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The big buzz in our weather world is the late season snowstorm that brings the threat of heavy wet snow to start the weekend. The system is still in the organizing phase but seems to be maturing as expected. There is now high confidence some part of my area will receive 6 inches, potentially more. However, confidence is only moderate in terms of location as differences remain between the U.S. based models and the EURO and GEM which are in a camp further northwest. The 100+ mile spread in tracks could make the difference between some of you shoveling a back breaking snow to little at all.

As for the storm itself, you can see it in the formative stages Thursday night by way of the mid-level water vapor loop. The sub-tropical jet feed off the Pacific is scooping up moisture and thrusting it at the Midwest while the upper air energy churns over the central Rockies. That is a very dynamic looking set-up

By the time we get to late Friday night PWAT's (water vapor) are shown on the EURO reaching 1.20 inches with the deformation zone (snow band) centered on EC Iowa. That's the moisture that would fuel the heavy snow band. Determining where that sets up will be the challenge over the next 24 hours.

For some perspective, that level of moisture is nearly 300 percent of what's normal.

That's about 4 standard deviations above the norm which is quite significant in terms of moisture.

Another hurdle with the system is the speed at which dynamic cooling takes place. Temperatures may be warm enough for precipitation to start as rain but as it intensifies evaporative cooling lowers the freezing level of the column allowing the full transition to snow. The sooner that happens, the more water vapor goes into snow production and the higher the end amounts will be.

At midnight Friday night the freezing line at 850mb 0 degrees C. (the snow threshold) is running from Cedar Rapids to Dubuque. NW of that line it's cold enough for snow, even if surface temperatures are a few degrees above 32.

With the storm intensifying, precipitation falling, and strong vertical velocities in place, 850 temperatures collapse Saturday morning. By daybreak the 0 line is well into central Illinois ensuring the transition to snow in any place where precipitation is falling NW of that line.

In the end though, the track will be the key and all other factors will fall in line around it. One thing that is certain is the fact there will be a sharp cut-off to the NW extent of the snow, wherever that ends up.

I mentioned earlier the GFS was trending NW closer to what the EURO was depicting. It's new run just in slipped a little further southeast. That really throws things into a tizzy, especially considering the latest EURO was spot on with its previous run. The EURO ensembles have been quite consistent for several days and I feel it's closer to reality. Many of its members are clustering around central Illinois as the likely spot for the surface low to track.

The GFS ensembles are significantly further east in NC Indiana. Even if you split the difference you get a track towards Gary, Indiana which is still prime for a heavy snow event in some part of my area.

After scouring over data all day Thursday looking for a clue, the key to success sometimes just doesn't turn the lock. My region is in a spot where there is little room for error in a set-up that's already full of unknown variables. I just can't say yet with great assurance what the outcome will be. It seems some part of my area is in line for 3-5 inches of slushy wet snow. If there is banding or convection involved, amounts could be heavier than that in a narrow band. The question remains where? Models are really throwing out some generous numbers but they can be deceiving in late season storms, especially when transitions are involved and ground temperatures are warm. Unless snow falls heavily, much of the accumulations occur on grassy and elevated surfaces. Additionally the snow melts from the bottom up so it can be hard to get an accurate assessment of how much actually falls due to settling. Needless to say, the harder it snows, the greater the chances for those accumulations of 6 inches or more. Last but not least, if the transition from rain to snow is slower than expected, several inches of accumulation is wasted as rain.

To show you the amount of doubt involved, here is the NWS forecast currently in place. It's what they believe is the most likely outcome based on the data available. There's a lot of range in here showing plenty of uncertainty.

Here's what the worst case scenario is on the NWS model, (a 1 in 10 chance of these high end amounts).

Here's the best case scenario on low end amounts. There's a 9 in 10 chance totals will be heavier than this.



That leads me to the latest models and I stress their solutions are very tentative and just raw model data. They also show some very big numbers in spots which is not reflected in the official forecast above. The NWS is very skeptical of these totals and track or they would have issued a winter storm watch by now. If confidence grows for them overnight that is still possible. Just remember, the totals I show are computer generated with no human intervention. I show them just to give you and idea of how intricate the process of forecasting snow really is. I've been doing this for decades and it never gets easier. Anyway, in my opinion the guidance I'm showing is the worst case scenario which is on the table and unlikely at the high levels shown. Additionally, if the track shifts Friday the amounts all move with it. There is nothing I dislike more than hype but the EURO and Canadian GEM have me concerned this storm could be more significant than many seem to think. I will be very interested to see where models trend Friday at the 11th hour. Here's what guidance suggests.

The deterministic run of the EURO

The EURO ensemble control is a bit further east with multiple members averaged in.

The GFS, well to the east of the EURO

The hi-res Canadian HRDPS

I will close saying this, we had a somewhat similar situation a few weeks ago where the GFS with its southward track actually beat the EURO. That rarely happens and because of that I am hesitant to get fully behind the EURO. However, it's won many battles for me over the years and my loyalty is strong. If I'm going to go down, I'm doing it with a trusted ally. Maybe a compromise works best in the end. Who knows? Expect the unexpected. Stay tuned as Friday should be the day where we see if the king has lost its power. Roll weather...TS


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