# How to interpret daily vs. hourly "% chance of rain" precipitation forecasts?

Looking at the National Weather Service forecast for daytime precipitation tomorrow, it shows a 70% chance of rain ("showers likely"). I interpret this as meaning that on average it will rain 7 out of 10 days with a similar forecast -- leaving 3 days of 10 with no rain.

Looking at the hourly breakdown of the same forecast it shows a 50% to 70% of rain for each daytime hour (a 12 hour period). Simplifying to 50%... if I flipped a coin at the beginning of each hour, the odds of a heads are the same as the odds for rain (50%).

Taking this further, that means that "no rain today" is as likely as "12 heads in a row" ... a far, far, smaller probability than the 30% implied by the 70% forecast for the day.

How can I interpret the hourly forecast percentage to be consistent with the daily forecast?

• I've wondered myself exactly how the events whose probabilities of occurrence are being provided are defined. That being said, you seem to be making an assumption of hourly rain occurrence probabilities as corresponding to independent occurrence of rain or not across consecutive hours in a day. That is likely far from valid for a variety of reasons, and incommensurate with how the forecast is made. Apr 27, 2016 at 23:51
• As an extreme case, suppose there is a 50% chance it is going to rain tomorrow; and if it rains, it will be from 10 A.M. to 10 P.M. Then rain probability for the day is 50%, and rain probability for each hour from 10 A.M. to 10 P.M. would be 50%. Apr 27, 2016 at 23:58
• @MarkL.Stone - Correct, I am pointing out that "making an assumption of hourly rain occurrence probabilities as corresponding to independent occurrence of rain or not" is obviously incorrect. What I'm struggling with is (a) a more precise definition of those hourly forecast percentages -- which make intuitive sense along with (b) how to convert a series of hourly forecasts to a daily one. Apr 28, 2016 at 10:59
• Looking further at my local NWS forecasts, it appears that the daily forecast % is equal to the maximum hourly percentage (rounded to nearest 10%). So, perhaps the interpretation is... if the conditions of this hour existed for 12 consecutive hours, there is a 50% probability of rain during one of those 12 hours. Apr 28, 2016 at 11:03
• How the weather varies from one hour to the next, or even one day to the next, is not like independent flips of a coin. You need a more sophisticated model to account for temporal correlation in conditions at nearby times.
– whuber
Aug 11, 2016 at 20:56