Winter Weather Advisory in effect for Winnebago, Boone, Ogle, Lee, DeKalb and McHenry counties. This replaces the Winter Storm Watch due to the fact this snow will be a prolonged event rather than receiving the snow in a short amount of time. We'll likely see the surrounding areas follow later this afternoon.
NOON UPDATE: Going over some of the morning model runs and the going forecast looks to be on track. The latest NAM has trended a little higher in snow totals while the GFS has trended down some in totals. As was mentioned in the earlier update, the system seems to be slowing some and is a bit stronger in the upper levels. The slowing of the trough may delay the onset of snowfall to our west late tonight/early Thursday morning. The fact that the upper level low will be slower and likely strengthening over northern Illinois may up totals in some spots but will likely pose more of a threat during the afternoon and evening rather than during the early morning. For now I'm keeping the 3"-6" range across north-central Illinois, but it may be extended further west to include all of northwest Illinois.
Snow to liquid ratios will still be a concern because they'll likely be changing throughout the duration of this storm. They still appear to remain on the high side which will create a fluffy snow. Remember...fluffy snow = greater accumulation. There could still be some locations that exceed six inches going into Thursday night. Winds continue to be another concern as the snow will blow around as it falls reducing visibility at times. Winter Storm Watches continue from Thursday going into early Friday morning. The reason why a Winter Storm Watch has been issued is not solely on snow totals. It was a combination of snow, wind and the cold to follow. It's possible that with the later updates from the NWS they may end up calling this a 'high end' Advisory event or even a low end 'Warning' event. This will be determined with the late afternoon updates.
A Winter Storm Watch has been issued for southern Wisconsin and northern Illinois beginning early Thursday morning and lasting through early Friday
A strong cold front will move through later this afternoon bringing with it cloud cover and a northwest wind. The front itself is nearing Des Moines, IA this morning but the snow and gusty winds are behind the front. The reason: a trough of low pressure is currently moving through the Upper Plains. That's why for today all we're really expecting are the clouds and wind shift. Late tonight and Thursday, however, will be a different story.
Temperatures will gradually fall behind the front later this afternoon and then likely take a dive into the 20s after midnight. The system has slowed a little from previous forecasts so I would anticipate a little light snow to move in west/northwest of us after midnight and have more of an impact on southern Wisconsin and northern Illinois perhaps by daybreak Thursday. With the slower upper level low this brings a chance for an all day, light to moderate snow event and with the light, fluffy nature totals may begin to add up.
Total precipitation amounts look to range anywhere between about 0.25" up to 0.50" at most. If we were using a 10:1 snow to liquid ratio that would yield about 2.5" to 5.0" (roughly) of snowfall. Temperatures will be cooling tonight and Thursday with the cold air pouring in behind the front so with the cooler and drier air mass in place the snow to liquid ratios will likely be higher. Say somewhere between 12:1 and 14:1. With that in mind there could be some totals that fluff up to 6" or higher. Gusty northwest winds Thursday afternoon may help to break apart some of the flakes and this may help keep those snow to liquid ratios from getting too out of control.
Average snowfall totals from our weather models this morning were giving about four inches across northern Illinois. Another factor to consider with the amount of snow we'll receive is that the dendritic zone (region in the atmosphere where temperatures range roughly from -12°C to -18°C which can efficiently produce snow) may be deep enough to keep snow production going.
Wind will also be a factor Thursday as a surface low strengthens while it moves into Michigan. Wind gusts could get as high as 35 mph (maybe 40 mph at most) during the day and this will cause reduced visibility when out traveling.
The snowfall forecast is a 'preliminary' forecast as we'll continue to do some fine-tuning with totals later today and tonight. Remember too, any deviation of the low could mean either higher or lower snowfall totals for the area. We'll continue to track this system as it moves through the Midwest today and continue to provide updates right here on the blog as well as facebook and twitter.