There's just too much warm air - 02/26/13
That giant sucking sound you might hear coming from the atmosphere over West Michigan today is the air going out of the forecast for 3-6" of snow. Instead, it's looking like maybe 1-2" of snow will fall along/south of I-94 with higher amounts possible along I-96. In the venacular of the operational meteorologist, this forecast would be called a "bust."
So, what is happening to cause the change? The short story: there's just too much warm air being transported into West Michigan along with the moisture. Nothing knocks down snow accumulations like a good rain/snow mix, and it's looking pretty convincing that this system will have too much rain mixing with the snow to come up with 3-6". That's why you'll likely see projections coming down to the 2-4" range -- or even 1-3" -- as the afternoon progresses.
This situation highlights how fine the line can sometimes be when trying to predict what type of precipitation will accompany a winter storm. If the storm track were a little farther south, our atmosphere would remain a little cooler, supporting more snow and less rain, meaning higher accumulations. As it develops this afternoon, it looks like more warm air will be accompanying the system than earlier anticipated. For example, max temps (surface) today along the I-94 corridor are projected to be in the middle 30s, perhaps even upper 30s. Yesterday's model projections ranged from the lower to upper 30s. Temps in the lower to middle 30s would -- obviously -- be much friendlier to accumulating snow than temps in the middle to upper 30s. Because the air will remain colder along the I-96 corridor (as opposed to I-94), the snowfall looks to be a little healthier there (2-4").
All this said, as I'm writing this (12:30 pm), there are reports of rain turning to a wet, heavy snow in northern Illinois/Chicago's western suburbs. I'm still anticipating a burst of wet, heavy snow late this afternoon/this evening, and it's not out of the question that the earlier forecast for heavier accums will verify. However, I'm not optimistic. As I like to say: stay tuned!