New Years Eve 2010 – A Midwestern Severe Event in the Making?
Severe weather is even more uncommon in the winter than it is any other season. This is not a news flash to anyone. What is generally a news flash is when it does decide to occur. Yet again another big storm is forecast to make its presence known just in time for the upcoming holiday. Interestingly enough, this system has some similarities to some of those rare winter severe weather events in Illinois, could this be another? Lets take a look.
Using 2 recent events, January 7th 2008 and December 27th also in 2008. We will compare what was going on then and what is forecast to happen December 31st, 2010 also known as new years eve. As mentioned before these setups are quite difficult to come by in the winter. Since there are only a few events to study and climatology does not favor severe weather, this adds an extra hindrance to the already difficult task of trying to predict these events. Lets take a look at the synoptic features for both events and see what, if any clues we may be able to pick out.
On both days one can clearly see a few things. Most importantly are the conditions ahead of the obvious cold fronts. If you look at the observations in IL you can see abnormally high temperature and dewpoint readings, especially on January 7th with surface temps in the mid 60s approaching 70 across IL. Both days however featured dewpoints in the mid 50s, something to keep in mind when we look at the forecast later. Lets take a peak at the 500mb chart now.
Both days here are showing a troughing pattern with intense jet streaks. This type of jet stream setup is one of the key ingredients for severe weather. We can see jet streaks in the 90-120kt range on both days. This would indicate the potential for impressive speed shear, yet a fast moving storm system. Now, one of the main reasons we don’t get the severe weather as often in the winter is the lack of energy. Heat from the sun is another key ingredient to severe weather and as we all know in the winter it is pretty cold. Some are always constantly hounding about lack of something called CAPE and thus severe weather not being possible. In the severe weather world the general threshold for CAPE leading to severe weather and tornadoes is considered to be about 1000. If you have no idea what any of that means…its basically the energy in the environment. You can’t have a storm without energy so the more the better. Taking a look at the CAPE values found on both days is a bit interesting.
On both days note where the strongest CAPE values are. We can see areas of 500 CAPE shown on the chart above. This is generally considered to be below the severe weather threshold, so why is it worth noting? Low CAPE events are tough to understand. It can likely be explained by the presence of the really strong jet streaks and associated shear as noted above. Sometimes this exceptionally strong shear can compensate for the lack of significant CAPE. I like to call such setups “Insane-O shear setups” and many times this is what winter events end up being. They are more driven by the strong shear, and need very little energy to produce severe weather. If we can get enough energy to spark and maintain a storm for just a little while, they can be surprisingly strong with such strong shear. Finally, the storm reports, notice the location of the tornado reports and compare with the charts above.
So what do we have coming into our future and why is there the potential for a round of severe weather in the same areas? Well lets take a look at some charts below. We’ve got the Strong 500mb jet streaks as shown by both the GFS model which resembles the January 7th event.
Both the GFS and the NAM are showing mid 50s dewpoints advecting into the area ahead of a sharp cold front. 
This is an eyebrow raiser given the time of year. Since moisture is one of several contributing factors to the ever important CAPE, each model is also showing cape values approaching the 500 level and even higher. 
Given everything shown above, if this forecast were to verify and based off past events it would seem reasonable to assume that some severe weather could indeed be possible across the area on Friday, December 31st. Now, there are many forces acting against the scenario shown above. Currently there is a respectable snowpack across the area. This could hinder the quality of moisture return. The fronts could either slow down, speed up, or the surface low could take an entirely different track than what is shown above.
One thing forecasters do which is probably the most important thing is to compare what is being shown by models to what is actually happening. Will these mid 50s dewpoints go as far north as expected? Observing current observations can sometimes offer insight. 
Looking at the graph above one can note deep moisture already in place over east Texas and the gulf states and when we look at a streamline chart…
…we can see that the gulf is wide open for business and this moisture should have no problems surging north. As the surface low begins to develop and the intense jet streak wraps around and heads into the midwest this should help pull all the ingredients northward that could potentially lead to some severe weather. At the very least folks across the region will enjoy a somewhat mild day with rain and general thunderstorms a solid bet.
Will the severe threshold be met and could Friday the 31st be another “surprise” off season severe weather event in a region that typically does not see it? It does seem possible, and certainly bears watching. Often times events of this caliber can go undetected by forecast models and even forecasters themselves, as they defy what we typically come to expect in the winter. Only by understanding past events, and carefully observing how future and current events pan out can we better our forecasting skills. Regardless of what happens, this is certainly an event that bears watching. Oh, one last odd, yet fun comparison for you all. The SPC has noticed everything above and introduced severe probabilities for Friday as shown below. 
Sure, it doesn’t look like much…but consider the outlooks for January 7th 2008 and December 27th 2008 and yet again, some similarities can be seen. 
In conclusion: Will history repeat itself and will the MO/IL area be rocked by severe weather to close out 2010? Or will this scenario succumb to climatology and not materialize? Only time will tell. Regardless of what happens, we here at Convective Addiction wish you all a safe and happy new years!
Look for a more detailed and traditional forecast about the setup coming soon.
-Adam
