The simple answer is it is almost impossible to tell, unless you live in the USA. The USA has a website, called ForecastAdvisor, that validates forecasts and observations for around 990 different locations. Forecasts are typically retrieved from providers’ public websites and APIs. Forecasts for a given location are retrieved at the same time from all providers, and location order is randomized each day.
It is notable though that there is no single forecasting service that is best for all locations. So, although AccuWeather claimed in 2020 to be the most accurate forecasts overall, it does not mean that in 2023 they still are, nor are they the best everywhere.
ForecastWatch calculates the accuracy, skill, and quality of weather forecasts. They collect over 40,000 forecasts each day from Accuweather, the National Weather Service, and others for 990 U.S. cities and 1300 international cities and compare them with what actually happened. But it appears that ForecastWatch does not make that data freely available to end users to do comparisons. Their customers include AccuWeather, WeatherTrends360, etc, so I'm assuming those services use it to tweak their own forecasts.
Something interesting that an app like the weawow app provides, is in its settings it shows a graphical comparison of forecasts from the various services it supports. So, you could go look at your own local weather forecasting service and then compare which of weawow's providers most closely match that.
So until the rest of the world gets a ForecastAdvisor service we'll probably have to keep guessing which is the best weather app for our own location.
See
Weather Forecast and Weather Forecast Accuracy for Your City5-day weather forecasts for the United States in a nice, easy to read format, AND the 1- to 3-day out accuracy of other weather forecasters (Accuweather, The Weather Channel, MyForecast.com, Intellicast, and the National Weather Service) including links to their forecasts.
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