December 25-28 Potential Significant Winter Storm

I have upgraded the title of this potential winter storm from 'Winter Storm' to 'Significant Winter Storm'.

**This post is dedicated to Rachel Davino, age 29, a victim of the Connecticut shootings.**

We'll start with today's 12z GFS model and its forecast for the 500mb height layer. This is the forecast for the evening of December 27th. We see a deep depression in the 500mb height layer in the Midwest, signifying our storm system. It looks to be centered in central Illinois. If this looks familiar to you from the upcoming Dec. 19-22 blizzard forecasts, then you're correct- but the buck stops there. There's one thing that I think will make this system fairly big: The Rex Block.

Theweatherprediction.com

This is an image of a Rex Block from the site of theweatherprediction.com. The Rex Block involves a low pressure system in the Southwest US, and a high pressure system in the Pacific Northwest. In response to this highly meridional flow (north to south wind pattern), storm systems in the presence of a Rex Block are strengthened, as shown in this example of a storm system in a Rex Block-turned-Omega Block during May 2005.

While we're on the topic of this storm system in May, let's see if this storm system has any similarities with the GFS forecast above, and the ECMWF forecast below.

Are you surprised? It's almost like a carbon copy of the May 2005 storm is now in this ECMWF forecast for the morning of December 26th. Putting aside the (eventually crucial) time differences, we see the Rex Block has now transitioned to an Omega Block, as was pictured in the May 2005 picture above.

It should be noted that the ECMWF is showing an Omega Block while the GFS is showing a fair Rex Block, with a system in the Southwest and slight ridge in the Pac. NW. The slight difference in these two, as small as it may seem, is actually quite significant, and something we will need to watch later on.

What does this mean? It means that the models are in rare agreement of high pressure setting up on the West Coast, leading to an Omega Block (possibly Rex Block) over the area that then gives the storm system a much more amplified feel. Naturally, as the Omega block/Rex block produces a much more meridional flow, we see the vorticity centers in the low pressure system enhanced by the meridional flow, and suddenly a much stronger storm is born. Another thing interesting is the level of agreement from the ECMWF and GFS on the placement of the storm. Given, there are some time issues that need to be worked out, but it's reassuring to see this storm now showing up on more than one model.

For those wondering what precipitation forecasts are coming out of the models, here's a few images:

Evening of Dec. 27th precipitation forecast. All precip.
above blue line is snow. Multiply value on legend with 10 to get snow.

Andrew

December 19-22 High-Impact Blizzard Event

**This post is dedicated to Rachel Davino, age 29, a victim of the Connecticut shootings.**

I am holding firm with yesterday's call as far as the track goes, but PLEASE DISREGARD THE SNOWFALL FORECAST. I have a new snowfall forecast below. I still expect the system to move from the Plains through the Midwest, most likely through north Indiana and southern Michigan. These areas will have a tougher time getting any precipitation, as dry slotting issues may occur. As the storm strengthens, it will wrap in air through its southern flank in response to very humid air (the precipitation) wrapping around its northern side. Wherever the low pressure system travels, dry slotting could cut down precipitation of any kind.

I'm still holding with my idea of a rain-to-snow situation in much of north Illinois, southern Michigan, Missouri and Kansas. The main snows for these areas will depend on the strength of the backside snow. Models were earlier indicating a heavy band of backside snow coming through the Midwest, and if such a solution happened, a few inches could fall.

The model consensus has aligned itself overnight with my track, leaving me with the logical option to hold this forecast in place.

Snowfall is expected to be heavy, and, combined with winds, will make travel dangerous (see Impact Scale below). I am anticipating amounts of 1 to 3 inches in the light blue colors. This is the tricky part, because this involves the areas right along the rain-snow line, something that troubles may forecasters and such rain-snow line forecasts are prone to 'busting', or failing. I am confident in a good 3-6 inch accumulation in the darker blue, and a 6-12 inch plus snowfall over southern Wisconsin, much of Iowa, extreme northwest Missouri, extreme southeast Nebraska and northeast Kansas. Amounts will increase as you move northeast through this forecast in response to the storm strengthening. Again, such a strengthening storm and the tricky rain snow line makes forecasts difficult to confirm. Watch for amounts over a foot in Wisconsin.

