Having just had a CSA army stuck (and then unsupplied) in the muds of New Mexico from Feb to April, it occurs to me that the game designers don't have a clue about the actual probabilities of bad weather in the U.S. And what is sad is that long-term monthly data on average temperatures and rain are so readily available on the internet for every area on the map by just choosing a nearby town or city.
Generally speaking, the desert southwest (such as southern Arizona and New Mexico) gets most of its rain in the summer during the summer monsoon. The winters are cooler and comparatively dry (about 3 cm/month in the Tucson area), meaning that is when most fighting should occur. And as a rule, because desert soils are so sandy, the hard-packed roads were routed in a way that mud is only an issue where they cross the few flowing streams in the region (which are often hundreds of km apart), or pass by a notoriously muddy formation such as the Mancos shale. In my opinion, operations in the desert southwest should never be affected by weather, unless it is the extreme heat of the summer.
It is also shocking to me how often blizzards hit the prairies in April, and how often Texas gets harsh snow conditions. Sure, a couple of cm of snow can fall in Texas in the winter, but it usually melts and is gone the next day or two. It certainly wouldn't seriously affect military or supply operations in the region over a two week period. In my opinion, severe thunderstorms were a much bigger factor in the Civil War, such as at Shiloh, and could be modeled as a random possibility at the battle level based on the region and time of year.
Will someone please look at monthly averages of rain/snow for each area or state and change the probabilities of the weather types to more realistic values.