Since there are historical rainfall records for most, if not all, US cities and if they are daily records, can't the calculation of probability work? Using the
Average Number of Days With Precipitation numbers for June from Weatherbase.com for Cleveland & L.A. I figure the following: For instance, in Cleveland the average for June was 11.1 days of rain. Therefore the probably of rain was 37%, 11.1 divided by 30. For L.A. 1 day in June the probably chance of rain for a single random day would be 1 divided by 30 or 3.3%. Those percentages are higher than what is in games weather details. I don't know what impact they would have on games. Also, if you could find the actually number of days of rain for each city by year, you could refine it more. I have search for that detailed data here on the interweb but have not yet found it.
Travel Weather Averages (Weatherbase)
How to Calculate Probability (with Cheat Sheets) - wikiHow