Green: Low Impact
Yellow: Moderate Impact
Red: Heavy Impact
Pink: Major Impact
Blue: Extreme Impact

We here at The Weather Centre want to put out forecasts that best interest you and your safety. For that reason, earlier this year, we developed our Exclusive Impact Scale, where I can identify areas where travel may be hazardous, among other things, depending on the season. My forecast for this system involves a Low Impact forecast for much of the region that will see some precipitation that is not wintry from this system. Low accumulation spots on the order of 1-3 inches then includes the Moderate Impact spot, where some people could see slippery roads and slightly hazardous travel weather. Heavy Impact status is expected in much of the 3-6 inch accumulation stripe, where there will be fair travel delays. Flights will have issues, and driving will be a 'challenge'. Unnecessary travel is not advised. Finally, I hesitantly put out a pink stripe of Major Impact status for the 6 inch+ areas. The only reason I do this is for the wind- very high winds will make whiteout conditions common in rural areas, and long-lasting travel delays will stem from those areas.

And now, our final graphic is for the kids. This is another tool recently developed here at The Weather Centre. I have made a forecast chance for a snow day based off of my previous snow day experiences and putting myself in the situation of a bus driver, administrator and parent, the big question being "Can kids safely get to school?" Although there are 10 categories, DO NOT use them as a 1/10, or 2/10 chance- that is not the intent.

I marked down generally low probabilities for much of the impacted region simply because of the lack of significant snow. I did put a 'Not Very Good' chance for the 3-6 inch range (you may have to adjust it west a tad, or clip off the southern edge of the yellow zone). The Medium forecast is again based off of blizzard-like conditions (or just plain blizzard conditions). I then went ahead and went up to 'Good' for Wisconsin (you should consider it likely). I did not put likely in there without loss of visuals of the counties.

You can also use a formula I made a year or two ago for snow day potential by clicking here.

Hunker down, folks, this one looks to be a nice introduction to winter. I will have more updates on the Facebook page, which you can find on the right-hand sidebar. If you haven't given it a 'Like', go ahead and do it- you can find my latest thoughts not seen here on the blog.

Andrew

December 26-28 Potential Winter Storm

**This post is dedicated to Daniel Barden, age 7, a victim of the Connecticut shootings.**

The potential for a winter storm in the Plains and general East US is present between the 26th and the 28th, as the ECMWF is now showing such a potential.

The above image is for the morning of December 25th. We see our system as a deep depression in the Plains, in a 'U' shaped depression. Such a deep depression gives me three clues: 

One, this system could be pretty strong when it ejects from the system.

Two, the deep-ness of the depression and neutral tilt tells me it could evolve into a closed low.

Three, a cold air outbreak could follow this system if cold air is available up north.

That ridge present in the eastern US displays a sort of positive PNA feel, with the typical positive PNA ridge on the West Coast also in play. This looks to be a Midwest/Plains storm transitioning to the coast, where the Mid-Atlantic and Northeast may be affected. No guarantees, however.

This is the Hour 240 forecast from the ECMWF, once again dealing with 500mb heights. We see that the storm system has now evolved into a very deep closed low, meaning it has been cut off from the jet stream. Closed lows are notorious for staying in place for days on end, but this does not look like it will happen. We have the system squeezed in between two high pressure systems. One is in New England, prohibiting the storm from going directly northeast, and the other ridge is in the Rockies. Considering the ridge in the Rockies looks much stronger, and the angle of the ridge favors a direct east path, I do not find the idea of the Midwest, Ohio Valley and Northeast getting a winter storm totally off the table. This closed low would indeed bring in a lot of cold air to the Midwest and Great Lakes.

The ECMWF also shows a very nice accumulation of snow in the Midwest from this system on the 26th, shown as the colored values above the dark blue 32 degree temperature line:

And the very high amounts of precipitation in the Mid-Atlantic some time later on Dec. 27 tell a possible severe weather story:

All in all, it's looking pretty interesting. While we are still a while away, we are in a negative PNA pattern favorable for an amplified storm track. If a steep negative NAO evolves, we could have a highly meridional flow more conducive for strong storms.

