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.

Prognosis

OWL

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.

Meteocentre

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:

PSU

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.

Prediction
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.

Meteocentre

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).

Andrew

Special Forecast Discussion - Christmas Snowstorm Threat

This is a special forecast discussion concerning the Christmas snowstorm threat.

Discussion prepared Monday (12/22), 06z
Purpose of discussion: To analyze the Christmas snowstorm threat


Prognosis
Model guidance continues to have unresolved problems with the upcoming potential of a high-impact Christmas storm system. Solutions range from a major snowstorm impacting Chicago, to snow falling in central and southern Ohio.

OWL

The attached image shows the American GFS model interpretation for the set-up of this storm. In this depiction, we see a positively-tilted trough positioned in the Southern Plains, with ridging set up on either coastline of the United States. The previous expectation saw the northern piece of energy slide north and out of the picture, allowing the southern energy to become negatively-tilted, and turn into a significant storm. Since then, guidance has converged on stronger northern energy, and slower progression of the southern energy. These two factors combine to force the trough to tilt more positively, and restrict strengthening.

Consequentially, precipitation impacts continue to be reduced on incoming model runs, as both the southern energy weakens and slows down, with continued persistence of the strong northern energy. Thus, a strong snowstorm is not necessarily expected at this time. However, should a decent precipitation shield form on the western flank of the low, dynamic cooling aloft may permit accumulating snow in a swath of the Great Lakes. Exactly where this snow may fall is uncertain. For that, we turn to ensemble guidance.

NCEP

Shown above is the ensemble spread track guidance from the government modeling agency. This graphic depicts American GFS model ensemble members, and their individual predictions for where this storm will go. For this discussion, we will only focus on the tracks leading from the Gulf of Mexico on northward. Analyzing this picture closely, you may see a brown line tracking north and east into western Ohio. That brown line is the GFS model itself; all the other green lines are ensemble members. Notice how the ensemble members are predominantly west of the operational model. It appears the ensembles are split into two groups: one that carries the storm in a similar track to the GFS operational model, and one that drags the storm through western Michigan, even through eastern Illinois. This means that we could see the GFS model correct westward with its track. It may very well not happen, as the trend has been for a weaker, slower trough, but it's worth keeping an eye on.

Meteocentre

Briefly adding in other ensemble guidance, we see the CMC ensembles favor a track through eastern Indiana and western Ohio as of their morning forecast run (12z). Since then, the situation has changed as more energy has come onshore, but this westward idea remains something to monitor (as does an eastward correction). It is worth noting that those same 12z CMC ensembles see the storm as being stronger than projected, and just slightly to the west of this afternoon's actual 12z CMC model forecast, shown on that red line above.

Caveats to Forecast
Significant caveats exist with continued ensemble support for a westward shift in the track, but operational guidance's refusal to take such a step. Additional problems arise with significant timing variability among ensemble and operational guidance, as well as temperature profile variability.


Prediction
The continued degradation of the trough by model guidance, as well as slowing progression and strengthening northern energy, ultimately leads me to believe a more eastern track will come to fruition. Ensemble guidance appears to foresee a westward shift in track, though I fail to see the basis for such a solution unless the southern energy ends up stronger and a bit more progressive than recently projected. Additional monitoring of both operational and ensemble guidance is needed, as continued disagreement among individual models, and even among their own ensembles, means only one solution can be right. I can see reason for trough ending up slightly stronger than advertised, and this could bring about a notable shift in track, and I am still struggling with continued ensemble support for a western shift in track, but for now it appears best to go with newly emerging model data.

Favored Track
My favored track as of this writing resembles a CMC/GFS (pink/red lines respectively) track, through western Ohio and central Kentucky. Confidence in this track is low.

NCEP

Next Update
The next update to this special forecast discussion is not currently planned. If an update is needed, one will be provided prior to 11:00 AM Central Time on Monday, December 22.

Andrew