I am finding the record to be surprisingly silent regarding the weather in Missouri for that general time period (notwithstanding the above-referenced Linn County History report, which did not say what the temperature was--only that there was snow on the ground in Carroll County, and the Missouri River was froze).

In looking through the O.R., there are a number of reports from troops in states surrounding Missouri about how awful cold it was, but I didn't run across any reports coming out of Missouri mentioning weather. The closest one was in regard to a hunt going on around Howard County, with it being reported that the absence of snow was making it difficult to track the bushwhackers (had it melted, or had it just not snowed there for a while? Take your pick.).

I went to the State Historical Society's website and, after much effort, was able to do a search of of the St. Louis Republican for the period in question (my god, does anybody else find the search engine for that site to be breathtakingly difficult to use? I cringe at the thought of trying to use it). Anyway, I was finally able to get to the issue dates in question, and using keywords of temperature, cold, warm, and snow, failed to come up with anything that would shed any light on the weather for the time period in Missouri. There were reports regarding weather out on the plains and in Tennessee, Kentucky, Indiana and Pennsylvania, but I didn't come up with anything for Missouri.

All of which would seem to suggest that the Missouri weather was non-newsworthy--which would tend to lend credence to your theory that it might have been balmy enough to bring the partisans out. Coupled with the still-frozen Missouri River giving them free reign to move back a forth and you might very well have the recipe for an upswing in guerrilla activity.