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The focus of our weather continues to be the pattern change that will bring cold and snow to the region next week. Uncertainty with the ultimate track of a significant storm next Tuesday is giving me and forecasters headaches. Currently, there is about a 300 to 400 mile wide spread in tracks between the ensembles of the EURO and GFS. If you are wondering, ensembles are what the deterministic models are comprised of. They represent a range of solutions regarding track and strength that show possible "future" states of the atmosphere.

Here you can see 25 of the 50 ensembles showing potential snowfall scenarios...all different.

The EURO ensembles are a series of 50 members, plus a control, that are used to present a range of possible outcomes for a given forecast. The EPS is run globally out to 15 days. The ensemble system works by running the same weather model repeatedly, slightly changing the initial conditions each time. Any errors in the model’s initial understanding of the atmosphere become exponentially larger through time, due to the chaotic nature of the atmosphere. Because the model doesn’t know what each and every air molecule is doing at the start of the run, there will inherently be some errors due to an incomplete understanding of what’s currently happening. The ensemble system takes a shotgun approach to this problem by starting the model many times with many different guesses for what the atmosphere could be doing at that time. For example, one run might have slightly higher humidity, while another will set the temperature a bit lower than the “best guess” set of observations.

Here you can see the range in surface low positions produced by all the members for next Tuesday's snow system at noon. That's out 126 hours. Notice the big spread, with one low in Mississippi and another in northern Wisconsin... clear outliers that are very unlikely. Clustering is strongest in central Illinois to WC Indiana, but there is still plenty of variability. As we draw closer to the event, the clustering should tighten up substantially, with members bunched tightly and consistently in the runs just before the storm.

How is the EPS data best used? You don’t look at any one ensemble member and expect a correct forecast. Because each ensemble member is deliberately altered, any one member will be less accurate than the master run. Thus, the EPS data is best taken altogether, as a portfolio of possible forecast outcomes. If the lowest ensemble member forecast for snowfall at your location is 5”, and the highest is 30”, you know that snow is likely, but because of the wide range of possible outcomes, uncertainty is high. Likewise, if all the members forecasts a snowfall between 10 and 14”, the range of possible outcomes is pretty narrow, and you can have high confidence in the forecast. This works for a range of possible outcomes including temperatures, storm tracks, precipitation, and more, all to gauge the uncertainty in many different types of forecasts. The master run is an average of all the possible solutions rolled into one, giving the most likely solution from that individual run of the model.

Here you can see the storm is still out in the NE Pacific Thursday night, not a data rich environment. This is a time when ensembles can produce clues and be of value. Once this energy gets over the mainland, the spread in the ensembles should be diminished later Friday. That should solidify where this storm is going.


Heading into Friday, the storm has the signs of being a good snow producer over some part of the Midwest. As always, the million-dollar question is where? The key factors in favor are:

  • Closed circulation mid-level trough with negative tilt

  • Coupled upper level jet providing vigorous lift

  • A deepening surface low

  • Ample moisture


One thing I think is a major factor and I would like to see more of for snow in my area, is cold air and phasing by way of the northern branch of the polar jet. Without the injection of cold air, the system is already short on that element and intensification that could bring the baroclinic boundary and storm northward is limited. That in itself is likely to be the reason the worst of this system stays east of my area.

Anyway, due to the doubt in the track that still exists, as well as the proximity of the snow band to my area, I'm going to try something a bit different with the existing snowfall forecasts. First, I will show the traditional deterministic outlooks I usually do. These are based on a single solution. Then I will show what the master trend of the ensembles shows. This is the mean of as many as 50 solutions. A summary will follow.

The GFS 6Z deterministic single run through Tuesday.

The GFS ensemble mean "master run"...the average of numerous ensemble solutions is higher, despite being based on the same model.


The deterministic single run of the 0Z EURO.

The EURO 0Z "master run"...the average of 50 ensemble runs is higher than the deterministic single run.


Both the ensembles of the GFS and EURO show higher snow totals than their single run deterministic model output. The reason why is there are still ensembles that are showing tracks further NW that would result in higher amounts. That is in large part due to the distance between now and the actual event. 4 days out, that is not uncommon. As a forecaster, that puts me in a position to determine if the ensembles as a whole have merit. Personally, I think the lack of phasing would rule out the more northerly solutions for heavier snow in my area. Thus, throwing them out, I think the lighter deterministic runs of the GFS and EURO are more accurate. That means the more substantial snows in my region are likely to be SE of the Quad Cities. Even there, amounts may be confined to the 2-5" range, heaviest over the far southeast. Amounts NW of the Quad Cites should remain in the 1 to perhaps 2" category. As I was thinking last night, it's a close but no cigar situation for a major storm in my area. Not to say there won't be some issues, but for January in the Midwest, nothing that can't be handled by slowing down and the use of a little elbow grease.


Whatever happens with Tuesday's system, if you want snow, there are additional chances next week in this energetic pattern. The GFS shows snow nearly every 2 days, the11th, 13th, 15th, and perhaps the 18th. Here's what the deterministic run indicates for snow through January 20th.

Amazingly, the EURO run also shows a snowy scenario but one that's further NW than the GFS

Embedded in the snow intervals is what appears to be a healthy blast of Arctic air sometime around the 17th. This has been consistently hinted at for some time by both the GFS and EURO. If the snow comes as it appears, that will really enhance the potential for harsh cold. Here's what the GFS shows for temperature departures from normal, January 17th. There's a minimum on the lower right-hand corner of the graphic of 47 below normal! Let's hope not.

These are lows in the GFS the morning of January 17th. -30 to -34 is showing up from NE Iowa to SC Minnesota.

Wind chills of 35 to 45 below are common.

Since this is still 10 days out, the severity may not hold in future runs (I hope not). But at least for now, I see how it could happen. Honestly, this is a really remarkable pattern with high impact winter weather on the table January 9th through the 20th. That's all for now. Roll weather...TS



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