Weekend Update: Here's What We Know So Far

10pm Update: Not going to make too many changes to the going forecast this evening.  Snow totals still look on track, although amounts trending slightly higher are becoming possible.  A Winter Storm Watch will remain in place for N. IL and S. WI and the Winter Storm Warning for NW IL.  Remember to get the latest with Meteorologist Kristin Cwynar Friday morning.

The talk at the grocery store and around town has been the expected snowfall this weekend.  And rightfully so as it will be the first accumulating snow of 2015.

Let's first talk about the watches and warnings in place.  A Winter Storm Warning has been issued for Stephenson, Jo Daviess, Carroll and Whiteside counties.  A Winter Storm Watch has been issued for Green, Rock and Walworth counties in southern Wisconsin and for Winnebago, Boone, McHenry, Ogle, Lee and DeKalb counties in northern Illinois.  Both of which will go into effect beginning Friday at 6pm.  Some of you have asked that since Winnebago County isn't in the warning, if that means we're not going to get that much snow.  The answer to that is no.  The Quad Cities National Weather Service feels a little more confident for the snow to reach their warning criteria.  However, that confidence fades further to the east.  Through tonight and Friday morning, it is possible that the warning will be extended further east.  But right now, we're sticking with the watch.

Now let's talk about storm track and possible snow totals.  I was on two conference calls today with the National Weather Service as we tried to hammer out the specifics with this storm.  The storm in question has now just moved onshore in the Northwest.  Over the past 24 hours, there have been a few small shifts in the storm track Friday night into Saturday.  Most of the model data has shifted the low a little further south, while a couple others have shifted it to the north.  Both of which have different effects on snow accumulations.  With this system being over land for almost 12 hours this evening, our evening and overnight models runs will have a better sampling of data to work with - hopefully fine tuning the track a little more.

But here's where we stand right now.  With the low expected to move pretty much over the heart of the area, highest snow accumulations would be placed in northern Illinois and far southern Wisconsin.  This means snow totals near six inches (or slightly higher) would be very possible.  However, before we start calling for over six inches of snow, I want to discuss some of the limiting factors - at this moment - that we're looking at.  The first is storm track.  There is still time for this low to shift and either move further north or south.  Since we've already seen that happen, I wouldn't be surprised if we see that again.  Also, if the dip in the jet stream isn't as strong as originally forecast this means the low won't be as strong.  Warm ground temperatures are going to play a factor at the onset, but if we get higher snowfall rates during the overnight that will be able to overcome the warmer ground surface.

Also, this is a small scale, fast moving system.  It's kind of like a clipper, but not completely.  Often times weather forecast models have a very hard time keying in on the finer details with these smaller systems.  So those are just a few of the things we're looking at for tonight and are hoping become a little more clear by Friday morning.

Here is what we're thinking/looking at so far regarding timing and preliminary amounts.  Late afternoon into the evening, the snow will start out west of Rockford - closer to the communities near the Mississippi River.  It will likely start off as all snow, maybe a wet mix initially.  As we get closer to 8pm/9pm the transition over to snow will begin towards Rockford.  Before 8pm, little snow accumulation is expected.  The heaviest of the snow is expected to fall overnight Friday into Saturday morning, and that's when most of the accumulation is expected to occur.  By Saturday morning, low pressure will be passing to the east and northeast with the snow gradually ending from west to east by Saturday afternoon.  It will end for areas west of I-39 in the morning and places east of there in the afternoon.

Preliminary snow totals right now have us in the 4" to 6" range, with locally higher amounts.  Keep in mind that this could change closer to Friday evening.  One of my concerns is that the snow to liquid ratio is expected to be low.  Meaning the snow would be more heavy and have more moisture with it.  As it falls, it would likely compact rather than fluff up like dry snow.  Roads will be wet at the onset because the snow would initially melt on contact.  Grassy areas may begin to see the snow accumulate sooner.

I want to stress that there is still time for this to change.  On the higher end, some forecasters are calling for close to 10 inches of snow.  At this moment, I just don't see that happening.  But we'll wait to see how this system evolves over the next 12 hours.