There is a thing call climatology, which basically says: what usually happens weather-wise is what will probably happen.
Lately, what usually happens is that whenever computer models show us getting winter weather, it manages to bust.
Usually this is manifested by a few tantalizing runs of computer models followed by dashed hopes.
In that respect, my confidence that we see winter weather is higher than it has been in 2 years -- the GFS model has been relentless in its depiction of an event. Now the storm is coming into range of the shorter range NAM model and it too shows an event. The only model not on board is the European, which shows our low driving north right over our head -- not a good track for snow.
This is a classic CAD (cold-air damming) event - one key will be the strength of the high pressure to our north that feeds in the cold air which will convert precip to snow. The latest run projected a 1029 mb high, up slightly from the previous run -- anything 1030 or higher is considered a strong high pressure system which can be relied upon for a nice feed of cold, dry air.
Let's compare the GFS to the NAM and see how close they are:
Here is the 0z GFS at in the wee hours of Thursday morning.
And here is the NAM in the same time frame.
Then here is the NAM by dawn Thursday.
You can see the NAM is about 6 hours slower, but depicts the same kind of set up -- low pressure of the La./Miss. Gulf coast with precip starting to ride over the CAD.
Here's the GFS by Thursday AM. Snow is falling with more precip on the way.
So, what can go wrong?
Well, if the low tracks even slightly more north and west, it could scour out our cold wedge of air more quickly and we could transition to rain sooner. Or, the low might divert south and give us lighter precip.
Also, the models have pulled back some on the total precip -- a couple days back more than 1 inch of precip was depicted (using 10-1 ratio, that would have meant 10 inches of snow). Now, we're in more of .4-.8 area of precip.
Still, that would be plenty if even half of that fell as snow.
So, here is the new breakdown:
No precipitation to speak of: 5 percent
All rain: 40 percent
Marginal winter event (dusting of snow/slight ice coating) 25 percent
Minor winter event (1-2 inches of snow/minor ice coating) 18 percent
Moderate winter event (3-5 inches of snow/moderate icing) 10 percent
Major winter event (6 or inches of snow/major icing) 2 percent
I actually dropped the "major" category, but the other "snow" categories go up. We're really getting in a window now where the changes in the modeling should be more subtle, so I feel pretty safe saying there will be SOME sort of system coming out of the Gulf late week.