Hi radiation1,
The Weather data provider sometimes fluctuates the data that it has. It depends in which City you are residing. As I live in Africa, some cities will display information and some won't. Some will display information today and not have the same city data available tomorrow. My guess is, that if it is a small city/town that does not have constant information flowing in - especially if it is not in America - it will only display the information when it has received it from the weather observers.
Aside: If I am correct, the Weather app from Apple uses Yahoo weather data so downloading Yahoo weather will not cause for any more updates than the Weather app has. Try something like AccuWeather which I've found to almost always have the data for the City I am in at the time.
Hope I helped.