As I have cataloged climates of places around the world, starting with the contiguous US, in order to categorize them according to the climate classification system that I have in mind (which I alluded to in my previous post [LINK]), I have noticed that climates in the contiguous US which have wet summer halves of the year & dry winter halves of the year, especially those in the northern Plains states as well as in the state of Montana east of the Rocky Mountains, have dry winter halves of the year because of cold air coming from the seasonal system of high pressure from the settling of cold air over the Great Basin or directly from the settling of cold air to form a system of high pressure over those specific regions. However, these climates have wet summer halves of the year for different reasons in different regions. Again, I am not a trained climatologist or meteorologist; I can't guarantee that this information is accurate, and I can only say that my intuitions seem through my limited understanding to align with superficial aspects of more detailed explanations. Follow the jump to see the explanation.
The whole summer half of the year features warm moist southerly winds coming over the middle longitudes of the contiguous US from over the Gulf of Mexico. This leads to frontal precipitation when the air over land is colder than the air coming from over the Gulf of Mexico & quasi-convective precipitation when the air over land is warmer (but not too much warmer, or else the air coming from over the Gulf of Mexico would stabilize the atmosphere) than the air coming from over the Gulf of Mexico. Precipitation over the northern Plains states is then symmetric over the course of the year about a maximum approximately around the summer solstice.
By contrast, Montana east of the Rocky Mountains is too far poleward (north) & west of the line of southerly winds coming from over the Gulf of Mexico, so Montana generally does not get its precipitation from that source. Instead, its precipitation is concentrated in the spring season, while the summer season is much drier. This is because throughout the summer half of the year, Montana gets mild moist air carried by the prevailing westerlies generated by the subtropical ridge over the Pacific Ocean, as those prevailing westerlies are not blocked by the seasonal system of high pressure in the winter half of the year from the settling of cold air over the Great Basin; the Pacific Ocean at those latitudes is much colder than the Gulf of Mexico throughout the year. In the spring season, there is significant frontal precipitation as the air over land is still colder & drier than the mild moist air carried by the prevailing westerlies generated by the subtropical ridge over the Pacific Ocean. In the summer season, the land & the air over it in Montana have warmed much more, so the mild moist air carried by the prevailing westerlies generated by the subtropical ridge over the Pacific Ocean is much colder and therefore inhibits precipitation by stabilizing the atmosphere.
Essentially, Montana east of the Rocky Mountains features a harsh transitional climate from the climate of northern Idaho in the west to the climate of the northern Plains states to the east. In particular, Montana east of the Rocky Mountains has a wet spring season & dry summer season like northern Idaho for the same reasons as northern Idaho; it does not get the symmetrically wet summer half of the year seen in the northern Plains states because it does not get warm moist southerly winds coming from over the Gulf of Mexico. By contrast, in the winter half of the year, Montana east of the Rocky Mountains along with the northern Plains states experience very cold & dry conditions due to the seasonal system of high pressure shedding cold air clockwise into those areas, while northern Idaho is shielded from this by the Rocky Mountains and can therefore get much more wet winters from the combination of frontal + orographic lifting precipitation coming off of the prevailing westerlies generated by the subtropical ridge over the Pacific Ocean.
