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

Atmospheric Trifecta Preparing to Deliver Cold, Snowy January

A trio of atmospheric signals are gearing up for what could be a rather cold, snowy January.

Research I completed last night showed significant (10"+) snowstorms in the Midwest are most favored under the negative phase of the Arctic Oscillation, the negative phase of the East Pacific Oscillation, the negative phase of the North Atlantic Oscillation, and the positive phase of the Pacific-North American index. We look to have at least three of these factors locking down the atmosphere to round out December and kick off 2015.

Tropical Tidbits

The image above shows the GFS ensembles' forecasted 500mb geopotential height anomalies over the Northern Hemisphere, valid on the evening of December 26th. On this graphic, we see a prevailing negative EPO, as highlighted in this graphic with ridging over the Gulf of Alaska. A negative NAO has also emerged just west of Greenland, combining with ridging in Eurasia to provoke a negative AO regime. I'm watching for a positive PNA signal, but as a ridge is passing over the region, I'm content on acknowledging a trifecta for now.

It doesn't end there, however...

Tropical Tidbits

By December 30th, we find more than a few big developments have taken place. We see our negative EPO ridge has blossomed into a massive swath of high pressure, now extending along the West Coast into Alaska... and then some. Ridging over Greenland still exists, paving the way for a continued negative NAO and negative AO tandem. As a result, we continue to observe below-normal anomalies in the East US, signaling a very cold period to end December and start January.

My thinking is we'll see those below-normal height anomalies slowly progress west, as a west-based negative NAO (where the ridge is displaced to the west of Greenland) typically favors cold weather more into the Central US. This would be amplified/supported by the strong -EPO ridge, slowly evolving into a +PNA signal, it appears.

Now, onto the snow...

Tropical Tidbits

The image above shows the GFS ensembles projection on December 28th, showing jet stream wind speed values. There are a few things to note here.
First off, check out the extended Pacific jet stream. We're seeing that occur in the very near future, and it's no coincidence that this extended jet stream into North America is happening at the same time some storm threats are arising. The extended jet allows for cyclogenesis in the Northern Pacific, where energy may be shunted east into the United States, leading to those winter storm chances.
Second, we see a notable subtropical jet stream (STJ), as depicted by the green colors extending from the northeast Pacific into Mexico and the South US. With a negative NAO regime in place, as well as an enhanced subtropical jet stream, it's possible the East Coast could get rocking and rolling with these storm chances.

As for the Central US, I alluded to the below normal height anomalies pushing east. As far as snowfall impacts, clipper systems look to make their return, as the strong ridge pushing into Alaska produces a sustained northwest flow pattern (where the winds are out of the northwest). This could lead to not only episodes of snow in the North US, but also lake effect snow episodes, as all Great Lakes are currently well below last year's ice levels at this same time.

To summarize:

- The atmosphere is preparing to shift from a generally mild December pattern to a rather harsh January pattern.
- Sustained cold weather is likely for most of the nation east of the Rockies.
- Enhanced chances of snowstorms will be seen both along the East Coast, into the Central US (depending on individual storm tracks, of course).
- Buckle up, things are about to get fun.

Andrew

December 19-21 Potential Winter Storm

Model guidance is supporting the risk of a winter storm in the December 19-21 timeframe.
I'm trying to decide how to start off this post, since simply going through model guidance won't work. I'll begin by explaining each graphic, doing a compare/contrast as we do so.

Tropical Tidbits

The latest GFS model has a trough pushing into the Midwest on December 20th, as the blue colors and depression of contour lines shows. This trough is beginning to close off after becoming negatively tilted, and is pushing northward as a result. The surface low takes a track through the Ohio Valley on this run, dropping appreciable snows from east Kansas into southern Michigan. North Missouri sees snow over 6" from this system.

In this overview of the pattern, we can diagnose a few items arguing for this more inland track, as future guidance I'll show you will depict an East US track. First of all, we have another strong trough dropping into the West US, trying to work its way southward. This will try and force a ridge to develop in the central and eastern Rocky Mountains, as we can already see above. However, as we see a deep upper level low over Greenland, this ridge won't be able to exert too much influence (that ULL over Greenland defines the positive phase of the North Atlantic Oscillation/NAO, notorious for keeping the jet stream very 'non-wavy')
What does make a ridge form, however, is the troughing in the Bering Sea into the Gulf of Alaska, a textbook positive East Pacific Oscillation (EPO) pattern, which will act to limit cold air reserves in Canada, but also try to direct storm systems northward. The latter influence is seen well in that ridge centered over New York, just east of the storm system in question. That same northward influence could happen along the East Coast, but for now, the GFS favors this solution.

Tropical Tidbits

Here's the ECMWF 500mb geopotential height anomaly forecast, valid at the same time as the GFS graphic. We see more than a couple significant differences here.

First and foremost, the storm system in question seems to be developing a second piece of energy, signified by that second dip in the contour lines along the Gulf Coast. Looking at the forecast following this timeframe. it looks as if that second piece of energy will act to pull the main trough east, and develop it into an East Coast system. This is far different from the GFS, which maintains a single trough.
Additionally, we see the entrance time of the second trough into Western North America has been slowed by about a day. This allows the ridge in the central and eastern Rockies to flourish well into Canada, where the ridge up there is also quite a bit stronger than its GFS counterpart. Consequentially, the storm system is suppressed to the south in the ECMWF forecast. Also, note the lack of a ridge to the east of the storm system in the ECMWF image, compared to the GFS. This, combined with the extra piece of energy along the Gulf Coast, appears to favor an East Coast solution. The solution results in this pattern, valid 24 hours after the image above:

Tropical Tidbits

Purely for comparison, here's the GFS snowfall forecast I mentioned earlier, expressing the solution in opposition to the ECMWF:

Instant Weather Maps

I've been looking back and forth between the ECMWF and GFS images I've shown above, trying to figure out which one I think is the most valid, and I can't decide.

On one hand, the ECMWF projection appears to be obeying the positive EPO signal, as exhibited by troughing along the west coast and a strong ridge in central Canada.

On the other hand, the GFS is doing well with the emergence of a ridge just east of the trough, possibly as a result of the negative PNA orientation out west, as the aforementioned second storm system drops into the West.

It will ultimately depend on the timing of when the trough drops into the West, just how strongly the atmosphere responds to the +EPO signal, and (of course) if that secondary piece of energy forms along the Gulf Coast. It's worth noting none of the model guidance is having any consistency with the ridge in Canada, purely by looking at run-by-run comparisons. Additionally, today's 12z ECMWF run (examined above) is the first one to have that secondary piece of energy develop to the south of the storm, something that does not bode well for any consistency that either the GFS or ECMWF may have built up. I'll pass on giving my opinion right now, because this is a truly grotesque set-up.

To summarize:

- A winter storm is possible for the Central or East US in the December 19-21 timeframe.
- Model guidance is expressing little to no consistency on a defined track for this storm.
- Cold air availability will eventually become a concern.
- Anomalously low confidence exists.

Andrew