would be willing to bet big money that 95% of the time no one receives measurable rainfall when there is only a 20% chance. Heck I remember at least 7 different times now, and I’m sure it’s more, that when they changed a winter storm watch into a winter storm warning, we missed those storms, at the last minute be it whatever reason. It blows the mind to think that 100% chance warning and still get it wrong. That’s why 20% gets my dander up.
I don’t think that is correct. 20% means there is a 20% that at least a single drop of rain falls at a specific spot in the area.
If for a specific area there is a 100% chance of rain (they are 100% sure it will rain somewhere in the region) , but the rain will most likely only cover 20% of that area, the chance is reported as 20%.
Basically it means that if you go outside in that area to a random location, you have a 20% chance of getting wet during the period the forecast is valid for.
@palmhead5 if something has a 20% chance of occurring, it think the description ‘possible’ feels appropriate. What you basically are saying is that you don’t believe the forecast, and that the value should be lower. That might be true, but that is something that shouldn’t be based on someone’s believe, but on hard data.
How about when it’s 21% ? Or 27% ? What number means skip it ?
U.S. National Weather Service
According to the U.S. National Weather Service (NWS), PoP is the probability of exceedance that more than 0.01 inches (0.25 mm) of precipitation will fall in a single spot, averaged over the forecast area.
The PoP measure is meaningless unless it is associated with a period of time. NWS forecasts commonly use PoP defined over 12-hour periods (PoP12), though 6-hour periods (PoP6) and other measures are also published. A “daytime” PoP12 means from 6 am to 6 pm.
The NWS also provides hourly forecasts. The hourly PoP can be similar to the daily PoP and vary little, or it can vary dramatically.
An example of an event where the hourly PoPs are not independent is a hurricane. In that case, there may be a 1 in 5 chance of the hurricane hitting a given stretch of coast, but if it does arrive there will be rain for several hours.
exactly… averaged over the forecast area. That’s what I said, if they are 100% sure half of the area gets rain (and the other half gets no rain at all), the reported chance is 50%, not 100% (they are 100% sure it will rain somewhere in the area)
hourly predictions can only be equal or lower than the daily prediction for the same area.
needs to be at last 30% to even start to get my attention