Wednesday, February 13, 2013

February 21-23 Potential Significant Winter Storm

The potential for a storm system between the 21st and 23rd continues to unravel as the time draws closer.

Pictured above is the ECMWF model (henceforth called the European model), forecasted for Day 9 (9 days away). The image shows mean sea level pressure values for this system, and the system is defined in the Midwest with a central minimum pressure of just above 1001 millibars. This is not a particularly strong system, but is still worth watching closely, as this model is projecting widespread amounts over 6 inches for many big cities in the Great Lakes and Midwest.


Now pictured above is the GFS Model (henceforth called the American model), also valid for Day 9. This time, however, the American model is projecting mean sea level pressures to be as low as 993 millibars, indicating a very strong storm system. Storm placement is relatively similar, with both models indicating the storm will be somewhere in the central half of Illinois. There is definitely that big strength difference, and this strength difference will eventually determine just where the storm goes and how much snow is produced. The American model prints out nearly a foot of snow in the Upper Midwest in response to this storm system, higher than the European model.


Now we see the American model's ensembles for the same timeframe. Now shown are 850 millibar temperature values and mean sea level pressure contour lines. The ensembles take the system to a very similar placement as the European model; not surprising given that most forecast models tend to follow the European model. Notice how the system is weak and spread out in this forecast. This denotes uncertainty among the models- typically, uncertainty in ensembles is denoted by a very broad low pressure system, and that is what is being observed here. Personally, the addition of this ensemble set to the pack of forecasting systems supporting such a snowstorm enhances confidence that this event will actually happen.

Now shown above are the European model's own ensemble set, forecasting the Day 9 forecast of 850 millibar temperatures and mean sea level pressure contours. The ensembles are slightly further south when matched up with the American model, American ensembles and European model. This is to be expected; all models and ensembles, no matter their forecasting accuracy will have problems in the long range. Nonetheless, its presence in the relative same area as the other models is indeed supportive of this event coming to fruition.

I could go on and on about other models and ensembles supporting such a solution, but we need to take a step back and analyze the atmospheric pattern that will be in place when this storm potentially occurs.

This is the American ensemble forecast, yet again for 9 days away from today. The colors show 500 millibar pressure anomalies, with blues signifying below normal heights (low pressure) and reds depicting above normal heights (high pressure). First and foremost, this system will be originating from the Pacific. If we look at the Pacific ocean, we find a very tight jet stream, shown as the 500mb pressure contours grouped very close together. This tightened jet stream adds increased energy to disturbances that flow through this jet stream. The pieces of energy may start off in the Bering Sea, where persistent low pressure is holding strong in this forecast. As they swing out from the Bering Sea, strong high pressure in the Gulf of Alaska tightens the Pacific Jet stream further. In response to the extreme above normal height anomalies in the Gulf of Alaska, low pressure would be tempted to develop in the Southwest. If we recall Newton's Third Law of Motion, every action produces and equal and opposite reaction. The presence of extreme high pressure in the Pacific will not only enhance low pressure in the Bering Sea, it will also do so in the Southwest. A concern from those two anomalies of pressure is that a Rex Block may set up. A Rex Block occurs when high pressure is stacked north of low pressure. We can see this trying to happen in the West US, with that Pacific high pressure apparently trying to progress into the Pacific Northwest region. Persistent low pressure in the Southwest would finish out the Rex Block. A Rex Block induces a more zonally-oriented (west to east) wind flow that is more favorable for warm temperatures and not big coastal storms. Fortunately, these concerns are alleviated when looking at other ensemble forecasts near this timeframe.


What is apparent is that a negative Pacific North American index (PNA) will be present during this timeframe. A negative PNA acts to produce persistent low pressure in the West US, and this then instigates high pressure formation in the Southeast US. This high pressure formation is a barricade to Nor'easters and any winter weather for the East US. We do see a pretty solid negative PNA in the forecast by February 21st as shown in the image above. If this does happen, we can expect high pressure formation in the Southeast US in response.

However, all hope is not lost for those in the Midwest. Rather, it is enhanced. This is the forecast from several global models and ensembles for the North Atlantic Oscillation (NAO). In the negative NAO, high pressure forms over Greenland and stays there. In response, the jet stream will dip south into portions of the US and produce an environment favorable for coastal storms (aka Nor'easters). We see a fair consensus of a negative NAO during the timeframe of this potential storm. The strength of this negative NAO is TBD, but the general idea of a negative NAO is in place.
The North Atlantic Oscillation then gets more specific with the West-based and East-based phases. The West-based negative NAO means that high pressure over Greenland is centered to the west of that land mass, hence west-based negative NAO. It is this specific phase that is best for Nor'easters. The East-based negative NAO means that high pressure is strongest to the east of Greenland, and this means storms and cold are preferred in the Midwest over the Northeast. Looking over more model forecasts, the negative NAO will be nearly evenly distributed among both regions, but the East-based is definitely a favored phase by the time this storm's timeframe rolls around.

