Capron & Friday's weathermaker

I just found out I was mispronouncing the town of Capron for this morning's Weather Kid! My apologies to those of you who may have been a bit bugged by that. I'll be saying it correctly in the future! :)

Adam blogged about the Thursday night/Friday weather system yesterday and, since it's pretty much the only upcoming weather of note in our area, I'll discuss it too. He discussed the surface features and the complications with them. I'm going to put my head in the clouds and take a look at the temperatures aloft and why they matter for Thursday night and Friday.

The two images are basically where the two main computer forecast models are showing the freezing line location at 850 millibars, or about 5000 feet up, for Friday morning. The reason I'm looking at the temperature almost a mile up in the atmosphere is that there's a simple rule that we were taught in meteorology classes that relates to it: if the temperature at this height is above freezing, we will not get snow. However, if the temperature is below freezing at this height, any type of precip can fall, depending on the surface and near-surface temperature. Now, your first thought may be that this rule doesn't really help us out that much, but in this springtime scenarios, they're one of the few things that can really help us try to narrow things down.

The left image is what the GFS model is showing. It keeps Rockford below freezing aloft for the entire duration of the event (although a 50 mile shift northward puts us very near or just above freezing aloft). The consequence is that any type of precipitation may fall and that the temperatures at the very lowest levels of the atmosphere will determine what actually hits the ground. Now, the image on the right is what the NAM model is showing, and it keeps the freezing line aloft well north of Rockford until Friday night. If the NAM were to verify, snowfall would be basically nil (especially considering the winter we had).

So where does that leave us? Well, it leaves us in a very tough-to-forecast situation. The GFS has been doing considerably better than the NAM (and its biases easier to figure out) so we've been strongly leaning in its direction on many systems that have come our way. The best prediction we can really make right now is for a wintry mix and try to iron out the details over the next day as the low develops and can actually be "seen" by the observation networks.