I have lived in the Midwest my entire life. I have spent every New Year's, as far east as Cleveland and as far west as Rockford, in the Midwest. Yesterday (New Years Eve) was rainy and in the 50's. I have seen New year's with bitter cold or heavy snow or something in between. I have never seen rain on New Year's Eve in the Midwest. I think God (if you prefer nature, or the elements, or random that's okay but I prefer God) was shedding tears for the lost year for the 99ers.
Ancient civilizations also saw rain as a symbol of good luck. I like that.