Below is a graph of the average monthly occurrences of troughs, left column, and ridges, right column, over the rough extent of the United States. Our dominate monsoonal ridge is well represented in the June-Aug data. July and August also show a well defined separation between troughs off the east and west coasts of the US. A large blocking pattern, like the one associated with the monsoonal ridge, would produce a pattern very similar to this one.
June and September both show a larger extent of both trough and ridge occurrences. Since these months can be are more closely related to the more dynamical Spring and Fall weather patterns of the northern hemisphere than July and August, we could expect to see more transient features and thus a larger area experiencing these features during June and September.
This is pretty close to what I commented on previously. It would be good to count the number of troughs/ridges at each point. This is different than the expression given above. You could count the trough as exceeding the 25 threshold used, give all values exceeding it a value of 1 and others a value of 0. This would allow you to answer the question: What is the probability of a trough off the West Coast in ___?
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