Andrew

December 19-22 High-Impact Blizzard Event

I have once again upgraded the title of this event from 'Potential Blizzard' to 'High Impact Blizzard'. The following represents my truest thoughts and NO hype. All posting from now on is directly related to what you should know to prepare yourself for this high impact blizzard event.

**This post is dedicated to Daniel Barden, age 7, a victim of the Connecticut shootings.**

"This is truly a battle of the models."

What you see above is storm tracks for the future from several models of the 12z suite. The pink lines are from the Canadian model, the brown lines from the United Kingdonm's UKMET model, the asparagus-colored line signifies the NOGAPS model, the neon green shows the NAM model, and the red belongs to the American GFS model. We are looking at the lines that go from Oklahoma to Illinois and Wisconsin- that is our storm.

Recently, the models have been converging on this solution of taking the system northeast into Chicago, possibly just north, giving the biggest snow to north Wisconsin and similar areas. In a bizarre twist, the NAM was the one that started this new north trend, and the storm isn't even inside it's domain!! If the storm isn't inside the domain, it means the model is essentially shooting in the dark- models have no idea what's going on until the system is within the forecast domain. That said, we can (temporarily) render the NAM model (and its ensembles, the SREF) out. Regardless, all of the models mentioned above are in the 'North Camp', where the storm goes into Chicago or nearby areas.

Here's where things get interesting, and this is why this truly is a battle of the models.

ECMWF Hour 72 forecast

ECMWF Ensembles Hour 72 forecast

The ECMWF and ECMWF Ensembles are taking the South Camp at this time. The South Camp involves the system going through southern Illinois, and that is just what the ECMWF/ECMWF Ensembles are going. Now, you may be thinking that I should disregard it just because it's the outlier. However, the ECMWF and ECMWF Ensembles have been incredibly consistent thus far with this system, whereas the GFS has moved north or south by at least 50 miles every run thus far. One must also remember that the ECMWF Ensembles are composed of 51 separate forecasts compared to the meager 16 or so ensemble members in the GFS Ensembles.

This is an image of anomaly correlations at the 500mb level from several global models. In the upper image we have the North Hemisphere, and the South Hemisphere is on the bottom image. On the bottom left we have the correlation statistics, and if we look closely, we see the numbers. The ECMWF beats out the GFS easily with 0.926 correlation on the ECMWF to 0.901 correlation on the GFS. The UKMET ties with the GFS. That said, there is reason to believe the ECMWF- something many are not doing at the moment. The ECMWF has historically done well with winter storms and is known for Catching onto solutions other models have not, so such a situation playing out here is not out of the question.

Analogs of situations similar to this one can also be helpful. This is a 72-hour snowfall accumulation of the top-ranked analog for this storm system. It was in 1996, and heavy snow fell in the Upper Midwest, something that would suggest a North Camp solution. Analogs can be deceiving, but it is worth mentioning a negative PNA and negative NAO were present for this storm system. I can't say I don't believe this analog could repeat itself this time around.

Frankly, it comes down to which is more significant at the moment- the consensus or the outlier. On one hand, we have several semi-reliable global models going for one solution, with one model and one ensemble set (both extremely reliable) going for another solution.

I'm not willing to pick a forecast preference, because this is an epic battle I feel will need to be settled in the day or two away from the system's impacts. However, no matter the track, there will be heavy snow and very high winds- such a combination would indeed produce blizzard conditions. I will fill you in more tomorrow morning and through the evening as more information becomes available.

Here's my new call for a forecast graphic:

Forecast Preference: Indifferent To All
Confidence: N/A

Andrew

December 19-22 Potential Blizzard Event (Updated 5:30 PM)

**This post is dedicated to Charlotte Bacon, age 6, a victim of the Connecticut shootings.**

This is an update to this morning's original blizzard post, which can be found by clicking here.

This update concerns the new models from the 12z suite, and not so much for teleconnections, which were covered in the morning post. Also, I have now established the two 'camps' for the models. The 'North Camp' involves models that take the system into north Illinois at Hour 96, and the South Camp has the system in central/southern Illinois at Hour 96.

