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While there's still some unanswered questions, it still looks like a significant winter storm will impact the central Midwest just in time to ring in the new year. The Weather Prediction Center (WPC) has all of my area under a heavy snow risk January 1st. They also indicate much below normal temperatures approaching from the west.

Currently, WPC shows a large swath from the southern Rockies to the Great Lakes where snow is likely to accumulate.

Here's a tighter view of my area depicting odds of 80-90 percent of an inch or more of snow. Those will go even higher as we get closer to the event (assuming the storm stays on track).

The Winter Storm Severity index has increased to include moderate impact potential over the southern half of my area Saturday.

While it hasn't happened yet, I expect Winter Storm Watches to be issued for at least the southern 2/3rds of my area in the next 24 hours. That said, there have been some important trends develop Wednesday and I will get to them below.

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Before I get to the latest on the storm, there was some snow around Wednesday. As expected it wasn't much in most areas but a band of 1-2" totals was seen extending from near Newton to Iowa City and on to Lowden where Steve Gottschalk reports 2.3" of new powder. As you can see on the map that stayed just NW of the Quad City metro.

The new trend that I'm seeing with Saturday's storm is that Arctic air which we new was going to get involved at some point, seems to be pressing further south altering the baroclinic boundary that the system will follow. That in turn forces the track further south as well. The issue here is that Arctic air is quite dense and often very shallow. Models have troubles dealing with its density and it can bully and ooze its way south by its shear weight. Guidance then has to play catch-up for a period until it gets integrated into the grids. This appears to have happened as models the past 24 hours have shown a noticeable trend to gradually drop the snow band south with each passing run.

The question now is when will this correction end and the southward drift stop? I would hope models are now in the loop and are locking in on solutions. Anyway, I won't speculate anymore. Here's the most recent snowfall output I can give you in this post. REMEMBER, these are not forecasts, just raw model output that we use to access trends. What we hope to see within 24 hours of the event is good consistency regarding the track, snow totals, and the placement of the snow band. Then we can get some hard numbers to throw at you. By the way, most of the snow falls New Years day before tapering off in the evening.

The snow forecasts I'm using are based on the Kuchera method. With an average snow system, 1 inch of liquid is the equivalent of 10 inches of snow. That's a 10:1 ratio. 15:1 gets you 15 inches. Due to the Arctic air in this storm, snow ratios may start at 10:1 but by the time it ends should be 15:1 (or lower) as readings grow colder. You can't bank on a specific ratio to get accurate snow totals unless temperatures are constant. Kuchera gives us a better estimate because it factors in the temperatures the snow is falling into which in this case are much colder to end than to start. Class dismissed! Here you go with snowfall guidance.

The EURO...Of all the models it has shown the biggest decrease in amounts and the most significant shift south on the snow band. That does not mean it's correct but it's a good model and deserves respect.

The GFS... It's similar on amounts in the north but has higher QPF (precip totals) further south. That increases amounts in that part of my area compared to the EURO. It tends to be overzealous so I suspect its high in that respect. Notice too how it's heaviest snowfall is now showing up well into Missouri, SE Iowa, and central Illinois due to colder thermal profiles.

The 12k NAM...What can you say, it nails most of my area.

The Canadian GEM

One thing I would have to say is that at least for now, the overall clustering of the the snow band positioning is pretty consistent which is encouraging. Hopefully no more major shifts from here on out! At this point, I think accumulations should fall into the range of 4-9 inches from north to south. Just an early estimate.

If there's one area of high confidence with this system it's with the cold air that gets drawn into it as it passes. The EURO still shows lows zero or below west of the Mississippi Sunday morning.

Wind chills fall into the range of 15 to 25 below. Those winds could also cause blowing and drifting snow, a factor that will be addressed when we are more certain of the exact track.

Needless to say, Thursday will be a big day in determining what the final impacts from this storm will be. I'll anxiously be awaiting the new pieces of the puzzle. Until then, roll weather...TS


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