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RIddle me this. What do you get when warm moist air over-runs cold dense arctic air? Warm advection snow! That is the set-up that will lead to our winter storm Friday afternoon and night.The arctic high with its frigid temps is directly over us Thursday. By Friday it advances into the Ohio Valley and the return flow on its backside sends a slug of mild juicy air up and over the chill before it can retreat. Bingo! You have yourself a mess,

Here's the surface parameters showing the south winds surging into the Midwest at noon Friday, about the time precipitation breaks out.

Here's the 850mb jet (5,000 feet just above the surface). A max of 85 mph is pushing from Iowa into Minnesota.

That rapidly transports moisture into the central U.S. Precipitable water vapor is over an inch into central Missouri.

That leads to available water vapor that is more than 300 percent above normal just to the south.

The key to how much snow we get is tied to the 850 temperatures. As long as they are at or below 0 degrees C. (32 F) we are in snow. At 9:00 pm Friday evening the GFS has the 0 (freezing line) extends from Marshalltown to Iowa City and on to Muscatine. Everything north of there should be falling as snow. South of that line you get a mix and eventually a transition to all rain.

The position of the freezing line is the wild card in this forecast. How far north it gets determines who gets all snow. How soon it passes a location determines how much of the moisture that would have been snow is lost to a mix of freezing rain, sleet, or even rain. If the timing is off a couple hours, that can be the difference between gaining or losing 2" of snow. Also, if the line ends up further south or north than projected that can seriously alter amounts as well. How the 850 line evolves is going to be the critical component of how much snow ultimately falls, especially in the southern half of my area...say HWY 30 south.

One thing that is different in this warm air advection event from the one of last weekend is that this time we will have cold air and cold ground temperatures in place at the onset of the storm. That was not the case last time and it led to high levels of ice and rain. In this situation, at least 60 percent of the precipitation should be snow in my southern counties (I-80 south) and more like 80-100 percent HWY 30 north. Cold is gold when it comes to snow.

Here are the most recent snowfall forecasts. This is just raw data and not a hard final call. You can see the differences that exist between the models.I should also point out these are 10:1 ratios. For a time ratios will be greater than that implying higher totals but I prefer to stay conservative at this point. I still like a general range of 3-5". Maybe a few spots in the far south with only 2" and the far north with up to 6".

The GFS 10:1

The EURO 10:1

Back to the issue of higher snow ratios. Just to give you an idea of what they look like. Here is what the EURO snowfall comes out like using the Kuchera method which accounts for ratios that at times are more than 10:1. In Cedar Rapids the total goes from 4.7" to 5.8".

The 12k NAM at 10:1