FLIPPING THE SWITCH TO COLD...
I've saw some things on the charts Wednesday that got me to set up in the chair a little higher than I usually do. Before I get into it I do want to caution that these are nothing more than trends in their early stages. I never like to make long range forecasts off forecasts but if the evidence is strong I'll certainly pass it along.
The first thing I will throw at you are the 500mb jet stream heights. Right now in Alaska they are very low indicating the existing presence of cold dense air. In less than 2 weeks the GFS forecasts them to explosively rise. Look at the difference side by side. 488 heights today...576 February 12th. That's a rise in heights of 88 decimeters which indicates a dramatic increase in temperatures.
How dramatic you say? Look at today's readings. Normal highs on the north slope are below zero, today readings are as much as 33 below average. These are the departures found there today.
February 13th, the forecast departures are as much as 45 degrees above normal. That's a reversal of of about 78 degrees!
The old timers told me many moons ago to watch what they called the Alaskan connection as a guide to what temperatures would be like here in the Midwest, especially in the winter. If it's cold there, chances are its relatively mild here. It works the other way too. So, if the GFS is on the right track moving that cold air out of Alaska over the 7-10 days, it has to go somewhere and the pipeline runs through us.
The 18Z GFS shows temperatures running 35-40 below normal February 13th. Trust me you don't want to know how cold that is but numbers 15-20 are floating around for those who do. This is just a single run of a model and whether or not it's the best one on the planet it is going to change more than 10 days out. It's what I consider an option on the weather menu, at least for now.
On top of that, it's important to take stock in the fact this is the GFS we are talking about, well known for going on trips to fantasyland at long ranges. For that very reason I would not put this out there if I did not have support from other credible models.
That leads me to the EURO EPS ensembles which I can tell you have a similar look. Check this out on February 12th. The ridge over Alaska has replaced the trough that's been a long term feature and Arctic air is pressing into the Midwest.
You can see it in the temperature departures which are growing at the 5,000 foot level.
That look in the upper air pattern, especially the ridge in Alaska is your negative EPO (Eastern Pacific Oscillation)which is critical to getting cold into the Midwest and holding it for any period of time. So let's see what the models are showing for the EPO. They are all going from a very elevated positive phase to some degree of negative departure.
This is the EURO EPO ensemble.
The GEFS ensemble
The GEM ensemble
Just as important as the EPO is the bias corrected NCEP MJO (Madden Julien Oscillation) which is trending out of the warm phase 6 look towards 1, 2, and 3 which are much friendlier analogs for cold.
There's also the chance that in the transition to colder weather a snow system will travel through some part of the central Midwest next Tuesday. A decent snow of more than 5" is possible somewhere but it's a little early to focus on location right now.
The way I'll leave it for moment is that the evidence is mounting for a colder pattern to materialize (potentially one that could include Arctic air). This winter has burned me twice with teases of similar changes that never materialized. For that reason I'm classifying this as a strong trend that's well supported but not in terms of length and intensity. We'll see where things go in the next few days. (Bottom line this is not yet a slam dunk, more like a mid to short range jumper)! Roll weather...TS