January 3-5 Potential Winter Storm

It's looking like the next threat of a winter storm will arrive in time for the first few days of the New Year.

TwisterData

On December 31st, a strong trough is expected to drop into the West US and close off, as the circular 500mb geopotential height contours show above. This cyclone closing off in the West slowly drifts eastward, and that's the first thing we must watch for. A common model bias involves guidance closing off these troughs too quickly. For those located in the Plains and Midwest hoping for a snowstorm, you want this trough to close off quickly. A later close-off of the trough would result in a further south track, which might be in the cards of this is an instance of that model bias. For now, however, the trend has been to close off the energy earlier and earlier.

Tropical Tidbits

This image shows the projected 500mb vorticity values for January 3rd, the same type of chart as above. Off of the latest GFS model projection, we can see the energy now located in Indiana, taking on a negative tilt, as the contours seem to be "pushing" in a southeast-ward direction. When the system initially exits the Southwest, it will have a positive tilt. In order for those north of Kentucky to have a shot at a good snowstorm, this storm needs to attain a negative tilt rather quickly, as the GFS has done above.

Tropical Tidbits

The 500mb geopotential height anomaly chart off the ECMWF, valid for January 3rd, shows a similar situation as the GFS is predicting. We see our energy in southern Minnesota and northern Arkansas attaining a negative tilt. Although I don't have access to the smaller-interval charts from the ECMWF, it's not a stretch to assume this storm takes a similar path as the GFS model, as well as the Canadian GGEM model (not shown).

There are a few, rather significant items standing in the way of this being a 'likely' winter storm for the Midwest.

Tropical Tidbits

The image above shows the projection for 500mb geopotential height values (colored regions) and MSLP values (contour lines) for December 29th over the West Pacific. We see a low pressure system has formed on the southeastern coast of Japan, and is moving east-northeastward along the coast. As time progresses, this low will eventually shift out to sea and away from Japan.

Our Christmas Eve storm was expressed well by this Typhoon Rule application, and this storm looks rather similar to the track the storm took in Japan preceding the Christmas Eve event. The only difference here is this time around, the low pressure system drifts out to sea instead of cutting up the eastern coast of Japan. This is a huge red flag, and tells us that this storm may very well go further south and east than current model guidance is suggesting. Given the success this predictor has had in the past few years I've used it, there is reason to believe this system could go further south and east.

Tropical Tidbits

Also of concern is that massive Arctic high pressure system to the north in Canada, pressing southward into the United States. It is well known that model guidance has a bias to be too slow with the progression of Arctic high pressure systems southward. Applying this to our storm here, this means that model guidance might be forecasting this low pressure system to be too far north. Consequentially, this would result in the storm going further south and east.

When you shed all the more 'uncertain' parts to this forecast, you end up with a model consensus favoring a northward track, and the Typhoon Rule in favor of a southward track. Usually, I would go in favor of the Typhoon Rule and predict a further south track, but we have a strong streak of mid-level winds rounding the base of the trough on the latest GFS, which is a big signal for a storm to go negative-tilt, and in this case, go north. Add up all the model biases and predicted phasing of the subtropical & polar jet streams for this event, as well as a suspicious timeframe for the system in Japan, and there's too much uncertainty to decide on one particular track at this time.

To summarize:

- There is the potential for a winter storm to impact the Central and Eastern United States between January 3-5 (good confidence).
- There is too much uncertainty to tell whether this storm will hit the Midwest and Great Lakes, or stay south in the southern Ohio Valley.
- With good Arctic air already in place prior to this storm, accumulating snowfall is a good bet at this time.

Andrew

January to Commence With Wintry Central US, Balmy East

The pattern to kick off January 2015 is looking cold for the central section of the country, while the East looks to bask in warmth.

Tropical Tidbits

The above image shows 500mb geopotential height anomalies, valid for December 28th, when we expect this pattern to start establishing itself. The atmosphere is quickly turning very La Nina-like, as we see atmospheric angular momentum values begin to circulate in anomalously low regions. This La Nina-like set-up will kick off with a strong ridge forming in the Gulf of Alaska, the primary indicator of a negative East Pacific Oscillation (EPO) pattern, which favors cold weather in the United States. Energy coming down the pipe from the Pacific will ride the jet stream and crash into the West US, setting up a nightmare scenario for the East US' cold weather fans.

NOAA

Shown above is the typical jet stream alignment for a La Nina pattern. Notice the aforementioned ridging in the Gulf of Alaska, as well as the polar jet stream arcing over that ridge and then pushing southward into the West. Due to all the energy crashing into the West, we will see a ridge form over the East US, shown by the push north in the polar jet stream over the eastern CONUS in the above image. What ends up happening is a dream scenario for snowstorm fans in the Central US, and a nightmare scenario for all winter weather fans in the East US.

SJSU

Energy that does crash into the West US will have the potential to become a substantial winter storm in the Plains and Midwest. This is due to what looks like phasing of the jet streams over the Central US. Notice how the polar jet stream shifts south from Canada and then mixes in with that other jet stream coming in from Mexico. This phenomenon supports potential strengthening of any storm systems that form and maintain their structure as they eject into the Plains, eventually progressing east-northeast into the Ohio Valley.

To summarize:

- A pattern change is currently in the works, and is expected to take hold in just a couple of days.
- This new pattern resembles a La Nina pattern.
- This pattern will favor cold and possibly snow in the Plains and Great Lakes, as well as very wet conditions in the West.
- Warmth and relative quiet will prevail to end 2014 and start the month of January in the East US.

Andrew

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