Global Forecast System is a weather prediction model maintained by the NCEP. Sometimes referred to as the "American Model". It's forecasts are most accurate in the 0-180hr range. It is widely thought of as the second best model for short-mid range forecasting behind the ECMWF.
A lot of "what ifs" about a system next week it seems, the GFS and Euro have both been showing it with various degrees and types of impacts each run. But the models have been showing it for a few days. Regardless of what it brings I am just looking forward to the warmup.
Thought it was interesting that the 00Z Canadian run had two camps for the weekend storm. Camp A aligns with GFS, camp B is off to the east and includes the operational. I wonder what the 12Z will look like
12GFS showing a fun snow event for this weekend. Working on a member blog discussing the uncertainty. 
southern trend in most models today following the EC's lead. At the surface most models are pretty close to the EC. The ensembles are leaning in that direction. At 850-700mb I see some things that need to be sorted out. Storm is coming on shore now. GFS CMC storm track is actually from Dubuque to Sheboygan, EC slightly south of that. 
Well that was an interesting run of the GFS lol 3.5 days of continuous snow and 23" later. I've seen a lot of crazy stuff that never happens in the 170 hr & beyond range. This is right up there with the best of them. Might have to hold onto this one.
Thought CIPS was interesting today for Friday-Saturday AM analogs. It seems this pattern more times than not produces accumulating snowfall across southeast Wisconsin. Obviously this was based on one run with no consistency. We're playing with fire a little bit. If 00Z models trend west we'll know whats coming. A low pressure track pointed at western Lake Erie is what we're looking for. The 18Z NAM for what it's worth, at the edge of it's range; is encouraging. 18Z GFS slipped east a bit from 12Z but close to NAM.
12Z models in agreement on low track across southwest Wisconsin thru Wednesday. This would bring the most snow across the central or northeast. NAM today showing QPF 0.4-0.8", NAMNEST 0.3-0.7", and GFS at 0.2-0.5". Snow ratios will be in excess of 11-1. I think the GFS is most realistic. The NAM is hinting at the snow bandy characteristics of the event, that some *spots* could be in excess of 7". Question is where??
Interesting discovery on the 06Z GFS for central Wisconsin between midnight and 6AM tomorrow morning. Very strong spike in forcing through the snow growth zone. The result is 3"/hr snowfall rates resulting in 6" of accumulation by day break. THIS IS PROBABLY NOT GOING TO HAPPEN. No other site but KOSH showed this but I wonder what the GFS trying to tell us? I think t-storms moving in from the south, abruptly changing to frozen precipitation over central or southern Wisconsin. The interface between t-storms in the south, snow north, and tanking surface temps; could get real messy early Friday morning. 
00z GFS vs. CMC low-track differences for Friday snowstorm. When you add in NAM & EC, GFS/NAM are on the northern side, EC & Canadian south. I prefer the northern cluster right now which would bring heaviest snows to northwest Wisconsin.
GFS & Canadian generally agree today on the track of a large storm system Tues-Weds next week. Thermal profile between all three major models keeps heaviest snows across northern Wisconsin and upper Michigan despite surface low track through Chicago. Dynamic system that will strengthen quickly as it pulls away, therefore I think the wrap-around will be interesting but it's early.