Monday, December 22, 2014

Special Forecast Discussion - High-Impact Christmas Snowstorm

This is a special forecast discussion concerning the potential for a high-impact Christmas snowstorm.

Discussion prepared Monday (12/22/14), 22z.
Purpose of discussion: To evaluate the increasing threat of heavy snow immediately prior to Christmas.


The attached image shows 500mb vorticity values over the United States from the American GFS model, valid on Christmas Eve morning. Model projections see a strong trough anchored over the Central US beginning to tilt more neutrally, eventually negatively as the storm pushes east. As the storm strengthens and the trough continues to attempt a negative tilt, the low will shoot northward, bringing the potential for heavy snow across a portion of the western Great Lakes and Midwest. Recent model guidance has favored a slight speeding up in the storm's progression into the Central US, as well as a strengthening of the trough as a whole. This, in turn, leads to a more westward track.

Global model guidance is coming into agreement on this westward track, as ensemble projections finally line up with operational model projections (something that had been listed as a red flag in this morning's special discussion). The thin red lines show individual Canadian model ensemble members, while the blue lines depict individual ensemble members from the American model. The dark blue line signifies the ensemble control forecast off the American suite, while the dark red line does the same for the Canadian suite. All in all, it is clear that model guidance has converged on what may be our final solution. This solution is split into two potential tracks.

The first track is favored by most of the Canadian ensemble members and a slight majority of the American members, and brings the storm through western Indiana and western Michigan. This, in turn, creates an environment favorable for accumulating snow in north-central to north-east Illinois, as well as a bit in southeast Wisconsin. Accumulations on the order of 6" may be anywhere from isolated to widespread, depending on how the warm layer projected to be present a few thousand feet off the ground reacts.
The second track, agreed upon by the remainder of American ensemble members and high-resolution NAM & regional GEM model guidance, takes the low through eastern Illinois. This drops significant snows (on the order of 12"+) on western Illinois to southern Wisconsin, in addition to eastern Iowa. This solution is observed in the latest high-resolution NAM model run below:

I don't have an image to illustrate the first track I described, but you can cut totals above in half and shift the entire band southeast to end just north of Chicago to get a good idea of what that solution entails.

Caveats to Forecast
Model disagreement on track remains a primary concern, though this concern has been somewhat alleviated in recent model runs. Placement of the low to a westward extent is rapidly becoming an important caveat, as the two solutions described above show.

Current prediction from my end is we will see Solution 1/the first track verify, probably nudged west a bit. I feel high resolution guidance and other projections on the western edge of the envelope are too far displaced, especially considering this high resolution guidance is notorious for over-strengthening storms. Also can find reason to lower snow totals from high-res guidance, so that maximum amounts throughout the event may just touch the 10" or 12" benchmark; anything beyond that seems a bit too far-fetched for my liking at this time. NCEP control forecast on the large ensemble image above appears the most reasonable at this time.

Favored Track
My favored track at this time is represented by the pink NCEP ensemble control forecast below.

Next Update
Another update to this special forecast discussion is not currently required. If one is needed in the short term, the next update can be expected prior to 1:30 AM central time (0630z).



Anonymous said...

Hey Andrew
Thanks for your hard work keeping us informed. Im in the Mid-South and its really looking like this winter is a wash. Unfortunately, it looks like NOAA and Accu-weather maybe right for a change with this winter being a warm winter. Nothing is lining up for much cold air this year. Last Christmas temps were 33/13 - this year almost 60 for a high. And no colder air to be found. Maybe Next Year...Merry Christmas

Jason F said...

If your favored track were to come to fruition, would this hit Chicago with the heavy band of snow?

Anonymous said...

What a pathetic winter, this is a far cry from what everyone expected to happen about a month ago. I just don't know what happened to the beautiful winter setup. I'm holding on to hope things will change in January (which probably isn't likely at this point) but who knows maybe we could get lucky. I miss last winter so much.

Anonymous said...

I'm in Chicago. Drive an Evo so I watch this blog during the winter like a hawk. Looks like the summer tires are coming off tomorrow.

Elizabeth said...

Thank you Andrew ...your blog is the BEST ! Keeping my fingers crossed for the pattern shift in late Dec. early Jan. ;it is our only hope of having a decent Winter! Do you subscribe to the view others have espoused concerning the snow amounts in NYC forecasting a snowy winter? The first time I saw this was on Accuweather - just curious!

Anonymous said...

Andrew, I wrote you on another post about the weather forecasts in my day as I became interested in weather in the late 50's and since. This storm to me has been a bust ever since it showed up on the computer models, to me some who were speculating this would represent a substance pattern change also appear to be misguided.

Computers are helpful, but can make a forecaster (speaking in general terms) look bad when they hang their hats on the 12 hour computer updates. If you study the patterns I find it more beneficial. Computers at best should be an alert, not the end all to a forecast. Many have egg on their faces on this storm.

Please keep up the good work, your posts are informative and offer learning opportunities.

Andrew please answer me one question that has bothered me for awhile. When I was in my teens in the early sixties we had storms develop in the four corners region then move to the Texas panhandle then drop to the Gulf, track along the Gulf coast to the east coast then hook up for true Nor'Easters, you don't see that that much today any idea why?.

Anonymous said...

Andrew, looking back through your several posts leading up to this Dec 22nd post about this storm system, it jumps out to me yet again what an excellent teacher you are.
I get to learn not only what looks to be approaching weatherwise but I also get to learn how that forecast was put together, the roles the various global and regional weather features played, and the roles that each of the models and ensembles played in presenting their own mathematical analysis and projections.
Many thanks again!