CLASSIC GFS LOOK FOR COLD AND STORMY...
Don't know how it will turn out but the GFS and its ensembles are trying to tell us something about another shot of cold air to start December. If you saw last nights post, I focused on the mixed messages regarding this trend in the available teleconnections. You can read the full post HERE but the conclusion I came to was this: When you see discrepancies in models like this (no mojo on the MJO) you know the pattern is in flux and it's a good idea to be cautious in the long term. I still think the EPO is going to drive the pattern this winter so as long as that is neutral to positive, I don't see the harsh cold getting back into the pattern for any extended period of time until after Thanksgiving.
OK, so what's the message (or trend) I've picked up on today. It's that the pattern is still in flux but I'm seeing signs on several models that the EPO (eastern Pacific oscillation) is set to go back into negative territory in early December. That is a cold signal. Below the GFS ensemble EPO forecast through December 6th.
Additionally, the MJO (Madden Julien Oscillation) associated with the GFS is also in sync. It rolls through phase 2 for about 7 days (that's a known mild phase this time of year), and then cuts back into 7, 8, and 1 (cold phases) about the time the EPO shows its crash. This type of progression would enable the negative phase of the EPO to develop. I very much like the consistency within the ensemble.
To give you an idea what the MJO depicts, phase 2 on the MJO this late in the month would be a blend of it's November/December temperature correlations which look like this.This is the short term trend.
When we get to phases 7, 8, and 1 which looks to be where we are heading, it's a complete reversal. Take a look at these temperature correlations in the month of December. The long term trend.
Another factor which has come into play is the GFS based AO (Arctic Oscillation) is also projected to go negative about the same time as the EPO.
The negative AO in conjunction with the EPO and MJO leads to a pattern that looks like this over North America. Essentially the Midwest gets cold.
Now the straw that could break the camels back! When the AO goes negative, it's a sign that the westerlies are weakening. Without their shearing effect it makes it easier for cold air masses to leave their source region (which is the Arctic) and dig into the U.S.
We also know that we're in a solar minimum (for sun spots) and that can open the door for what we call stratospheric warmings. That's something you look for to get the polar vortex dislodged and relocated over the northern U.S. Now here's the kicker, and this could be really big with major implications. When the AO goes negative it allows the heights to rise over the polar regions.That is forecast to happen on the GEFS in early Dec. This is at 10mb the highest reaches of our atmosphere. The heights are greatest near the pole.
When the heights go up, that forces the cold air that's normally there to be forced or displaced south. These are the 10mb temperature anomalies and you can see that relative to average, it's warmer in Siberia and the North Pole than it is over Florida and the entire United States!
To be clear, if the GEFS is right a stratospheric warming is underway and we could be in for some really frosty weather in some part of early December.
Not only that, the 500mb pattern (jet stream) is projected to look like this. That huge ridge poking into Alaska (in red) is the manifestation of the negative EPO and that looks COLD! Not only that, the jet is in a position where the pattern could be stormy and snowy.