4:00 PM CDT | November 13, 2017
The Arctic Oscillation is neutral and is likely to drop into negative territory soon. Some of the guidance shows it dipping well into negative territory several days from now. There is often a lag between when it dips and when we see the effects of the dip. What could this mean later this month? As discussed at the start of the month, when the AO becomes negative, it creates blocking WELL to our north, usually forcing the jet stream farther south. This often makes areas east of the Rockies very susceptible to Arctic air. The first graphic shows the Arctic Oscillation - what it has been over the last few months (black) and where the latest models are forecasting it to be (red). The second graphic shows the typical pattern when the Arctic Oscillation is in negative territory.
Since our weather pattern is still developing and we have not seen signs of it repeating, we are unable to use the recurring cycle theory. This means that we are limited to watching other factors such as the Arctic Oscillation and model guidance. I often look at the ensemble models when trying to see where the guidance is trending. Each ensemble model is slightly different, so some can pick up on features that others may miss altogether. If they are in good agreement, that can signal to meteorologists and forecasters that there is a good chance of something happening... or not happening. The following graphic shows what the GFS (American model) ensembles are predicting Saturday morning. The 500 mb level is the "steering level" of our atmosphere, and most weather systems follow these winds. Looking at the 500 mb charts give us insight into what features are in the mid-levels of our atmosphere. As you can see, they are in pretty good agreement for 5 days away.
It is a different story by the time we get to Thanksgiving morning. They are all over the place, but there is a trend toward an eastern trough. This is certainly plausible if we see the AO continue to drop (see second graphic above).
This last graphic shows the 850 mb temperatures on Thanksgiving morning. Some of the models show a trough digging farther west and some COLD air pushing into the Southern Plains. Now, it is too early to say what will happen around and after Thanksgiving. The purpose of this post is to discuss the trends with the Arctic Oscillation and some of the latest model guidance. This is where the value of the recurring cycle theory would come into play. If the pattern was set up and its cycle length determined, we would know which of these solutions could be thrown out and which would be possible. Until then, we monitor the weather pattern, the model guidance, and what impact the Arctic Oscillation has on the jet stream and our weather later this month. You can get the latest on the blog and Facebook page.