The negative PNA will set off the storm originally going through the Southern US, but high pressure in the Southeast will try to force it north into the Northern Plains, a common storm track this winter. However, the east-based negative NAO will suppress that high pressure to the point that the storm system is able to stay on a more eastward track that then goes through the Midwest and Lower Great Lakes, delivering snow to those areas. For those wondering if the Southeast ridge could disappear and the storm goes up the coast, chances are nil. The negative PNA, having a stronger influence on the US weather as it is upstream from the nation, will make the Southeast ridge prevail to an extent that matches my description in the first few sentences of this paragraph.

Here's my current prognosis:

Andrew

13 comments:

Greg Carbin said...

Very nice update Andrew, however; unfortunately this storm system IS rather strong, with some models bringing the central low pressure down as low as 987mb, which is quite respectable in nature. Being a NWS/SPC meteorologist for 23 years now, systems like this can really bring some big snows, and some serious lake-effect behind it. Would not be surprised, that if these model solutions persist as they are, we could see a swath of a foot or more somewhere, most likely across the southern/central Great Lakes and into the IA/MO regions. This band of 12"+ might be rather narrow, but again, could still happen. Obvisouly it would greatly depend on how it evolves and where it tracks, but as you've noticed, most of the models are favoring a track roughly from southern/central KS to near St. Louis and into central IL/IN. We've seen consistancy which is very interesting. Some of our biggest storms have been the most accurate, even a week ahead of time. Much can happen, but always interesting to monitor the evolution of forecasts of systems like this in an era when models are catching and accurately predicting systems of this type with increased reliability-among the many cases in point, the Feb 2011 blizzard, picked up by model more than a week ahead of time, Superstorm Sandy, accurately predicted more than a week ahead of time and last week's Northeast blizzard--also predicted more than a week ahead of time.

Ray T. said...

So you're best chance for accumulating snow area looks perfect! Keep up the good work my friend. The country, and us here at the Storm Prediction Center greatly appreciate you taking your time to help alert the pulbic on potential hazardous weather conditions. Just ignore the ones who put you down, they have no clue what meteorology is about and what is consists of. Have a wonderful evening!

Anonymous said...

Will the Ohio Valley receive any snow, or is it limited to areas to the west?

Anonymous said...

this is great news!! so could this storm that is crossing kansas before heading to the lakes also be dropping that foot of snow as mentioned above in the first comment
BRING IT ON, WE WILL TAKE IT AND MORE

Anonymous said...

Not gonna happen

Anonymous said...

I find it hard to believe that TWO SPC meteorologists just happened to find this blog at the exact same time. In fact, there was a half hour distance between the comments. Something is sketchy!!!

Andrew said...

Anonymous at 9:51: I can assure you nothing is sketchy- Ray T has commented here for a while now, and has been a great contributor to the discussion. This is Mr Carbin's first comment, with hopefully many more to follow. To both meteorologists: A pleasure to see your comments!

Andrew said...

Anonymous at 8:41: It's still TBD, I will update tomorrow.

Storm-Chaser Wx said...

Wow, if you take a look at the latest gfs and gem 00z runs, we could be talking about the possibility of a storm of the century! If most models are already tuning in on this at this far out, I can't imagine what might actually happen. Keep us informed andrew and keep up the hard work!

Anonymous said...

And yet again the Shenandoah Valley does not get any snow. Oh well, I'm starting to feel that this is not our year....but winter is not over yet! Thanks Andrew for such a in-depth report.

bjenks said...

Looks to be a large storm...also looks like the Ohio River could be possible track which would only give the Lou.KY area cold rain. I hate cold rain. Kentucky (all of KY) is so overdue for a SNOW Storm. We get plenty of Severe storms, but never a SNOW STORM...

Ray T. said...

9:51 reply from last evening:
I and Greg have been in this profession for 20+ years, and have been good friends that long. He works along with me here and I referred him to this page. That is why the two of us have responded.

On the other hand, lets get back to the topic that is SUPPOSED to be on this page, and back to the big storm on the way! The models continue to develop a huge storm, with little change in track from yesterday. The GFS and GEM in particular is very robust about this and we even see smaller-scale low pressure systems around it by Friday. This could be quite concerning. If you recall, the reason the northeast storm was so big is because one large system and a smaller one became two peas in a pod, and cause a real mess. This is a big one coming and need to be watched closely. Anyone living along and north of St. Louis to Ft. Wayne line needs to be on the lookout with this storm!!

Anonymous said...

Do you see any chance for snow over the Cumberland Plateau area in Tennessee out of this one? We will sometimes see snow even if it doesn't appear on the radar I guess due to the upwards flow of the moisture...Thanks for all of your work I really enjoy reading it !! I am just an amateur but I love snow :-) Thx, Elizabeth