Sunday, February 17, 2013

February 21-23 High-Impact Significant Winter Storm

Have removed the 'Potential' from the storm title, as the event is within the timeframe of reasonable certainty.

Satellite imagery shows the storm system of interest is in the Gulf of Alaska and nearly onshore in Alaska at this time. Analysis of the infrared field determines that the center of low pressure is on the coast of Alaska, with a very ragged comma shape still present in association with this storm. Surface analysis indicates the storm is in the 990 millibar range, so it is a fairly decent storm at this point in time.

Model uncertainty still remains a priority to deal with at this time, as it does appear there are two possible solutions evolving. Shown above are 500 millibar height anomalies for Hour 120, or 5 days away. Cold colors suggest the presence of low pressure, while warm colors favor areas of high pressure. The bottom image shows 850 millibar temperatures and mean sea level pressure for Hour 132, meaning it is 5 days and 12 hours away. This model is the North American Ensemble Forecasting System (NAEFS), a set of ensembles which I understand is derived from the Canadian and American ensemble forecasting systems. The NAEFS has done very well in the past two years for predicting temperature anomalies during the winter, this will be my first time seeing how it fares with storm systems. The NAEFS is predicting the storm system to ride east-northeast through the Plains and into the Midwest. This is not what recent model trends have been showing- the latest forecast models have taken it further north.

The issue with the models is how strong that high pressure system will be in both Canada and the East US. If we look at the top image of the NAEFS, we can see that the high pressure system is strongest from Canada into Greenland. Models and ensembles differ as to where the center of this high pressure system will be. The NAEFS proposes the high pressure system to be centered in north Canada, which then allows the storm system to progress further east and into the Midwest. The European model, as you will see below, takes a much different track.

This is the most recent forecast from the typically-reliable European model. It is called the ECMWF model in the weather world, but for practical purposes we will refer to it as the European model. This forecast also has two images: the top image once again shows 500 millibar height anomalies, while the bottom image depicts 850 millibar temperatures and mean sea level pressure values. The 500 millibar chart is valid for Day 5 (Hour 120), while the bottom image of mean sea level pressure is for Hour 144, or 6 days away. If we do a comparison between the NAEFS and this European model, it's pretty obvious that the European model is stronger with the high pressure in the East than the NAEFS. We see more oranges stretching down south from Canada and into the Eastern US than the ensemble system. The reason for this is a much weaker connection between the high pressure in Greenland and high pressure in Canada. Almost like a bridge, the European model nearly breaks off the connection between these two masses of high pressure, which would allow the Canadian high pressure system to then push further south and allow the European solution to prevail. If the European model solution did prevail, one would expect lower snow totals with any snow further to the north. Quite an odd solution in my opinion, and we will evaluate it further later on in this post.

Something that needs to be brought up is that strong high pressure system in the Pacific. That big high pressure system, combined with the low pressure in the West US creates what's called a negative Pacific North American index, or a negative PNA. The low height anomalies in the West provoke above normal height anomalies (high pressure) to arise in the East US, which typically can push storms into the North Plains in a similar fashion to the European model. The NAEFS ensemble system also has this high pressure system in the Pacific, but is more widespread than the European model. The more widespread it is, the stronger the high pressure is. As a result of a stronger high pressure, the 500mb heights must become lower and this would push the storm track further to the south. However, we now go back to the previously-mentioned high pressure over Greenland. When high pressure is established to the east of Greenland, it is called and East-based Negative NAO; 'East based' for being east of Greenland, and the negative NAO (North Atlantic Oscillation) implies high pressure over Greenland. The negative NAO provokes the subtropical jet stream to become more active and pushes the overall storm track south. It also suppresses the high pressure in the Southeast, which is why I'm not confident in the north-bound European model. Both images above show the east-based negative NAO, but both have different solutions. It all comes down to which factor has the stronger influence. Because the negative PNA is upstream of the Midwest, it will have a stronger effect than the negative NAO, which is far up in Greenland. However, by the time the storm system is able to reach the Plains, the negative NAO will hold some influence and ought to be able to keep the storm system south, definitely further south than the European model's solution.

I can understand why there have been trends north. Shown above are 500 millibar heights (not anomalies, just the regular values) valid for Hour 96 (4 days away). We can see our low pressure system just beginning to emerge from the Rockies. The issue from here is where it comes out of the Rockies. The NAEFS forecast in this image has the low pressure system in far southeast Colorado. When you get storms coming from that area, there is a decent possibility that high pressure could push it north, and I believe this is where the European model is getting the idea of a northward trend from. Additionally, the storm will be reaching what's called a negative tilt by the time it is exiting Colorado. A negative tilt implies that a storm system's highest vorticity values are pointing towards the southeast, and this is where we find the big difference between the southern solution (NAEFS forecast) and the northern solution (European model).

I put together the NAEFS and ECMWF (European) models together for the same timeframe. They are both forecasting 500 millibar values for 4 days away. The black line illustrates the tilt of our storm system, while the red line illustrates the tilt of the high pressure system in the Central US that is preceding this storm. The issue between the northern and southern solutions is that the European model wants to bring the system to a negative tilt faster than the NAEFS ensembles do. This is shown by the ECMWF image's black line tilted to the southeast, while the NAEFS is more neutral-tilted, meaning the highest vorticity centers are not tilted in any particular direction. In response to the negative tilt by the European model, the storm system digs to the Southeast more. This situation then responds similar to if you were to push some form of a wedge (shovel, for example) into the ground- the shovel (storm system) tries to dig down further south, but the ground (high pressure system) is provoked to move north in response, and this is how the European model gets its northern solution.

Considering there are three different ensemble systems rooting for a southern solution and just one model and one ensemble system rooting for a northern solution (the most recent American model jumped south a bit), I'm going to have to go with the southern solution. I am a bit hesitant, because the European model and its ensemble set are very prestigious and reliable forecasting systems, so it's a very tough forecast to make when it's essentially Europe vs. the world.

Despite this, I am willing to stick with my prediction that was made yesterday for this system. To me, there are three stages of the models. The first is the Long Range track, where the models get a consensus roughly 10 days away from the event. The second stage is what I like to call the '60 Hours of Chaos', where the storm gets within 8 to 5 days away (Hours 180-120 on the models) and the models go in every which way. The third and final stage is the final track, where the models converge on a solution within 5 days of the storm. This final track can sometimes be nearly the same as the first Long Range track. We already observed the first stage, and are now clearly in the second stage. The system will get on land in the next few days, so model solutions should clear up as the time comes closer. I should mention that the Chicagoland area into southern Wisconsin and north Indiana and southwest Michigan may see that 'Accumulation Possible' dropped if dry air manages to start that region with virga (precipitation is being produced, but evaporates in a dry layer of air before it hits the ground) as some sources are suggesting. There will be a lot of dry air in place just a day or two before the event, and this could very well hinder the already-lowered totals. I will most likely have my first accumulation map out either tomorrow or Tuesday; confidence remains low.