Wednesday Morning: Evidence of More Stratospheric Warming & It's Effect on December Weather

Good morning once again everyone. As we all experience these bitter cold temperatures, we are getting a taste of what can happen when warming occurs at the highest levels of the atmosphere. Many factors contributed to this cold outbreak, but a big driver was the warmer than normal temperatures we saw build up in the stratosphere (very high level of the atmosphere) late last month. As a reminder, when the stratosphere warms, it compresses the troposphere below it because warm air expands and the atmosphere cannot escape into space. When you compress the air below it causes the air to cool. In addition, it puts a tremendous amount of stress on the polar vortex which is basically a constant low pressure system over the pole where the coldest air in the northern hemisphere is held. At times, if the warming is significant enough it can cause this vortex to get knocked off it's axis or even split in to multiple parts. This is the phenomenon that causes major cold out breaks throughout winter. 

The warming that occurred in late October/early November was impressive, but was not classified as a Sudden Stratospheric Warming. A Sudden Stratospheric Warming in simple terms is stratospheric warming on steroids and is what normally causes this vortex to split. What is my point in explaining all of this? Well evidence is building that we will have a least another episode of stratospheric warming develop over the next few weeks. Whether it is significant enough to be Sudden Stratospheric Warming remains to be seen. In my Winter Forecast and my post back on November 3rd I discussed some of the drivers that increase the probability for stratospheric warming to occur. Some of these include, low sun activity which increases ozone in the upper atmosphere and wind patterns that favor the transport of heat from the tropics to the poles.

Lets start off by taking a look at the current transport of heat..

Just look at the red line and where is its in comparison to the green dotted line. If it is above the green dotted line that means there is an above normal transport of heat. This is the first clue that can tell us the stratosphere has the potential to warm because heat is being transported  up to the poles.

This is also supported by ozone continuing to build..

Now that we have a few factors established, we shift gears from current observations to model projections of strasopheric temps. 

Just focus on the area where you see the warm colors labeled by the "H". This is the projected temps currently in the first image and 240 hours from now in the second. Notice how the warm area expands and gets larger. This means the model is projecting the stratosphere to warm at this particular level of the atmosphere.  I can take you though the other levels but the message would be the same- models projections support the building up of warming.

So whats the bottom line for December? There is a 2-3 week lag from the time the stratosphere warms to its effect on our weather. Depending on the significance of this projected warming event will determine the severity but by the time we get to the 2nd week of December, overall warming favors below normal temperatures for most of our region. If warming event ends up being severe enough, it can lock in a prolonged cold pattern for winter due to the split in the polar vortex.

The fact we are seeing these factors develop so early this year is really building evidence that this winter might be no joke. In fact, I think we can all agree this current cold outbreak backs up that statement. 

A lot of forecasts are based on what the long terms models are projecting at the surface for temperatures. However, the model performance has been awful over the past month. According to the models, November was supposed to be a warmer than normal month. The opposite occurred and we are now flirting with one of the colder Novembers in history. That is why its important to look at other drivers besides the models. Looking at the stratosphere back in late October would not have led you wrong and I think looking at the strasophere now for Decmeber should result in the same. Lets see how this evolves in a few weeks.

Temps stay very cold until this weekend where we get a nice warm up. By the time we approach thanksgiving, things cool down again. I do not see any storm threats to jump on at this time. I expect the pattern to reload a little over the last week of November and even the 1st week of December. This means temperature can moderate (evidence of this supported by MJO) before round two of cold occurs around the 2nd week of December. 

Thanks for checking in.