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

Christmas Potential High-Impact Winter Storm

Model guidance is still wreaking havoc on forecaster confidence, meaning this post will be laced with numerous caveats.

Instant Weather Maps

The image above shows the 12z GFS model forecast for snowfall totals from this Christmas storm. That thin swath of snow in northwest Indiana into western Michigan is the accumulation from this storm. That's a far cry from the projections of 6"+ earlier this week, even twenty-four hours ago.

This big change came about late last night, when the energy associated with this system finally crashed into North America, and the weather balloon network could take measurements of the system. This process, called 'sampling', as the weather balloons sample what the system is doing and how strong it is, gives weather models more confidence in a solution, as there are no weather balloon launches out in the Pacific.
At least, that's usually how things go. Late last night, when this first sampling took place, model guidance took a significant jump eastward and greatly weakened the overall storm, bringing about minimal snowfall accumulations. The theory among forecasters was that this would be the final solution.

But the drama appears far from over.

Instant Weather Maps

The above image shows the 18z GFS forecast for snow from this storm, the most recent forecast we have to work with. In this projection, the storm jumped west again, like model guidance was favoring about twenty-four hours ago. In this forecast, we see amounts generally of 3-6" draped across east Missouri into west and northern Illinois, with a sliver of southeast Wisconsin getting into the action.

Now's the time when I would give my personal analysis of what will happen here. But to be frank, I have no clue. This is one of the worst performances of model consistency I've seen in my five years of forecasting, if not the worst performance. I'm preparing to go about this storm on a now-casting basis, where the forecast is made as the storm actually happens.

For now, most of the population living in St. Louis on eastward will likely have to contend with travel difficulties, as this storm system still looks to be a strong one. Snow forecasts are useless at this juncture, as it's possible people could get a foot, or could get nothing. On top of that, location of this snow is nearly-impossible to predict at this point in time.

Andrew

Christmas Eve Potentially Significant Snowstorm

Model guidance continues to struggle on the concept of a winter storm around Christmas Eve.

Tropical Tidbits
Click images to enlarge

The image above shows the ECMWF model's projection of 500mb geopotential height values (shaded colors) and mean sea level pressure values (contours) for the morning of Christmas Eve. On this image, we see a deepening low pressure system pushing northward into western Ohio, as a strong trough (seen by the blue colors) begins to negatively tilt (seen in this image as those blue colors "pointing" southeastward) and drag the storm northward.

This forecast solution is one of (quite literally) tens of ideas for this storm that have come out of recent model runs, so individual model projections are being weighted unusually low, due to unusually low confidence. Though I don't have the maps to confirm, this projection from the ECMWF would likely deliver a hefty snowstorm, with strong winds, to portions of the Upper Midwest and Great Lakes, particularly Michigan.

Tropical Tidbits

We now move on to the Canadian GEM model's projection, once again showing 500mb geopotential height values and MSLP contours, and once again valid for the morning of Christmas Eve. In this forecast, the storm system in question becomes negatively tilted quicker than the ECMWF model, which then leads to this more westward solution. Here, the GEM has the storm in central / west-central Indiana, instead of the ECMWF's west-Ohio idea.

Tropical Tidbits

As a consequence of this change, the GEM places a swath of accumulating snow in a north-south orientation, from eastern Wisconsin down through Illinois, barely scraping far western Kentucky. Additional snowfall hits Ohio and Indiana, but the amounts in Illinois are the most significant, where totals surpass the 6" benchmark. Recent model guidance has been looking more into this westward shift, but there's no consensus at this time.

Tropical Tidbits

The last model projection we can analyze, owing to an ongoing massive NOAA data outage, is the Parallel GFS model. Once again, the forecast shows 500mb geopotential height and MSLP values, and this projection is once again valid on the morning of Christmas Eve. This forecast is similar to that of the GEM model, in that we see quicker strengthening of the storm system, leading to a more westward solution.

It should be noted that ensemble guidance is a bit further east of these western tracks.

All in all, we have a very difficult forecast still evolving, which might not be resolved for another few days. For now, those across the entire Central and Eastern US should be prepared for potentially significant weather, especially if you are traveling immediately prior to, or after Christmas.

To summarize:

- Model guidance continues to suggest a powerful storm moving through the Central/Eastern US around Christmas Eve.
- Guidance remains very inconsistent, with little to no consensus currently built around a certain track.
- Regardless of where this storm tracks, significant and adverse weather remains possible, if not likely.
- Travel plans may need to be re-evaluated.

Andrew

Christmas Eve Potentially Significant Winter Storm

I'm watching the increasing potential for a significant winter storm in the Central US, created by what could be one of the strongest low pressure systems in the last few years, if not longer.

Instant Weather Maps

The above image shows 500mb geopotential height contours over the United States from the prestigious ECMWF model, valid for December 23rd. In this image, we see two pieces of energy in the Central US. One is a closed low, placed in the Northern Plains, while the other is a deepening trough in the Southern Plains, shown by the depression in contour lines. In a situation similar to that I described in my December 10th post, we see this trough begin to lift northward into the Central US, but not before the "bomb" goes off...

Instant Weather Maps

Just 24 hours later, on the morning of Christmas Eve, we find that the two pieces of energy have combined, and the trough as a whole has now attained a negative tilt, indicating it has reached its mature phase. As a result, the storm undergoes rapid strengthening, very near the criteria of bombogenesis. Bombogenesis is a meteorological term used to describe the phenomenon when extratropical cyclones rapidly strengthen, and their minimum central pressure values decrease by 24 millibars in a 24 hour period. Switching between December 23rd and 24th, we find that pressure values from the ECMWF lower by about 19.3 millibars, only a little ways off from being a true 'bomb'. Even though this storm doesn't fit the criteria, the strengthening is nothing to shake a stick at.

This solution is a tricky one. I don't have access to pay-to-view weather model graphics (yet), so I cannot see the precipitation pattern from this system. However, from what others are discussing, it appears that not only will the nearly-due-northward movement of this system foul up the precipitation shield, but temperature profiles are above freezing in many spots that would otherwise see snow. This could be placed on model error, or it may be a legitimate forecast. For now, it's just too far out to tell one way or another.

Weather Online

What is interesting, however, is the difference between the ECMWF model and the ECMWF ensembles. The above image shows mean sea level pressure values, valid on Christmas Eve (the same time as the second image we discussed). By simple comparison, note how the ECMWF model places the center of this storm somewhere in southern Ohio, while the ECMWF ensembles put northeastern Indiana in the center of this cyclone.

A further east track of this storm system could result in more of a snow impact to the Great Lakes, instead of primarily wind-driven snow in the Midwest and Ohio Valley from current runs of the ECMWF. But as I said earlier, we are still quite a while away from nailing down these details.

To summarize:

- Model guidance is beginning to sniff out a very strong storm system impacting most of the Central and East US on Christmas Eve into Christmas Day.
- This would severely impact travel.
- Snow would be confined primarily to the north-central Great Lakes into Canada.
- Very high uncertainty still exists.

Andrew