We start with the GFS model, known as one of the better forecasting models that is out there today. The GFS is forecasting the storm system to be in north Indiana on the morning of December 20th. The solid blue line is the rain/snow line- any precipitation to the north of this line is theoretically snow, and precipitation to the south is rain. The storm system has a central minimum pressure of 994 millibars, meaning it is fairly strong. The forecast calls for snow to be ongoing in Illinois, Wisconsin and Michigan, with a potential severe weather situation ongoing in Kentucky and Tennessee. I will not make an assumption on if the GFS is right of wrong until I review the other models below. Needless to say, the GFS is in the North Camp.

This is the European's ECMWF model forecast, once again valid for the morning of December 20th. The ECMWF is widely regarded as the best forecasting model used for daily weather forecasting, like this. The model predicts that the storm system will actually be in south Illinois at this time, thus being further south and slower than the GFS. The central minimum pressure is down to 995 millibars, roughly the same strength as the GFS. The ECMWF solution has shifted south from recent forecasts, not a surprising thing to happen in the winter. I am cautiously optimistic with this forecast, as I think the teleconnections fit in nicely here, but models are notorious for shifting south in this time period, only to shift back north later on. The ECMWF is in the South Camp at this time.

This is the forecast from the ECMWF Ensembles, also called the ECMWF EPS. The ECMWF EPS can be thought of 51 separate ECMWF model forecasts, each with slightly different starting parameters. This forecast, once again valid for the morning of December 20th, has the storm system in southern Illinois, actually a tad farther south than the ECMWF model itself. The minimum strength is at 1002 millibars, meaning it is fairly weak. However, there is something slightly more significant. The fact that an average of 51 different forecasts, all originally based off of the best forecasting model known to man, came up with such a solid solution is very supportive at this point for the South Camp.

The current overview of the models is this:

North Camp: GFS, GFS ENS (2 models)
South Camp: ECMWF, ECMWF EPS (2 models)

We now move to the Canadian's GGEM model. It shows the storm system in central Illinois with a central pressure of 1000 millibars. The blue rain/snow line remains in North Illinois, but far enough south for cities like Des Moines, Chicago and Milwaukee to get snow at this point. Considering the GGEM is taking a cold solution to the mentioned cities, I feel like it belongs in the South Camp.

This is the forecast for precipitation, 850mb temperatures and the freezing line from the GGEM Ensembles (GGEM ENS). Although the Canadian model is not the best, ensembles are always better than models. The GGEM ENS believe that a solution not all that different from the ECMWF will evolve, with the storm system taking a path through south Illinois and snow given to the previously mentioned cities. Heavy rain in TN/KY suggests a possible linear storm event could be ongoing. This is indeed a South Camp storm.

Another model overview at this point in time tells us the following:

North Camp: GFS, GFS ENS (2 models)
South Camp: ECMWF, ECMWF EPS, GGEM, GGEM ENS. (4 models)

This is the image of the GFS Ensembles. Yes, I know the ensembles were already listed in the North Camp, this is just for you all to see what it says. The GFS Ensembles (GFS ENS or GEFS) shows a major snow event in much of the Midwest after the storm system blows through. Rain is ongoing in southeast Michigan, and again that linear storm threat appears with the states of Kentucky and Tennessee.

Here's the final total after reviewing some other models:


North Camp: GFS, GFS ENS, JMA NOGAPS ENS,  (4 models)
South Camp: ECMWF, ECMWF EPS, GGEM, GGEM ENS, FIM. (5 models)

Analysis: This is not as difficult as you may think. Despite the input of 9 models and ensembles, only a select few are worth making note of. The GFS/GFS ENS should be watched carefully, as should the ECMWF/ECMWF EPS. The JMA and NOGAPS ENS are worthless; I threw them in there for some diversity. As for the FIM, it has historically verified slightly better than the GFS, so it can be taken note of. I would watch the Canadian model and ensemble set, but not as closely as the American and European sets.

Forecast Preference: ECMWF/ECMWF EPS
Confidence: 60%

Snow amounts, totals and eventual track will need to be determined at later dates, so please do not ask me for snow totals for your city, because I just don't know yet.

Andrew